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Multicomponent fuel droplet evaporation using 1D Global Rainbow Technique
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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 process involved in many engineering applications such as spray drying, fire extinguishing, internal combustion engines, etc. ∗
Corresponding author. Fax: +33 2 32959780. E-mail addresses: [email protected], [email protected] (S. Saengkaew).
During the combustion of liquid fuels, evaporation directly controls the combustion. Accordingly, the evaporation of droplets has been studied in many kinds of experimental configurations, which can be divided into two categories: suspended droplets and moving droplets. Studies on suspended droplets (on a wire [1], acoustically levitated [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.
http://dx.doi.org/10.1016/j.proci.2016.08.010 1540-7489 © 2016 by The Combustion Institute. Published by Elsevier Inc.
Please cite this article as: J. Promvongsa et al., Multicomponent fuel droplet evaporation using 1D Global Rainbow Technique, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.08.010
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As evaporation has an impact on several parameters such as size, temperature and composition of droplets, a large number of experimental techniques have been developed to measure one or several of these characteristics. Without being exhaustive, the following approaches can be cited: • • • •
Imaging technique [2] Phosphorescence [5] Fluorescence [6] Morphological dependent (MDR) [7] • Raman scattering [8].
resonances
Alternatively, rainbow techniques are attractive because both critical pieces of information, the droplet size and refractive index (which depends on droplet temperature and composition), can be measured simultaneously from the light distribution around the rainbow angle. Rainbow techniques exist in two main configurations: one is the Standard Rainbow Technique (SRT); the other is the Global Rainbow Technique (GRT). The SRT approach is based on the assumption that only one particle (or identical particles) creates the rainbow signal. Accordingly, signal-processing strategies are based on single-scattering theory (i.e. Airy, Nussenzveig, Debye and Lorenz-Mie’s theory). However, the main difficulties are due to the presence of highfrequency fringes, called ripples, which are created by the interference between pure rainbow light (p = 2) and the externally reflected light (p = 0). The ripple structure is very sensitive to any change in the refractive index, size, and shape of the droplet. Then, to carry out accurate measurements, the ripple structure must be taken into account in the signal processing, as demonstrated by Saengkaew et al. [9]. In contrast, the GRT approach is based on analyzing the global rainbow created by a large number of particles of different sizes. The summation of a large number of rainbow signals issued from different particles removes the ripple structure. Consequently, the sensitivity of the measurements to the particles’ shape is dramatically reduced. Accordingly, the developed processing strategy, based on matrix inversion with constraint and minimization [10], allows the accurate extraction of the refractive index value, and therefore the droplet temperature, to an accuracy of about 1 °C. The GRT can be applied at high pressure and high temperature without adding any additive to the liquid. Therefore, this attractive technique can be used to characterize a wide range of spray applications [11–13]. The GRT measures the size distribution and averaged refractive index of the droplets at a defined “point”, which is spatially selected by a system of lenses and pinholes [11,14]. Typically, the size of the measurement volume is about 1 mm3 , according to optical configuration. In the particular case of a line of monodispersed droplets, the measurement of size and refractive index can
be as accurate as 0.01 μm for diameter and the fourth decimal for the refractive index by combining the measurement at the rainbow angle with forward scattering pattern [10]. Recently, an extension of GRT called One-Dimensional Global Rainbow Technique (1D-GRT), using slit apertures and a laser sheet, has been introduced by Wu et al. [15]. With this configuration, it is possible to measure the evolution of droplet size and refractive index with high precision, from a single recorded rainbow image. The aim of this paper is to demonstrate that this 1D-GRT configuration allows the extraction of relevant parameters, describing multicomponent droplet evaporation (i.e. size, temperature, composition, evaporation rate) from a single rainbow image. The technique has been developed and validated using N-heptane before being applied to a multicomponent gasoline. 2. Experimental setup 2.1.. 1D GRT configuration The 1D-GRT setup is modified from the classical global rainbow technique to extend the technique from a point measurement (0D) to a onedimensional measurement (1D). It was introduced by Wu et al. [15] for measuring the evolution of the refractive index of fuel spray along a line of view. Figure 1 is a schematic representation of the optical setup. The control volume can be assimilated to a line, defined by a combination of lenses and slits. The rainbow patterns scattered from the droplets at the different vertical locations along the control volume are recorded simultaneously in a single image. Two calibrations are required: a length calibration (y-axis) and angular calibration (x-axis). For the length calibration, the correspondence between the vertical position on the CCD camera of the scattered light and the relative location of the droplets in the control volume is correlated. For the angular calibration, a mirror is fixed on an accurate rotating goniometer. The location of the reflected laser beam on the CCD combined with the mirror orientation permits an accurate angular calibration. In a single image, each line represents a rainbow signal corresponding to different locations of droplets, which is represented schematically in Fig. 1b. Consequently, by processing the rainbow signals at different CCD lines, the spatial evolution of diameter and refractive index of the droplets along the linear control volume can be measured. The experimental setup is based on a continuous laser beam with a wavelength of 532 nm. The cylindrical beam is transformed into a vertical laser sheet, which coincides spatially with the line of monodispersed droplets. To collect the scattered light, a horizontal slit is installed in front of the first lens. Accordingly, the position and dimensions of the measurement volume can be specified. The
Please cite this article as: J. Promvongsa et al., Multicomponent fuel droplet evaporation using 1D Global Rainbow Technique, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.08.010
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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-convex lenses, of 150 mm focal length and 75 mm diameter. The size of horizontal slit is 1 mm x 75 mm. The length of the control volume is 8 mm. The camera used in this experiment is a JAI RM 4200 CL camera (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 droplet stream is generated by a TSI 3050 droplet generator. With the appropriate flow rate and frequency, the droplets can be generated with equal space and size. In this experiment, the droplet generator is used with a 50 μm diameter pinhole. The liquid flow rate is equal to 0.6 mL/min, and the excitation frequency is equal to 18.3 kHz. With these operating conditions, the diameter and velocity of
droplets are approximately 100 μm and 5 m/s, respectively. As displayed in Fig. 2, a cavity chamber is installed 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 (Ti ) at steady state is measured by a thermocouple. The measurement point is located 23 mm away from the nozzle orifice. Experiments have been carried out at two different temperatures of the air in the cavity (Ti ), 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-
Please cite this article as: J. Promvongsa et al., Multicomponent fuel droplet evaporation using 1D Global Rainbow Technique, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.08.010
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Table 1 Some properties of the studied gasoline (surrogate: npentane 36%: iso-octane 46%: n-undecane 18% in volume). Density at 15 °C, (kg/m3 ) Vapor pressure, (mbar) RON C/H ratio, (% Mass) Initial boiling point, (°C) 5% Vol distillation, (°C)
751.7 565 95.5 6.58 30 48
undecane 18% in volume) are prepared. Some properties of the gasoline are given in Table 1. One bottle is left closed, while the other one remains opened at room temperature. On day 0, the initial properties of the fuel are measured. Two days later (day 2), the gasoline in the opened bottle is investigated. On the eighth day (day 8), fuel from both bottles is measured. A series of 200 images is recorded and then processed for each measurement.
3. Signal behavior and signal processing 3.1. Signal behavior Lorenz-Mie theory is used to numerically investigate rainbow behavior. The rainbow signal depends on the size and refractive index of the liquid droplet. The droplet size essentially affects the shape and the intensity of the rainbow signal, while the refractive index essentially affects its angular location. Figure 3a compares the rainbow scattering diagrams for three N-heptane droplets of different sizes (100, 101 and 110 μm), with a constant refractive index value of 1.3850. The change of the rainbow pattern can be observed when the
Fig. 4. Rainbow signal sensitivity to refractive index value.
droplet diameter increases from 100 to 110 μm. For the largest droplet, the main peak is narrower and the intensity is significantly higher. However, when the 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 to measure. Figure 3b displays the ripple shift when the particle diameter changes from 99 to 100 μm. The relationship between the phase difference and the diameter change is discontinuous. Five periods are observed. The ripple phase shift must be continually measured to avoid 2π ambiguity in the relationship between the measured angular phase shift and a diameter variation. Moreover, the change in the refractive index affects 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 angular rainbow position can be detected even for a tiny change in the refractive index (up to the fourth decimal). For an N-heptane particle of 100 μm diam-
Fig. 3. Rainbow sensitivity to particle size.
Please cite this article as: J. Promvongsa et al., Multicomponent fuel droplet evaporation using 1D Global Rainbow Technique, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.08.010
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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 phase change of one period corresponds to a diameter change of 0.2 μm, or a refractive index change of 0.0041. Figure 5 displays simulated 1D-GRT images corresponding to six different lines of droplets. For each image, each column corresponds to a scattering angle, and each row corresponds to the rainbow issued from droplets located along the linear measurement volume. The left and right axes represent the diameter and refractive index, respectively, while the horizontal axis is the scattering angle. The rainbow signals from droplets with a continuous linear variation of diameter and/or refractive index are compared to the rainbow signal from droplets with a constant diameter and refractive index (Fig. 5a). The simulated rainbow images show
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 = ϕn + ϕd
(1)
The size and refractive index contributions to the shift of the ripple are independent. Consequently, the small change in droplet size can be extracted from the ripple-phase change when the refractive index evolution is known.
Please cite this article as: J. Promvongsa et al., Multicomponent fuel droplet evaporation using 1D Global Rainbow Technique, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.08.010
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Fig. 6. 1D-GRT image recorded for two different temperatures in the cavity.
3.2. Signal processing
4. Experimental results and discussion
For each recorded 1D-GRT image, two calibrations are required: length and angular calibration. The length calibration gives the relationship between the droplets’ location in the measurement volume and the “vertical” pixel position on CCD. In this experiment, as illustrated in Fig. 1b, nine bands are selected, corresponding to 8 mm length measurement volume (the measurement step is equal to 1 mm). Each band contains 21 rows and is averaged along the columns to obtain the intensity vector. The angular calibration is used to transform the pixel column into the scattering angle. The signal processing strategy is composed of two steps. The first step is to measure the refractive index and the droplet size with micrometric accuracy. The second step is to measure the droplet size evolution with nanometric accuracy.
The rainbow images recorded at different temperatures of the air cavity are shown in Fig. 6. These images are characterized by a large bright spot (corresponding to the mean peak of the primary rainbow (p = 2)), with inclined fringes (corresponding to the ripple). The inclination angle of the ripple fringes is proportional to the particle change (i.e. size and refractive index). The change of ripple phase is larger at higher temperature. The angular shift of the main rainbow position cannot be observed easily. However, the variation of the refractive index can be measured from the extracted light distribution. Accordingly, the rainbow light distributions are extracted for each level and processed. Figure 7 displays the extracted refractive index of gasoline droplets at two different temperatures of the air cavity. In all cases, the refractive index increases with the travelled distance of the droplets. Moreover, the value of the refractive index also increases with the bottle opening time, i.e. the measured refractive index values for measurements with a liquid extracted from a bottle opened for 8 days are larger than those for a bottle opened for 2 days, which are again larger than those for a “fresh” bottle. This behavior of the refractive index can be attributed to the fact that the light components (more volatile) evaporate faster than the heavy components. The mixture becomes heavier during the evaporation time (i.e. the density increases). Furthermore, it is well known that the refractive index value decreases when the temperature increases. This behavior explains why the refractive index values are smaller in Fig. 7b than in Fig. 7a. In the opened bottle, the fuel was left to evaporate and the composition changed over time. The refractive index of the gasoline increased as the days passed. On the other hand, closing the bottle inhibited evaporation. The composition did not change during storage. The extracted refractive index in the closed bottle did not change significantly. These results underline the high sensitivity of the technique to tiny changes of the refractive index value.
• For the first step, the inversion code based on Nussenzveig’s theory, developed by Saengkaew and co-workers [11], is used. The refractive index and size of the droplets are extracted by searching for the best fit between recorded and simulated signals allowing the extraction of refractive index with an accuracy to the fourth digit, as well as the size with an accuracy of about 0.2 μm. • For the second step, the ripple phase shift is measured using the Cross Spectrum Density (CSD) approach [16], permitting to measure continually the ripple phase shift along a trajectory. The contribution to the phase change, due to the change of refractive index (ϕ n ), is calculated from the refractive index and size obtained from the first step of processing. The phase shift due to the change of diameter (ϕ d ) is obtained by removing the phase shift due to the refractive index (ϕ n ) from the measured phase shift (ϕ), allowing the diameter change to be measured with a nanometric accuracy. This precision is necessary to be able to compute the local evaporation rate.
Please cite this article as: J. Promvongsa et al., Multicomponent fuel droplet evaporation using 1D Global Rainbow Technique, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.08.010
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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 droplet size, can be obtained by subtracting the phase change, due to the change of refractive index, from the total phase change extracted from the rainbow signal. Using the correlations provided by the Lorenz-Mie simulated signals, the diameter change of droplets is evaluated. For droplets of about 100 μm in diameter, diameter changes as small as 20–70 nm are measured for a travel distance of 1 mm. By summing the diameter changes, the diameter evolution versus the travel distance has been measured, as plotted in Fig. 8. These curves show that the evaporation is very sensitive to the liquid composition. Moreover, when the droplet velocity is known, the diameter evolution can be straightforwardly transformed into evaporation rate values from the difference of mass between two locations divided by the flight duration. For example, Table 2 compiles the evaporation rates deduced from measurements at 17 and 18 mm for air cavities at 20 °C and 40 °C, for a particle velocity of 5 m/s.
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. temperature 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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J. Promvongsa et al. / Proceedings of the Combustion Institute 000 (2016) 1–8 Table 2 Measured evaporation rates.
Diameter Diameter change Evaporation rate
20 °C, day 8
20 °C, day 0
40 °C, day 8
40 °C, day 0
99.52 μm 0.30 μm 1.7 μg/s
99.27 μm 0.40 μm 2.3 μg/s
99.38 μm 0.30 μm 1.7 μg/s
99.17 μm 0.70 μm 4.0 μg/s
tained from these data. The results show that the refractive index increases while the evaporation rate decreases, due to the loss of the light components, demonstrating the high sensitivity of this technique to the characterization of droplets; e.g. composition, temperature and size. The next steps are the extension of this approach to droplets in more hostile environments (e.g. high temperature, high or low pressure, flames), using a pulse laser, as well as comparing the experimental results to numerical simulations. These two developments are currently in progress. Acknowledgments The financial support provided by Thailand Research Fund (TRF) through the Golden Jubilee Ph.D. Program (Grant No. PHD/0272/2551), the European Program INTERREG IVA-E3C3 and ANR-Astrid DEVACOL are gratefully acknowledged. References [1] D. Sagawa, M. Yoshida, S. Nakaya, T. Kadota, Proc. Combust. Inst 31 (2007) 2149–2156. [2] G. Brenn, L.J. Deviprasath, F. Durst, C. Fink,, Int. J. Heat Mass Transfer 50 (2007) 5073–5086. [3] R. Holyst, M. Litniewski, D. Jakubczyk, et al., Rep. Prog. Phys 76 (2013) 034601.
[4] P. Chylek, J.T. Kiel, M.K.W. Ko,, Phys. Rev. A 18 (5) (1978) 2229–2233. [5] A. Omrane, G. Juhlin, F. Ossler, M. Alden,, Appl. Opt. 43 (17) (2004) 3523–3529. [6] G. Castanet, P. Lavieille, F. Lemoine, et al., Heat Mass Transfer 45 (2002) 5053–5067. [7] R.J. Hopkins, R. Symes, R.M. Sayer, J.P. Reid,, Chem. Phys. Lett. 380 (2003) 665–672. [8] G. Chen, Md.M. Mazunder, R.K. Chang, J.C. Swindal, W.P. Acker,, Prog. Energy Combust. Sci. 22 (1996) 163–188. [9] S. Saengkaew, T. Charinpanikul, C. Laurent, et al., Exp. Fluids 48 (2010) 111–119. [10] S. Saengkaew, Development of novel global rainbow technique for characterizing spray generated by ultrasonic nozzle, Ph.D. thesis., University of Chulalongkorn, Bangkok, Thailand, 2006 University of Rouen. [11] C Letty, B. Renou, J. Reveillon, S. Saengkaew, G. Grehan, Combust. Flame 60 (2013) 1803–1811. [12] S. Saengkaew, V. Bodoc, G. Lavergne, G. Grehan,, Opt. Commun. 286 (2012) 295–303. [13] X.C. Wu, Y.C. Wu, S . Saengkaew, et al., Meas. Sci. Technol 23 (2012) 125302. [14] J.P.A.J. van Beeck, D. Giannoulis, L. Zimmer, M.L. Riethmuller, Opt. Lett. 24 (1999) 1696–1698. [15] X.C. Wu, H.Y. Jiang, Y.C. Wu, et al., Opt. Lett. 39 (2014) 638–641. [16] J. Domnick, H. Ertel, C. Tropea, in: Proc. 4th International Symposium on Applications of Laser Techniques to Fluid Mechanics, LisbonPortugal, 1998 paper 3.8.
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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 process involved in many engineering applications such as spray drying, fire extinguishing, internal combustion engines, etc. ∗
Corresponding author. Fax: +33 2 32959780. E-mail addresses: [email protected], [email protected] (S. Saengkaew).
During the combustion of liquid fuels, evaporation directly controls the combustion. Accordingly, the evaporation of droplets has been studied in many kinds of experimental configurations, which can be divided into two categories: suspended droplets and moving droplets. Studies on suspended droplets (on a wire [1], acoustically levitated [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.
http://dx.doi.org/10.1016/j.proci.2016.08.010 1540-7489 © 2016 by The Combustion Institute. Published by Elsevier Inc.
Please cite this article as: J. Promvongsa et al., Multicomponent fuel droplet evaporation using 1D Global Rainbow Technique, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.08.010
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As evaporation has an impact on several parameters such as size, temperature and composition of droplets, a large number of experimental techniques have been developed to measure one or several of these characteristics. Without being exhaustive, the following approaches can be cited: • • • •
Imaging technique [2] Phosphorescence [5] Fluorescence [6] Morphological dependent (MDR) [7] • Raman scattering [8].
resonances
Alternatively, rainbow techniques are attractive because both critical pieces of information, the droplet size and refractive index (which depends on droplet temperature and composition), can be measured simultaneously from the light distribution around the rainbow angle. Rainbow techniques exist in two main configurations: one is the Standard Rainbow Technique (SRT); the other is the Global Rainbow Technique (GRT). The SRT approach is based on the assumption that only one particle (or identical particles) creates the rainbow signal. Accordingly, signal-processing strategies are based on single-scattering theory (i.e. Airy, Nussenzveig, Debye and Lorenz-Mie’s theory). However, the main difficulties are due to the presence of highfrequency fringes, called ripples, which are created by the interference between pure rainbow light (p = 2) and the externally reflected light (p = 0). The ripple structure is very sensitive to any change in the refractive index, size, and shape of the droplet. Then, to carry out accurate measurements, the ripple structure must be taken into account in the signal processing, as demonstrated by Saengkaew et al. [9]. In contrast, the GRT approach is based on analyzing the global rainbow created by a large number of particles of different sizes. The summation of a large number of rainbow signals issued from different particles removes the ripple structure. Consequently, the sensitivity of the measurements to the particles’ shape is dramatically reduced. Accordingly, the developed processing strategy, based on matrix inversion with constraint and minimization [10], allows the accurate extraction of the refractive index value, and therefore the droplet temperature, to an accuracy of about 1 °C. The GRT can be applied at high pressure and high temperature without adding any additive to the liquid. Therefore, this attractive technique can be used to characterize a wide range of spray applications [11–13]. The GRT measures the size distribution and averaged refractive index of the droplets at a defined “point”, which is spatially selected by a system of lenses and pinholes [11,14]. Typically, the size of the measurement volume is about 1 mm3 , according to optical configuration. In the particular case of a line of monodispersed droplets, the measurement of size and refractive index can
be as accurate as 0.01 μm for diameter and the fourth decimal for the refractive index by combining the measurement at the rainbow angle with forward scattering pattern [10]. Recently, an extension of GRT called One-Dimensional Global Rainbow Technique (1D-GRT), using slit apertures and a laser sheet, has been introduced by Wu et al. [15]. With this configuration, it is possible to measure the evolution of droplet size and refractive index with high precision, from a single recorded rainbow image. The aim of this paper is to demonstrate that this 1D-GRT configuration allows the extraction of relevant parameters, describing multicomponent droplet evaporation (i.e. size, temperature, composition, evaporation rate) from a single rainbow image. The technique has been developed and validated using N-heptane before being applied to a multicomponent gasoline. 2. Experimental setup 2.1.. 1D GRT configuration The 1D-GRT setup is modified from the classical global rainbow technique to extend the technique from a point measurement (0D) to a onedimensional measurement (1D). It was introduced by Wu et al. [15] for measuring the evolution of the refractive index of fuel spray along a line of view. Figure 1 is a schematic representation of the optical setup. The control volume can be assimilated to a line, defined by a combination of lenses and slits. The rainbow patterns scattered from the droplets at the different vertical locations along the control volume are recorded simultaneously in a single image. Two calibrations are required: a length calibration (y-axis) and angular calibration (x-axis). For the length calibration, the correspondence between the vertical position on the CCD camera of the scattered light and the relative location of the droplets in the control volume is correlated. For the angular calibration, a mirror is fixed on an accurate rotating goniometer. The location of the reflected laser beam on the CCD combined with the mirror orientation permits an accurate angular calibration. In a single image, each line represents a rainbow signal corresponding to different locations of droplets, which is represented schematically in Fig. 1b. Consequently, by processing the rainbow signals at different CCD lines, the spatial evolution of diameter and refractive index of the droplets along the linear control volume can be measured. The experimental setup is based on a continuous laser beam with a wavelength of 532 nm. The cylindrical beam is transformed into a vertical laser sheet, which coincides spatially with the line of monodispersed droplets. To collect the scattered light, a horizontal slit is installed in front of the first lens. Accordingly, the position and dimensions of the measurement volume can be specified. The
Please cite this article as: J. Promvongsa et al., Multicomponent fuel droplet evaporation using 1D Global Rainbow Technique, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.08.010
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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-convex lenses, of 150 mm focal length and 75 mm diameter. The size of horizontal slit is 1 mm x 75 mm. The length of the control volume is 8 mm. The camera used in this experiment is a JAI RM 4200 CL camera (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 droplet stream is generated by a TSI 3050 droplet generator. With the appropriate flow rate and frequency, the droplets can be generated with equal space and size. In this experiment, the droplet generator is used with a 50 μm diameter pinhole. The liquid flow rate is equal to 0.6 mL/min, and the excitation frequency is equal to 18.3 kHz. With these operating conditions, the diameter and velocity of
droplets are approximately 100 μm and 5 m/s, respectively. As displayed in Fig. 2, a cavity chamber is installed 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 (Ti ) at steady state is measured by a thermocouple. The measurement point is located 23 mm away from the nozzle orifice. Experiments have been carried out at two different temperatures of the air in the cavity (Ti ), 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-
Please cite this article as: J. Promvongsa et al., Multicomponent fuel droplet evaporation using 1D Global Rainbow Technique, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.08.010
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Table 1 Some properties of the studied gasoline (surrogate: npentane 36%: iso-octane 46%: n-undecane 18% in volume). Density at 15 °C, (kg/m3 ) Vapor pressure, (mbar) RON C/H ratio, (% Mass) Initial boiling point, (°C) 5% Vol distillation, (°C)
751.7 565 95.5 6.58 30 48
undecane 18% in volume) are prepared. Some properties of the gasoline are given in Table 1. One bottle is left closed, while the other one remains opened at room temperature. On day 0, the initial properties of the fuel are measured. Two days later (day 2), the gasoline in the opened bottle is investigated. On the eighth day (day 8), fuel from both bottles is measured. A series of 200 images is recorded and then processed for each measurement.
3. Signal behavior and signal processing 3.1. Signal behavior Lorenz-Mie theory is used to numerically investigate rainbow behavior. The rainbow signal depends on the size and refractive index of the liquid droplet. The droplet size essentially affects the shape and the intensity of the rainbow signal, while the refractive index essentially affects its angular location. Figure 3a compares the rainbow scattering diagrams for three N-heptane droplets of different sizes (100, 101 and 110 μm), with a constant refractive index value of 1.3850. The change of the rainbow pattern can be observed when the
Fig. 4. Rainbow signal sensitivity to refractive index value.
droplet diameter increases from 100 to 110 μm. For the largest droplet, the main peak is narrower and the intensity is significantly higher. However, when the 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 to measure. Figure 3b displays the ripple shift when the particle diameter changes from 99 to 100 μm. The relationship between the phase difference and the diameter change is discontinuous. Five periods are observed. The ripple phase shift must be continually measured to avoid 2π ambiguity in the relationship between the measured angular phase shift and a diameter variation. Moreover, the change in the refractive index affects 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 angular rainbow position can be detected even for a tiny change in the refractive index (up to the fourth decimal). For an N-heptane particle of 100 μm diam-
Fig. 3. Rainbow sensitivity 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 phase change of one period corresponds to a diameter change of 0.2 μm, or a refractive index change of 0.0041. Figure 5 displays simulated 1D-GRT images corresponding to six different lines of droplets. For each image, each column corresponds to a scattering angle, and each row corresponds to the rainbow issued from droplets located along the linear measurement volume. The left and right axes represent the diameter and refractive index, respectively, while the horizontal axis is the scattering angle. The rainbow signals from droplets with a continuous linear variation of diameter and/or refractive index are compared to the rainbow signal from droplets with a constant diameter and refractive index (Fig. 5a). The simulated rainbow images show
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 = ϕn + ϕd
(1)
The size and refractive index contributions to the shift of the ripple are independent. Consequently, the small change in droplet size can be extracted from the ripple-phase change when the refractive index evolution is known.
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Fig. 6. 1D-GRT image recorded for two different temperatures in the cavity.
3.2. Signal processing
4. Experimental results and discussion
For each recorded 1D-GRT image, two calibrations are required: length and angular calibration. The length calibration gives the relationship between the droplets’ location in the measurement volume and the “vertical” pixel position on CCD. In this experiment, as illustrated in Fig. 1b, nine bands are selected, corresponding to 8 mm length measurement volume (the measurement step is equal to 1 mm). Each band contains 21 rows and is averaged along the columns to obtain the intensity vector. The angular calibration is used to transform the pixel column into the scattering angle. The signal processing strategy is composed of two steps. The first step is to measure the refractive index and the droplet size with micrometric accuracy. The second step is to measure the droplet size evolution with nanometric accuracy.
The rainbow images recorded at different temperatures of the air cavity are shown in Fig. 6. These images are characterized by a large bright spot (corresponding to the mean peak of the primary rainbow (p = 2)), with inclined fringes (corresponding to the ripple). The inclination angle of the ripple fringes is proportional to the particle change (i.e. size and refractive index). The change of ripple phase is larger at higher temperature. The angular shift of the main rainbow position cannot be observed easily. However, the variation of the refractive index can be measured from the extracted light distribution. Accordingly, the rainbow light distributions are extracted for each level and processed. Figure 7 displays the extracted refractive index of gasoline droplets at two different temperatures of the air cavity. In all cases, the refractive index increases with the travelled distance of the droplets. Moreover, the value of the refractive index also increases with the bottle opening time, i.e. the measured refractive index values for measurements with a liquid extracted from a bottle opened for 8 days are larger than those for a bottle opened for 2 days, which are again larger than those for a “fresh” bottle. This behavior of the refractive index can be attributed to the fact that the light components (more volatile) evaporate faster than the heavy components. The mixture becomes heavier during the evaporation time (i.e. the density increases). Furthermore, it is well known that the refractive index value decreases when the temperature increases. This behavior explains why the refractive index values are smaller in Fig. 7b than in Fig. 7a. In the opened bottle, the fuel was left to evaporate and the composition changed over time. The refractive index of the gasoline increased as the days passed. On the other hand, closing the bottle inhibited evaporation. The composition did not change during storage. The extracted refractive index in the closed bottle did not change significantly. These results underline the high sensitivity of the technique to tiny changes of the refractive index value.
• For the first step, the inversion code based on Nussenzveig’s theory, developed by Saengkaew and co-workers [11], is used. The refractive index and size of the droplets are extracted by searching for the best fit between recorded and simulated signals allowing the extraction of refractive index with an accuracy to the fourth digit, as well as the size with an accuracy of about 0.2 μm. • For the second step, the ripple phase shift is measured using the Cross Spectrum Density (CSD) approach [16], permitting to measure continually the ripple phase shift along a trajectory. The contribution to the phase change, due to the change of refractive index (ϕ n ), is calculated from the refractive index and size obtained from the first step of processing. The phase shift due to the change of diameter (ϕ d ) is obtained by removing the phase shift due to the refractive index (ϕ n ) from the measured phase shift (ϕ), allowing the diameter change to be measured with a nanometric accuracy. This precision is necessary to be able to compute the local evaporation rate.
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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 droplet size, can be obtained by subtracting the phase change, due to the change of refractive index, from the total phase change extracted from the rainbow signal. Using the correlations provided by the Lorenz-Mie simulated signals, the diameter change of droplets is evaluated. For droplets of about 100 μm in diameter, diameter changes as small as 20–70 nm are measured for a travel distance of 1 mm. By summing the diameter changes, the diameter evolution versus the travel distance has been measured, as plotted in Fig. 8. These curves show that the evaporation is very sensitive to the liquid composition. Moreover, when the droplet velocity is known, the diameter evolution can be straightforwardly transformed into evaporation rate values from the difference of mass between two locations divided by the flight duration. For example, Table 2 compiles the evaporation rates deduced from measurements at 17 and 18 mm for air cavities at 20 °C and 40 °C, for a particle velocity of 5 m/s.
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. temperature 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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J. Promvongsa et al. / Proceedings of the Combustion Institute 000 (2016) 1–8 Table 2 Measured evaporation rates.
Diameter Diameter change Evaporation rate
20 °C, day 8
20 °C, day 0
40 °C, day 8
40 °C, day 0
99.52 μm 0.30 μm 1.7 μg/s
99.27 μm 0.40 μm 2.3 μg/s
99.38 μm 0.30 μm 1.7 μg/s
99.17 μm 0.70 μm 4.0 μg/s
tained from these data. The results show that the refractive index increases while the evaporation rate decreases, due to the loss of the light components, demonstrating the high sensitivity of this technique to the characterization of droplets; e.g. composition, temperature and size. The next steps are the extension of this approach to droplets in more hostile environments (e.g. high temperature, high or low pressure, flames), using a pulse laser, as well as comparing the experimental results to numerical simulations. These two developments are currently in progress. Acknowledgments The financial support provided by Thailand Research Fund (TRF) through the Golden Jubilee Ph.D. Program (Grant No. PHD/0272/2551), the European Program INTERREG IVA-E3C3 and ANR-Astrid DEVACOL are gratefully acknowledged. References [1] D. Sagawa, M. Yoshida, S. Nakaya, T. Kadota, Proc. Combust. Inst 31 (2007) 2149–2156. [2] G. Brenn, L.J. Deviprasath, F. Durst, C. Fink,, Int. J. Heat Mass Transfer 50 (2007) 5073–5086. [3] R. Holyst, M. Litniewski, D. Jakubczyk, et al., Rep. Prog. Phys 76 (2013) 034601.
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Please cite this article as: J. Promvongsa et al., Multicomponent fuel droplet evaporation using 1D Global Rainbow Technique, Proceedings of the Combustion Institute (2016), http://dx.doi.org/10.1016/j.proci.2016.08.010