Hannah E. Bower, Scott B. Leask, and Vincent G. McDonell ... · High Speed Cinematography High...
Transcript of Hannah E. Bower, Scott B. Leask, and Vincent G. McDonell ... · High Speed Cinematography High...
ILASS Americas 27th Annual Conference on Liquid Atomization and Spray Systems, Raleigh, NC, May 2015
Study of Sprays Generated by Impinging Liquid Jets from Unlike Doublet Injectors
Hannah E. Bower, Scott B. Leask, and Vincent G. McDonell*
UCI Combustion Laboratory
Mechanical & Aerospace Engineering
University of California, Irvine
Irvine, CA 92697-3550 USA
Abstract
This paper explores the application of optical diagnostics for the study of the structure of sprays generated by unlike
doublet injectors. The cold-flow spray characteristics can be compared to corresponding hot-fire tests on prototype
thrusters. A set of conditions, including differing injector impingement angles, were tested using high speed cinema-
tography, laser diffraction, and a novel dual wavelength laser induced fluorescence system (DLIF). The DLIF system
involved the development of appropriate dye/solvent mixtures that can be optically isolated through use of two exci-
tation wavelengths and corresponding optical filters for isolating the fluorescence from the liquid stream from each of
the two injectors. Additional analysis was required to assess the extent to which wavelength dependent scattering by
the droplets impacted the relative intensity of the fluorescence from each stream. This was accomplished using cal-
culated extinction coefficients from Mie scattering theory for each excitation wavelength combined with the measured
size distribution from laser diffraction. The high speed cinematography captures the highly dynamic process induced
by the impinging jets. The periodic nature of the atomization process for this type of injection strategy is clearly
indicated and quantified in terms of dominant wavelengths and frequencies. High speed video is also used to quantify
the resulting spread angle of the spray plume and the average spray breakup generated by the impingement. Addi-
tionally, the laser diffraction size distribution results indicated larger Sauter Mean diameters and volume distribution
diameters for smaller impingement angles. The visualization results from the DLIF system indicated the effects of
impingement angle on overall mixing of the two liquid streams. The results from the various optical measurement
methods clearly demonstrate the strong dependence of the spray structure on impingement angle.
Keywords: Spray, atomization, measurement, drop size, imaging diagnostics
*Corresponding author: [email protected]
Introduction
Unlike doublet injectors are commonly used for liq-
uid rocket combustion technologies. Unlike doublet in-
jectors are used most often for propellant combinations
that have approximately equal fuel and oxidizer injection
orifice areas and momentum ratios [1]. Unlike doublet
injectors also produce finer atomization elements and
have higher performance efficiencies, but usually pro-
duce less stable flames [1]. However, experiments
aimed at understanding the cold flow (i.e., atomization,
dispersion, mixing, and evaporation) behavior of avia-
tion and rocket fuel using impinging jets have not been
sufficiently studied to date. Most tests on rocket injector
engines, such as the Lunar Module Decent Engine
(LMDE) and the SENTRY Pinch and Yew engine con-
sist of only hot fire tests, as mentioned in the “TRW Pin-
tle Heritage and Performance Characteristics” paper by
G.A. Dressler [2]. Few hot fire tests have been coupled
with cold flow tests. R.J Santoro and C.L. Merkle per-
formed cold flow tests to determine spray angles as TMR
varied and coupled them with hot fire test to examine ig-
nition and combustion characteristics [3]. Furthermore,
J. Houseman performed a study on mixing of hypergolic
propellants in unlike doublet injectors, which displayed
large concentration gradients in the chamber 5 inches
downstream from the impingement point. J. Houseman’s
study also depicts images of the relative mixing of the
fuel and oxidizer along with the corresponding combus-
tion efficiencies for penetrated, mixed and separated liq-
uid propellants [4]. However, Santoro and Merkle’s and
Houseman’s cold flow tests did not examine any other
fuel characteristics, such as droplet size and velocity,
which could have further enhanced their study of the RP-
1 fuel. Therefore, further research in the flow character-
istics of unlike doublet injectors could lead to better de-
signed and more efficient combustion systems. This pa-
per explores multiple experimental techniques to better
classify and describe the spray structure produced from
two unlike doublet injectors, to help advance combustion
technologies for many applications, such as the aero-
space or automotive industries.
Approach
The approach taken was to experimentally study the
structure of sprays from two unlike doublet injectors. A
test rig was constructed to allow the injectors to be posi-
tioned at precise locations, creating different spray im-
pingement angles, while advanced diagnostic measure-
ments were taken to characterize the spray structure.
High speed cinematography, Malvern laser systems, Mie
scattering modeling and Dual Wavelength Laser Induced
Fluorescence (DLIF) techniques were used for spray
characterization. Water was used for all experiments as
a replacement for RP-1 and other liquid propellants.
Experimental and Analytical Methods
Mie Scattering
A Mie scattering code (Light Lab: Far Field Mie
Scattering) was used to model extinction coefficients in
relation to droplet size. The parameters used to perform
the Mie scattering calculations are displayed in Table 1.
Laser Diffraction
To study the droplet size distribution of the spray, a
Malvern RTS-100 laser system was used. The Malvern
encompasses a line of sight laser diffraction method to
measure and calculate droplet size distributions of a line
within the spray. For the Malvern droplet distribution
tests, three different injector impingement angles (50°,
70° and 90°) were tested. Data taken for all three angles
were taken at a constant Z-axis position, 6mm down-
stream from the impingement point, and Y-axis position,
with varying X-axis positions ranging from -6.000mm to
8.000mm by increments of 2.000mm, with the X-posi-
tion 0.000mm at the center of the spray cone, as dis-
played in Figure 1. The flow rates for the oxidizer and
fuel were kept at 15.3kg/hr and 11.5kg/hr, respectively.
In order to determine the scattering efficiency de-
pendence on wavelength, the Malvern distribution was
analyzed using a Rosin-Rammler distribution function,
which then was coupled to the Mie scattering data. A
MATLAB tool, created by Ivan Brezani, et.al., was used
to transform the Malvern Gaussian size distribution to fit
a Rosin-Rammler distribution function [5]. Coupling the
calculated Mie scattering extinction coefficients with the
size distribution allowed for the calculation of the extinc-
tion coefficient for each wavelength (447nm and 655nm)
at each tested X-axis position to be calculated. Finally,
ratios of the calculated extinction coefficients for each
laser were compared and the dependence of wavelength
on scattering efficiency was determined.
Table 1. Parameters used for Mie scattering calculations
Parameter Value Droplet Diameter Variable
Range of Droplet Diame-ter
0.0 to 300 m
Index of Refraction- Real Part
1.33
Index of Refraction- Im-aginary Part (oxazine)
7.19x10-5
Index of Refraction- Im-aginary Part (fluores-
cein)
3.24x10-8
Wavelength of Beam 0.447um, 0.655um Beam Waist Diameter 1.00E6 um
Scattering Angle 0.00 Degrees Number of Data Points 1500
3
Figure 1. Malvern measurement schematic
High Speed Cinematography
High speed shadowgraph videos were captured us-
ing a Vision Research Phantom v7.1 monochrome digi-
tal high speed camera. Videos were recorded in 256x256
pixel resolution at 26143 frames per second. Snapshot
images were taken from the videos and used to depict the
structure of the sprays at the three nozzle impingement
angles tested (50°, 70° and 90°).
MATLAB was utilized for image analysis from the
data gathered by the high speed cinematography. The
code was written to analyze the pixel intensity distribu-
tions of the spray videos which then in turn produced av-
erage spray structure characteristics. The average diverg-
ing spray angle and periodic spray breakup points were
the characteristics considered. The average diverging
spray angle was calculated by producing linear lines
which corresponded to the spray plume edges. These
lines were found for 100 frames of the high speed video
which were then averaged. The angle between these lines
resulted in the average diverging spray angle. The peri-
odic spray breakup points were found through vertical
line intensity profiles. These profiles located the maxi-
mum and minimum intensity locations which produced
the distances between breakups.
LaVision DaVis 7.2.2.429 software was used to ex-
tract velocity information from the high speed videos of
the sprays. The video files were imported into the soft-
ware while maintaining the correct pixel size and time
between each frame via import options. With the im-
ported data, a PIV time series operation was utilized to
produce a vector field for each frame of the video. Proper
Orthogonal Decomposition (POD) was then applied to
the PIV time series data which produced an average vec-
tor field for each impinging jet angle.
DLIF
The two lasers used in the DLIF system were a
447nm OEM MDL-III-447 laser and a 655nm OEM
MRL-III-FS-655 laser. Each beam was collimated at the
laser exit using an Opto Engine 400 micron multimode
fiber coupler SMA905 connector. The fiber couplers
were then attached to an Ocean Optics Split400-UVVIS
400uM Fiber splitter/combiner. Using the combiner, the
447nm and 655nm beams were combined to a single fi-
ber. A 40mm collimating lens was then used at the end
of the fiber to collimate both beams together. For this
experiment, line measurements were taken to gain un-
derstanding about the relative concentration of the liquid
originating from each of the doublet injectors. An Andor
i-Star Intensified CCD camera was placed at a 90° angle
from the DLIF lasers and a 675nm -10nm and a 532 -
2nm optical bandpass filters were placed on the camera
to isolate the concentrations of both the oxazine (“oxi-
dizer”) and fluorescein (“fuel”) could be determined, re-
spectively. Figure 2 displays the DLIF instrumentation
schematic.
Figure 2. DLIF instrumentation schematic
Furthermore, the DLIF setup was used to confirm
the experimental scattering efficiency dependence on
wavelength. The initial intensity (I0) of each laser was
measured via a power meter. Then the incident laser in-
tensity of the laser beam after it traveled through the
spray plume (I) was measured. The I0/I at each X-axis
position were calculated and the two lasers were then
compared to each other.
Results
Mie Scattering
In order to model the Mie scattering of the spray, the
imaginary part of the index of refraction was calculated.
The absorption coefficient for each wavelength was
found from tests performed using the two lasers that
make up the DLIF system. Two calibration curves were
constructed, one for each laser used. The absorption co-
efficient for each wavelength is described by the equa-
tion
𝛾−1 =𝜆
4𝜋𝑛′ (1)
where 𝛾 is the absorption coefficient, λ is the laser wave-
length and n’ is the imaginary part of the index of refrac-
tion [6,7].
A series of solutions of differing concentrations of
oxazine in water and fluorescein in water were prepared
for the 655nm and 447nm calibration curves, respec-
tively. The oxazine and fluorescein solution concentra-
tions tested are displayed in Table 2.
4
Table 2. Fluorescein and oxazine concentrations used
for the calibration curves.
Fluorescein solutions (mM) Oxazine solutions (mM)
2.13 1.37 2.19 1.95 5.33 2.93
10.65 4.88 15.97 9,.75 21.3 14.6
- 19.5
For each concentration, the solution was placed in a
quartz cuvette 6mm from the laser. A power meter was
placed behind the cuvette and recorded the intensity of
the light exiting the cuvette, as displayed in the experi-
mental schematic shown below in Error! Reference
source not found.. The initial intensity of each laser was
recorded via the power meter in the absence of the cu-
vette. Furthermore, a “blank”, containing deionized wa-
ter, was measured before each concentration data point
was collected for each wavelength.
Figure 3. Laser system schematic for the creation of the
fluorescein and oxazine calibration curves.
Calibration curves were then constructed from the
data points collected. Two additional calibration curves
using the same solutions were created using a Shimadzu
UV-1700 Spectrometer. The UV-Vis spectrometer cali-
bration curves were then used in comparison with the la-
ser induced fluorescence (LIF) curves to determine the
accuracy and precision of the LIF calibration curves.
Furthermore, using the calibration curves and Beer’s
Law, the absorption coefficient was determined for both
oxazine and fluorescein.
Calibration curves to determine the absorption coef-
ficient of both the 447nm laser with differing concentra-
tions of fluorescein and the 655nm laser with differing
concentrations of oxazine were constructed. Figure 4 dis-
plays the oxazine calibration curve from the LIF system
with concentrations ranging from 1.37 M to 19.5 M.
Three calibration curves were constructed at 655nm
using the LIF method and the same solutions. An aver-
age absorption coefficient (slope) was calculated, equal
to 0.0725.
Figure 4. Laser induced fluorescence calibration curve
for oxazine.
An additional calibration curve was constructed us-
ing the Shimadzu UV-1700 Spectrometer to determine
the precision and accuracy of the LIF systems. The UV
Vis spectrometer calibration curve for oxazine is dis-
played in Figure 5.
The UV Vis spectrometer calibration curve for oxa-
zine exhibits an absorption coefficient equal to 0.0744,
which differs from that of the LIF system by 2.55%.
Figure 6 displays the fluorescein calibration curve
from the LIF system with concentrations ranging from
2.13 M to 21.3 M. Three separate calibration curves
were made using the laser induced fluorescence method
and the same solutions, for precision. An average absorp-
tion coefficient (slope) was calculated, equal to 0.0091.
An additional calibration curve was constructed for
the fluorescein solutions using the UV Vis spectrometer
at 446nm, which is represented in Figure 7. The UV Vis
spectrometer calibration curve for fluorescein depicts an
absorption coefficient equal to 0.0126, which differs
from that of the LIF system by 27.8%.
Figure 5. UV Vis spectrometer calibration curve for
oxazine.
y = 0.0724x + 0.0111R² = 0.9999
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 5 10 15 20 25
Ab
sorb
an
ce
Concentration
Absorbance vs. Concentration (uM) of Oxazine at 655nm-LIF system
y = 0.0744x - 0.0037R² = 0.999
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 5 10 15 20 25
Ab
sorb
an
ce
Concentration (uM)
Absorbance vs. Concentration (uM) of Oxazine at 655nm- UV Vis Spectrometer
5
Figure 6. Laser induced fluorescence calibration curve
for Fluorescein
Figure 7. UV Vis spectrometer calibration curve for flu-
orescein
The calculated absorption coefficients for both the
oxazine and fluorescein experiments were used in the
imaginary part of the index of refraction calculations.
The imaginary part for 447nm and 655nm were deter-
mined via Equation 1 to be 3.24E-8 and 7.19E-5, respec-
tively. The calculated Mie scattering results from the
Light Lab software were then coupled with the Malvern
distribution data that was transformed using a Rosin-
Rammler distribution function to determine the extinc-
tion coefficients for each laser at the X-axis positions
tested. The ratio of the calculated 447nm and 655nm ex-
tinction coefficients at the varying X-axis positions are
displayed in Table 3.
Based off of the data collected and analyzed, all
three impingement angles tested display ratios close to
1.0 for all X-axis positions tested. Based on these results,
no correction factor is needed to account for the differ-
ences in scattering efficiency when using the two wave-
lengths in the DPLIF system.
Table 3. Calculated extinction coefficient ratios
(447nm/655nm)
X-axis Po-sition
Impingement Angle (degrees)
50 70 90 -6 1.00 1.05 1.06
-4 1.05 1.05 1.06
-2 1.04 1.05 1.06
0 1.04 1.05 1.06
2 1.07 1.05 1.06
4 1.04 1.05
1.06
6 1.05 1.06 1.06
8 1.05 1.06 1.06
Experimental data were collected by measuring the
initial intensity and the intensity of the laser after it trav-
eled through the spray for each laser at a 90° impinge-
ment angle. The ratio of I/I0 for each laser at each X-axis
position were then compared and are displayed in Table
4. The experimental data and modeled Mie scattering
data displays extinction coefficient ratios near 1.0 for all
X-axis positions tested. Therefore, the experimental data
and calculated results support the wavelength having no
dependence on the scattering efficiency.
Table 4. Experimental extinction coefficient ratios
(447nm/655nm)
X-axis Position Extinction Coeffi-cient Ratio
(447nm/655nm)
-6 1.04
-4 0.99
-2 0.99
0 0.99
2 0.98
4 1.01
6 0.98
8 0.97
High Speed Imaging
Video results were obtained for each impingement
angle tested. In order to understand the dynamic struc-
ture of the sprays, the video recordings were taken at
26143 frames per second. As a result, snapshot pictures
are shown to illustrate and support the conclusions drawn
from the videos. Figure 8 displays pictures from the three
impingement angles tested taken at the camera angles of
0°, 45° and 90° in relation to the impinging unlike dou-
blet injectors’ plane.
y = 0.0093x + 0.0024R² = 0.9944
0
0.05
0.1
0.15
0.2
0.25
0 5 10 15 20 25
Ab
sorb
an
ce
Concentration (uM)
Absorbance vs. Concentration (uM) of Fluorescein at 447nm-LIF System
y = 0.0126x + 0.0037R² = 0.9992
0
0.05
0.1
0.15
0.2
0.25
0.3
0 5 10 15 20 25
Ab
sorb
an
ce
Concentration (uM)
Absorbance vs. Concentration (uM) of Fluorescein at 446nm-UV Vis Spectrometer
6
Impinge-ment an-gle
Camera An-gle= 90°
Camera Angle= 45°
Camera Angle= 0°
50°
70°
90°
Figure 8. Images from the three impingement angles
tested taken at differing camera angles
The videos taken at the 0° camera angle for the three
respective impingement angles were read into the
MATLAB code to calculate the average spray angle. The
results from the code are found in Table 5
Table 5. Spray angle versus impingement angle.
Impingement Angle Average Spray Angle
50° 49.9° 70° 72.5° 90° 51.5°
It can be noted from this that the 70° impingement
angle produces the largest average spray angle.
Following this, the same impingement angles were
then read into the code for the average distance between
spray breakup points. The results are found in Table 6
and illustrate that the breakup distance increases with
larger impingement angles.
Table 6. Average distance between spray breakup
points at different impingement angles.
Impingement Angle Average Breakup Dis-tance (microns)
50° 297.44 70° 325.16 90° 358.60
The videos taken at the 0° camera angle for the three
respective impingement angles were imported into the
PIV software and POD was implemented on each video.
Figures 9-11 show the average vector field for the 50°,
70° and 90° impinging jet angles, respectively.
Figure 9. Average vector field for the 50° impingement
angle.
Figure 10. Average vector field for the 70° impinge-
ment angle.
Figure 11. Average vector field for the 90° impingement
angle.
7
Laser Diffraction
To help further understand the spray structure at the
differing impingement angles, droplet size measure-
ments were taken. Profiles of the Sauter mean diameter
(SMD, D32), DV90, and the distribution span ([Dv90-
Dv10]/Dv50) were calculated and are represented in Fig-
ures 12-14.
Note that the SMD trends for the impingement an-
gles tested display similar trends, with an increase in
droplet diameter near the 0mm X-axis position and
smaller SMD diameters at the -6mm and +8mm X-axis
positions. The 50° impingement angle also demonstrates
the largest SMD diameters at all X-axis positions. Also,
all three impingement angles tested show similar volume
distribution trends to that of SMD. The span distribution
for the 70° impingement angle exhibits the widest distri-
bution of droplet sizes compared to the other impinge-
ment angles tested.
Figure 12. The Sauter Mean Diameter distributions for
the spray at various impingement angles.
Figure 13. The volume distributions for the spray at var-
ious impingement angles.
Figure 14. The span distribution for the spray at various
impingement angles.
DLIF
To further understand the mixing of the fuel and ox-
idizer, DLIF line measurements were performed. Images
from each laser at each X-axis position were collected.
Since the camera was placed at a 90° angle from the laser
beam entering the spray, a Y-axis intensity profile was
constructed via a MATLAB code. Furthermore, the max-
imum intensities of pure oxazine and fluorescein solu-
tions at 655nm and 447nm, respectively, were measured
and used for normalization. The relative intensities of the
oxazine to fluorescein at a 50° impingement angle were
calculated and are demonstrated in Figure 15.
The plot shows a steep peak between the Y-axis po-
sitions of 100-150 pixels for each of the X-axis positions
tested. Note the low trough displayed between Y-axis
positions 150-200 pixels for each line tested. These
troughs suggest that the blue laser (fluorescein) is domi-
nant in this region. This could be due to the fact that the
injector spraying fluorescein is present at the 200 pixel
Y-axis position.
Also, take note that the relative intensity values be-
tween 50-100 pixel Y-axis positions are close to 5. This
suggests that the red laser (oxazine) is dominant in this
region, which could be due to the oxazine injector’s
placement at the 0 pixel Y-axis position. The differences
in relative intensities between the 50-100 pixel and 150-
200 pixel Y-axis positions could also be due to a slight
offset of the jets in the Z-axis direction.
Finally, this plot could suggest that the optimal mix-
ing of the fluorescein and oxazine is at the full-width-
half-max of the displayed peaks. Further testing and data
analysis will be done to confirm this.
50
70
90
110
130
150
170
190
210
230
-7 -2 3 8
Dro
ple
t D
iam
ete
r (u
m)
X-axis Position
Sauter Mean Diameter Distribution for Varying Spray Impingement Angles
90 degrees
70 Degrees
50 Degrees
150
250
350
450
550
650
750
-7 -2 3 8
Dro
ple
t D
iam
ete
r (u
m)
X-axis Position
Volume Distribution for Varying Spray Impingement Angles
90degrees
70Degrees
50Degrees
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
-7 -2 3 8
Dro
ple
t S
pa
n
X-axis Position
Span Distribution for Varying Spray Impingement Angles
90Degrees
70Degrees
50Degrees
8
Red to Blue Laser Relative Intensities for the 50° Impingement
angle
Figure 15. 3D relative intensity plots for the 50° im-
pingement angle.
Summary
An experimental investigation of spray structure
from unlike doublet injectors was conducted. Based on
the high speed cinematography data analysis performed,
the 70° impingement angle displayed the highest spray
cone angle, while the 90° impingement angle displayed
the largest average sheet breakup distance. Furthermore,
the drop size data collected suggests that the 50° im-
pingement angle produces the largest droplet Sauter
Mean diameter and volume distribution droplet size for
each X-axis position tested, demonstrating a dependence
of the spray structure on impingement angle. Addition-
ally, the Mie scattering calculations coupled with the
Malvern and light intensity data suggest that there is no
wavelength dependence on scattering efficiency
throughout the experiments ran. Lastly, the DLIF data
collected suggests that there may be unequal mixing of
the oxidizer and fuel throughout the spray plume.
Additional experiments need to be conducted to help
better understand the dynamic spray structure produced
by two unlike doublet injectors. In the future, planar
measurements will be taken with the DLIF system. Also,
image splitter instrumentation will be incorporated in the
DLIF system so that the concentration/intensity of both
fluorescein and oxazine can be measured at the same
time, reducing systematic and human errors. Addition-
ally, more droplet size and high speed cinematography
data will be collected to help better understand the dy-
namics present in the sprays. All of these results can help
to better interpret the role of fuel/oxidizer injection on
thrust performance results in associated hot fire tests.
Acknowledgements
The authors would like to acknowledge the assis-
tance of the University of California, Irvine Spectros-
copy Laboratory and BioTel Laboratory for access to
their laboratories. The authors and Mr. Leask in particu-
lar also acknowledge the University of Glasgow Engi-
neering Department for assisting in the collaboration
with the University of California, Irvine. Mr. Leask is at
UC Irvine as part of an international education oppor-
tunity made possible by the University of Glasgow.
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