PLIF Experiments on Evaporating Isolated Droplet and Droplets Array

AIAA 2016-4311 AIAA Aviation 13-17 June 2016, Washington, D.C. 46th AIAA Thermophysics Conference

PLIF Experiments on Evaporating Isolated Droplet and Droplets Array Hafiz Laiq-ur Rehman, Abdelouahab Mohammed-Taifour*, Julien Weiss† and Patrice Seers‡

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École de Technologie Supérieure, Montréal, Canada, H3C 1K3

Fuel droplet evaporation involving heat and mass transport holds key interest due to its industry wide applications. This paper presents a novel experimental investigation of an isolated fuel droplet and tandem droplets array evaporation. Acetone droplets of 2120μm diameter were supported on 300μm diameter glass fiber to investigate under hot forced convection ambience. Planar laser induced fluorescence (PLIF) was used to estimate the vapor concentration around the evaporating isolated droplet and droplets array. An attempt was made to estimate the vapors concentration around and behind the evaporating droplets. The depletion of vapor concentration for isolated droplet was found to be more than the leading droplet in droplets array. The wake of the trailing droplet was longer than the isolated droplet wake. The effect of leading droplet was evident on the trailing droplet life time and size.

Nomenclature C d do df Δhvap M md mf n P R r r/do S T U V ρ

= concentration = droplet diameter varying with time = initial droplet diameter = support diameter = latent heat of vaporization = Molecular mass = droplet mass = mass transfer between droplets = number of moles = saturated vapor pressure = Universal gas constant = downstream distance = inter-droplet spacing = fluorescence intensity = air temperature = air velocity = volume = density

Subscript back f

= background = droplet support



Graduate research assistant, École de Technologie Supérieure, Montréal, Canada, H3C 1K3 Associate Professor, École de Technologie Supérieure, Montréal, Canada, H3C 1K3 ‡ Professor, École de Technologie Supérieure, Montréal, Canada, H3C 1K3 †

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Copyright © 2016 by Hafiz Laiq-ur Rehman, Abdelouahab Mohammed-Taifour, Julien Weiss and Patrice Seers. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

iso sat ∞

= isolated droplet = saturated state = ambient condition

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I. Introduction Fuel spray injection systems are widely used in gas turbines and internal combustion engines. Droplet evaporation involving heat and mass transfer is a vital link between proper air-fuel mixture and its combustion. Extensive experimental, analytical and numerical research have been conducted on droplets arrays and sprays vaporization [2-10] along with single droplet evaporation [1, 11-25]. Isolated droplets are relatively simple to replicate in experimental and numerical testing environments in comparison to complex spray systems and they better resemble the far field dilute region in the air-fuel mixture where aerodynamic transport is more dominant [22]. The near field of fuel atomizer is crowded with the droplets of different sizes and varying inter-droplet spacing which cover the majority of the combustor volume [22]. The studies on fuel droplet evaporation have accounted for various droplet vaporization rate controlling factors such as ambient temperature, pressure, inter-droplet spacing, ratio of droplets radii, gravity, transport properties of air surrounding the droplet, the droplet properties, the ambient air velocity and whether the air flow is laminar or turbulent. Majority of the experimental research work conducted on estimating the vapor concentration of evaporating droplet is based on the planar laser induced fluorescence (PLIF) technique. PLIF is used to measure the fuel droplet and vapor concentration simultaneously since it is nonintrusive in nature and allows collecting the instantaneous information. The other advantage of using PLIF technique is that it allows discriminating between liquid and vapor phases of the fuel [26]. Kurosawa et al. [27] measured the spatial droplet distribution, size, velocity and vapor concentration in acetone droplets spray by using interfero-metric laser imaging for droplet sizing (ILIDS) and PLIF technique. Vapor concentration was estimated by using the correlation proposed by Fujikawa et al. [28]. Sahu et al. [29] applied both ILIDS and PLIF techniques on acetone droplets stream to measure the fuel droplet and vapor concentration simultaneously. They [29] also proposed a correction method for droplet halation which helped in predicting the droplet size accurately and estimating the vapor concentration while moving away from droplet. These studies [26-32] were performed on acetone droplets stream or spray of extremely small droplets sizes. The vapors concentration behind the evaporating isolated droplet and droplets array is an important phenomenon and it was therefore important to see the flow behavior around evaporating droplets. The primary objective of this study is to measure the vapors concentration behind suspended isolated droplet and tandem droplets array. The secondary goal was to check the difference between the evaporation time of isolated droplet and droplets array based on this vapor concentration. This was achieved experimentally by varying geometrical and flow characteristics such as ambient temperature and ambient velocity.

II. Experimental Setup Experimental results were collected in the combustion laboratory at École de technologie Supérieure. The detailed description of experimental set-up can be found in Rehman et al. [1]. The experimental setup comprised of a vertical 10cm-diameter, 1.5m-long pipe at the base of which a variable speed fan was installed to generate laminar hot air flow field as shown in fig 1. The ambient air temperature (303 – 403K) was maintained at the desired value through a heater controlled by PID controller. A rheostat was used to change the fan speed which can vary the air speed in test section from 0.4 – 2.7m/sec. The experimental setup was equipped to suspend tandem and staggered array of fuel droplets by using thermocouples and glass fibers of different sizes. A volume of 5µL of acetone yielded the initial diameter do=2120µm which was suspended on the glass fiber with the help of a precision syringe (Socorex 825.0020). The repeatability of the droplet mass was ensured by using a micro scale capable of reporting value accurately up to four decimal places of a gram. Experimental arrangement for the PLIF measurements can be seen in fig 2. The ND:YAG laser (60mJ/pulse at 266nm, beam diameter 50sec are not shown as the droplet was in the final stages of its life time and hence those profiles collapsed on each other except in the vicinity of droplet.

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Figure 5. Temporal variation of isolated acetone droplet evaporation process at U∞=0.4m/sec and T∞=303K

Figure 6. Temporal variation of concentration profiles in the wake of evaporating isolated acetone droplet at U∞=0.4m/sec and T∞=303K B. Droplets Array 1. Temporal Droplets Evaporation Figure 7 shows the temporal variation of evaporating acetone droplets tandem array at ambient temperature of T∞=303K and ambient air speed of U∞=0.4m/sec. The initial diameters for both the leading and trailing droplets are 2120μm corresponding to the acetone volume of 5μL. The initial center to center distance between two droplets is kept to r/do=2.5.

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Figure 7. Temporal variation of acetone droplets array evaporation process at U∞=0.4m/sec and T∞=303K This value of r/do=2.5 is selected as it is near the lower bound of the correlations proposed by the Sirignano [33, 34] for two droplets tandem array. The life spans for leading and trailing droplets are approximately 70sec and 90sec respectively for this particular case. The droplets shown in these images exhibit the spherical nature except the last phase of droplet life time where the sphericity is compromised due to the adhesiveness between acetone and glass fiber. The leading droplet is exposed to the ambient air speed containing no acetone vapors and hence causing the quicker evaporation. This effect can also be seen by observing the smaller leading droplet. The trailing droplet is exposed to the wake of the leading droplet. The air passing through the trailing droplet contains the acetone vapors from the leading droplet which brings the local ambient temperature down [7] and hence increasing the droplet life time. Another important factor can be the concentration gradient which is more for leading droplet than the trailing droplet. The wake for trailing droplet is wider which is due to bigger trailing droplet. 2. Droplets Concentration Profiles Figure 8 shows the temporal variation of concentration profiles of evaporating droplets array of acetone at T∞=303K and U∞=0.4m/sec. All the vapor concentration profiles started from high values similar to the case of isolated droplet and their magnitudes depleted with the downstream radial distance. The wake of the leading droplet looked qualitatively similar to the isolated droplet but the presence of trailing droplet changed its trend. The values of concentration profiles behind the leading droplet are slightly higher than the isolated droplet at a fixed radial position downstream for the same ambient conditions. This is due to the difference in concentration gradient for isolated droplet and droplets array. The wake behind the leading droplet mixed with the trailing droplet halation which stopped the further depletion of vapor concentration strength. Similar phenomenon was observed when comparison was drawn between leading and trailing droplet where the depletion of vapor concentration was more for trailing droplet than the leading droplet. The wake of trailing droplet however was elongated due its relatively bigger size. The characteristics of the profiles did not change much in the earlier phase of droplet life time (t ≤ 20sec) so only t=10sec plot is shown in the fig 8. The wake of the trailing droplet is longer and wider due to its relatively larger size. The droplet size shrank over the course of its life time in addition to losing the concentration magnitude as 8 American Institute of Aeronautics and Astronautics

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represented by the curves. The leading droplet evaporated around 70 seconds mark but still showing concentration profile at t=90sec. This is due to the luminosity emitting from the glass fiber. The curves of t=70sec and t=90sec are collapsed on each other before reaching the trailing droplet. The difference between these two curves appeared when flow reached the leading end of the trailing droplet and later shed into the wake. The concentration magnitude downstream of trailing droplet appeared close to zero for t=90sec plot which suggested the completion of evaporation process.

Figure 8. Temporal variation of concentration profiles in the wake of evaporating acetone droplets tandem array at U∞=0.4m/sec and T∞=303K The experimental results are incapable of presenting small diameter values due to small droplet to glass fiber diameter and high degree of uncertainty associated with those values. Despite of these limitations, the results presented provided a method to measure the concentration around the isolated droplet and droplets array which can be used to measure the mass transfer between evaporating droplets.

V. Conclusion An experimental study on the acetone isolated droplet and two droplets tandem array was performed in the hot forced convection ambience using PLIF technique. Glass fiber was used to suspend the droplets while ICCD camera captured the droplets behavior in the presence of laser sheet. An attempt was made to estimate the vapor concentration around the isolated evaporating droplet and droplets array. Vapors concentration was related with the PLIF fluorescence intensity counts after performing the suitable calibration provided in the literature. Vapor concentration was found to be changing over the course of droplet life time. The vapor concentration behind isolated droplet was depleting quickly for isolated droplet than the leading droplet in droplets array. The wake behind the trailing droplet was observed to be longer than for isolated droplet. This study enabled to examine the vapor concentration around the droplets under different flow conditions and will be helpful in understanding the droplet evaporation in fuel droplets arrays.

Acknowledgments The authors are grateful to Mr. Simon Laflamme for his invaluable help and support in carrying out PLIF experiments.

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