An Optical Pattemator For Quantitative And On-line Spray ... · extinction measurements to evaluate...
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An Optical Pattemator For Quantitative And On-line Spray Diagnostics by Rama Deljouravesh A thesis submitted to the Department of Mechanical Engineering in conformity with the requirements for the degree of Master of Science Queen's University Kingston, Ontario, Canada October 1997 copyright O 1997 R DeIjouravesh
Transcript of An Optical Pattemator For Quantitative And On-line Spray ... · extinction measurements to evaluate...
An Optical Pattemator For
Quantitative And On-line Spray Diagnostics
by
Rama Deljouravesh
A thesis submitted to the Department of Mechanical Engineering in conformity with the requirements for
the degree of Master of Science
Queen's University Kingston, Ontario, Canada
October 1997
copyright O 1997 R DeIjouravesh
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Abstract
Quantitative measurements of the spatial distribution and symmetry of rnass
concentration and flux delivered by spray atomizers is valuable in many industrial
applications which involve sprays and spray processing. Such measurements are
motivated by engineering research, quality assurance in the manufacture of noules.
and monitoring and control of spray processes.
Although a number of mechanical and optical pattemation techniques for the
quantitative measurement of spray patterns have been devised and used, none are
thought to be suitabie for use in on-line monitoring of sprays. This work covers the
development of instmmentation hardware and software for quantitative analysis of
spray pattems baszd on prior theory. The pattemator uses light scattenng and
extinction measurements to evaluate the uniformity and symmetry of the liquid
distribution within a spray, and provides a high-resolution, non-intrusive, and
quantitative measurement of spray patterns that can be carried out in a quick.
automated, and low-cost fashion.
Initial test results show that the system displays good performance under
repeatability criteria and therefore has potential for industrial use in quality assurance
and monitoring of spray processes.
Acknowledgments
I would like to thank rny research supervisor Dr. R.W. Sellens of the
Department of Mechanical Engineering for his guidance. encouragement, and financial
support through the duration of this project. A special thanks goes to Mr. G. Wanz of
the Department of Mechanical Engineering at the RMC whose constant friendly advice
was vital to the completion of this project.
I would also like to thank Mr. O. Oosten, Mr. A. McPhail, Mr. A. Pappas. and
Dr. J. Garner in the Mechanical Engineering Department for their technical suppon. as
well as Mr. D. Bouma and Ms. K. MacKinder of the Physics Department for providing
me with the various pieces of test equipment required during the early stages of this
project.
I wish to express my sincere gratitude to my fnends and colleagues S. Allicock,
and S. Langstaff as well as al1 of my office-mates in the past three years for their
friendship and support.
Finally, I wish to thank my mother and R. Goodwin without whose love and
suppon this work may have been completed long ago! Thanks for believing in me.
This research was financially assisted by Queen's Graduate Awards. a Dean's
Scholarship, and NSERC research grants held by Dr. R.W. Sellens.
Contents
ABSTUCT ............................................................................................................ II
......*......... .... ACKNOW LEDGMENTS ......... . r . . o . . . . . . , . . . . . . III
CONTENTS ............~.............................................................................................. IV
LIST OF FIGURES ..... *....................... ............................................................ VI 1
NOMENCLATURE ..................................................................... .... .................... LX
................................................................................................ Backgroztrrd.. f
7 Spray characterizalion by patternation.. ....................................................... - Objectives of pattemation meanrrements .................................................. 3
..................................................................................... Research Objectives 3
........................................................................................... Thesis over-view 6
.................................. ..................... ................... CHAPTER 2 ......... .................. 7
............................................ CURRENT TECHNIQUES FOR SPRAY PA?-iERN ANALYSIS 7
................................................................................. 2. I Mechanical techniqzies 7
2.2 ûpzcal&-bared nzec~st~rernen~ techniques.. ................................................ I 2
............................................................................. 2.2. i Laser Dïfiac~orneters 13
.................................................................... 3.2.1 Phme Doppkr mzernomrtt y. I 5
................................. 2 - 2 3 Photogrnphic c r r d holopphic itnctgitzg techriiqzc<cs 18
........................................................................ 2 2 . 4 Laser i@t sheet imag-ng 1'1
7 7 2.2.5 Proof of coitcept for the optical paiterizator ............................................ --
....................... CHAPTER 3 ... ........................................................................... 24
...................... LASER LIGHT SHEET IhWGiNG AND NON-UNtFORMITY CORRECTIONS 34
................................................... 3.1 Orr scatterirzg nbsorptioil . arld extinctior 24
...................................... 3.2 A formal approach for rioti-unflorrnity correctiorz 28
............................................ 3.3 Non-unrfotmity corrections in polar geornetry 30
................................................... 3.4 Prrsprc t ive currrcliotz ......................... .... 36
........................................... 3.5 image ar~aiysis fo i. paiierrlaiio~~ rneosiiremerzts 40
............................................................. ....................... CHAPTER 4 ...................... 43
...................................................... EXPERIMENrAL APPARATUS AND PROCEDURES 43
......................................................................... 4.1 Erperimental orrangerner~t 43
.................................................. 4.2. i Light source and sheet producing optics 461
............................................................................. 4.2.2 Optical detector array 50
.................................................................................. 4 7 . 3 CCD video cornera 52
........................................................................... 4 2 . 4 Presslrre-srvirl atomizer 53
............................ ... 4-25 PC- based instmrnentatiort and &ta acqziisitiotz ... j j
....................... CHAPTER 5 ... ........... .......... ......................................... 57
..................................................................................... ~ S L J L T S AND DISCUSSION 57
..................................................................................... 5.1 Over-view of tests 37
................................................................. . 5 2 Remlts siimmary and discussion 58
............................................................................ 5.3 Instrxmentatioir so f ~ a r e 67
........................................................................ 5.4 Geometric traizsform rrrors 72
................... BiBLIOGRAPHY ... ............ .... .................................................. 80
APPENDIX A . DESIGN DRAWLNGS ........... ..................... ...................m........ 81
............ APPENDlX B . PERFORMANCE AND DESIGN SPECLFICATIONS 86
APPENDIX C O TEST SUMMARY ............................................... ... 89
. ....................................................... APPENDIX D SAMPLE CALCULATION 90
VITAE ................ ........... .............................................................................*....... 92
. .
List of figures
Figure 2 . I Typical graduated vessel arrangement for the measurement of circurnferential
.......................................................................................................................... patternation 121 Y
................................... Figure 2.2 Schematic of collection vesse1 divided into sectors and annuli 9
................................... Figure 2.3 Probe layozrt in a high-pressure mechanical pattemator [6/ 10
Figue 2.4 Array Oextractive probes in a high pressure mechanical pattemator [6/ ............... 10
........................................ Eigire 2.5 Drflractometer optical arrangement [9/ .. ........................ 14
....................................... Figure 2.6 Optical arrangement for Malvern-based tomography [9] l j
............................................................ Figure 2.7 Optical arrangement of a PDA systern [I 21 l 7
Figure 2.8 Eurmple of spray pattern skewing due io non-iintform illzrmination (light enfers
the scattering zone from bottom of the image) [17J ................................................................ 21
......................................................... Figure 2.9 Cornparison of optical and mechanical radial -73
...................................................................... and cirarmferential patternaîion reszilts [19.20/. 23
............................................................... Figure 3.1 Single scattering event ..................... ... 23
Figure 3.2 Schematic of Cartesian geomrtry optical patternator by Wang et al . [I 7/ ............... 29
...................................... Figure 3.3 Application of conservation of energy to a control vohime 30
........................................................ Figure 3.4 Polar discretization of the scattered light field JI
.................. Figure 3 . j Fonvnrd transmission . extinction . and scattering along a radiai sector 32
......................................................................... Figure 3.6 Side vlew of the optical focal plane 37
Figure 3.7a Perspective corrected (cropped Image) Fipire 3 . 76 Original image .................... 38
......................... Figure 3- 8 a and b Cornparison perspective-corrected image with the original 40
Figure 3.9 Schematic of'sectorization for circumferentinl patternation measurements ............. -41
............................................. Figure d l Side-view schematic of the experimental arrnngement 44
Figure 4.2 Top-view schematic of the radial optical detector array ................................ .. . . 45
................................................................ Figure 4.3 Erperimental apparatus .................... .. 46
...................................................................................... Figure 4.4 Data acqziisition compter 47
............................................................................. Figure 4.9 Vertical woter spray wind tunnel 47
....................................................................... Figure 4.6 Laser and g l m rod seticp ... ........ 49
Figure 4 7 Opticnl deiector nrray and CCD camera mozinting .................... .... ................ jl
Figure 4.8 FIo w arrangement in a ppical pressure - i l atomizer (1 2/ ................................... -14
Figure 4.9 Mtrogen tank and wafer container .................................................. .... ................ 3
Figure 5.1 Sprq pattern from a rnalfunctioning 2.50 60 O.4 nozzle. ........................... .... ............. 5 9
Figure 5.2 Spray pattern from a 3.00 60 ' A nozzle .................... ..... .................................... 59
....................................................................... Figure 5.3 Spray pattern before n o d e rotation 60
Figure 5.4 Spray pattern afrer rotation a 180 Orotation of the nozzle .......................................- 60
Figure 5.6 Repeatability testing for 2.75 80 "A nozzie .................... ... .............................. 62
Figure 5- 7 a) Background subtraction image from rorational repeatabiiity test .
6) Background subtraction image from repearabiiity with time test .......................... ..... ...... 63
Figure 5.8 Obsntration e#ects on spray pattern from a 3.00 60 "A nozzle ............. .. .............. 64
Figure 5.9 Spray pattern of 3.00 60 "A nozzle wirhozir obscnration .......................... .... ............... 64
Figure 5 10 Eflects of intensiîy stnatiovls along rhe light sheet ..................... ....... ................ 65
Figure 5.1 I Glnss rod is adjrrsted to reduce striation ..... .. ........................................................ 66
Figrrre 5.12 Radiai distribution from 2.75 80 * A nozzle at I O0 PSI Iine pressure Cfrom image
in Figure 5.5). ........................................ .. .................................................... .... ..................... 67
Figirre 5-13 Radiai distribution fvam 3.00 60 O A nozzle at 70 P . S.I. line pressure Cfrom
................................................................................................................ image in Figure 5.9). ri 7
Nomenclature
Pattemation Index
Minimum/Maxirnum collected volume per sector
ratio
Spray Uniformity Index
Extinction cross-section
Absorption cross-section
Scattering cross-section
Incident light intensity
Attenuated light intensity
Turbidity
Light path length
Droplet number density
Mean extinction efficiency
Droplet area mean diameter
Incident light power per angular strip
Attenuated light power per angular strip
Local light power per angular strip
Total scattered power per sector
Sum of pixel values per sector
Geometric correction factor per angular strip
Particdate surface area concentration per unit
volume
Target distance from video camera
Target size
Subtended angle with respect to the near side of the
target and the target bisector
Subtended angle with respect to the far side of the
target and the target bisector
Normalized total volume collected per sector
Normalized average volume collected over al1 sectors
TotaI number of sectors
Chapter 1
Introduction
1.1 Background
Atomization is the process by which a volume of liquid is convened into droplets. The
use of atomizers (noules) and sprays can be seen in industnal applications such as :
Combustion processes. such as industrial fumaces. gas turbines. and reciprocating
interna1 combustion engines.
Spray processing industries. with applications in evaporative cooling, spray coatins.
injection molding, and spray drying.
Agriculture (crop spraying with pesticides).
In the above instances the fùnctions of a novle can be outlined in three ways :
Delivery of a precise amount of liquid at a predetermined time (this is especially true in
automotive applications [ 1 1).
Disintegration of bulk liquid into droplets.
Distribution of liquid in a specified manner (Le. hollow-cone or solid spray structure.
with a uniform and symmetric spray pattern).
Spray novles accomplish the conversion of liquid from bulk into drops in various
ways. They use either a high velocity differential between the dispersed phase (liquid) and
the continuous phase (surrounding gas), the liquid's own kinetic energy. or a combination
of both to disintegrate the liquid. which proceeds from the nozzie exit into fine dropiets
The former approach is seen in air-blast atomizers. whereas the latter is used in pressure-
swirl, and rotaiy atomizers. An example of the combined approach is seen in fuel injection
systems used in modem gas turbine engines, where a pressure-swirl pilot nowle is used to
initiate efficient atomization and combustion dunng startup and low load mnning
conditions (when the Fuel flow rate is low), and under normal engine loads (high fuel flow
rates) most of the fuel is then diverted to an air-blast atomîzer.
1 -2 Spray characterization by pattemation
The measurement of the uniformity and symmetry of the liquid distribution in a
spray is referred to as patternation. Patternation measurements are made radially and
cicumferentially. In the latter case a rneasure of the uniformity and symrnetry of the liquid
distribution about the periphery of the spray is obtained, whereas in the former the liquid
distribution uniformity is measured as a tùnction of the radial distance from the noule's
axis of symmetry . The statistical figures O btained from circumferential and radial
pattemation measurements are used to characterize the overall quality of the spray pattern-
The basis of these statistical figures of merit and how they relate to the spray patterns are
discussed in Chapter 3 .
Spray patternation measurements are most commonly obtained through mechanical
patternation techniques. These techniques will be discussed in more detail in Chapter 2,
1.3 Objectives of pattemation measurements
The objectives for making patternation measurements are three-fold :
Research and development.
Quality control, and detection and reduction of manufacturing defects.
On-line, 211 sim state monitoring and control of spray processes.
The quantitative measurement of spatial distribution and syrnrnetry of the mass flux
and concentration delivered by spray atomizers is of importance fiorn the point of view of
industrial research and development. In gas turbine cornbustors for instance. atomization
is required for stable ignition and combustion. Once the fuel is atomized, its surface area
increases drastically. therefore increasing evaporation rates. The evaporated fuel can then
mi. with the intake air in the desired equivalence ratio to prornote efficient combustion.
Improper distribution of the fuel through poor pattemation will result in the formation of
fuel iich zones within the combustor, in which high levels of particulates (soot) and
unbumed hydrocarbons are found. This decreases the combustion efficiency (since not al1
of the fuel is bumed) and increases the pollutant emissions through production of exhaust
smoke and unbumed hydrocarbons. Alternatively, in the fuel lean zones (created by poor
pattemation) high temperature regions are found in which oxides of nitrogen are
produced, resulting in higher pollutant ernissions. The existence of such "hot spots" is
also darnaging to the combustor liner and the turbine blades. Pattemation, which is a
global spray characteristic, can therefore contribute to the quality of combustion. In
combustion research, the measurement of global spray characteristics such as the
injection-timing. transient response. spray targeting (alignment). spray cone angle. and
pattemation. are performed with the goals of increasing the combustion etficiency while
reducing pollutant emissions.
In quality control, the objective of perforrning quantitative pattemation
measurements is to see to what extent a nozzle approaches a given standard or ideal
behavior, and hence to detect manufacturing defects. For instance. it has been
demonstrated that the inside surface finish of the final discharge orifice. and misalignments
between the swirl chamber and the final discharge orifice can have severe effects on the
spray pattern produced by pressure-swirl atornizers [2 , 31. Such problems can be detected
through patternation measurements at the time of manufacturing. The detection technique
should be quick, so that production is not slowed down, and automated and low-cost. so
that the need for a human operator to perform visual inspection of spray patterns is
removed, hence allowing for a more consistent measurement and defect detection.
Quantitative spray charactenstics such as the patternation index, minimum/maximum flow
per sector ratio, and the spray uniformity index can be used in an automated environment
to provide better and more consistent defect detection.
On-line and periodic monitoring of spray processes (such as paint sprays) is required in
order to asses the performance of a particular nozzle, given its intended application, and
hence to provide guidelines for the replacement intervals for noules. This is critical since
various nozzle orifices do tend to deteriorate with time and usage. In spray processing
applications such as spray painting, these factors can result in non-uniformities in the spray
pattern, which will result in non-unifonn spray coatings (Le. added production time and
costs). The method of monitoring should be on-Iine and non-intnisive, so that the
operation of the noule is not interfered with and production level is not reduced due to
off-line nozzle testing procedures. The same requirements for automated and low-cost
operation (of the monitoring technique) also exist here, for essentially the same reasons.
1.4 Research Objectives
The objectives of the present research were to develop an optical pattemation system
which has potential to address industrial quality assurance, and state monitoring and
control concems. In keeping with the requirements of the tasks to be performed. the
design criteria for the system to be developed were as follows :
Good spatial resolution in the rneasurement, to allow detection of small localized spray
behavior. such as streaking.
Quick rneasurement and analysis.
Low system costs.
Minimum input from an operator, so that the testing procedure c m proceed
automatically.
A non-intrusive measurement technique to allow on-line operation and applicability for
monitoring and control situations.
1 -5 Thesis over-view
Current techniques for making pattemation measurements are discussed in Chapter 2 .
A new laser-based optical spray pattern analyzer has been developed to overcome the
disadvantages of conventional pattemators. The pattemator uses light scattering and
fonvard attenuation measurements. to evaluate the syrnmetry and uniformity of the liquid
distribution within a spray. and to provide a non-intrusive. high-resolution. cost-efficient.
and quantitative measure of spray patterns that can be carried out quickly and
automatically. Fundamentals and the theory behind the determination of the extinction
cross-section, based on forward extinction measurements will be discussed in Chapter 3.
In Chapter 4, the experimental arrangement and the various pieces of hardware and
software used in the construction of the optical pattemator. have been presented. Radial
and circumferential pattemation results for a selection of semi-hollow cone nozzles. tested
using this technique are shown in Chapter 5 and a discussion of these results follows. This
optical pattemation technique is capable of providing reliable spray pattern analysis and
figures of ment for a given spray. and the approach, being optically-based and non-
intrusive, shows great potential for use in manufacturing quality control and automated.
on-line monitoring of spray processes.
Chapter 2
Current techniques for spray pattern analysis
The two major methods of making quantitative spray pattern measurements are
outlined and discussed in the following sections. Mechanical. or intrusive techniques
require the insertion of extractive probes or collection vessels in the flow field of interest.
The non-intrusive methods discussed here can be divided into two groups : imaging
techniques, and those based upon light difiaction and scattering. The potential OF each
technique for achieving on-line monitoring tasks is assessed based on its merits and
shortcomings, and in keeping with the design criteria mentioned in the Iast chapter.
2.1 Mechanical techniques
Mechanical techniques were the first to be developed and used. Mechanical
patternators have been used for decades to obtain spray patterns by collecting the sprayed
liquid, in pan or whole, into partitioned collection vessels or arrays of extractive probes.
Liquid volume (or mass) collected by the individual extractive probes or the vanous
sections of the collection vessel, over a given perïod of time, is then measured to
determine the spray pattern based on the localized liquid volume (or rnass) flux.
Figure 2.1 Typical graduated vesse1 arrangement for the measusement of circumfercntial pattcrnation 12 1.
In the case of sectorized collection vessels, such as shown in Figure 2.1, the entire
liquid spray is collected by the pie-shaped radial sectors of the collection vessel and a
measure of the circumferential pattern is obtained along with maximum and minimum tlow
per sector [3, 41. The criteria for the calculation of the unifomity and symmetry of
circumferential distributions are outlined by Tate 131, and will be discussed in more detail
in Chapter 3. The number of secton typically varies frorn 6 to 12. depending on the
manufacturer, however it has been shown that a larger number of sectors creates a more
strict measure of the circumferential pattemation by providing better spatial resolution [3]
To allow measurement of the radial liquid distribution in sprays, the sectonzed
collection vessel arrangement has been modified through the addition of equal area annuli.
The typical number of annuli in this arrangement, shown in Figure 2.2, is 4 [SI. The liquid
volume collected by each collection bin (Le. A I , B 1, C 1, etc.) over a given penod of time
is then compared with other bins and the liquid volume (or mass) flux distribution is found
as a function of the annulus number (radial patternation) and the sector position
(circurnferential pattemation).
Collection vessel arrangements shown in Figures 2 1 and 2.2 are not suitable for use
with air-blast atomizers because of the splashing of the fluid collected in the collection
vessels and recirculation of the flow field caused by the collection vessel itself which can
alter the spray pattern, distorting the measurement results.
Figure 2.2 Schematic of collection vessel divided into sectors and annuli.
Pattemation testing of air-blast atornizers is better accomplished with extractive
probes, which collect only a portion of the liquid contained within the spray. Extractive
probes can be arranged in arrays, such as shown in Figure 2.3. The array can then be
rotated about its a i s in order to provide a measure of the circumferential liquid
distribution and pattemation. This measurernent technique also ailows for a direct
measurement of the radial and circumferential liquid volume and mass flux.
Figure 2.3 Probe layout in a high-pressure mechanical patternator [6] .
Figure 2.4 A m y of extractive probes in a high pressure mechanical patternator [6 ] .
Although as can be seen in Figure 2.3, the spatial resolution of the measurement
is improved over the graduated vesse1 arrangements of Figures 2.1 and 2.2, the extractive
probe array is still not capable of very high spatial resolution measurements. The appeal
of mechanical patternators is in t heir availabiiity. since mechanical patternat ion techniques
have been in use for many years.
Mechanical pattemation measurements, as mentioned earlier, are extractive and
intrusive by nature and present disadvantages because of the practical problems
encountered in noule testing. which make them unsuitable for use in monitoring and
control applications, for instance :
Insertion of mechanical collectors affects the flow upstream and downstream of the
collectors. Careful control is required to minimize perturbations to the two-phase flow
tield due to the presence of the sampling probes. This is important especially in dealing
with air-blast atornizers where perturbations made to the carrier phase (gas) by the
sampling probes and the collection vessels will cause drastic changes in the spray
pattern. causing it to be distoned. To minimize the effect of the presence of the
sampling probes isokinetic sampling techniques are used [6, 71. Isokinetic sarnpling
techniques usually involve the elimination of the pressure or velocity differential
between the test section and the collection tubes, through use of numerous sensors and
electro-mechanical valves 171, which can be time consuming and cost intensive to
render in design. From the point of view of instrumentation and automation.
mechanical patternators have proven to be extremely cumbersome.
Limited spatial resolution is offered by mechanical techniques. This cm effectively
mask-off and hide fine localized spray non-uniformities, such as streaking.
On-line operation is not feasible due to the intrusive nature of the measurement.
Measurements are slow, as enough liquid needs to be collected over time and
sometimes the collection probes need to be rnoved during the rneasurement (such as
wouid be the case with collection probe arrays). This problem is more evidenr when
deaiing with low density sprays.
2.2 Optically-based measurement techniques
Sampling errors caused by the presence of collection vessels or the extractive probe
amay and the necessity for isokinetic sampling are avoided through use of optically-based
techniques, which do not require the insertion of mechanical collectors in the flow field.
The biggest advantage of optically-based systems is in their applicability in on-line
situations, where atomization can be studied and measurements can be made in real
situations. This is of particular benefit in perfonning diagnostics in spray combustion
processes, which are often accompanied with high pressures and temperatures.
Optical techniques are also referred to as indirect techniques. as the obtained
measurernents are not based on direct mechanical measurements of the local mass flux.
Major advances in the areas of digital and optical instrumentation have been made in the
past few decades. Signal collection and analysis is largely handled by cornputers, hence
facilitating the task of automation. Commercially available lasers have made it easier to
measure velocity, temperature, species concentrations, and particle size and have al1 but
replaced extractive probes for measurernents in single-phase and two-phase turbulent
flows.
Optically-based measurement techniques can be divided into two categories
Imaging techniques such as flash photography, pulsed holography, and laser
imagine.
light sheet
Techniques based upon the principles of light diffraction (MastersizerO by Malvern
Instruments) and light scattering ( L D 4 PDA).
Imaging techniques allow for the measurement of the particle size. size distribut ion.
and velocity through image analysis. Light difiaction and scattenng techniques allow for
the determination of the drop size andfor velocity distnbution in a spray through
measurements of propenies of the scattered light such as intensity. phase and frequency.
and extinction. The particle size and/or velocity. are determined indirectly through signal
analysis. and extensive computation.
The importance of particle size measurement (offered by some optical methods) in two
phase-flows can not be over-stressed. In some medical applications sprays are used for
dnig delivery via the patient's respiratory tract through the use of metered dosage
inhalers. To ensure delivery of exact amounts of medication the drop size distnbution
needs to be measured globally so that a knowledge of total volume or mass of the
delivered drug may be gained [8]. In spray combustion applications the governing
equations for various physical phenomena such as the fluid dynamics. chernical kinetics.
and particle dynamics al1 involve the droplet diameter as a parameter. The particle size
and distribution are required for the validation of most industrial designs or operations [ 9 ] .
2.2.1 Laser Difiactorneters
Malvem particle sizing and Malvern-based tomography allow for real-time
measurernent of droplet size and distribution, indirectly, through Iow angle laser light
scattering (LALLS).
Particle Beam f icld expander f
momchromic Fourier I
tight transforrn Detector in lems focal plane
of tens
Figure 2.5 Diffractomercr opticai arrangement [9 1.
The basic setup consists of an expanded laser beam which produces a diffraction
pattern afier falling incident upon the measurement zone. The diffraction pattern
generated by the drops consists of a series of concentnc fnnges of light. where the
undifiacted light is at the center of the plane defined by the fnnges (Fraunhofer
Diffraction). A Fourier transforrn (or range) lens is utilized to focus the difiaction fringes
ont0 a photodetector array, which consists of a number of concentric ring-diodes. The
determination of the drop size distribution is amved at through data inversion procedures
and rigorous solutions of the Lorenz-Mie equations [S, 101.
The technique of Malvern-based tomography, which is an extension of LALLS has
been developed for the measurement of drop size distribution and concentration in volume
elements within an axisymmetric region. The principle of operation is similar to medical
X-ray brain and body scanner (compter assisted tomography or CAT scan), where a
standard Malvem Particle Sizer is used instead of an X-ray tube. The measured scattered
light from the different regions of the spray are Abel transformed to provide a two
dimensional distribution of the droplet size in a cross-sectional plane across the spray [ 9 ] .
Figure 2.6 Optical arrangernent for Malvern-based tomography [9I.
LALLS. being an optically-based approach. is a non-intmsive and low-cost technique
allowing for real-time determination of volume (and mass) concentration, with reasonably
fine spatial resolution. through droplet size measurements. A full discussion of the
practical problems encountered in using LALLS is beyond the scope of this discussion. In
monitoring and control applications, an overall representation of the spray pattern is
sought through pattemation measurements. For this purpose pattemation measurernents
based on point by point drop size and volume distribution measurements (such as obtained
by LALLS) are far more detailed than necessary for simple state-monitoring and quality
control applications. A point by point measurement by using LALLS can give an accurate
representation of the spray pattern, but this quite tirne-consuming and therefore not
desirable in monitoring and control situations.
2.2.2 Phase Doppler anemometry
Ln the recent years PDA has been recognized as a robust and reliable method of real-
time. simultaneous drop size and velocity measurement. PDA is an interferornetric
particle size and velocity measurement technique. similar to LDA. in which two non-
parallel laser beams originating fiom the same source (coherent) are crossed to form a
small measurement volume within the particle field of interest. At the intersection of the
two beams an interference fringe pattern is created. These fnnges are caused by
constructive and destructive interference fiom the two crossed beams. Drops which cross
the measurement volume traverse these interference fnnges and cause scattering. The
scattered intensity is best described as pulses of light or a "Doppler burst" which is
measured using two or more sensitive off-axis optical detectors. Once the measurement
volume is moved within a spray, droplet sizes and velocities can be obtained for an?
spatial location within the spray and From this the mass flux distribution is determined
The setup consists of a laser light source, transmitting optics, signal processors. and data
analysis and collection software.
Detemination of the droplet velocity is made through analysis of the Doppler burst
signal in frequency and tirne domains in conjunction with the known fringe spacing, which
determines the frequency of the Doppler burst signal and is related to the velocity
component of the particle in the plane of the fringes. The droplet size is determined bv
measunng the phase difference between signals taken at two or more locations, assuming
that the pariicle is spherical. This is based on the fact that the length of the optical path
followed by light scattered by a particle at slightly different angles is dependent on the
scattenng angle and the radius of curvature of the surfaces [ I l ] . This slight difference in
the optical path length (on the order of the wavelength of the laser light) will cause a
phase shifi in the received signals.
Pin photodiodes
Diode Ieser plate
Figure 2.7 Optical arrangement of a PDA system [ 121.
Although determination of the spatial distribution of mass flux can be useful in
research environments the detailed measurements provided by PDA are not necessary for
most quality assurance and monitoring and control applications in which an overall or
macroscopic representation of the spray pattern is often adequate. [n addition to high
cost, PDA systems display other practical problems, which rnake them not suitable for
quaiity assurance, and state monitoring and control applications :
As with LALLS, the size of the measurement volume is srna11 in relation to the total
volume spanned by the spray. This presents one of the practical drawbacks in dealin3
with PDA systems : documenting an entire spray is a time-consuming task. This is an
important factor since it immediately renders PDA systems undesirable for quality
assurance and monitoring applications. A coarser measurernent grid could be used to
reduce the time of measurement, but this has the disadvantage of effectivelv hiding fine
local spray structures such as streaks.
High operational costs in terms of computational intensity and the required signal
receiving and analysis hardware render PDA undesirable.
Most currently used PDA instruments are based on gas lasers which in addition to
being expensive, are also rather bulky and fragile. and create the danger of electrical
discharge through sparks from high voltage power supplies. This problem has been
addressed through the use of commercially packaged laser diodes. which are
inexpensive and small. but require optical corrections to be made to the incident beam
[l;].
2.2.3 Photographic and holographie imaging techniques
Photographic techniques were the earliest of the non-intmsive techniques to have been
developed and used. The development of LALLS and PDA has to some extent made
photographic imaging techniques obsolete. In double flash photography and particie
image velocirnetry (PIV) measurement of the particle velocity is made by companng the
relative position of individual drops within the focal plane of the camera lens as a function
of time. Droplet size and distribution are determined From the obtained photographs
through inversion (creating a negative) and automated image analysis. Pulsed holography
is an extension of flash photography, which allows three dimensional representations of
the flow field, as opposed to photographs in which depth and distance are not preserved.
The processing of holographic images is done in much the same way as with tlasli
photographs. with the exception that the viewing plane may be traversed in depth.
The main practical problem in dealing with photographie and holographic irnagin-
techniques is determining which drops are within the focal plane of the camera lens (i.e
are in focus) and hence need to be counted and measured. This problem is caused by
particles of larger diameter. which can appear in focus over a greater distance. To
minimize counting errors a large number of drops need to be counted. This means that
several pictures need to be obtained and the time of post-processing is long.
2.2.4 Laser light sheet imaging
The technique of laser light sheet imaging has been used in a wide variety of tlow
visualization applications to obtain qualitative information regarding the overall structure
of the flows considered. This technique requires the use of cylindrical optics to produce a
planar region of illumination From a laser beam. Objects crossing this region of
illumination scatter light in different directions through refiaction and reflection. The
scattered light can be captured and recorded by using a video camera. In dealing with
two-phase flows, such as sprays, the technique of light sheet imaging is frequently used
along with visual inspection of the general "shape" of the spray. Care must be taken to
avoid image saturation through proper choice of shutter speed and the lens aperture
setting. lncreasing saturation will result in a very high contrast image in which it is
difficult to see any of the spray stmcture. Laser light sheet imaging is a convenient and
reasonably low-cost method of obtaining spatially and sometimes temporally resolved
images of planar regions within a flow. This may not be sufficient in characterizini b
cornplex three dimensional flows, however for pure two dimensional flow-field
visualization (such as would be the case with most state monitoring and quality assurance
testing applications for sprays) light sheet imaging is often sufficient.
Qualitative analysis however, is not suficient for quality assurance and automated
monitoring applications where quantitative measurements are required for more consistent
measurements and better quality control. The technique of planar laser-induced
fluorescence (PLIF) has to some extent addressed this issue by providing quantitative
pattemation measurements based on the global mass concentration distribution. The
theory behind PLIF has been discussed by McMillin et al. [14]. A pulsed laser light source
is used as an excitation source, tuned such that its frequency coincides with the rotational-
vibrational transitional energies of a species of interest with which the spray fluid may be
doped. Upon excitation a transition back to the lowest allowable energy state takes place
and light with a frequency characteristic of the transition energy is emitted. This is
referred to as fluorescence. In general. the fluorescence intensity depends on the pressure.
temperature, species concentration, and the initial laser intensity.
This technique as utilized by Sankar et al. [15] and Arellano et al. [16] does not take
into account the illumination non-uniforrnities that exist within the planar region of
illumination. These non-uniformities are produced in two ways : by the light source itself.
and the scatterers in the path of the incident light. Because the light source (laser) has a
Gaussian intensity profile a Gaussian intensity variation is produced in the transverse
direction, which remains in the sheet even after collimation. The second cause of
illumination non-uniformities along the illuminated region is the shadowing effect caused
by the drops (scatterers). In the presence of scatterers in the path of the incident Light the
intensiry of the incident light decreases m the forward direction. This attenuation of the
incident iight mtensity in the forward direction is caused by scattering and absorption.
Cleariy, such non-unifonnities need to be accounted for before any evduation of the
local mass concentration based on the scattered light intensity can be made. Unfortunately
in ail past instances light sheet Unaging and PLE have been used without any formal
approach to correct for the presence of severe non-uniformities (in some cases) w i t h the
illuminated region. In high flow capacÎty situations (Le. al-blast atomizers) non-
uniformities caused by f o m d mtensity attenuation can be quite severe and result in a
skewed representation of the spray pattern (see Figure 2.8).
Figure 2.8 Example of spray pattern skewing due to non-uniform illumination (light enters the scattering zone Eom bottom of the image) [lq.
The basis of operat ion of the opticai pattemator which is presented in this work is to
"correct" the scattered light intensity for illiimination non-uniformities, through
measurements of the forward attenuation znd CO rrection calculations based on Iocal
illumination values
2.2.5 Proof of concept for the optical pattemator
A forma1 approach for non-uniformity corrections in planar regions of illumination has
been developed by Wang et al. [17]. The approach is based on the conservation of energy
principle and the application of the control volume fonnalism to a planar region of
illumination. Optical pattemation results have been compared with rnechanical
measurements obtained under identical spray conditions. The results of this cornparison
are shown in Table 2.1. The spray patterns in al1 instances (rnechanical and optical
pattemation) have been characterized on the basis of the pattemation index (P.I.) and the
MinMax collected volume per sector ratio [2], as well as the spray uniformity indes
(S.U.I.) [18]. By companng the P.I.. MinMax, and S.U.I. it can be seen that good
agreement exists between results from mechanical pattemation measurernents and
pattemation results from analysis of spray images obtained through this technique [S. 18.
19. 201. This concept and its application in polar geometries are described in detail in
Chapter 3 . The approach is essentially identical to that used by Wang et aI.[17] in
Cartesian geometnes.
Table 2.1 Cornparison of Pattemation Parameters [ 19. 20 1.
1 Patternaior 1 P.[.(%) 1 M i M a u Ratio
Mec hanical atP& W
Mec hanical at RMC opticai al RMC
8.36
8.75
73.20
70.58
12.10
- - - -
9.87
11.15
12.13
73.13
0 Mechanical Pattemator at P & W 8 Mechanical Pattemator at RMC
Optical Pattcmator at RMC I
A B C D E Sector
Figure 2.9 Cornpanson of optiul and mechanical radial
and circurnferential patternation results [ 19.20 1.
Laser light Sheet irnaging and non-unifomity corrections
In the following sections, a e r a brief introduction to elastic scattenng and concepts
such as extinction and absorption, an analytical approach is described, which enables
quantitative pattemation measurements to be made based on scattered light intensities
along a laser-illuminated sheet by "correcting" and accounting for the effects of
illumination non-unifonnities present within the light sheet. This technique makes use of
measurements of the forward light power attenuation to eliminate the effects of
illumination non-uniforrnities and calculate the corrected scattenng cross-section.
3.1 On scattering, absorption, and extinction
A discussion of inelastic scattenng (the type of scattering where the frequency
component of the scattered light varies fiom that of the incident) is irrelevant for the
purposes of this work, since there is virtually no interaction between the incident Iight and
the scattenng drops on the sub-molecular level. Hence discussion of scattering in the
following sections is limited to elastic instances. The quantity of interest in characterizing
spray patterns is the intensity of the scattered light, since this is a direct measure of the
drop surface area concentration, as will be explained shortly. For this reason a discussion
of the phase relations of the scattered light has been ornitted.
abject Light Source
Figure 3.1 Single scattering event.
In a single scattering event the incident light is attenuated in the direction of
transmission. By extension, the intensity attenuation of light traversing a medium
consisting of particles of arbitrary size is referred to as extinction. This attenuation is
caused either by scattering or absorption, or a combination of both. Scattering and
absorption remove energy frorn the incident light, causing its intensity to be attenuated as
it traverses the medium. The scattered intensity varies along different directions of
observation and the total scattered power is expressed in terms of the product of the
incident light intensity and the scattering cross-section, which is representative of the
particulate cross sectional area through which the incident light is scattered. In a similar
manner the total absorption of light is determined through an absorption cross-section.
which is representative of the particulate cross-sectional area through which the incident
light is absorbed. Application of conservation of energy would necessitate that the
summation of these two cross-sections for a given reference area be equal to the
extinction cross-section, corresponding to the same reference area, as defined by :
C extinction=Cwttcred+Cabsorbcd
In other words, the difference of energy between the incident and transmitted beams must
equal the surn of the scattered and absorbed energies. The relative magnitude of each of
these parameters depends on the particles' physical dimensions and propenies Light
liquid hels and water-based solutions and slumes, often used in most spray processes. are
largely transparent to visible and near-visible light, therefore it is reasonable to assume that
the efTect of absorption is negligible in dealing with such fluids [17]. The attenuation of
the intensity of the transmitted light in the fonvard direction (extinction) is therefore
caused by scattering alone.
This forward intensity attenuation is descnbed by the Beer-Lambert law [ 1 O]:
where Io and 1 are the incident and attenuated intensities, respectively, and their ratio is
referred to as the transmittance. L is the path length through the particulate medium and r
is variously referred to as the extinction coefficient, the turbidity, or the attenuation
coefficient. Turbidity can be represented in terms of the droplet area rnean diameter. DZ,,
and the droplet nurnber density, C. [17] :
where Qmi is the mean extinction efficiency and its value has been given for large
(compared to the wavelength of the incident light) sphencal scatterers by van de Hulst
[IO] as 2. Equation (3.3) can be re-written as :
From this, it can be seen that turbidity is directly proportional to the droplet cross-
sectional area concentration per unit volume, which is proportional to the total droplet
surface area concentration per unit volume. By extension of this, it can be seen from
equation (3.2) that transmittance varies inversely with the particdate surface area
concentration, hence for larger droplet surface area concentrations. less iight is transrnitted
in the fonvard direction, dong with more scattenng. The surface area concentration can
therefore be used as a measure of the spray pattern. The obtained measurement, while not
of the traditional spatial mass and volume flux type, is an equally useful and valid
indication of the spray pattern for monitoring and control applications. Although the
spatial distribution of volume and mass flux (obtained through mechanical measurements)
controls the heat release rate From the drops in the spray, the surface area concentration
controls the local evaporation, mass transfer, and thus reaction rates at the base of the
flame, which are equally important parameters in spray combustion applications. Optical
pattemation measurements provide valuable information regarding the overall spray
pattern, which can be useful in monitoring and control of most spray processes.
The Lorenz-Mie theory is the general theory for the evaluation of the extinction cross-
section. The exact solutions for the Lorenz-Mie theory have been discussed by van de
Hulst [IO]. The theory starts by obtaining solutions of Maxwell's elecrrornagnetic
equations with specific initial and boundary conditions under the plane wave assumption.
and arrives at exact solutions for cross-sectional efficiencies for individual scatterers. The
Lorenz-Mie solution predicts that the cross-sectional efficiencies for extinction (which are
indicative of the scattered intensity) depend on the particle size, the ratio of the refractive
indicies of the dispersed and continuous media, as well as the angle of observation.
Obtaining exact solutions for the Lorenz-Mie theory is computationally demanding, hence
geometric optics (or ray optics) are often used as an approximation with generally good
results for al1 but the smallest of drops. The solution of the geometric optics
approximation approaches the Lorenz-Mie solution asymptotically for larger drops. and
shows the same dependence on the particle size and angle of observation for the intensity
of the scattered light. Geometric solutions, which are much easier to obtain. require that a
sufficient difference exist between the refractive indicies of the continuous and dispersed
phases in the flow. This is ofien the case in sprays, and for this reason instruments such as
PDA are based upon geometric solutions of the scattering problem.
In the sprays encountered in industrial applications (such as in gas turbine combustors)
droplet diameters are generally large compared to the wavelength of the incident light
(632nm or He-Ne) with drops under 5pm representing a very small fraction of the total
number of drops [17]. Geometric optics can show that the scattered intensity is therefore
a function of the angle of observation alone independent of the drop size, since the effect
of absorption will be negligible [17]. This basic principie is an important factor in the
design of the optical pattemator as it indicates that the variations in the intensity of the
scattered light with changes in the angle of observation will be the sarne for al1 drops.
3 -2 A formal approach for non-uniformity correction
As indicated in the previous section, conservation of energy requires that the
summation of the absorption and scattering cross-sections equal the extinction cross-
section. To treat the non-unifonity problem in the illuminated region, through use of the
energy conservation principle. a knowledge of the incident and transmitted illumination
power is required. Given these two measurabie quantities. the extinction cross-section is
calculable.
The application of energy conservation to a planar region of illumination and
scattering of small thickness (equal to the diameter of the laser beam which is on the order
of 0.5 mm) is best done through discretization of the scattering plane. This technique has
been successfully employed by Wang et al. [17], while using a Cartesian discretization
approach along with a large collimator to produce a wide rectangular sheet. Initial
pattemation results obtained tlirough this technique were presented in Chapter 1 and
generally show good agreement with mechanical pattemation measurements which were
performed during the same study.
- Figure 3.2 Schcmatic of Cartesian geometry optical pattemator by Wang et al. [ 171
While the choice of an appropriate discretization scheme is dependent on the
geometry of the light sheet, the basic formalism of the approach remains the same
regardless of the discretization geometry. In the interest of cost reduction for the optics
and the elimination of alignment difficulties in the present approach the collimator has
been removed. The produced sheet fans out radially and necessitates the use of a polar
rather than a Cartesian discretization approach, along with a radial array of discrete photo-
detectors.
3 -3 Non-unifonnity corrections in polar geometry
The discretization approach, regardless of geometry, relies on the control volume
formalism in order to deterrnine the extinction cross-section, which is a measure of the
particulate surface area concentration.
A simple application of the control volume formalism is shown in Figure 3 .3 , where
Scattered '/. Controt. ~.oluriic
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
the sum of the scattered and forward transmitted light energies (system output) must equal
the incident light energy (system input). The control volume method can be applied to a
Iarger system, consisting of many smaller control volumes. The typical geometry is shown
in Figure 3.4, where the field of the light sheet has been divided into imaginary angular and
Incident Light
....................................
Figure 3.3 AppIication of conservation of energy to a control volume.
c , Fonvard transmission
radial sectors. Treatment of individual control volumes contained within this field is done
through analysis of images captured by a CCD (charge-coupled device) video canera. In
the present approach. due to the use of a 16-channel analog to digital converter 16 radial
and angular sectors were detined. Each of the 16 discrete detectors in the radial detector
array has been positioned so as to span 2 O angular displacements.
Spray region -,
Figure 3.4 Polar discrctization of the scattered Iight field.
Consider scattering and attenuation of the incident light in the forward direction, along
a single angular strip, as shown in Figure 3.5. The incident light power (Po) is attenuated
to P., after passing through the spray. This attenuated value can be measured by using
photo-detectors to give a time-averaged rneasure of the attenuated light power, hence the
total extinction across the angular strip.
f',, ,m--]-i;-rF] eV-
- - - - -
Figure 3.5 Fonvard transmission. cstinction. and suttering along a radial scctor.
From this total extinction and the image data, local transmittance values across the angular
strïp may be calculated. The overall transmittance for each angular strip consists of the
contributions 6om al1 radial sectors (control volumes) within the stnp. Application of
conservation of energy to a single sector (or control volume) in the strip yields :
where Sr,o is the total scattering power From the corresponding sector. and P,.U is the
transmitted local light power. In an application involving digitized images. the values of
pixels contained in each sector can be summed to represent the total local scattering
power from a sector, but since the angle of observation is fixed the carnera sees only a
portion of the total scattered light. To overcome this problem a geornetric correction
factor should be introduced, so that equation (3.4) can be re-written as :
where oie is the sum of the pixel values in the corresponding sector, which is
representative of the local scattering power. Hence for a single angular strip :
KI- The geornetric correction factor, serves a double purpose here. It represents the ratio
of the total scattered light from each angular strip to the portion seen by the CCD video
camera. a constant dependent only on the camera view angle and solid angle of collection.
It also functions as a conversion factor, since the forward light power rneasured bv the
photo-detectors is in units of V/W and the local scattered intensity is measured in S-bit
digits of arbitrary units, which can be read from the CCD image. The value of this
correction factor is purely dependent on the physical position of the CCD camera. so that
at different viewing angles different values for the correction factor would be found. This
is caused by the fact that the scatter of light is directionally dependent so that the intensity
of the scattered light varies with the direction of viewing.
The first step in correcting the scattered image is to sum the pixel values in the initial
image (Le. integrate the scattered intensities) along sector-wise defined regions within the
image. Special care must be taken to ensure preservation of the exact distance between
the cylindrical lens, which is the origin of the radial light sheet, and the view field of the
camera. Conversion from laboratory coordinates to pixel coordinates in the image
becomes a straight-forward task, with the origin of the sheet conveniently chosen as the
origin of the coordinate system.
A 1 6 x 1 6 rnatrk, representative of the sumrnation of the pixel values is obtained from
the image. These values are sorted in the order of the radial and angular positions which
are spanned by the summed pixels, and the total scattering along each of the 16 angular
strips is found by adding the pixel value sum in radial sectors along the stnp. The
geometric correction factor for a single angular strip is given by :
Once this geometnc correction factor has been determined along the vanous angular
strips, the local fonvard transmitted light power, based on the local scattenng power. can
be calculated. This is done through a system of linear equations. which can be quickly
solved
Refemng back to equation (3.4), the local scattered power ) is a fbnction of the
total particulate surface area (i.e. the extinction cross-section) and the local transmitted
light intensity :
S., 75.&4,
where A, is the surface area per particle in the sector. r is the sheet thickness, and rwde
represents the area element normal to the direction of propagation of the incident light.
Equation (3.9) can be re-arranged and written in tems of the particulate surface area
concentration per unit volume in the sector, a,:
where duwd8 is the sector area, hence drxtwd0 represents the sector volume and a, the
local surface area concentration can be given as :
(3. I l )
Substituting for S,.owe get :
but K and dr are both constant so that :
for a collection of pixel values and for a single pixel value we have :
Pixel C'dire a P S .U
The local pixel value can readily be obtained from the digitized image. and the corrected
pixel value (intensity) is defined as :
Corrected Pixel i'nlzrr = Origirzal Pixel Vaiire
where Po.,.,, the initial transmitted light power in the center of the light sheet has been
used as a normalization factor so that the units of the corrected pixel values match the
arbitrary units of the 8-bit pixel values seen in the image. The corrected pixel intensity is
representative of the local surface area concentration and is therefore also referred to as
the extinction cross-section.
This correction accounts for the effects of intensity (and power) non-uniformities
along all angular stnps. Equation (3.15) shows that for regions of low light power the
original pixel value is corrected to a greater extent. Conversely, and in accordance with
the conservation of energy principle, the amount of this correction in regions of higher
illumination power (regions of the image nearest to the light source) will be minimal.
3.4 Perspective correction
In keeping with the cost criterion, a relatively low-cost Helium-Neon _jas laser
with a nominal output of 5mW was chosen for the present work. The output intensity is
low and side scatter is in general very weak. For this purpose it is not possible to position
the CCD camera perpendicular to the plane of scattering. Scattering, however is strongest
in the fonvard direction, hence with a forward OR-axis camera a bright scattering image
can be obtained while the camera itself can be safely positioned outside the spray cone and
its holding and positioning assembly can be integrated with the optical detector arrav
This is especially convenient as it makes possible a compact and truly non-intrusive
arrangement.
In the interest of compactness and integration with the optical detector array the
camera was positioned at 24.5" with respect to the plane of scattering. This shallow angle
of viewing causes the obtained image to appear distorted, so that geometric perspective
corrections are required to transforrn the image into what would be seen as viewed frorn a
direction perpendicular to the plane of scattering. Perspective corrections of the image are
necessary before the image can be corrected for illumination non-uniformities and
analyzed for pattemation results. Some of the optical lenses commonly used in today's
imaging equipment (wide angle lenses, for instance) result in optical distortions in addition
to the geometric perspective distortions. Through appropriate choice of a lens and diopter
combination however, it is possible to obtain an image in which the optical perspective
distortions are negligible.
In the present work, a software-based technique for perspective correction has
been developed and used, which provides a reasonably accurate and quick transformation
of the time-averaged scattering image. The algorithm is based on the assumption that the
subtended angles with respect to the target image bisector do not Vary appreciably. This
was done in order to sirnpliG the transformation and to keep the time of processing to a
minimum amount. As can be seen in Figure 3.6, the values of S' and Sv, the subtended
angles with respect to the target bisector can be assumed equal for geometries where the
target distance, D is sufficiently larger than the target size, S. as is the case here. This
assumption while generally valid will result in small errors in the transformation, which
will be identified shortly and later discussed
Optical focal plane r and quantified in Chapter 5 .
S.
Lm position
D Targct Target distance objet
Figure 3.6 Side view of the optical focal plane.
Given this assumption however, the transformation is a simple task. Prior to
executing the geometric transformation a spatial calibration procedure is perfomed d u h g
which the pixel locations of the corners of a 2 dimensional square target of known
dimensions are recorded and used as parameters in the transformation. Several targets of
varying sizes were initially used with the final choice of a 7.5x7.5 cm target as the
optimum calibration. since the spray diameter is not likely to exceed this limit if
measurements are kept to the 2.5-5.0 cm region downstrearn of the noule. The
transformed image represents the area contained within the four calibration points (corners
of the target).
In executing the transformation, a 2-dimensional region (480x480 pixel image) is
allocated to store the cropped (perspective-corrected) image (Figure 3 7a) . The cropped
image is sized. so that the aspect ratio of the target is preserved. For a given point in the
perspective corrected image (i, j), the corresponding point on the original 640x480 pixel
image will lie at the intersection of the two lines, defined by y=j and x=i. mapped ont0 the
original image (Figure 3.7b). We can interpolate linearly to find the corresponding
location (i', j') in the original image. This is done in two steps :
First. find the endpoints for lines defined by O, j and X, j rnapped onto AD and BC.
respectively.
Interpolate to find (i ' , j'), the point in the original image, rhat is i/X of the distance
between those end points.
X
Figure 3.7a Pcrspcctivc correctcd (cropped Image)
I 1
Figure 3.7b Original irnagc
The point ( i 7 . j') is a point somewhere on the original image. no< necessarilv
centered on a single pixel, hence to find a pixel intensity value. an averaze value based on
linear interpolation of the intensities in the surrounding pixels is obtained. This is done in
two steps :
Find the pixel vertex closest to ( i ' j ' ) .
Take a weighted average of the four pixels shanng that vertex.
The weighted average is obtained by calculating the position of the pixel centroid in the
original image corresponding to a position in the transformed image. From this. the pinel
over-lap with the top and bottom as well as the left and right halves of the neighborins
pixels can be found. The amount of this over-lap determines the amount of contribution
(or weighting) from the neighboring pixels. The total pixel value in the geornetncally
transformed image can be expressed in tenns of the weighted average of the four
surrounding pixel values (for instance the contribution €rom the top lefi neighboring pixel
is determined by multiplying the top and left over-lap values by the value of the top lefi
neighboring pixel in the original image).
The result of this simple transformation on a 7.5x7.5cm calibration target have
been shown in Figure 3.8b. The pixel locations of the corners of the target image, seen in
Figure 3.8a were found. This process has been referred to as the spatial calibration for the
geometric transfom. The cross (+) in Figure 3.8b indicates the actual center of the image
which should correspond with the target center as determined by the intersection of the
diagonals. This off-set seen in the image is caused by the aforementioned assumption
about the subtended angles.
Figure 3.8 a) Figure 3.8 b)
Figure 3.8 a) and b) Cornparison perspective-corrected image with the original.
3.5 Image analysis for pattemation measurements
Figures of merit, ofien used for spray characterïzation are : the pattemation index
(P.I.) , and the minimum/maximum per sector ratio [3], which will be defined shortly. In
addition, the image centroid as well as the spray uniformity index were obtained fkom the
corrected images. In al1 tests, 35-50 frames were obtained and averaged. The time-
averaged image was perspective-corrected, and then corrected for illumination non-
uniformities. The final image was analyzed for pattemation results.
Pnor to testing, the camera is positioned so that the center of the geomeaically
transformed image is aligned with the geometric axis of the n o d e . The image is indexed
and sectorized, with its center as the origin. In the present approach, 8 angular sectors
were allocated. The pattemation index, which is a measure of the circumferential
uniformity and symrnetry of the spray is defined as the normalized variance fkom the
expected mean, surnrned up for al1 sectors :
s - l
where W, is total Iiquid volume collected per sector, normalized with respect to the total
volume coilected by al1 sectors, and n is the number of angular sectors (in this case equal
to 8). Analogously, in the present approach intensities From all pixels were summed to
provide a measure of the total scattenng from the spray. The image was then integrated in
8 sector-wise defined regions to obtain the total scattering per sector. This was nomalized
with respect to the total scattering from the spray. The maximum and minimum
normalized - total scattered intensities per sector - were determined. and from this the
minimurn/maxirnurn ratio was obtained. This value is a rneasure of the circumferential
non-uniformity in the spray pattern.
Spray region
Min
Figure 3.9 Schematic of sectorization for circumferentiai pattemation measurernents.
The centroid for the image was determined by evaluating the pixel intensity
weighted average for the image in both x and y directions :
r = l . / = l celllroid, = , =-Ig0*,=4,&
Pixel b~lrmity(i, j ) t = I . j = I
For a well behaved spray, this value should be close to the image center (point (0.0)). The
centroid is a measure of the targeting accuracy of a nozzle and gives a good indication of
how well the swirl chamber and the final discharge orifice are lined up in a pressure-swirl
The spray uniformity index is a more stringent measure of the circumferential
pattenation. It is a measure of the standard deviation of the normalized total scatterin9
per sector, Rom the mean :
where is the calculated mean for al1 sectors.
The radial distribution of liquid in sprays is of equal importance to the
circuderential distribution. This measure is easily obtained by scanning the final
corrected image dong lines defined by 0 = O", 0 = 45", 0 = 90°, and 0 = 135". while
recording the corrected scattered intensity as a function of the pixel distance from the
origin.
Spray patterns for a small collection of noules under different test conditions have
been evaluated based on the above criteria, and the results have been correlated and
discussed hrther in Chapter 5.
Chapter 4
Experimental apparatus and procedures
In the following sections the experimental apparatus and instrumentation hardware
and software used in the construction of the optical pattemator are described. A low-cost
and non-intrusive system capable of making quantitative, high-resolution pattemation
rneasurements quickly and autornatically is presented. The system was constmcted
almost entirely fiom commercially available parts.
4.1 Experimental arrangement
Side and top view schematics of the experimental apparatus are s h o w in Figures 4.1
and 4.2. As illustrated in Figure 4.1 the optical pattemator consists of two sub-systems.
one for the acquisition and recording of the forward light power and one for the capture
and digitization of video images through a CCD canera. These sub-systems are
controlled by a PC (personal cornputer) and their simultaneous operation is coordinated
through instrumentation software written for the optical pattemator. The instrumentation
software serves a double purpose here. It enables the measusement process to proceed
autornatically, while providing an interface through which an operator c m input
commands and view in real-time the results of pattemation tests.
CPU
Angle of viewing - (Z4.Y)
Cylindrical lens
He-Ne Laser
DC Power S ~ P P ~ Y t
Keithly Metsabyte DAS 1402 16-channel
ADC
Matrox Meteor
4
Calibntion bos
PCI frarne grabber
Figure 4. t Side-view schematic of the experimental arrangement.
4 CCD camera
controller
r Calibration box
Radial optical detrctor array
, Rectangu~ar cross- section test section
Spray zone
Cy l indrical lens
Figure 4.2 Top-view schematic of the radia1 optical detector array.
Tests were conducted using the experimenral apparatus shown in Figure 4.3. This
apparatus consists of :
The radial detector array (consisting of 16 optical tubes).
CCD camera and its mounting assembly.
He-Ne laser.
DC power supply for the laser.
Cylindrical lens.
Micro-positioning hardware for the laser.
The CCD camera controller, DC power supply for the PIN photodiodes, and the data
acquisition cornputer were remotely located and can be seen in Figure 4.4.
The nozzles used for testing were located centrally within the clear rectangular test
section of the vertical water spray wind tunnel as described by Ahrnadi [12] and as shown
in Figure 4.5. The clear glass test section, optical detector array, and CCD carnera were
covered (as seen in Figure 4.5) to block off extemal light.
Figure 4.3 Experirnental apparatus.
46
Figure 4.4 Data acquisition cornputer.
Figure 4.5 Vertical water spray wind tunnel.
4.2.1 Light source and sheet producing optics
A 632.8nrn Melles Griot cylindrical Heliurn Neon laser was used for illumination.
The unit has a nominal output of 5mW in the TEM, mode and produces a beam which is
0.8rnm in diameter (1/e2). The unit is light and compact and is placed on a micro-
positioning plate to allow alignment with the g l a s rod used to produce the light sheet. It
is powered by a compact DC power suppiy, which consists of a step-up transformer and a
voltage regulator/rectifier and does not require any adjustments once turned on.
To produce a light sheet a 1.8mm diameter glass rod was utilized. A section of a
glass rod with nominal diarneter of 3.0rnm was melted and stretched to reduce its
diameter. The diameter of the glass rod was reduced to increase the included angle of the
light sheet to about 45'. Several attempts were made at producing a glass rod with no
surface imperfections and scratches. M i l e the glass rod which was used as a part of the
present apparatus displays no visually detectable surface scratches, small striations were
still visible in the produced sheet. These striations are caused by small surface scratches
as well as difiaction from impurhies within the g l a s rod itself and appear as radial lines
of lower intensity along the light sheet. Striation will be m e r discussed in Chapter 5 in
relation with the test results dong with suggestions for fûture improvements in the
quality of the sheet-producing optics.
The produced sheet fans out radially and its intensity is reduced at the outer edges
of the sheet due to the Gaussian intensity profile of the incident beam. The effects of the
intensity reduction at the outer edges of the light sheet are avoided by producing a sheet
which has a Iarger than required included angle ( 4 5 O ) . The brightest region along the
middle of the sheet, which has the most uniform intensity distribution is aligned with the
radial optical detector array, which spans a 3 2 O included angle. This is done in two steps:
first the cylindrical lens is adjusted until the sheet is horizontally aligned with the optical
detector array, then the micro-positioning plate is used to move the laser horizontally
until the bright region of intensity in center of the sheet is aligned with the collection
lenses of the optical detector array. This procedure is reasonably quick and easy to
perform. Figure 4.6 shows the laser and the cylindrical lens mounted directly in front of
it (right in the picture).
The laser, its positioning hardware, and the cylindrical lens holding assembly
were mounted on a section of %" thick alurninum plate, which will be referred to as the
laser mounting assembly. A 1 meter length of 2"x2" steel tubing is used to attach the
laser mounting assembly to the optical detector array base-plate.
Figure 4.6 Laser and glas rod setup.
4.2.2 Optical detector array
As mentioned in the last chapter, the elimination of collimating optics has made
necessary the use of polar geometry and discretization in treating the illumination non-
uniformity problem. To make the optical path length equal for al1 light emanating fiorn
the cylindrical lens a radial detector m a y was constnicted.
This arrangement, seen in Figure 4.7, consists of a base-plate and 16 optical tubes
and collection lenses to focus the incident light on PM photodiodes. The design
drawings for the base-plate and optical tubes are presented in Appendix A and the design
configuraticns as well as performance specifications for the PM photodiodes used are
given in Appendix B. The collection lenses on the optical detector array (as indicated in
Figure 4.2) span in total a 32' arc of a circle of radius 47.3 cm. Plano-convex g l a s
lenses were used for focusing. These lenses, supplied by Melles Griot, have a diameter of
15mm with a focal length of 100 mm. For easy disassembly, replacement, and alignrnent
of the diodes the optical tubes were fitted with removable end-caps containing the PIN
photodiodes. The design drawings for the end-caps are presented in Appendix A.
Alignment of discrete optical elements can be time consurning and at times
fnstrating. The arrangement of radial grooves on the base-plate eliminates the need for
any alignrnent procedures for the optical tubes. Each optical tube containing a collection
lens and a PIN photodiode is individually mounted on the base-plate by two positioning
screws, which are accessible through the top of the opticai tube. This arrangement offers
several advantages :
As indicated. alignment procedures are completely unnecessary. The base plate has
been constructed with strict tolerances and the arrangement of the radial grooves
along with the mounting holes on the base-plate provides excellent alignment for the
optical tubes.
The use of modular optical components makes handling the collection optics much
easier. In the case of component failure each of the optical tubes can quickly be
replaced.
Figure 4.7 Optical detzctor array and CCD camera mounting.
PIN photodiodes are small and inexpensive semiconductor transducers which
'fer an excellent dynamic range with great sensitivity and response (slew rate). The
:vice essentially behaves as a current switch which closes once light energy falls
incident upon its collector-base junction. The amount of this current is linrad>-
proportional to the incident light intensity. A small fonvard-biasing voltage (2.0 V ) was
applied and the current from each photodiode is put across a potentiometer and a
precision metal film resistor. This increases the responsivity on the output side and
allows for individual calibration of the photodiodes, while bringing the output voltages
up to the 0.5V range, which can be measured easily and accurately with good resolution.
The calibration circuits for the photodiodes were integrated on a PC board and enclosed
within an instrument box for protection. The circuit diagram for the calibration circuit of
one of the PIN photodiodes is shown in Appendix A.
4.2.3 CCD video camera
8-bit gray-scale (black and white) images of the scattered light fiom the spray
were captured by using a Sony XC-77RR CCD video camera module, consisting of the
head unit or the controller, a 2-dimensional CCD array, and a VCL-MY-M focusing
zoom lens. The head unit allows selection of shutter speeds, while sending out an NTSC
video signal of the seen image to the Frame grabber.
The focal plane of the carnera lens intersects the scattenng plane. Regions of the
spray nearest and farthest from the lens need to be in focus so that bluriness in the image
(caused by the image being out of focus) does not mask any of the spray structure. To
minirnize the depth of field and limit focusing to the region of intersection of the spray
with the light sheet large aperture settings were used. With the aperture selected, an
appropriate shutter speed was found to give the best contrat and the l e s t saturation in
the image. To b h e r irnprove focusing a 49rnrn 2x diopter lens was used. The diopter
lens rnounts directly ont0 the zoom lens and allows reasonabiy good focus over the spray-
light sheet intersection region. Performance specifications for the CCD carnera system
used are given in Table B.3 in Appendix B.
As mentioned, the CCD carnera is positioned at a forward off-axis location. To
hold and position the camera a compact mounting assembly was designed and
constructed. The camera rnounting assembly is attached to the base-plate so that the
entire unit (consisting of the base-plate, the carnera mounting assembly. and the laser
mounting assembly) can easily be mounted on a three-way traverse outside the test
section (described by Ahmadi [12]) or transported to be used at a different test site.
4.2.4 Pressure-swirl atomizer
A small collection of pressure-swirl semi hollow-cone nozzles were selected for
use during testing. The nozzles were supplied by Delavan Inc. [21] and ranged in flow
capacity from 2.50 to 3.00 gallons per hour with varying cone angles. These nozzies are
intended for use in domestic oil h a c e s .
Pressure-swirl atomizers are used in a wide variety of combustion applications
and offer good mechanical reliability and the ability to sustain combustion at lean heuair
mixtures. Simple design, low cost, and effective atomization make pressure-swirl
atomizers the ideal choice in low flow capacity instances. The spray cone angle is
dependent on different variables. Many studies have been conducted to demonstrate the
effects of such variables as the ambient and fuel pressure as well as the final discharge
orifice geometry on spray patterns from pressure-swirl atomizers [2. 22. 23. 24. 251.
Liquid properties such as viscosity and surface tension are also of importance in
determining the spray cone angle although not to the extent that pressure and nozzle
geometry have been shown to affect this variable [2,25.26].
K' (m5-I Wir 1
Sm - u o g p h m
Figure 4.8 Flow arrangement in a typical pressure-swir1 atomizer [12].
Figure 4.8 shows the basic design of Deiavan oil burner nozzles. The nozzles
used during testing had rnesh strainers as opposed to the sintered filter which is seen in
Figure 4.8. The swirl motion which is imparted upon pressurized liquid fuel entering the
swirl chamber creates a radial pressure gradient. This causes a hollow core to be formed
at the nozzle exit and the liquid proceeds fiom the final discharge orifice in an expanding
concentric film, which disintegrates into fine droplets. This type of spray profile is
particularly useful in combustion applications. since the majority of the drops are
concentrated at the periphery of the spray, ensuring rapid evaporation, mixing, and
combustion.
Water was used as the sprayed medium and to pressurize it compressed nitrogen
was used. The line pressure was adjusted by using the pressure regulator on the nitrogen
tank. An aluminum container was used to store the pressurized water. The nitrogen tank
and water container used are s h o w in Figure 4.9.
Figure 4.9 Nitrogen tank and water container.
4.2.5 PC-based instrumentation and data acquisition
To digitize and record the forward light power a 16-channel Keithly MetrabyteB
DAS- 1402 analog to digital data acquisition board was used. For convenient application
development in the Visual Basic prograrnming language DriverLMXNB 4.0 (Scientific
Software Tools Inc.) custom drivers were used. The board was calibrated and configured
to operate with 16 single-ended unipolar inputs and a common ground. Scan rates of up
to 100 kHz are achievable. The performance specifications for this board are provided in
Table B. 1 in Appendix B.
The capture and digitization of video images was accomplished through use of a
Matrox Meteor@ PCI h e grabber. The MIL-Lite package, which is a subset of the
Matrox tmaging Library was used for the processing of the images and the obtained
images were pseudo-colored. Video image capture and manipulation is handled entirely
by the MIL-Lite library of functions in Visual Basic. The performance specifications for
this board are provided in Table B.2 in Appendix B.
The main program to control the simultaneous operation of the two boards was
written in Visual Basic. This program enables the user to see live pseudo-colored images
of the spray during testing, displaying the time-averaged result at the end. It also
provides an interface for user commands to start or stop testing and to input parameters
such as scan rates and the total number of video images to capture. Up to 127 images cm
be obtained and averaged over time. while during the capture of each image the detector
array is sampled up to 4000 times at a user-specified sampling rate (typically 80 kHz) to
provide a time-averaged measure of the forward light power for d l 16 channels. Once
the acquisition is finished perspective and non-uniformity corrections are performed on
user command through the instrumentation interface. The final image is then analyzed
for pattemation results. Performance specifications for the data acquisition computer
used are given in Table B.4 in Appendix B. The use of the instrumentation program is
demonstrated in Chapter 5.
- . .-
Chapter 5
Results and discussion
The optical patternator was used to test a small collection of pressure-swirl
atomizers under different test conditions and to determine the figures of ment for the
produced spray pattern in al1 cases. Use of the instrumentation software and the user-
interface designed for this pattemator has been demonstrated in the following sections and
the obtained results ftom the tests are correlated to show the consistency and relevance in
the obtained measurements. The system's capability and performance potential were
tested under repeatability critena. Some commoniy encountered problems such as
obscuration and intensity striations are discussed and suggestions for future improvements
have been made.
5.1 Over-view of tests
The tested noules, the conditions of testing, and the test results are shown in
Table C. 1 in Appendix C. In al1 cases 35-50 images were captured and time-averaged.
The line pressure was reduced in some cases to reduce the obscuration level through a
reduction in the flow rate from the nozzle. The distance of measurement downstrearn of
the nouie was reduced for measurements involving the wider spray cone angle noule. in
order to obtain a better measure of the spray pattern and structure, and also to limit the
spray cross-sectional area to the 7 5 7 . 5 cm region of spatial calibration for the jeometric
transform.
Pattems from a good noule and a malfunctioning one were obtained and analyzed
for comparison. Two repeatability tests were performed to assess the spray pattern. afier
a rotation of the noule (about its axis of symmetry). and under identical initial test
conditions at a later tirne. Radial distributions from al1 tested noules displayed semi-
hollowness in the spray structure, indicating that the majority of the liquid contained in the
spray is concentrated in an annular region about the center of the spray, so that the center
itself is "hollow". This is especially noticeable in sprays with wider cone angles.
5 -2 Results summary and discussion
Seen in Figures 5.1 and 5.2 are results of pattemation testing of a malfunctioning
noule and one which displays a more symrnetnc and uniform pattern. The images
represent the spray cross-section at a distance of 5.0 cm downstream of the noule The
spray pattern of the rndfùnctioning n o d e displays streaking in three regions. Streaking is
likely caused by blockage in the final discharge orifice or leakage around the discharge
orifice disk. Streaking seems to be a cornmon problem with many older noules, since as
has been mentioned eariier the vanous nozzle orifices tend to deteriorate with tirne and
usage. The existence of these streaks in Figure 5.1 explains the relatively low min./ma.. .
ratio, as well as the reasonably high values for the pattemation index and the spray
uniformity index (seen in Figures 5.1 and 5.2) which are al! indicative of the non-
uniformity and the asyrnrnetry in the spray pattern.
Figure 5.1 Spray pattern t o m a maffiinctioning 2.50 60' A nozzie.
Figure 5.2 Spray pattern fiom a 3.00 60" A nozzle.
The results Eom repeatability testing under rotation have ken shown in Figures
5.3 and 5.4. The tests were performed at a distance of 2.5 cm dowllstzeam of the nozzle.
A 2.75 80' A nozzle was used and the line pressure was reduced (to 40 P.S.I.) until a l es
uniform spray pattem with a distinguishable structure (horse-shoe shape) became visible.
This is shown in Figure 5.3. The nozzle was then rotated 180" and the test was repeated.
59
Figure 5.3 Spray pattern be fore nozzle rotation.
Figure 5.4 Spray pattern &a rotation a 180° rdatim of the nozzie.
It can be seen in Figure 5.4 that the horse-shoe pattern m the spray has been
rotated dong with the smaller regions of higher scattering mtensity (lobes). P.I. and
S.U.I. were calculateci to be 18.93% and 19.81% respectively before rotation. The
calculateci values for P.I. and S.U.I. after the rotation were 19.16% and 21.2%, so that on
average the pattemation results before and d e r the rotation differ by less than 1.5%. The
glass test section needed to be removed before the rotation of the nozzle. Although the
test apparatus was re-aligned pnor to conduction of the test (fier rotation) a small off-set
may have remained in the alignment of the axis of symmetry of the n o d e with the center
of the view field of the camera. This is thought to account for the ditference in the
patternation resdts before and &a the rotation
Results Eorn repeatability test ing under identical init id conditions have ken
show m Figures 5.5 and 5.6. The same 2.75 80° A nozzle was used and the images
shown represent the spray cross-section at a 2.5 cm distance downstream of the nozzie.
The line pressure was hcreased to 100 P.S.I. for these 2 tests and while the horse-shoe
pattern is no longer present, the lobe near the top of the image rernains and has grown m
size since the flow rate is higher than before due to the increased iine pressure. The two
tests were perfomed 24 hours apart. P J. varies by 1.25% and S .U.I. is seen to vary by
2.5%, so that on average the patternation results agree to wahin 1.8 %.
1
l
I
Figure 5.5 Repeatability testing for 2.75 80°A node.
Figure 5.6 Repeatability testing for 2.75 80°A nozzle.
To better assess the degree to whicfi the spray patterns remaineci the same in the
repeatability tests background subtract ion calculat ions were perfonned on the corrected
spray images. In the nrst instance (repeatability with rotation) the image &er node
rotation was rotated back by 1 80' and then subtracted fiom the ht image. The result
~ o m this c m be seen in Figure 5.7 a In a smiilar manner, spray images fkom the
repeatability with t h e test were subtracted nom each other and the remit has been shown
in Figure 5.7 b.
For two identical images background subtraction will result in an image which is
completely black (zero ciiffietence in the pixel values over the entire Eage). Hence the
gray regions in the subtracted images are indicative of the Merences in the subtractzd
images. The highest pixel values m the subtracted images were less than 25 (out of 255)
so that no contours could be seen in the pseudo-colored background subtraction images,
thus we can see that all portions of the comparable images d8er by les tha. 1 contour
intemai. The images in Figures 5.7 a and b are gray-sale black and white images (Le.
they have not been pseudo-colored).
Figure 5.7 a) Figure 5.7 b)
Figure 5.7 a) Background subtraction image fiom rotational repeatability test. b) Background subtraction image from rcpeatability ~4 th tirne test.
The effects of obscuration are demonstrated in Figures 5.8 and 5.9. Obscuration.
caused by multiple scattering, is a problem encountered by many optical systems. This
problem anses when at small distances between neighboring drops (for instance in very
dense sprays) the scattered light fiom one drop gets scattered off another neighboring
drop. Particle-particle scattering (Le. multiple scattering) from drops above the planar
region of illumination has caused the near and far edges of the image in Figure 5.8 to
appear jagged. In order to reduce obscuration, the line pressure was reduced From 100
P.S.I. to 70 P.S. 1. Reduction of the line pressure reduces the flow rate of the nozzle and
results in a less dense spray. The onset of obscuration is s h o w in Figure 5.9 as evidenced
by the jaggedness of the far edge (top) of the spray pattern, while the near edge is well
defined. This is caused by the longer optical path length of the scattered light fiom the far
edge of the image to the camera lens. which increases the possibility of multiple scattering
as the scattered light fiom the sheet traverses the spray cone. With the effect of multiple
Scattering reduced a signifïcant improvement is seen in the P.I. and S.U.I. seen in Figure
5.9 (P.I.=17.91%, SS.U.I.=18.52% in Figure 5.8 Ys. P.I.=13.39%, S.U.I.=15.05% m
Figure 5.9).
J
Figure 5.8 Obscuratioa &emi on spray pattern f?om a 3.00 60°A o d e .
Figure 5.9 Spray pattern of 3.00 60°A node without obsanatim.
The problem of intemity striations in the ilhiminated sheet was mentioned earlier in
Cbapter 4. An example of this problem c m be seen in figure 5.10. The 2.75 80° A nozzle
wtiich was used m the repeatabiliry testing was used once again. A line pressure of 100
P.S.I. was applied and the measurement was perfomed at a distance of 3.75cm
dow~lstrearn of the nozzie.
Figure 5.10 Effects of intensity dong the light sheet.
The jagged edges of the contours in the image are caused by non-uniform
illumination dong regions in the laser sheet affecteci by striation. Lines of striation
onginate at the cylincûicai lem (origin of the sheet) and f i out radially w3.h the sheet. To
reduce striation, the cyhdrical lem (glass rod) was rotated and moved vertically until a
position was found that resulted in the lest amount of striation (Figure 5.11). Intensity
non-unifodies in the region of illumination caused by striation resuit in the distortion of
the scattered image, so that the spray pattern is distorted and the obtained pattemation
meaSuTernents are monmus (P.I.=22.46%, S .U.I.=24.52% in Figure 5.1 0 Ys.
P.L=31.15%, S.U.I.=34.46% in Figure 5.1 1).
65
Figure 5.1 1 G l a s rod is adjustexi to reduce striation.
Radial distriiutions eom 80° and 60° semi-hoîiow cone nozzles are compared in
Figures 5.12 and 5.13. These disûi'but ions were O bt ained by scanning the final corrected
image from left to nght (or bottom to top) along the mdicated lines. The large lobe in the
upper region of the spray pattern m Figure 5.5 can be seen as a maximum along in
the radial distribution plot of Figure 5.12. Figure 5.13 shows the radial distriiution plot
for the spray pattern m Figure 5.9. It c m be seen kom the two plots that the corrected
normalized mtensity (i.e. the n o m W extinction cross-section) drops off much fista m
the case of the 60' nozzie, bdicating a srnaller effective cone angle. The asymmetry of the
spray patterns m Figures 5.5 and 5.9 and can also be seen in the radial distribution plot.
The regions of l o d minima in the plots are mdicative of the location of the hoffow cone.
Figure 5.12 Radial distribution fiom 2.75 80" A nozzle at 100 PSI ihe pressure (fiun image in Figure 5.5).
- 136 aairr l'El
Figure 5-13 Radial disrnition korn 3.00 60° A n o d e at 70 P.S.I. line pressure (fiom image in Figure 5.9).
5.3 Instrumentation software
Routines for the acquisition and digitization of images and the fonvard Light power
were integrated into a main program, d e n in the Visual Basic prognimmmg language.
[mare correction and analysis (for pattemation) routines were also integrated into this
main prosram. Frame by frame images of the spray. as well as the results of the
transformation and non-uniformity corrections can be viewed through a display window
In the following section the use of this program has been demonstrated for a sample test
on a 2.75 80" A nozzle.
The procedures for a single test are as follows :
Before spraying, the detector array is scanned to record the background illumination
by obtaining the average initial light power received by each optical detector (5000
points are averaged for each channel at a preset scan rate of 20 kHz). This procedure
is performed by the "Initialize" command button on the interface.
The number of frames to acquire and average are specified by the user through the
appropriate selection window. In al1 tests 35 to 50 images were captured and time-
averaged. Although more averaging provides for a better measurement. in this
instance the test time was to be kept short so that the least number of images to give
the best overall time-averaged image was chosen. By visual inspection there were no
improvements seen in the stability of the time-averaged image beyond 50 samples.
The sampling rate for the acquisition of the fonvard light power during the
simultaneous operation of the fiame grabber and the A/D board is selected by the user
through the interface. An 80 kHz scan rate was used in al1 test cases.
With the spray staned, simultaneous acquisition and digitization of images and light
power levels can begin by pressing the "Grab" command button on the interface.
Pseudo-colored images of the spray can be viewed frame by h m e in real-time through
the display window. For every digitized image a total of 500 intensity values were
6 a
recorded and averaged through each channel of the optical detector array. The timz-
averaged and pseudo-colored image was displayed at the end of the "Grab" operation :
By cornparison of the time-averaged values of the fonvard attenuation to initial
mtensities captured at the time of initiakation the extinction percentage across the
spray c m be calculated. These extinction percentages are displayed through the
interfice :
1 Channel Nuniber 1
In al1 tests, the f o w d extinction did not exceed 3%. This is because the
pressure-swul nodes used have d flow capacities (3 .O0 GPH for the largest nonle).
This &O explains why the corrected final image does not Vary greatly fiom the
wrrected image. These forward extinction values tapered off to les than 1% for the
outermost angular stnps in ail test cases.
The tirne-averaged image is geometridy transfonned. This requires user input of the
pixel locations of the four corners for the caliiration target. DefiuIt values have been
determined for a 7 3 7 . 5 cm target. The trausformation is performed on user
command through the interface :
The geometricdy transformecl image is corrected for illumination non-uniforxnity
effécts. This requires that the value of K, the geometric correction factor be found
along the angular stnps in the image. A complete set of sample calculations for the
spray in Figure 5.3 have been inchided m Table D.1 and D.2 in Appendix D and the
calculated geometric factors for this spray c m be seen m Figure 5.14.
1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 S 1 6
Angular ?os ition
Figure 5.14 Geomctric correction factors along the angular sirips.
Values of this geometric correction factor Vary among the dif3erent angular strips
due to the varying scattering geometry along each strip. As can be seen from Figure 5.14.
variations in the geometric correction factors increase for the outermost angular strips
The geometric correction factors for these regions are calculated based on forward
extinction values which are often less than 1%, and hence errors in the measurernents of
the forward extinction could account for the variations of the correction factor for the
outer angular stnps. In other words the uncertainty and error associated with these
correction factors increases with decreasing extinction. Geometric correction factors for
the centrally located angular strips, where extinction is highest remain reasonably constant
since the rneasurernent errors in these regions are likely to be minimal.
The corrected image is analyzed for pattemation results. The results are displayed afier
cornpletion of the test through text boxes on the interface :
At this pomt another test can be perfonned. The total t h e for one test (50 images) is just
short of 5 minutes.
5.4 Geometric transform errors
The subtended angles were assumed constant with increasuig distance. To validate
this assumption and m the interest of quant- any errors in the geometric
transformation which r e d t f?om this assumption, the square calibration target was
marked along its diagonals to mark as center. Pixel position of this marker was recorded
after the transformation. A 20 pixel vertical offset fiom the image center was observed.
This offset, shown in Figure 3.8 b (in Chapter 3) arnounts to a 4.2% error in the
transformation.
As evidend by the relative& low error percentage, the assumpt ion of constant
subtended angles is reasonably valid. For this purpose the algorithm was not modined
turther to account for the variation of subtended angles with increasing distance
5 .5 Recommendations
As demonstrated, minor striations are visible in the produced sheet. Better sheet
producing optics are required. The glass rods used in the present arrangement are not of
optical quality and the use of optical quality cylindncal lenses or a line projector in future
applications of the optical pattemator is strongly recommended.
The problem of optical obscuration, mentioned earlier, could pose a limitation in
tems of the applicability of the optical pattemator in some practical situations. Pressure-
swirl noules used in the present tests are designed to perfom over a wide range of line
pressures, typically fiom 50 to 150 P.S.I. [2 11. Nthough the line pressure was reduced in
order to reduce the obscuration Ievel, the test line pressure of 70 P.S.I. is still within the
design operating pressure range for the tested noule. The easiest way to ensure minimal
optical obscuration is to position the CCD camera such that the scattered light collected
by the camera traverses the lowest density region of the spray (Le. farther downstream of
the nozzle discharge orifice). In the present expenmental arrangement the camera could
be positioned at a -24.5" fonvard off-axis position with respect to the plane of scattering.
Since in this position the scattered light seen by the camera would traverse the lower
density region of the spray farther downstream of the noule discharge orifice, the amount
of optical obscuration should be reduced, and the onset of obscuration should occur at
higher spray densities. This will be investigated in future tests of the pattemator.
Although the errors from the geometric transformation are small. in al1 applications
involving spray pattern analysis it is crucial that the pattern from a spray not be distorted
by the transformation. While the assumption regarding the subtended angles is a good
approximation in geometries where the target distance exceeds the target size by at least
an order of magnitude, better transformation resuits can be obtained by accounting for the
minor variations of the subtended angle. To minimize transformation errors and provide
an exact geometric transformation the algorithm can be modified to account for the srnaII
variations of subtended angles with increasing distance. The geometric transformation
algorithm which was developed based on the constant subtended angle assumption was
seen as adequate for the present work.
For commercial applications more compact packing is required and recommended.
The current arrangement of the optical guides and the camera holding and positioning
assembly are ideai. however the collection lenses, CCD camera and iens, and the
calibration box should be sealed off and protected from the potentially harmful
environment of industnal testing, especially in the presence of combustible fluids.
To fùrther reduce costs, a less expensive CCD camera could be used. The
replacement of the He-Ne gas laser and the cylindncal sheet producing optics with a less
expensive diode laser line projector has the benefits of cost reduction, as well as making
the instrument more robust. Diode lasers can be used in a battery or array, offering higher
illumination intensities, which could be usefûl in making measurements in denser sprays.
The instrumentation software and interface which were designed for this optical
pattemator have made the task of automation possible. For automated and on-line testing
and defect detection in an industrial environment the instrumentation code and user
interface could be modified in a nurnber of different ways to improve the interface. and to
provide the user with an automated diagnosis for the tested noule based on such figures
of merit as the P. 1.. midmax ratio, S. U. 1.. and the image centroid.
5.6 Closure
The technique of laser light sheet imaging with non-uniformity corrections has
been demonstrated to be a quick and reliable method for the quantitative measurement of
spray patterns in a low cost and non-intnisive experimental arrangement. Better
packaging of this instrument will facilitate its transportation and use in different tesr
environments. The spatial calibration for the geometric transforrn is a task which needs to
be performed before measurements c m be made. With the calibration out of the way.
noule testing is a reasonably quick and effortless task requiring on average 5 minutes per
nonle (if 50 images are averaged). This is a major irnprovement over conventional
mechanical pattemators which require 30-45 minutes for the testing of a single nozzle.
Add to this the applicability for on-line monitoring of sprays in spray processes offered by
the non-intrusive nature of the measurement. and the potential for industrial use is
immediately obvious.
In the present study sprays of low flow capacity were used to demonstrate the use
of the aforementioned illumination non-uniformity correction technique. and to
demonstrate the use of the optical patternator as an instrument, showing the validity and
relevance in the measurements of spray patterns obtained by using this instrument. As lias
been demonstrated, in flow situations where the forward extinction is less than 1% the
uncertainties in the calculated geornetric correction factors increase. The application of
illumination corrections are still necessary to account for non-uniformity in the laser Ji-ht
sheet.
References
[I l Winklhofer E; Ahmadi-Behi B; Wiesler B; Gresnoverh G. "The influence of
injection rate shaping on diesel fuel sprays - an experimental study" Proc Irtsm
Mech. Eligrs., 206: 173-1233, 1992.
[2] Chen SK; Lefebvre AH; Rollbuhier J. "Factors influencing the circumferential
liquid distribution frorn pressure-swirl atomizers". Jozrrnczl of D>gi,~rer~rzg fo,.
Gas Trrrbines and Power. 115447452, 1993.
[3] Tate RW. "Spray patternation, a significant variable in fuel combustion and
chemical processes utilizing atomizing noules". Eqztiprnrnt C Z F ~ desig~.
50(10):49A-55q 1960.
[4] Jones RV; Lehtinen JR; Gaag M. "Testing and characterization of spray nozzles
: The manufacturer's view point". Parker Hannifin Corporation, 17325 Euclid
Avenue, Cleveland, Ohio 44 1 12.
[SI Wang G; Sellens RW; Olesen MJ; Bardon MF. "Spray pattern evaluation for
pressure atomizers using an optical pattemator". RMC Department of
Mechanical Engineering in conjunction with Queen's University, Department of
Mechanical Engineering, Kingston, Ontario K ï L 3N6, Canada, 1994.
[6] Cohen JM; Rosfjord TJ . "Spray pattemation at high pressure". J. Propitlsiori.
7(4):48 1-487, 199 1.
[7] McVey JB; Russel S; Kennedy JB. "High-resolution pattemator for the
characterization of fuel sprays". J. Propzrfszott, 3(3):202-209, 1987.
[8] Mitchell P. "Medical aerosols : Techniques for particle size evaluation". Paper
presented at I U S S 9 7, May 1 8-2 1, 1 997.
[9] Chigier N. Combzrstiori Measzrrcmettts. Hemisphere, New York, 199 1.
[ 1 O] van de Hulst HC. Light scattering by mal2 particles. Dover, New York,
1981.
[ 1 1 ] SeIlens RW "A derivation of the phase Doppler measurement relations for an
arbitrary geometry" (.Gpcrimrtrfs br Fl~îid~. 8: 165- 168, 1989.
[12] Ahmadi M. "A simpl~JedMEFdropsi=e distribrrfioti for sprays". MSc. thesis
submitted to Queen's University. Department of Mechanical Engineering. 1 990
[l3] Sellens RW. "A compact. laser diode based phase Doppler system".
Drprriments itr Fhids. 9: 1 53 - 1 5 8, 1990.
[14] McMillin BK; Lee MP; Hanson RK. "Planar laser-induced fluorescense imaging
of shock-tube flows with vibrational nonequilibrium". AlAA .loimal.
30(2):436-443, 1992.
[ 1 S] Sankar SV; Maher KE; Bachalo WD. "Time-resolved measurement of liquid
mass distribution in a fùel injector spray using an optical pattemator". Paper
presented at ILAS'S 97. May 18-2 1, 1997.
[ 161 Arellano L; Ateshkadi A; Fukushima H; Mc Donell VG; Samuelsen GS. "Effect
of mixer geometry on spray distribution : A multivanate experiment approach".
University of California at Irvine. UCI Combustion Laboratory, Irvine. CA
92697-3550. . Paper presented at I U S S 9 7, May 18-2 1, 1997.
[17] Wang G; Sellens RW; Olesen MJ; Bardon MF. "Preliminary work on an optical
pattematoi'. RMC Mechanical Engineering Report No. 93 100 1, Kingston.
Ontario, October, 1993.
(1 81 Wang G; Sellens RW; Olesen MJ; Bardon MF. "PW300 Air-blast atornizer
spray pattemation using an optical spray pattern analyzer". RMC Department
of Mechanical Engineering in conjunction with Queen's University, Department
of Mechanical Engineering, Kingston, Ontario K7L 3 N6, Canada, 1 995.
[ 191 G. Wang, R. Deljouravesh, R.W. Sellens. M.J. Olesen, and M.F. Bardon. "An
Optical Spray Pattern Analyzer". Presented at ILASS Americas '97, Ottawa.
Canada, May 1 8-2 1, 1997.
[20] G. Wang, R. Deljouravesh, R.W. Sellens, M.J. Olesen, and M.F. Bardon. "An
Optical Spray Pattern Analyzer'' Presented at The Combustion Institute.
Canadian Section, Halifax, Canada, May 25-28, 1997.
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spray angle". iiatrstrcfiota of the ASME. 56:607-6 1 5. 1 95 7.
[23] Ortman J; Lefebvre AH. "Fuel distributions from pressure-swirl atomizers". .J.
Proptrlszon. L ( I ) : I 1-15, 1985.
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Trrrhirzes and Porver-. 1 14: 97- 1 03, 1 993.
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Bi bliography
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[ 3 ] Menkirch W. Flow Glsrialization. Academic Press, New York, 1974.
Appendix A - Design drawings
a s e - p l a t e Material : %" Aiurninum Plate Dwg. by : Rama Deljouravesh, 0 1/08/96
Notes : 1 ) Al1 dimensions in miliimeters. unless othenvise specified. 2) Al1 tolerances arc within 110. I mm. 3 ) Drauing not to scalc.
1 O~tical Tube Assemblv 1 p~aatëÏial : 1/2" scheduie 80 aluminum
Dwg. by : Rama Deljouravesh 04/07/96
Notes : 1 ) Al1 dimensions in rnillimeters. unless other-wise specified. 2) Al1 tolerances are wiihin CO. 1 mm. 3) Drawing not to scale.
- - 8.0 dia
, , _ -- +,-. - - 4 - . , -- - - - - - - - - - - - - - - - - - - - - - - - - -. - - - - . -
I - - . -- 13.3 dia.
-- -
1 8.0 dia
Notes : 1 ) Al1 dimensions in millimeters. unless othenvise specified. 2) Ail toletances are within +O. 1 mm. 3) Drawing not to scaie.
1 Camera holding assembly 1 Material : %" and 3/8" Aluminum plate Dwg. by : kirna Deljounvesh. 20/02/97
Notes : 1. Al1 dimensions in centimcters. unless othenvise stated. 2. Tolerances +/- 0.025 cm or as indicated. 3. Drawing may not be to scale.
a -- -
Figure A. I Circuit diagram for single PIN photodiode. mctal film rcsistor. and potcntiornercr
Appendix B - Performance and Design Specifications
Tablc B. 1 Analog input spccifications for Keithley Meuabyte DAS-1402 board.
Number of channels Switchconfigurable as eight differential or 16 single- ended
input mode
Resolution Range (at unity gain) Settling tirne (at unity gain) Throu.&p ut AbsoIute accuracy Lineariw Acquisition time i n ~ u t im~edance
.- -- - -- -- - 1 Interrupt levels 1 2.3.4, j.&Jnd7
Switchconfigurablt: as unipolar or bipolar
12-bits ( 1 part in 4096) 0.0 to + I O V for unipolar
l 0 p 100kHz for al1 gains
_+ 1 LSB + 1 LSB t . 4 p Greater than 25Ml2
- input over-volta& DMA channels
Table B.2 Performance specifications for Matros Meteor PCI frame grabber
-
+7 5 -0 V continuous power 1 and3
~- -
Acquisition
Interface and connectors m
Software-seIectable video input (up to 4 channels) Standardcoiorormonochromevideo(NTSC/PAL/SECAM. RS- 1701CCIR Composite or WC) Pixel jitter : Bris
Real-time transfer rates up to 45 MB/sec PCI 32-bit interface. Video input connector (DB9 for composite or RGB inputs)
Tablc B.3 Pcrfomancc specifications for Sony XC-77RR CCD video camem modulc.
CCD vertical drive frequency 15.734 kiiz 1 1% CCD horizontal drive Freauencv 14.3 18 MHz
Pickup dcvice Picture dernents
Interline transfer CCD 768 (Hl x 493 O
-- -- --
Ccll size Chip size Lens mount Horizontal resolution Vertical effective lines Sensitivitv
Tablc B.3 Performance specifications for the data acquisition cpmputer.
-
1 1 (Hl x 13N)p-n 10.0 (H) x 8.2 (V) mm C mount 570 TV lines 2 : I interlace 485 lines 400-lus with F4.0
-
S N Shutter mode S huttcr spced
1 Computing
56 dB Switch-selectable normal or DONPISHA shutter mode Normal shutter mode : 1/63 to 1/358000 sec. DONPISHA shutter mode : l/3 -6 to 112200 sec.
Pentium 120 MHz 64 Mb RAM 2.1 Gb disk space Platform : Windows 95 MIL Lite deveiopment Ianguage DriverLiNXM3 4.0 board driver and custom controls Visual Basic 4.0
Design specifications for the P I N diodes :
Sm421
Angular displacan mt - degrees
Appendix C - Test Summary
TabIe C. 1 Summan of tests and test results.
P.I. O h 1 S.U.I. % 1 rnin/mas % 1 Centroiri 1 Coniriicrit';
position 20.13927 23.92340 47.2553 1 243, 232 huit! rionlc 17.9 1392 18.5227 1 59.45 199 249.124 obscuratiori
* Image ccnter is located at pisel position 240. 240.
Appendix D - Sample calculation
