Three-color LIF thermometry applied to the mixing of two non-isothermal sprays

Transcription

1 Three-color LIF thermometry applied to the mixing of two non-isothermal sprays Alexandre Labergue 1,2*, Alain Delconte 1,2, Fabrice Lemoine 1,2 1: Université de Lorraine, LEMTA, UMR 7563, Vandoeuvre-Lès-Nancy, F454, France 2: CNRS, LEMTA, UMR 7563, Vandoeuvre-Lès-Nancy, F454, France * Corresponding author: alexandre.labergue@univ-lorraine.fr Abstract The present paper deals with the application of the two-color laser induced fluorescence (2cLIF) applied to droplets temperature measurements in the case of polydisperse sprays. It is first briefly described how the 2cLIF, initially used and validated in the case of single calibrated droplets, is modified for reliable performing measurements in sprays. It leads to the so called three-color LIF (3cLIF). In the present contribution, temperature measurements have been performed in the case of the mixing of two water sprays. The liquid of one spray is heated whereas the second spray liquid is kept and injected at ambient temperature. Moreover, 3cLIF measurements were combined with a phase Doppler anemometer in order to derive droplets temperature per size class. By maintaining a constant flow rate for the heated spray, the effect of the flow rate of the cold spray was studied. The mean temperature evolution is first analyzed along the centerline of both sprays and along a radial direction. In both explorations, results demonstrate a significant decrease of the mean temperature when the cold spray is mixed with the heated spray. Obviously, increasing the cold spray flow rate should enhance the droplets cooling. Combined LIF and PDA measurements show that the droplet cooling is greater when the droplet diameter is smaller. Furthermore, for a given diameter, this cooling seems being enhanced when the flow rate of the cold spray is increased. However, it is difficult to determine from these measurements if heat transfers occurs between both sprays. In order to address to this issue, a second set of experiments was undertaken by seeding only one of the two sprays with the fluorescent tracer. The results demonstrate clearly that heat transfer between the heated and the cold droplets remains marginal. 1. Introduction In the current context, the characterization of the heat and mass transfer in polydisperse sprays is an important challenge in engineering applications like internal combustion engines [1-3] or cooling of hot surfaces [4;5]. In particular, the optimization of such applications requires measuring the temperature, the size distribution, the mass flux, and the concentration of droplets. Both last parameters can be now accurately determined using Phase Doppler technique [6] whereas the temperature measurements still remain quite difficult. Few techniques are able to measure properly the droplets temperature. The Global Rainbow Thermometry allows quantifying the refractive index of a droplet and hence its temperature based on the angular position of the rainbow created by the light scattered by droplets in the vicinity of the rainbow angle [7;8]. Nevertheless, although this technique provides simultaneously the diameter and the temperature for a droplet, the direct correlation between both parameters was reported in very works [9;1]. In another way, temperature-sensitive fluorescent dyes excited by a laser can alternatively used as a tracers seeded in the liquid: some of these techniques use an exciplex [11;12] or alternatively two fluorescent tracers [13;14]. In the present study, the two-color LIF (2cLIF) is used and combined with a phase Doppler anemometer (PDA) measurements in order to obtain droplets temperature per size class [3]. The 2cLIF used a single dye but the fluorescence is collected on two spectral bands in order to derive a ratio that depends, in principle, only on the liquid temperature. The technique was successfully applied and validated in the case of single evaporating or combusting droplets [15;16]. However, it was demonstrated that the direct application of the 2cLIF in the case of polydisperse sprays is not - 1 -

2 straightforward [17;18]. First, a significant dependence of the fluorescence ratio on the droplet size was highlighted, especially for droplets diameter lower than 1 µm [17], and secondly, it was also demonstrated that a too large depth of field of the fluorescence collection optics induces a bias [18]. The use of a third spectral band of detection leading to the 3cLIF and a long distance microscope, with a lower depth of field, permit to decrease significantly the consequences of both drawbacks. In this work, the 3cLIF technique is applied and more thoroughly tested in the case of the mixing of two non iso-thermal water sprays, i.e. a spray injected at ambient temperature is mixed with a heated spray. In a first part, the main outlines of 2cLIF are given before to introduce and describe in details 3cLIF. The second part is devoted to the experimental set-up such as the sprays facilities and the principles of the combined 3cLIF/PDA measurements. Finally, experimental results are discussed and analyzed. 2. Principles of the three-color LIF thermometry 2.1 The two-color LIF thermometry Only the mean features of the 2cLIF thermometry are presented in this section. More details are available in previous publications [15;16]. The liquid is preliminary seeded by a tracer at a low concentration, which is temperature sensitive. Here, the liquid is deionised water seeded by sulforhodamine B at a concentration of mo.l -1. The fluorescence is induced by the green line of an argon ion laser (λ = nm). The fluorescence signal presents a broadband spectrum which has a temperature-dependant shape. Thus, the fluorescence intensity I fi collected and integrated on a spectral band [λ i1 ; λ i2 ] is expressed by [15]: I!! (T) = K!"#,! K!"#$,! V! I! Cf! (T) (1) where I is the laser excitation intensity, C the tracer concentration, T the absolute temperature and V c the collection volume of the fluorescence photons. This later is defined as the intersection of the illuminated part of the droplet volume and the field of view of the fluorescence collection optics. K opt,i and K spec,i are parameters influenced by the optical layout and the fluorescence properties of the tracers. The function f i (T) describes the temperature dependence of the fluorescence on the spectral band i and it can be generally approximated by [16]: f! (T) e!!!!!!!!, (2) where a i and b i are two coefficients that characterize the temperature sensitivity of the fluorescence signal on the spectral band i. The parameters C, V c and I are difficult to determine and therefore must be removed. This can be achieved by collecting simultaneously the fluorescence signal on two spectral bands, respectively I f1 and I f2 and by calculating their ratio R 12 : R!" =!!! = #,!#$,!! (!) =!,!#$,! e!!"!!!! (3)!!! #,! #$,!! (!) #,! #$,! where a 12 and b 12 denote respectively a 1 a 2 and b 1 b 2. The ratio R 12 appears solely dependent on temperature. In the present study, the selected spectral bands are: Band 1: [535 nm; 545 nm] Band 2: [615 nm; 75 nm]. These bands allow obtaining a high sensitivity of the fluorescence ratio to the temperature. A calibration, conducted in a similar process as described in [19], leads to a temperature sensitivity of about.9%/k. Finally, a single reference R 12 measurement at a known temperature T allows eliminating both constants K opt,i and K spec,i in eqn (3). The normalized ratio R 12 /R 12 is given by: =!!! (1) - 2 -

3 2.2 The droplet size and depth of field effects Recently [19], a size effect linked to the droplet diameter, and not included in eqn (3), was highlighted. This effect corresponds to a significant distortion of the fluorescence spectrum. The deviation from the previous model is more important when the droplets are small, typically under D = 1 µm. Fig. 1, extracted from [17], depicts the evolutions of the normalized fluorescence ratio of single calibrated droplets of diameter D, under isothermal conditions and in the case of sulforhodamine B dissolved in water (C = mol.l -1 ). A significant increase of the fluorescence ratio is observed when decreasing the droplet diameter, otherwise the normalized fluorescence ratio tends to 1 for the large droplets. To account for the effect of the droplet size on the fluorescence ratio, an empirical function g 12 (D) is added [17]: =! g!!!!" D (5) The determination of the function g 12 (D) will be described in a futher section. Moreover, a study presented in [18] shows how a too large depth of field of the fluorescence collection optics can induce a second bias in the fluorescence ratio R 12 measurements. This second phenomenon was studied by using the combined LIF/PDA measurements previously developed and tested in [3]. Fig. 2 depicts the evolution of the normalized ratio R 12 /R 12 plotted as a function of the droplet diameter class measured in a spray (Danfoss burner) for three injection pressures. The shift observed when the pressure increases is due to a fluorescence contribution in the off field of the collection optics. This contribution increases with the droplet concentration and the size of the depth of field. Furthermore, the contribution of the off-field fluorescence comes mainly from small droplets which tend to have a higher fluorescence ratio value. The use of a long-distance microscope (QM; Questar ) having a lower depth of field has allowed to minimize the unwanted fluorescence contribution Mono-disperse droplets In the spray for P inj =2.5 bars In the spray for P inj =4.5 bars In the spray for P inj =7.5 bars R 12 /R 12o R 12 /R 12o Figure 1: Evolution of the normalized fluorescence ratio as a function of the diameter of calibrated single droplets Figure 2: Results of LIF/PDA measurements obtained with an achromatic doublet: evolution of the fluorescence ratio as a function of the droplet diameter 2.3 The three-color LIF thermometry A comprehensive survey of the three-color LIF (3cLIF) is described in a previous paper [18]. A function that takes into account the non-linear effect of the droplet size on the fluorescence ratio, g 12 (D, was introduced. To determine this unknown function, an empirical approach was undertaken. The idea was to use a third spectral band to determine a second normalized fluorescence ratio: =! g! D (6)! The temperature dependence function f 32 (T) has a lower temperature sensitivity (<.1 %/ C) than - 3 -

4 f 12 (T). A similar reference R 32 is taken, i.e. in the same cell and same temperature T as for R 12 measurement. Moreover, g 32 (D) has also a different sensitivity than g 12 (D) on the non-linear effect of the droplet size. However, an empirical relationship between the functions g 12 (D) and g 32 (D) can be determined experimentally by measuring the corresponding normalized fluorescence ratio for several droplet diameters under isothermal conditions. It leads to a monotonous evolution which can be easily interpolated by a second order polynomial: g!" = αg!"! + βg!" + γ (7) Finally, combining eqns (5), (6) and (7) an equation depending only to the temperature is obtained:! = α!!!!!!!! + β!!!! + γ (8) Knowing α, β and γ, eqn (8) can be solved to determine the temperature from the measurements of the ratios R 12 and R Application to the mixing of two non-isothermal sprays 3.1 Sprays mixing facilities Two different water sprays, in terms of flow rates and droplets concentration for a same injection pressure, are used. Both sprays, which are full cone, are generated by Danfoss swirled nozzles, commonly used for burners, and supplied by independent pressurized tanks. The pressure for both sprays can be adjusted in order to fix the flow rate. In this study, one of both sprays, referred as spray (1), is preheated whereas the water of the other spray, referred as spray (2), is kept at ambient temperature. Both sprays are injected downwards with an angle of 1 in the vertical direction (Fig. 3). The origin of the X-Y-Z axis is taken at the intersection point of the inside edges of both sprays. P(D) Spray (1); Q 1 =2 ml/min Spray (2); Q 2 =6 ml/min Both sprays Figure 3: Sprays mixing facilities and optical devices for 3cLIF and PDA detection Figure 4: Typical droplet size distributions measured in the center (i.e. X = Y = ) at Z = 3 mm for spray (1) and sprays (2), and when both are mixed. 3.2 Sprays characteristics The vertical component of the droplet velocity (vertical along the Z-axis) and the droplet diameter were measured using a commercial Phase Doppler Anemometer (PDA) manufactured by Dantec- Dynamics. The system includes a classic reception optics and a P8 signal processor. The laser excitation volume is formed using a LDA transmitter probe (Dantec-dynamics Fiber-Flow probe)

5 The laser source is an argon ion laser tuned at λ = nm, also used for fluorescence excitation. Table 1 summarizes the characteristics of the PDA optical configurations. Fig. 4 gives typical distributions of droplets sizes for the spray (1) injected alone, for the spray (2) injected alone with Q 2 = 6 ml/min and when both sprays are mixed together. These distributions are measured at Z = 3 mm in the centerline (i.e. X = Y = ) at room temperature and under isothermal conditions. The distributions are peaking at about 3 µm and 45 µm for the spray (1) and the spray (2) respectively. Their respective mean diameter D 1 is about 6 µm and 39 µm and it is 41 µm in the case of the mixing. Moreover, measurements show that droplet concentration of spray (1) that is about the tenth of this of spray (2). Emission focal length, f e (mm) 31 Beam spacing, b s (mm) 6 Reception focal length, f r (mm) 5 Scattering angle angle, ϕ r ( ) 45 Maximum detectable droplet diameter, D max (mm).18 Laser excitation volume size based on 1/e² (mm) Size in Z and Y directions a z = a x.152 Size in X direction a y Table 1: Characteristics and optical configuration for PDA measurements 3.3 LIF optical set up and data processing The fluorescence signal is collected at right angle by using a long-distance microscope (QM; Questar ) at a working distance of 3 mm (Fig. 5). The collected signal is guided by an optical fibber to a high pass filter (Chroma, HQ 522 LP) in order to remove the light scattered at the laser wavelength. The remaining fluorescence signal is split into the three spectral bands mentioned in sections 2.2 and 2.3 by means of a set of dichroic and interference filters (Fig. 5). The fluorescence signal is detected by means of three photomultiplier tubes and digitalized with a frequency of 5 MHz for real time processing. A threshold is fixed significantly above the noise level of the channels of detection. If a sequence has more than ten consecutive samples above this threshold, it is considered coming from droplets. The fluorescence ratios R 12 and R 32 are then calculated as follows: R!" = = (9)!!!!,!!!!!!!!,!!!! where I fi,k and I fj,k are respectively the fluorescence intensities integrated on the k th droplet crossing the probe volume. n is the number of droplets detected during the acquisition period. N i and N j are the average dark noise values on the corresponding spectral bands. 3.4 Combined LIF and PDA measurements The optical arrangements and acquisition chain for combined LIF and PDA measurements are presented in Fig. 5 and the corresponding process is described in details in [3]. The combined 3cLIF/PDA system provides two data files recorded with the same time base, one corresponding to the droplet velocity and diameters measured by the PDA and the other one to the fluorescence - 5 -

6 intensities integrated on the droplet transit in the probe volume for the three spectral bands of detection. Droplets detected simultaneously by PDA and 3cLIF are identified on the basis of their arrival time and their transit time. Then, a fixed number of droplet size classes are defined and the fluorescence intensity on each channels, I f 1and I f 2, is averaged following eqn. (1). As a result, an average fluorescence ratio can be associated to a given droplet size class. Moreover, a main key issue of this technique is the common droplet size range that can be detected by both techniques. Indeed, when a droplet crosses the common LIF and PDA probe volume, the signal collected by the PDA is roughly proportional to D 2 whereas a study for LIF, based on ray tracing, has demonstrated that the fluorescence signal is proportional to D 2.8 [18]. Thus, the wider range of the LIF limits the combined measurements in terms of droplets sizes. Only a range of the droplets distribution can be simultaneously detected by LIF and PDA. In [18], it is shown how the LIF dynamic range detection can be determined and optimized. Here, the maximum droplet diameter D max is about 11 µm and the minimum droplet diameter D min is 2 µm. Figure 5: Combined LIF/PDA experimental set-up (top view) and LIF optical set-up. 4. Study of the mixing of the sprays using combined 3cLIF/PDA device 4.1 Mean droplet temperature Temperature measurements have been performed for an injection temperature T inj of the spray (1) of about 6 C and a constant flow rate Q 1 = 2 ml/min for all the development of the present study. The second spray is injected at ambient temperature and two flow rates values is applied: Q 2 = 4 or 6 ml/min. From combined LIF and PDA results, a mean temperature T m can be derived as following [16]: T! =!!!!!!!.!!!!!!!!!.!!!!!!! where p is the droplet size class index, N c the number of size classes, N p the number of droplets per size class and T p the temperature corresponding to the p th class. Fig. 6 depicts the variation of the mean temperature relatively to the injection temperature T inj for the spray (1) alone, for both sprays with Q 2 = 4 ml/min and Q 2 = 6 ml/min, along the Z-axis in the centerline (X = Y = ). In the case of spray (1) alone, a cooling of the droplets, on the order of 5 C, is observed. The effect of the cold (1) - 6 -

7 spray (2) appears clearly with an increased of the cooling with increasing the flow rate. Fig. 7 presents similar results as Fig. 6 but along the radial direction Y at Z = 3 mm. The position Y = 17 mm corresponds to the outer edge of the heated spray (1) and Y = 2 mm corresponds to the outer edge of the cold spray (2). For spray (1) alone, a temperature gradient is observed between the center of spray (1) (Y = 1 mm) and its edges, the droplets temperature being about 5 C lower than in the center. No data exist after Y = since it corresponds roughly to the right edge of spray (1) and thus, very few fluorescence signal can be collected. When the cold spray (2) is added, a decrease of the droplets temperature appears mainly between Y = 1 and Y = mm. The superimposition of datas at Y = 17 mm demonstrates that mainly droplets from the hot spray (1) are detected. In a similar way, the roughly constant temperature values between Y = 1 mm and Y = 2 mm shows that the collected droplets come mainly issued from the cold spray (2). Spray (1) heated alone Both spays with sprays (2) for Q 2 = 4 ml/min Both spays with sprays (2) for Q 2 = 6 ml/min Z (mm) Figure 6: Evolution of the mean droplet temperature along the Z axis for spray (1) heated and injected alone, and for spray (1) heated mixed with the spray (2) for two flow rates at ambient temperature. -4 Spray (1) heated alone Both spays with sprays (2) for Q 2 = 4 ml/min Both spays with sprays (2) for Q 2 = 6 ml/min Y (mm) Figure 7: Evolution of the mean droplet temperature along the Y axis at Z = 3 mm for spray (1) heated and injected alone, and for spray (1) heated mixed with the spray (2) for two flow rates at ambient temperature. The effect of the cold spray (2) can be confirmed by the PDA measurements. Fig. 8 and Fig. 9 present the evolution of the mass flux corresponding to preceding experiments presented in Fig. 6 and Fig. 7. It is clear that increasing the flow rate increases the local mass flux. Therefore, statistically, the number of detected cold droplets increases with the flow rate, and then, tends to decrease the mean temperature derived from eqn (1). This interpretation assumes that there is no heat transfer between both sprays due to droplets coalescence. This assumption will be discusses in the last section of the paper. q m (g/cm 2.s) Spray (1) alone Both sprays whith spray (2) for Q 2 = 4 ml/mn Both sprays whith spray (2) for Q 2 = 6 ml/mn Z (mm) Figure 8: Evolution of the mass flux along the Z axis for the spray (1) alone and for both sprays mixed for two flow rates of spray (2). q m (g/cm 2.s) Spray (1) alone Both sprays whith spray (2) for Q 2 = 4 ml/mn Both sprays whith spray (2) for Q 2 = 6 ml/mn Y (mm) Figure 9: Evolution of the mass flux along the Y axis for the spray (1) alone and for both sprays mixed for two flow rates of spray (2)

8 4.2 Droplet temperature per class size The droplet temperature evolution relatively to T inj is represented as a function of the droplet diameter class at Z = 3 mm in Fig. 1a and at Z = 6 mm in Fig. 1b for three cases: the heated spray (1) alone, both sprays with Q 2 = 4 ml/min and both sprays with Q 2 = 6 ml/min. As expected and for each case, the temperature decrease is higher for the smallest droplets than for the bigger one. It also noticed that, as for the mean temperature, the cooling of droplets increases with the flow rate except at Z = 3 mm for the droplet less than 35 µm in diameter. -4 Spray (1) alone heated =4ml/min =6ml/min (a) (b) Figure 1: Evolution of the droplet temperature as a function of the droplet diameter for both spray mixed at ambient temperature, for spray (1) heated injected alone and for spray (1) heated mixed with the spray (2) at two flow rates: (a) at Z = 3 mm and (b) at Z = 6 mm. Fig. 11 presents the same experiments as in Fig. 1 but along the Y-axis at Z = 3 mm for Y = 17 mm (Fig. 11a) and for Y = 1 mm (Fig. 11b). At Y = 17 mm, the three cases are almost superimposed since at this location (outer edge of the heated spray (1)) mainly heated droplets from the spray (1) are detected. For Y = 1 mm (i.e. on the main axis of the spray (1)), the effect of spray (2) appears and tends the mean droplets temperature similarly whatever the value of Q 2. These two statements are in agreement with Fig. 7. Spray (1) alone heated =4ml/min =6ml/min -4 Spray (1) alone heated =4ml/min =6ml/min Spray (1) alone heated =4ml/min =6ml/min (a) (b) Figure 11: Evolution of the droplet temperature as a function of the droplet diameter for spray (1) heated injected alone and for spray (1) heated mixed with the spray (2) at two flow rates: measurement for Z = 3 mm at Y = 17 mm (a) and Y = 1 mm (b)

9 4.3 Heat transfer between the sprays In the previous section, the potential coalescence of a hot and a cold droplet leading to heat transfer between both sprays was discussed. In this last section, experiments will be conducted in order to check this assumption in the specific case of Q 2 = 4 ml/min. For that purpose, only one spray is seeded with the fluorescent tracer whereas the second spray is only composed with deionised water. In that way, only the droplets coming from the seeded spray can be detected. However, before performing these experiments, it is necessary to check if the concentration of the dye remains constant. Indeed, some coalescence occurs between a seeded droplet and pure water droplet a dilution of the tracer can occurs. According to eqn (1), under isothermal conditions, the fluorescence intensity I fi will be modified. Therefore, the fluorescence intensity collected on a spectral band can be simply plotted as a function of the droplet diameter for the two following cases, when a spray is injected alone and when both sprays are injected together. Fig. 12 shows an example of such results obtained at Z = 3 mm and Y = 1 mm. In this location, the droplets concentration is the highest when both sprays are mixed and therefore, this situation maximizes the coalescences phenomena. It can be observed in Fig. 12, that both plots are superimposed which proves that the fluorescence tracer concentration remains constant. Moreover, Fig. 13 compares the mean distance l m between two droplets, derived from droplet concentration PDA measurements, and the mean droplet diameter D 1 along the Y-axis at Z = 3 mm, when both sprays are injected simultaneously. It appears clearly that the mean distance is widely greater than the mean diameter which tends to minimize the probability of collisions. I f2 (u.a.) Spray (1) alone 25 Both sprays with spray (2) without tracer Y (mm) Figure 12: Evolution of the fluorescence intensity on the second band of detection I f2 for the spray (1) injected alone and for both sprays mixed when the spray (2) is not seeded in fluorescence tracer. Figure 13: Comparison between the mean distance between two droplets and the mean diameter along the Y axis at Z = 3 mm. Fig. 14 describes the evolution of the mean temperature along the Z-axis and along the Y-axis for five cases: 1) Spray (1) seeded by the fluorescent tracer, heated and injected alone 2) Spray (1) seeded by the fluorescent tracer, injected alone at ambient temperature, i.e. without pre-heating of the water 3) Spray (1) heated and spray (2) injected simultaneously and both sprays are seeded by the fluorescent tracer 4) Spray (1) heated and spray (2) injected simultaneously but only spray (1) is seeded by the fluorescent tracer; normally only hot droplets from spray (1) have to be detected. 5) Spray (1) heated and spray (2) injected simultaneously but only the spray (2) is seeded by the fluorescent tracer; normally only hot droplets from spray (2) could be detected. D 1 (µm) D 1 l m l m (µm) - 9 -

10 The analysis of these measurements leads to three statements. First, the superimposition of curves of cases 1) and 3) demonstrates that spray (2) has no influence on spray (1). Secondly, cases 1) and 2) give similar results as those observed in Fig. 6 for Q 2 = 4 ml/mn. It can now be deduced that the temperature decrease is due to the averaging process that takes into account cold and hot droplets. Thirdly, the superimposition of the curves of cases 4) and 5) shows that the spray (1) has also no influence on spray (2). Finally, Fig. 15 presents the results obtained along the Y-axis at Z = 3 mm for three cases: 1) Spray (1) seeded, heated and injected alone 2) Spray (1) heated and sprays (2) injected simultaneously and both sprays are seeded by the fluorescent tracer 3) Spray (1) heated and spray (2) injected simultaneously but only the spray (1) is seeded by the fluorescent tracer; normally only hot droplets from spray (1) could be detected. As in the previous results of Fig. 14, the almost same temperature values obtained for the cases 1) and 3) shows again that spray (1) is not influenced by the presence of spray (2). It can be concluded that heat transfer between the two sprays remains probably marginal. T-T in j Spray (1) alone heated Spray (1) alone at ambien temperature Both sprays mixed and seeded in with fluorescente tracer Spray (1) alone heated Both sprays mixed and only spray (1) is seeded Both sprays mixed and seeded Both sprays mixed and only spray (2) is seeded Both sprays mixed and only spray (1) seeded Z (mm) Y (mm) Figure 14: Evolution of the mean droplet temperature along the Z axis for spray (1) alone (heated or at ambient temperature) and for both sprays mixed when both or only one are seeded in fluorescence tracer. Figure 15: Evolution of the mean droplet temperature along the Y axis for spray (1) heated injected alone and for both sprays mixed when both are seeded in fluorescence tracer and only when spray (2) is seeded in fluorescence tracer. 5. Conclusions Three color laser induced fluorescence (3cLIF) thermometry combined with phase Doppler measurements was applied in polydisperse sprays. This technique allows us to obtain droplet temperatures by droplet size class. This combined 3cLIF/PDA device was tested in the case of two miwed non-isothermal sprays. The liquid of a first spray is pre-heated before being injected through a second spray maintained at ambient temperature. The influence of the flow rate of the second spray is also analyzed. Main results are following: As expected, a decrease of the droplet temperature is observed along the main axis of the spray and this decrease is all the more important that the droplet size is small. Similar behavior is observed along a radial direction. higher decrease of the droplet temperature is obtained by increasing the flow rate of the cold spray A more defined set of experiments allows determining that the observed apparent cooling effect is mainly caused by the local averaging process of droplets of different temperatures. Therefore, it appears that the influence of one spray on the other cannot be highlighted and then, that the heat transfer between the two sprays due to droplets collisions remain probably marginal

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