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Available online at www.sciencedirect.com

Proceedings of the Combustion Institute 000 (2016) 1–8 www.elsevier.com/locate/proci

Multicomponent fuel droplet evaporation using 1D

Global Rainbow Technique

Jantarat Promvongsa

a , c , Pumyos Vallikul b , Bundit Fungtammasan

c , Annie Garo

a , Gerard Grehan

a , Sawitree Saengkaew

a , ∗

a CORIA-UMR6614 Normandie Université, CNRS, INSA et Université de Rouen, Av de l’Université, 76800 Saint Etienne du Rouvray, France

b Department of Mechanical and Aerospace Engineering, King Mongkut’s University of Technology, North Bangkok , Thailand

c Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, Bangkok, Thailand

Received 3 December 2015; accepted 3 August 2016 Available online xxx

Abstract

The accurate characterization of droplet evaporation requires measurement of both the size variation

(evaporation rate) and the temperature/composition evolution. To obtain the evaporation rate of moving droplets, a nanometer diameter change must be quantified on droplets of several dozens of microns, moving at a few meters per second. This paper used an innovative optical technique, 1D Global Rainbow Technique, to characterize locally the evaporation of gasoline droplets by measuring the droplet evaporation rate and

refractive index (i.e. temperature and composition). The information on the refractive index and the droplet size are extracted from the angular rainbow position and from the rainbow shape, respectively, while the evaporation rate is extracted from the ripple structure angular shift. © 2016 by The Combustion Institute. Published by Elsevier Inc.

Keywords: Optical diagnostics; Global Rainbow Technique; Liquid droplet temperature; Droplet evaporation

1. Introduction

The evaporation of liquid droplets is a key pro-cess involved in many engineering applications suchas spray drying, fire extinguishing, internal com-bustion engines, etc.

∗ Corresponding author. Fax: + 33 2 32959780. E-mail addresses: sawitree_s@coria.fr ,

s.sawitree76@gmail.com (S. Saengkaew).

http://dx.doi.org/10.1016/j.proci.2016.08.010 1540-7489 © 2016 by The Combustion Institute. Published by E

Please cite this article as: J. Promvongsa et tion using 1D Global Rainbow Technique, Procehttp://dx.doi.org/10.1016/j.proci.2016.08.010

During the combustion of liquid fuels, evapo- ration directly controls the combustion. Accord- ingly, the evaporation of droplets has been stud- ied in many kinds of experimental configurations, which can be divided into two categories: sus- pended droplets and moving droplets. Studies on

suspended droplets (on a wire [1] , acoustically levi- tated [2] , electrically levitated [3] , optically levitated

[4] ) focus essentially on the measurement of the evaporation rate, while the measurement of moving droplets is dedicated to the droplet temperature.

lsevier Inc.

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As evaporation has an impact on several pa-ameters such as size, temperature and compositionf droplets, a large number of experimental tech-iques have been developed to measure one or sev-ral of these characteristics. Without being exhaus-ive, the following approaches can be cited:

• Imaging technique [2] • Phosphorescence [5] • Fluorescence [6] • Morphological dependent resonances

(MDR) [7] • Raman scattering [8] .

Alternatively, rainbow techniques are attractiveecause both critical pieces of information, theroplet size and refractive index (which depends onroplet temperature and composition), can be mea-ured simultaneously from the light distributionround the rainbow angle. Rainbow techniques ex-st in two main configurations: one is the Stan-ard Rainbow Technique (SRT); the other is thelobal Rainbow Technique (GRT). The SRT ap-

roach is based on the assumption that only onearticle (or identical particles) creates the rainbowignal. Accordingly, signal-processing strategies areased on single-scattering theory (i.e. Airy, Nussen-veig, Debye and Lorenz-Mie’s theory). However,he main difficulties are due to the presence of high-requency fringes, called ripples, which are cre-ted by the interference between pure rainbow light p = 2) and the externally reflected light ( p = 0). Theipple structure is very sensitive to any change inhe refractive index, size, and shape of the droplet.hen, to carry out accurate measurements, the rip-le structure must be taken into account in theignal processing, as demonstrated by Saengkaewt al. [9] .

In contrast, the GRT approach is based on an-lyzing the global rainbow created by a large num-er of particles of different sizes. The summa-ion of a large number of rainbow signals issuedrom different particles removes the ripple struc-ure. Consequently, the sensitivity of the measure-ents to the particles’ shape is dramatically re-

uced. Accordingly, the developed processing strat-gy, based on matrix inversion with constraint andinimization [10] , allows the accurate extraction of

he refractive index value, and therefore the dropletemperature, to an accuracy of about 1 °C. TheRT can be applied at high pressure and high

emperature without adding any additive to theiquid. Therefore, this attractive technique can besed to characterize a wide range of spray applica-ions [11–13] . The GRT measures the size distribu-ion and averaged refractive index of the dropletst a defined “point”, which is spatially selectedy a system of lenses and pinholes [11,14] . Typi-ally, the size of the measurement volume is about mm

3 , according to optical configuration. In thearticular case of a line of monodispersed droplets,he measurement of size and refractive index can

Please cite this article as: J. Promvongsa et tion using 1D Global Rainbow Technique, Procehttp://dx.doi.org/10.1016/j.proci.2016.08.010

be as accurate as 0.01 μm for diameter and thefourth decimal for the refractive index by combin-ing the measurement at the rainbow angle with for-ward scattering pattern [10] . Recently, an extensionof GRT called One-Dimensional Global RainbowTechnique (1D-GRT), using slit apertures and alaser sheet, has been introduced by Wu et al. [15] .With this configuration, it is possible to measurethe evolution of droplet size and refractive indexwith high precision, from a single recorded rain-bow image. The aim of this paper is to demonstratethat this 1D-GRT configuration allows the extrac-tion of relevant parameters, describing multicom-ponent droplet evaporation (i.e. size, temperature,composition, evaporation rate) from a single rain-bow image. The technique has been developed andvalidated using N-heptane before being applied toa multicomponent gasoline.

2. Experimental setup

2.1.. 1D GRT configuration

The 1D-GRT setup is modified from the clas-sical global rainbow technique to extend the tech-nique from a point measurement (0D) to a one-dimensional measurement (1D). It was introducedby Wu et al. [15] for measuring the evolution of therefractive index of fuel spray along a line of view.Figure 1 is a schematic representation of the opti-cal setup. The control volume can be assimilated toa line, defined by a combination of lenses and slits.The rainbow patterns scattered from the dropletsat the different vertical locations along the controlvolume are recorded simultaneously in a single im-age. Two calibrations are required: a length cali-bration ( y -axis) and angular calibration ( x -axis).For the length calibration, the correspondence be-tween the vertical position on the CCD camera of the scattered light and the relative location of thedroplets in the control volume is correlated. For theangular calibration, a mirror is fixed on an accu-rate rotating goniometer. The location of the re-flected laser beam on the CCD combined with themirror orientation permits an accurate angular cal-ibration. In a single image, each line represents arainbow signal corresponding to different locationsof droplets, which is represented schematically inFig. 1 b. Consequently, by processing the rainbowsignals at different CCD lines, the spatial evolutionof diameter and refractive index of the dropletsalong the linear control volume can be measured.

The experimental setup is based on a continu-ous laser beam with a wavelength of 532 nm. Thecylindrical beam is transformed into a vertical lasersheet, which coincides spatially with the line of monodispersed droplets. To collect the scatteredlight, a horizontal slit is installed in front of thefirst lens. Accordingly, the position and dimensionsof the measurement volume can be specified. The

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Fig. 1. Schematic representation of 1D-GRT technique.

Fig. 2. Schematic view of droplet generator assembly.

collecting unit is composed of two plano-convexlenses, of 150 mm focal length and 75 mm diame-ter. The size of horizontal slit is 1 mm x 75 mm. Thelength of the control volume is 8 mm. The cameraused in this experiment is a JAI RM 4200 CL cam-era (2048 × 2048 pixels), with a pitch of 7.4 μm.The exposure time is 67 ms, and sampling rate is∼15 fps.

2.2. Droplet generator setup

In this study, a linear monodispersed dropletstream is generated by a TSI 3050 droplet gener-ator. With the appropriate flow rate and frequency,the droplets can be generated with equal space andsize. In this experiment, the droplet generator isused with a 50 μm diameter pinhole. The liquidflow rate is equal to 0.6 mL/min, and the excita-tion frequency is equal to 18.3 kHz. With these op-erating conditions, the diameter and velocity of

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droplets are approximately 100 μm and 5 m/s, re- spectively.

As displayed in Fig. 2 , a cavity chamber is in- stalled above the nozzle. The temperature in this small cavity is controlled. The droplets travel from

the nozzle, pass through that 15 mm height cavity before being exposed to the room conditions. The temperature of the air in the cavity ( T i ) at steady state is measured by a thermocouple. The measure- ment point is located 23 mm away from the nozzle orifice.

Experiments have been carried out at two differ- ent temperatures of the air in the cavity ( T i ), equal to 20 °C and 40 °C. The measurement position is moved by increments of 5 mm between 23 mm and

41 mm from the nozzle with an overlap of 3 mm

between each increment. The experiment is devoted

to the quantification of the droplet behavior during evaporation. Accordingly, two bottles of gasoline (surrogate: n-pentane 36%/isooctane 46%/n-

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Table 1 Some properties of the studied gasoline (surrogate: n - pentane 36%: iso-octane 46%: n -undecane 18% in vol- ume).

Density at 15 °C, (kg/m

3 ) 751.7 Vapor pressure, (mbar) 565 RON 95.5 C/H ratio, (% Mass) 6.58 Initial boiling point, ( °C) 30 5% Vol distillation, ( °C) 48

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Fig. 4. Rainbow signal sensitivity to refractive index value.

ndecane 18% in volume) are prepared. Someroperties of the gasoline are given in Table 1 .ne bottle is left closed, while the other one

emains opened at room temperature. On day 0,he initial properties of the fuel are measured.wo days later (day 2), the gasoline in the openedottle is investigated. On the eighth day (day 8),uel from both bottles is measured. A series of 200mages is recorded and then processed for each

easurement.

. Signal behavior and signal processing

.1. Signal behavior

Lorenz-Mie theory is used to numerically in-estigate rainbow behavior. The rainbow signal de-ends on the size and refractive index of the liq-id droplet. The droplet size essentially affects thehape and the intensity of the rainbow signal, whilehe refractive index essentially affects its angularocation. Figure 3 a compares the rainbow scatter-ng diagrams for three N-heptane droplets of dif-erent sizes (100, 101 and 110 μm), with a con-tant refractive index value of 1.3850. The changef the rainbow pattern can be observed when the

Fig. 3. Rainbow sensitivi

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droplet diameter increases from 100 to 110 μm. Forthe largest droplet, the main peak is narrower andthe intensity is significantly higher. However, whenthe droplet diameter changes by as little as 1 μm,the change in the rainbow signal is not visible.Hence, small changes in droplet size are difficult tomeasure. Figure 3 b displays the ripple shift whenthe particle diameter changes from 99 to 100 μm.The relationship between the phase difference andthe diameter change is discontinuous. Five periodsare observed. The ripple phase shift must be contin-ually measured to avoid 2 π ambiguity in the rela-tionship between the measured angular phase shiftand a diameter variation.

Moreover, the change in the refractive index af-fects the angular position of the rainbow signal,and also the phase of the ripple, as shown in Fig. 4 .The change in the ripple phase and the angularrainbow position can be detected even for a tinychange in the refractive index (up to the fourth dec-imal). For an N-heptane particle of 100 μm diam-

ty to particle size.

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Fig. 5. Behavior of 1D GRT image to size and refractive index variations.

eter and a refractive index equal to 1.3850, a phasechange of one period corresponds to a diameterchange of 0.2 μm, or a refractive index change of 0.0041.

Figure 5 displays simulated 1D-GRT imagescorresponding to six different lines of droplets. Foreach image, each column corresponds to a scatter-ing angle, and each row corresponds to the rainbowissued from droplets located along the linear mea-surement volume. The left and right axes representthe diameter and refractive index, respectively,while the horizontal axis is the scattering angle. Therainbow signals from droplets with a continuouslinear variation of diameter and/or refractive indexare compared to the rainbow signal from dropletswith a constant diameter and refractive index( Fig. 5 a). The simulated rainbow images show

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that the ripple phase changes with the changes in

droplet size and refractive index, but the angular position of the main rainbow depends only on

the refractive index value. Accordingly, when

the droplet diameter and refractive index change together, the ripple-phase shift ( �ϕ) is the sum of the phase change, due to the diameter evolution

( �ϕ d ); and the phase change, due to refractive index modification ( �ϕ n ), according to Eq. (1) .

�ϕ = �ϕ n + �ϕ d (1)

The size and refractive index contributions to

the shift of the ripple are independent. Conse- quently, the small change in droplet size can be ex- tracted from the ripple-phase change when the re- fractive index evolution is known.

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Fig. 6. 1D-GRT image recorded for two different temperatures in the cavity.

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.2. Signal processing

For each recorded 1D-GRT image, two calibra-ions are required: length and angular calibration.he length calibration gives the relationship be-

ween the droplets’ location in the measurementolume and the “vertical” pixel position on CCD.n this experiment, as illustrated in Fig. 1 b, nineands are selected, corresponding to 8 mm lengtheasurement volume (the measurement step is

qual to 1 mm). Each band contains 21 rows and isveraged along the columns to obtain the intensityector. The angular calibration is used to transformhe pixel column into the scattering angle.

The signal processing strategy is composed of wo steps. The first step is to measure the refractivendex and the droplet size with micrometric accu-acy. The second step is to measure the droplet sizevolution with nanometric accuracy.

• For the first step, the inversion code basedon Nussenzveig’s theory, developed bySaengkaew and co-workers [11] , is used.The refractive index and size of the dropletsare extracted by searching for the best fitbetween recorded and simulated signalsallowing the extraction of refractive indexwith an accuracy to the fourth digit, as wellas the size with an accuracy of about 0.2 μm.

• For the second step, the ripple phase shiftis measured using the Cross Spectrum Den-sity (CSD) approach [16] , permitting to mea-sure continually the ripple phase shift alonga trajectory. The contribution to the phasechange, due to the change of refractive index( �ϕ n ), is calculated from the refractive indexand size obtained from the first step of pro-cessing. The phase shift due to the change of diameter ( �ϕ d ) is obtained by removing thephase shift due to the refractive index ( �ϕ n )from the measured phase shift ( �ϕ), allow-ing the diameter change to be measured witha nanometric accuracy. This precision is nec-essary to be able to compute the local evapo-ration rate.

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4. Experimental results and discussion

The rainbow images recorded at different tem-peratures of the air cavity are shown in Fig. 6 .These images are characterized by a large brightspot (corresponding to the mean peak of the pri-mary rainbow ( p = 2)), with inclined fringes (corre-sponding to the ripple). The inclination angle of theripple fringes is proportional to the particle change(i.e. size and refractive index). The change of ripplephase is larger at higher temperature. The angularshift of the main rainbow position cannot be ob-served easily. However, the variation of the refrac-tive index can be measured from the extracted lightdistribution. Accordingly, the rainbow light distri-butions are extracted for each level and processed.

Figure 7 displays the extracted refractive in-dex of gasoline droplets at two different temper-atures of the air cavity. In all cases, the refractiveindex increases with the travelled distance of thedroplets. Moreover, the value of the refractive in-dex also increases with the bottle opening time, i.e.the measured refractive index values for measure-ments with a liquid extracted from a bottle openedfor 8 days are larger than those for a bottle openedfor 2 days, which are again larger than those for a“fresh” bottle. This behavior of the refractive in-dex can be attributed to the fact that the light com-ponents (more volatile) evaporate faster than theheavy components. The mixture becomes heavierduring the evaporation time (i.e. the density in-creases). Furthermore, it is well known that the re-fractive index value decreases when the tempera-ture increases. This behavior explains why the re-fractive index values are smaller in Fig. 7 b than inFig. 7 a.

In the opened bottle, the fuel was left to evapo-rate and the composition changed over time. Therefractive index of the gasoline increased as thedays passed. On the other hand, closing the bot-tle inhibited evaporation. The composition did notchange during storage. The extracted refractive in-dex in the closed bottle did not change significantly.These results underline the high sensitivity of thetechnique to tiny changes of the refractive indexvalue.

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Fig. 7. Extracted values of the droplet refractive index.

Fig. 8. Normalized diameter evolution.

The phase change, due to the change of dropletsize, can be obtained by subtracting the phasechange, due to the change of refractive index, fromthe total phase change extracted from the rain-bow signal. Using the correlations provided by theLorenz-Mie simulated signals, the diameter changeof droplets is evaluated. For droplets of about100 μm in diameter, diameter changes as small as20–70 nm are measured for a travel distance of 1 mm. By summing the diameter changes, the di-ameter evolution versus the travel distance has beenmeasured, as plotted in Fig. 8.

These curves show that the evaporation is verysensitive to the liquid composition. Moreover,when the droplet velocity is known, the diame-ter evolution can be straightforwardly transformedinto evaporation rate values from the difference of mass between two locations divided by the flightduration. For example, Table 2 compiles the evapo-ration rates deduced from measurements at 17 and18 mm for air cavities at 20 °C and 40 °C, for a par-ticle velocity of 5 m/s.

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5. Conclusions

The newly developed One-Dimensional Global Rainbow Technique (1D-GRT) allows one to

follow the thermo-physical evolution of droplets along a line of view from the recording of the light distribution around the rainbow angle. This paper presents simultaneous measurements of size change and refractive index evolution (i.e. tem- perature and composition evolutions) of moving gasoline droplets, using 1D-GRT. In a first step, the detailed rainbow behavior was investigated

by systematic simulations, based on rigorous Lorenz-Mie theory, to determine the correlation

between the ripple-phase change and the changes in droplet size and refractive index. By processing recorded rainbow signals with a numerical code based on Nussenzveig’s theory, the refractive index and particle size were instantaneously extracted. By processing a pair of rainbow signals, the ripple-phase evolution was measured, enabling the quantification of the droplet size change at a nanometric scale. Taking into account the droplet velocity, the local evaporation rate is easily ob-

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Table 2 Measured evaporation rates.

20 °C, day 8 20 °C, day 0 40 °C, day 8 40 °C, day 0

Diameter 99.52 μm 99.27 μm 99.38 μm 99.17 μm

Diameter change 0.30 μm 0.40 μm 0.30 μm 0.70 μm

Evaporation rate 1.7 μg/s 2.3 μg/s 1.7 μg/s 4.0 μg/s

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ained from these data. The results show that theefractive index increases while the evaporationate decreases, due to the loss of the light compo-ents, demonstrating the high sensitivity of thisechnique to the characterization of droplets; e.g.omposition, temperature and size.

The next steps are the extension of this ap-roach to droplets in more hostile environmentse.g. high temperature, high or low pressure,ames), using a pulse laser, as well as comparinghe experimental results to numerical simulations.hese two developments are currently in progress.

cknowledgments

The financial support provided by Thailand Re-earch Fund (TRF) through the Golden Jubileeh.D. Program (Grant No. PHD/0272/2551 ), theuropean Program INTERREG IVA-E3C3 andNR-Astrid DEVACOL are gratefully acknowl-

dged.

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