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Influence of surface roughness on a spray cooling system with R134a. Part II: Film thickness measurements
Experimental Thermal and Fluid Science 48 (2013) 73–80
Contents lists available at SciVerse ScienceDirect
Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Influence of surface roughness on a spray cooling system with R134a. Part II: Film thickness measurements Eduardo Martínez-Galván a, Juan Carlos Ramos a,⇑, Raúl Antón a, Rahmatollah Khodabandeh b a b
TECNUN – University of Navarra, Paseo de Manuel Lardizábal 13, 20018 San Sebastián, Spain Royal Institute of Technology (KTH), Brinellvägen 68, Stockholm 10044, Sweden
a r t i c l e
i n f o
Article history: Received 22 November 2011 Received in revised form 27 August 2012 Accepted 18 February 2013 Available online 5 March 2013 Keywords: Spray cooling Surface roughness Film thickness measurements
a b s t r a c t Experimental measurements in a spray cooling test rig were carried out for two different heater surface roughnesses and for two different types of nozzles with the dielectric refrigerant R134a. In this paper, results of the sprayed refrigerant film thickness measurements are presented. The influence of the volumetric flow rate, the surface roughness and the type of nozzle (through the spray parameters) on the total average film thickness is analyzed and discussed. In a companion paper, results of the heat transfer measurements are presented. It has been found that there is a relation between the variations of the average Nusselt number and of the film thickness along the spray cooling boiling curve. The heat transfer regimes along that curve are related not only to a variation in the average Nusselt number but also to changes in the film thickness. The qualitative analysis of those variations served to better understand the heat transfer mechanisms occurring during the spray cooling. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Spray cooling is a powerful technique used to remove large amounts of heat, mainly by means of the combined effect of forced convection and nucleate boiling, while keeping a low operating temperature. This technique has many applications, such as electronic cooling, combustion technology, human skin cooling during laser therapy, and metallurgical processes. The spray cooling technique has shown better cooling performances in electronic cooling compared with other techniques that also combine sensible and latent heat [1]. But this technique is quite complex due to the interaction of many parameters in the process of cooling by atomization. Some spray parameters such as spray density and the mean droplet diameter and velocity have a large effect on the critical heat flux (CHF). According to Chen et al. [2] and Estes and Mudawar [3], the best combination to obtain high efficiency in spray cooling are low spray density, low droplet diameter and large droplet velocity. However, with respect to maximum heat transfer, a high CHF is reached with large droplet velocity and density values [2]. Estes and Mudawar [3] also concluded that in order to increase the CHF there are three alternatives: increasing the flow rate, increasing sub-cooling or decreasing the drop size.
⇑ Corresponding author. E-mail addresses: [email protected] (E. Martínez-Galván), jcramos@tecnun. es (J.C. Ramos), [email protected] (R. Antón), rahmatollah.khodabandeh@energy. kth.se (R. Khodabandeh). 0894-1777/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.expthermflusci.2013.02.010
Apart from the droplet characteristics, the film thickness – of the liquid deposited and the vapour generated over the heater – influences the spray cooling performance and its variations may be another indicator of the heat transfer mechanisms that takes place over the heater. Therefore, film thickness plays a role in the spray cooling performance and has been studied in the literature. There have been several attempts to measure the film thickness formed by a spray. One of these techniques is based on the principle of Fresnel diffraction, and it is used to measure the maximum time-averaged film thickness [4]. The experiments reported in [4] were done with water as the test fluid, with an air-assisted atomizer and under adiabatic conditions, where the film topography was measured with an interference holographic technique. Since this technique is based on the refractive index, which is temperature-dependent, and since in spray cooling there can be large temperature gradients within the film, all the film thickness measurements in [4] were made without heat flux. The results showed that the film was flat and thin and its thickness ranged between 85 and 235 lm, while water flow rates ranged between 0.0166 and 0.066 l/min, and the air pressure difference was 1.38 bar. An analytical model to determine the thickness of a liquid film deposited on a flat surface by a spray under the influence of a secondary gas stagnation flow field was built by Yang et al. [5]. The analytical model shows that the liquid film thickness remains constant under adiabatic conditions, over a flat surface. They also found that the liquid film thickness does not depend on the radial coordinate; it only depends on the liquid’s properties and the
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Nomenclature CHF d32 H N_ Nu Q q00
critical heat flux (W/cm2) Sauter mean diameter (lm) total average film thickness (lm) spray droplet density (drops/s) Nusselt number () volumetric flow rate (m3/s) heat flux (W/cm2)
volumetric flow. The analytical model was developed for water as the refrigerant in an air-assisted atomizer nozzle. Yang et al.’s analytical model [5] has been used to formulate a Nusselt number correlation as a function of the film thickness in the nucleate boiling regime [6]. This is an example of the importance of determining an estimation of film thickness in the spray cooling technique. The correlation was obtained for spray cooling using air-driven spray nozzles with water as the working fluid. Another technique that has been employed to obtain film thickness is a non-invasive optical technique that uses a laser beam to measure the angle of the total internal reflection of light at the interface between materials with two different refraction indices [7,8]. The film thickness measurements made by Pautsch and Shedd were carried out under adiabatic [7,8] and non-adiabatic [8] conditions. The nozzle was oriented so that the droplets were projected upwards and draining was gravity-assisted. The film thickness measurements with a heat flux of 15 W/cm2 were taken at two points: at the edge of the spray impact region and in the film flow region. The liquid used in the measurements of the film thickness under non-adiabatic conditions was gas-saturated FC-72 and the refrigerant flow rate was 0.0518 l/min. The results showed that for adiabatic conditions, film thickness outside the spray cone increases in the radial direction. In the non-adiabatic case, they found that the heat transfer is dominated by single-phase convection at the point just at the edge of the spray impact area, while at the point outside of the spray contact area was dominated by a two-phase flow with the liquid lifted from the surface. The dielectric properties of refrigerant R134a (dielectric constant of 7.2) makes it suitable for electronic cooling, but there are few papers that report its use in this field [9–11]. Hsieh and Tien [9] measured film thickness of R134a in the non-boiling regime and found that the thickness ranged between 0.93 and 1.35 mm for spray mass fluxes between 1.33 and 1.4 kg/m2 s. This refrigerant is commonly used in refrigeration installations and it has been also successfully used in cryogen spray cooling during laser dermatologic surgery [12,13]. In this field, Aguilar et al. have put their efforts into optimizing the cooling efficiency by means of the characterization of the spray droplet size and velocity distributions [12] and by means of the analysis of the influence of the angle between the nozzle and the target surface [13]. Martinez-Galvan et al. [14] employed a high speed camera equipped with a long distance microscope to measure film thickness in order to characterize the heat transfer regimes that take place in the spray cooling technique. The film thickness measurements were made in a closed loop spray cooling system with refrigerant R134a under non-adiabatic conditions. The results reported by Martinez-Galvan et al. permitted four heat transfer regimes along the boiling curve to be distinguished as a function of the Nusselt number and film thickness. The heat transfer and film thickness measurements were performed for four Weber numbers and one nozzle geometry. Surface roughness is another parameter that has an important effect on the spray cooling technique due to the pores created by
Ra V
average roughness (lm) droplet mean velocity (m/s)
Greek symbols DP pressure difference through the nozzle (bar)
the surface roughness, which serves as regions of vapour generation and entrapment. These pores enhance heat dissipation due to the phase-change process and keep the surface temperature at lower values. In [15,16], the authors reported the effect of roughness on the boiling curves in a spray system. An air-assisted atomizer nozzle was used to generate the spray, which uses water as working fluid. In both studies the smoother surface was obtained with a 0.3 lm grit polish and the rougher surface with a 14 lm and 22 lm grit polish, in [16] and [15], respectively. The results showed that in the boiling regime, the smaller the surface roughness, the lower the surface temperature for a given heat flux and the larger the heat flux for a given temperature difference. The results from Ortiz and Gonzalez [17] showed the opposite trend to those from [15,16]. The heater surface was roughened using either 600 grid SiC grinding paper or a 0.25 mm polycrystalline diamond suspension. The results showed that the higher the surface roughness, the more the heat fluxes dissipated, and therefore the higher the CHF. Finally, as the volumetric flow rate was increased, the difference between the boiling curves for both the rough and the smooth surface was less noticeable. In the present paper, the experimental results of film thickness measurements in a closed loop spray cooling system with refrigerant R134a as the working fluid are presented for two different values of the heater surface roughness and for two different types of nozzles. The influence of the volumetric flow rate, the surface roughness and the type of nozzle (through the spray parameters) on the total average film thickness is discussed. Moreover, it has been observed that there is a relation between the heat transfer measurements (presented in Part I) and the film thickness measurements. A correlation between the variations of the Nusselt number and of the total average film thickness along the spray cooling boiling curve with the heat transfer regimes that take place along it was found. A qualitative analysis of those variations served to better understand the heat transfer mechanisms occurring during the spray cooling technique. 2. Test rig description and spray characterization Fig. 1a shows a sketch of the test rig, which consists of the test chamber with the nozzle and the heater, the refrigerant flow loop, and the sub-cooling and the condenser water systems. This test rig was also employed to obtain the results presented in a previous article [14]. The heater consists of a copper block heated by a thin resistance. The surface of the heater facing the spray has a square shape with a 12.7 mm side length (see Fig. 1b). Two surfaces with two values of roughness have been tested: firstly, the experiments were carried out with the heater with its original roughness from the factory, an average roughness of Ra = 0.56 lm, and then after a polishing procedure, an average roughness of Ra = 0.04 lm. The refrigerant R134a was sprayed over the heater (the target surface) with the same full cone nozzle models used in the thermal tests presented in Part I, TG 0.4 and TG 0.7. All the elements of the test rig, the measurement systems and the operating conditions
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one pressure transducer and two thermocouples, one in the liquid phase and the other one in the vapour phase, were used. There was also another thermocouple inside the refrigerant pipe, close to the nozzle exit, to measure the temperature. Although there was a subcooling system installed, it was only used to sub-cool the refrigerant before it reached the pump in order to avoid cavitation. After passing through the pump, the coolant was heated to the saturation temperature corresponding to the chamber pressure. In order to measure the film thickness, a high speed camera, equipped with a long distance microscope, was used. A cool light was used for lighting in order not to cause changes in the heater surface temperature. Fig. 2 shows a photograph of the complete test rig with the high speed camera. The nozzles used in the tests have been characterized by means of three spray parameters, the droplet mean diameter and velocity and the spray droplet density, with the aim of analyze the influence of the geometry of the nozzle on the variations of the film thickness along the spray cooling curve. The experimental procedure and the analytical equations to obtain these spray parameters are presented in Part I paper. The results are presented in Table 1.
3. Experimental methods
Fig. 1. Test rig diagram (a) and detail of the heater (b).
are the same that the ones described in the accompanying paper, Part I, where the heat transfer measurements are presented and analyzed. The pressure drop across the spray nozzle is the parameter used to fix the volumetric flow rate. A pressure transducer was connected upstream of the nozzle to measure the working pressure and in order to measure the saturation condition in the chamber,
Fig. 2. Test rig installation.
Film thickness is defined as the thickness of the liquid film that is formed in the atomization process. However, when phase change appears, the thickness of the film is not referred only to the liquid phase but also to the vapour phase of the bubbles inside it. According to that definition, there will be an increase in the film thickness when two-phase is reached. The sprayed refrigerant film thickness was measured in a zone just outside of the spray cone but over the square heater, as represented schematically in Fig. 3a and shown in the photograph in Fig. 3b. The length of this zone where the film thickness is measured is 3.3 mm. In this picture, a reference target that illustrates how many micrometers correspond to each pixel is placed close to the heater. Due to use of the long distance microscope, the size of the pixels of the images is in between 6 and 9 lm, depending on the test. The film thickness measurements were made in this zone because the high density of the spray prevented us from distinguishing the film from the spray cone in the impact zone. Although the measure zone is outside the spray cone, it was assumed that film thickness analysis in this zone could give a first approximation and help to better understand the heat transfer mechanisms in the spray cooling technique. The film thickness was measured directly from photographs taken with a high speed camera. All the images were taken at 2000 frames per second with 752 376 pixels of resolution. Details of the film thickness measurements and image processing are presented in [14]. The average film thickness uncertainty, for all the tests, was ±11.6%. In Fig. 4, an example of the film thickness boundary (the film edge and the heater edge) can be seen. The heater edge had been previously determined from a picture without spray. As can be observed in Fig. 4, the border of the film is not flat. It has waves, valleys and protuberances that vary over time due to the impingement of the spray and to the renovation movement of the film. For this reason, the film thickness presented is an average over time from five photograph series taken at different times for all tests, where each series takes into account 2000 frames [14]. The local average film thickness over time has been spatially averaged in order to have only one value, which characterizes the film thickness.
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Table 1 R134a spray characteristics. TG 0.4
DP (bar) 1.05 2.05 3.10 a b c
TG 0.7 Q (l/min) 0.15 0.21 0.25
a
d32 (lm) 120.8 106.7 74.4
V (m/s)
b
13.1 18.3 22.5
N_ (drops/s)c 6
2.7 10 5.4 106 1.9 107
DP (bar)
Q (l/min)
d32 (lm)a
V (m/s)b
N_ (drops/s)c
1.07 2.08 3.00
0.28 0.38 0.49
118.9 97.0 58.2
13.1 18.4 22.1
5.4 106 1.3 107 7.9 107
Data obtained from the GSV measurements. Data obtained from Eq. (1) in paper Part I. Data obtained from Eq. (2) in paper Part I.
4. Results and discussion 4.1. Film thickness measurements Fig. 5 shows the total average film thickness, H, as a function of the heat flux for both nozzles, both surface roughnesses and at different volumetric flow rates. Film thickness in the test with the
Fig. 3. Measurement zone of the film thickness: (a) sketch of the heater base and spray cone impact area; (b) picture of the spray cone impinging over the heater.
Fig. 4. Example of the film thickness measurement.
heater surface of greater roughness (Ra = 0.56 lm) and the maximum flow rate using nozzle TG 0.7 (Fig. 5b) could not be measured because the amount of refrigerant expelled from the heater after the impingement was so high that the video measurements were difficult to obtain with good accuracy. In general, it can be observed that, for a given flow rate, the larger the heat flux, the thicker the film. This is coherent with the increase of the number of vapour bubbles generated inside the film over the heater as the applied heat flux is increased. Similar behaviour is found regarding the flow rate for the cases with a rough heater surface: the larger the flow rate, the thicker the film. In this case, it is expected that the film thickness grows with the amount of refrigerant sprayed. But when the heater has a polished surface this variation of the film thickness with the volumetric flow rate is not so clear. The reason for this could be attributed to friction: the pressure drop due to friction is proportional to the flow rate, but if the friction is negligible that proportionality is reduced. So for cases with a polished heater surface, the thickness of the film is more or less the same independently of the volumetric flow rate. This is true for the three volumetric flow rates for nozzle TG 0.4 and for two of the volumetric flow rates for nozzle TG 0.7. Regarding the influence of the heater surface roughness on film thickness, from the results presented in Fig. 5, a general rule of behaviour cannot be observed. It could be expected that for a rough heater (Ra = 0.56 lm) the film thickness would be higher and so, as it was previously pointed out, the roughness favours the nucleation sites, which implies more bubbles inside the film and therefore a thicker film. This is true for the two higher values of the flow rate for nozzle TG 0.4. However, for the low flow rate, in the nucleate boiling region there is no difference in the film thickness for the two values of the heater surface roughness. The explanation could be that for low flow rates the residency time of the liquid over the heater is longer, and therefore an important heat transfer mechanism is surface evaporation (at the liquid–gas interface of the film). So for low values of the volumetric flow rate, a thicker film is expected over a rough surface (more bubbles due to more nucleation sites), counteracted by the increase in friction, which means lower velocities, more residency time of liquid on the surface and more surface evaporation (resulting in a thinner film). In this paragraph, the discussion is focused on the behaviour of nozzle TG 0.7, in which the film thickness is greater for the smaller surface roughness (Ra = 0.04 lm). On the one hand, for this nozzle the roughness means more nucleation sites (creating a thicker film), but on the other hand, nozzle TG 0.7 has volumetric flow rates that are double those of TG 0.4 (for a given pressure drop in the nozzle). Since the velocity reduction is proportional to the friction coefficient (roughness) and to the flow rate, in this case for the tests with a greater surface roughness there will be an increase in the residency time due to a reduction of the velocity due to friction and probably due to the impact of the droplets. Nozzle TG 0.7 has droplets with a larger d32 (for a given flow rate) than those produced by nozzle TG 0.4, and these larger droplets may have the capacity to slow down the flow over the surface. The
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Fig. 5. The total average (time and space) film thickness curves as function of the heat flux for nozzles TG 0.4 (a) and TG 0.7 (b).
deceleration will have more influence at lower velocities since the droplets have the same inertia independent of the surface roughness. That means that since the velocity with a higher surface roughness (Ra = 0.56 lm) is lower than with a surface with small roughness (Ra = 0.04 lm), the extra velocity reduction due to bigger droplets (higher d32) will be greater. This in the end means more residency time, and more surface evaporation in a way that reduces the film thickness even more than in the case of a surface of smooth roughness. The effect of the impact of bigger droplets is diminished with greater flow rates. This is expected since greater flow rates mean there is a greater amount of liquid on the heater surface and greater inertia of the fluid that flows on the surface. That is a reasonable cause why with higher flow rates, the thickness increases again with a surface of a high roughness value. In this case, the contribution of the increase of nucleation sites may be the most important, while the residency time (and the surface evaporation) is lower and may not be so important. Anyway, this is a simple hypothesis to explain the issue that needs to be validated by measurements or estimations of number bubble nucleation sites, exit film temperature and velocity, three parameters that could influence in the presented explanation.
4.2. Correlation between the heat transfer and the film thickness measurements In order to get a better understanding of the heat transfer mechanisms that take place in the spray cooling technique, the thermal
measurements presented in Part I and the film thickness measurements of this paper are compared in this section. In Figs. 6 and 7 the H (total average film thickness) and the Nu curves are shown together, as functions of the heat flux for three different volumetric flow rates, for the two types of nozzles and for the two values of heater surface roughness. These plots have been made in order to see the effect of the surface roughness on the spray cooling performance and the clear relationship between the Nusselt number and the H variations during the different heat transfer regimes that take place in the spray cooling technique. The Nusselt number is defined according to Eq. (1):
Nu ¼ h L=k
ð1Þ
where L is the heater length, k is the thermal conductivity of the liquid and h is the average heat transfer coefficient which is defined as:
h ¼ q00 =DT
ð2Þ
Figs. 6 and 7 show that the changes in the slope of the Nu and H curves occur at approximately the same values of the applied heat flux. Therefore, the variation in both parameters is related and it is possible to identify different zones, as the heat flux increases. More details about this identification can be found in [14]. Firstly, there is a zone I, which can be seen in all the figures, where the H can be considered constant and the Nusselt number increases slightly as the heat flux increases. As mentioned earlier, in this zone the heat transfer regime is mainly through
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Fig. 6. Nusselt number and H as functions of the heat flux for nozzle TG 0.4 and both roughnesses.
single-phase forced convection, and therefore there is no generation of vapour bubbles inside of the film over the heater surface, though some evaporation in the liquid–vapour interface of the film is expected. As the heat flux increases, the onset of the nucleate boiling regime is reached, comprising zone II. This onset can be inferred from Figs. 6 and 7 by the abrupt increase of the H and the Nu, immediately after zone I. In the nucleate boiling regime, surface roughness plays an important role because it fosters vapour generation, as it was mentioned earlier. Therefore, once the nucleate boiling regime starts, for the rough surface case (Ra = 0.56 lm) three zones (zones II, III and IV) with different nucleate boiling regimes can be distinguished, but for the smooth surface roughness (Ra = 0.04 lm) only one zone (zone II) can be distinguished before reaching the CHF. This distinction can be done according to the changes in the slope of both curves, the Nusselt number and the H, in Figs. 6 and 7.
For the rough surface case, the three zones in the nucleate boiling regime can be described as follows. In zone II, the number of sites over the heater where the vapour nucleation takes place increase as the heat flux increases. As commonly acknowledged, the phase change removes more heat (latent) than the single-phase forced convection (sensible heat), which produces an increase in the Nusselt number. The increase in the number of nucleation sites, and therefore in vapour formation inside the film, makes the H increase as well. Beyond zone II, more and more bubbles are produced as the heat flux increases. Therefore, zone III is where the Nusselt number has reached the maximum value and the H is more or less constant as the heat flux increases. This phenomenon is caused by the fact that the amount of nucleation sites has reached its maximum, that is, it becomes difficult to find more sites to produce more bubbles. Hence, greater bubble production implies an increase of the
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Fig. 7. Nusselt number and H as functions of the heat flux for nozzle TG 0.7 and both roughnesses.
Nusselt number, but more vapour-phase in contact with the surface implies a lower Nusselt number. Finally, the third nucleation zone, zone IV in Figs. 6 and 7, corresponds with the moment in which bubble production is so high that as the heat flux increases so does the H, but unlike the other zones, the Nusselt number decreases. The reduction in the Nusselt number occurs because the high amount of vapour over the heater makes liquid film regeneration more difficult and also because the thermo-physical properties of the vapour-phase are worse than those of the liquid-phase. Finally, the CHF is reached when a vapour film totally covers the heater surface and the sprayed refrigerant is not able to rewet it. This means that the average heat transfer coefficient drops sharply and the average heater temperature increases drastically. In this zone, film thickness increases because of the higher amount of vapour over the heater.
For the case of heater surface with low roughness, the behaviour of both curves, the Nusselt number and the film thickness in the nucleate boiling regime (zone II) is explained as follows. Firstly, nucleation is delayed compared to the rough surface due to the lower number of the nucleation sites. But once nucleation starts, the process is faster and shorter. As explained when analyzing the spray cooling boiling curves, the reason for this behaviour could be that for the polished surface the separation of the vapour bubbles from the heater is faster than for the rough surface. When the surface of the heater has low roughness, the vapour bubbles generated on the nucleation sites can be easily separated from the sites (due to their small depth) by the film’s moving liquid. In this way, a new vapour bubble can be generated in a small period of time at the same nucleation site. This results in the lack of zones III and IV: once the nucleation starts, the transition to the regime of vapour columns
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and jets is immediate and, in the same way, the transition to the formation of the vapour film over the heater is immediate, resulting in a continuous increase of the Nu and the H until the CHF is reached. 5. Conclusions Film thickness measurements were taken using a closed-loop spray cooling set-up with R134a as a working fluid. Variations in the volumetric flow rates, in the values of the heater surface roughness and in the spray droplet characteristics (through the nozzle type) were carried out in order to get a deeper insight into the spray cooling performance. Although, it has been hypothesized some factors as relevant ones (the volumetric flow rate, the relation of surface roughness with friction, the relation of friction and droplet size with the liquid residency time over the surface, the relation of the surface evaporation heat transfer mechanism with the residency time of the liquid over the component), there is not a clear influence of the former parameters on the average total film thickness variations along the spray cooling curve. However, it has been observed that the variations of the film thickness along the spray cooling boiling curve are related to the heat transfer regimes that occur during the process. It was found that there is a relation between the variations of the Nusselt number and of the total average film thickness, H, along the spray cooling boiling curve, and that both variations are correlated with the heat transfer regimes developing along it. For the heater of high roughness (Ra = 0.56 lm), three zones can be defined in the nucleate boiling region of the curve as a function of the heat flux in which both the Nu and the H change in a related way. However, for low surface roughness (Ra = 0.04 lm) only one zone can be defined in the nucleate boiling regime before the CHF. The analysis of the film thickness variations outside the sprayed region and the average Nusselt number (over the whole heater) variations during the spray cooling boiling curve provides a good qualitative explanation of the heat transfer mechanisms that take place over the heater. Due to the found relationship, the measurement of the film thickness in the region where the spray is absent may be used as a tool to distinguish the different regions of the spray cooling boiling curve (over the whole heater), as for example the onset of the nucleation. Acknowledgement The authors wish to acknowledge the financial support of Cátedra Fundación Antonio Aranzábal – University of Navarra.
References [1] K.A. Estes, I. Mudawar, Comparison of two-phase electronic cooling using free jets and sprays, Advances in Electronic Packaging, ASME 10 (2) (1995) 975– 987. [2] R.-H. Chen, L.C. Chow, J.E. Navedo, Optimal spray characteristics in water spray cooling, International Journal of Heat and Mass Transfer 47 (2004) 5095–5099. [3] K.A. Estes, I. Mudawar, Correlation of Sauter mean diameter and critical heat flux for spray cooling of small surfaces, International Journal of Heat and Mass Transfer 38 (16) (1995) 2985–2996. [4] J. Yang, L.C. Chow, M.R. Pais, Liquid film thickness and topography determination using Fresnel diffraction and holography, Experimental Heat Transfer 5 (1992) 239–252. [5] J. Yang, L.C. Chow, M.R. Pais, An analytical method to determine the liquid film thickness produced by gas atomized spray, Transactions of the ASME Journal of Heat Transfer 118 (1996) 255–258. [6] J. Yang, L.C. Chow, M.R. Pais, Nucleate boiling heat transfer in spray cooling, Transactions of the ASME Journal of Heat Transfer 118 (1996) 668–671. [7] A.G. Pautsch, T.A. Shedd, Correlation of heat transfer data to film thickness data of the thin film found in spray cooling, in: International Mechanical Engineering Congress and Exposition, Anaheim, California, USA, November 2004. [8] A.G. Pautsch, T.A. Shedd, Adiabatic and diabatic measurements of the liquid film thickness during spray cooling with FC-72, International Journal of Heat and Mass Transfer 49 (2006) 2610–2618. [9] S.-S. Hsieh, C.-H. Tie, R-134a spray dynamics and impingement cooling in the non-boiling regime, International Journal of Heat and Mass Transfer 50 (2007) 502–512. [10] S.-S. Hsieh, T.-C. Fan, H.-H. Tsai, Spray cooling characteristics of water and R134a Part I: nucleate boiling, International Journal of Heat and Mass Transfer 47 (2004) 5703–5712. [11] S.-S. Hsieh, T.-C. Fan, H.-H. Tsai, Spray cooling characteristics of water and R134a. Part II: Transient cooling, International Journal of Heat and Mass Transfer 47 (2004) 5713–5724. [12] G. Aguilar, B. Majaron, W. Verkruysse, Y. Zhou, J.S. Nelson, E.J. Lavernia, Theoretical and experimental analysis of droplet diameter, temperature, and evaporation rate evolution in cryogenic sprays, International Journal of Heat and Mass Transfer 44 (2001) 3201–3211. [13] G. Aguilar, H. Vu, J.S. Nelson, Influence of angle between the nozzle and skin surface on the heat flux and overall heat extraction during cryogen spray cooling, Physics in Medicine and Biology 49 (2004) N147–N153. [14] E. Martinez-Galvan, J.C. Ramos, R. Anton, R. Khodabandeh, Film thickness and heat transfer measurements in a spray cooling system with R-134a, Journal of Electronic Packaging 133 (2011) 011002-1–011002-11. [15] M.R. Pais, L.C. Chow, E.T. Mahefkey, Surface roughness and its effects on the heat transfer mechanism in spray cooling, Journal of Heat Transfer 144 (1992) 211–219. [16] M.S. Sehmbey, M.R. Pais, L.C. Chow, Effect of surface material properties and surface characteristics in evaporative spray cooling, Journal of Thermophysics and Heat Transfer 6 (3) (1992) 505–512. [17] L. Ortiz, J.E. Gonzalez, Experiments on steady-state high heat fluxes using spray cooling, Experimental Heat Transfer 12 (3) (1999) 215–233.
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Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Influence of surface roughness on a spray cooling system with R134a. Part II: Film thickness measurements Eduardo Martínez-Galván a, Juan Carlos Ramos a,⇑, Raúl Antón a, Rahmatollah Khodabandeh b a b
TECNUN – University of Navarra, Paseo de Manuel Lardizábal 13, 20018 San Sebastián, Spain Royal Institute of Technology (KTH), Brinellvägen 68, Stockholm 10044, Sweden
a r t i c l e
i n f o
Article history: Received 22 November 2011 Received in revised form 27 August 2012 Accepted 18 February 2013 Available online 5 March 2013 Keywords: Spray cooling Surface roughness Film thickness measurements
a b s t r a c t Experimental measurements in a spray cooling test rig were carried out for two different heater surface roughnesses and for two different types of nozzles with the dielectric refrigerant R134a. In this paper, results of the sprayed refrigerant film thickness measurements are presented. The influence of the volumetric flow rate, the surface roughness and the type of nozzle (through the spray parameters) on the total average film thickness is analyzed and discussed. In a companion paper, results of the heat transfer measurements are presented. It has been found that there is a relation between the variations of the average Nusselt number and of the film thickness along the spray cooling boiling curve. The heat transfer regimes along that curve are related not only to a variation in the average Nusselt number but also to changes in the film thickness. The qualitative analysis of those variations served to better understand the heat transfer mechanisms occurring during the spray cooling. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Spray cooling is a powerful technique used to remove large amounts of heat, mainly by means of the combined effect of forced convection and nucleate boiling, while keeping a low operating temperature. This technique has many applications, such as electronic cooling, combustion technology, human skin cooling during laser therapy, and metallurgical processes. The spray cooling technique has shown better cooling performances in electronic cooling compared with other techniques that also combine sensible and latent heat [1]. But this technique is quite complex due to the interaction of many parameters in the process of cooling by atomization. Some spray parameters such as spray density and the mean droplet diameter and velocity have a large effect on the critical heat flux (CHF). According to Chen et al. [2] and Estes and Mudawar [3], the best combination to obtain high efficiency in spray cooling are low spray density, low droplet diameter and large droplet velocity. However, with respect to maximum heat transfer, a high CHF is reached with large droplet velocity and density values [2]. Estes and Mudawar [3] also concluded that in order to increase the CHF there are three alternatives: increasing the flow rate, increasing sub-cooling or decreasing the drop size.
⇑ Corresponding author. E-mail addresses: [email protected] (E. Martínez-Galván), jcramos@tecnun. es (J.C. Ramos), [email protected] (R. Antón), rahmatollah.khodabandeh@energy. kth.se (R. Khodabandeh). 0894-1777/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.expthermflusci.2013.02.010
Apart from the droplet characteristics, the film thickness – of the liquid deposited and the vapour generated over the heater – influences the spray cooling performance and its variations may be another indicator of the heat transfer mechanisms that takes place over the heater. Therefore, film thickness plays a role in the spray cooling performance and has been studied in the literature. There have been several attempts to measure the film thickness formed by a spray. One of these techniques is based on the principle of Fresnel diffraction, and it is used to measure the maximum time-averaged film thickness [4]. The experiments reported in [4] were done with water as the test fluid, with an air-assisted atomizer and under adiabatic conditions, where the film topography was measured with an interference holographic technique. Since this technique is based on the refractive index, which is temperature-dependent, and since in spray cooling there can be large temperature gradients within the film, all the film thickness measurements in [4] were made without heat flux. The results showed that the film was flat and thin and its thickness ranged between 85 and 235 lm, while water flow rates ranged between 0.0166 and 0.066 l/min, and the air pressure difference was 1.38 bar. An analytical model to determine the thickness of a liquid film deposited on a flat surface by a spray under the influence of a secondary gas stagnation flow field was built by Yang et al. [5]. The analytical model shows that the liquid film thickness remains constant under adiabatic conditions, over a flat surface. They also found that the liquid film thickness does not depend on the radial coordinate; it only depends on the liquid’s properties and the
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Nomenclature CHF d32 H N_ Nu Q q00
critical heat flux (W/cm2) Sauter mean diameter (lm) total average film thickness (lm) spray droplet density (drops/s) Nusselt number () volumetric flow rate (m3/s) heat flux (W/cm2)
volumetric flow. The analytical model was developed for water as the refrigerant in an air-assisted atomizer nozzle. Yang et al.’s analytical model [5] has been used to formulate a Nusselt number correlation as a function of the film thickness in the nucleate boiling regime [6]. This is an example of the importance of determining an estimation of film thickness in the spray cooling technique. The correlation was obtained for spray cooling using air-driven spray nozzles with water as the working fluid. Another technique that has been employed to obtain film thickness is a non-invasive optical technique that uses a laser beam to measure the angle of the total internal reflection of light at the interface between materials with two different refraction indices [7,8]. The film thickness measurements made by Pautsch and Shedd were carried out under adiabatic [7,8] and non-adiabatic [8] conditions. The nozzle was oriented so that the droplets were projected upwards and draining was gravity-assisted. The film thickness measurements with a heat flux of 15 W/cm2 were taken at two points: at the edge of the spray impact region and in the film flow region. The liquid used in the measurements of the film thickness under non-adiabatic conditions was gas-saturated FC-72 and the refrigerant flow rate was 0.0518 l/min. The results showed that for adiabatic conditions, film thickness outside the spray cone increases in the radial direction. In the non-adiabatic case, they found that the heat transfer is dominated by single-phase convection at the point just at the edge of the spray impact area, while at the point outside of the spray contact area was dominated by a two-phase flow with the liquid lifted from the surface. The dielectric properties of refrigerant R134a (dielectric constant of 7.2) makes it suitable for electronic cooling, but there are few papers that report its use in this field [9–11]. Hsieh and Tien [9] measured film thickness of R134a in the non-boiling regime and found that the thickness ranged between 0.93 and 1.35 mm for spray mass fluxes between 1.33 and 1.4 kg/m2 s. This refrigerant is commonly used in refrigeration installations and it has been also successfully used in cryogen spray cooling during laser dermatologic surgery [12,13]. In this field, Aguilar et al. have put their efforts into optimizing the cooling efficiency by means of the characterization of the spray droplet size and velocity distributions [12] and by means of the analysis of the influence of the angle between the nozzle and the target surface [13]. Martinez-Galvan et al. [14] employed a high speed camera equipped with a long distance microscope to measure film thickness in order to characterize the heat transfer regimes that take place in the spray cooling technique. The film thickness measurements were made in a closed loop spray cooling system with refrigerant R134a under non-adiabatic conditions. The results reported by Martinez-Galvan et al. permitted four heat transfer regimes along the boiling curve to be distinguished as a function of the Nusselt number and film thickness. The heat transfer and film thickness measurements were performed for four Weber numbers and one nozzle geometry. Surface roughness is another parameter that has an important effect on the spray cooling technique due to the pores created by
Ra V
average roughness (lm) droplet mean velocity (m/s)
Greek symbols DP pressure difference through the nozzle (bar)
the surface roughness, which serves as regions of vapour generation and entrapment. These pores enhance heat dissipation due to the phase-change process and keep the surface temperature at lower values. In [15,16], the authors reported the effect of roughness on the boiling curves in a spray system. An air-assisted atomizer nozzle was used to generate the spray, which uses water as working fluid. In both studies the smoother surface was obtained with a 0.3 lm grit polish and the rougher surface with a 14 lm and 22 lm grit polish, in [16] and [15], respectively. The results showed that in the boiling regime, the smaller the surface roughness, the lower the surface temperature for a given heat flux and the larger the heat flux for a given temperature difference. The results from Ortiz and Gonzalez [17] showed the opposite trend to those from [15,16]. The heater surface was roughened using either 600 grid SiC grinding paper or a 0.25 mm polycrystalline diamond suspension. The results showed that the higher the surface roughness, the more the heat fluxes dissipated, and therefore the higher the CHF. Finally, as the volumetric flow rate was increased, the difference between the boiling curves for both the rough and the smooth surface was less noticeable. In the present paper, the experimental results of film thickness measurements in a closed loop spray cooling system with refrigerant R134a as the working fluid are presented for two different values of the heater surface roughness and for two different types of nozzles. The influence of the volumetric flow rate, the surface roughness and the type of nozzle (through the spray parameters) on the total average film thickness is discussed. Moreover, it has been observed that there is a relation between the heat transfer measurements (presented in Part I) and the film thickness measurements. A correlation between the variations of the Nusselt number and of the total average film thickness along the spray cooling boiling curve with the heat transfer regimes that take place along it was found. A qualitative analysis of those variations served to better understand the heat transfer mechanisms occurring during the spray cooling technique. 2. Test rig description and spray characterization Fig. 1a shows a sketch of the test rig, which consists of the test chamber with the nozzle and the heater, the refrigerant flow loop, and the sub-cooling and the condenser water systems. This test rig was also employed to obtain the results presented in a previous article [14]. The heater consists of a copper block heated by a thin resistance. The surface of the heater facing the spray has a square shape with a 12.7 mm side length (see Fig. 1b). Two surfaces with two values of roughness have been tested: firstly, the experiments were carried out with the heater with its original roughness from the factory, an average roughness of Ra = 0.56 lm, and then after a polishing procedure, an average roughness of Ra = 0.04 lm. The refrigerant R134a was sprayed over the heater (the target surface) with the same full cone nozzle models used in the thermal tests presented in Part I, TG 0.4 and TG 0.7. All the elements of the test rig, the measurement systems and the operating conditions
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one pressure transducer and two thermocouples, one in the liquid phase and the other one in the vapour phase, were used. There was also another thermocouple inside the refrigerant pipe, close to the nozzle exit, to measure the temperature. Although there was a subcooling system installed, it was only used to sub-cool the refrigerant before it reached the pump in order to avoid cavitation. After passing through the pump, the coolant was heated to the saturation temperature corresponding to the chamber pressure. In order to measure the film thickness, a high speed camera, equipped with a long distance microscope, was used. A cool light was used for lighting in order not to cause changes in the heater surface temperature. Fig. 2 shows a photograph of the complete test rig with the high speed camera. The nozzles used in the tests have been characterized by means of three spray parameters, the droplet mean diameter and velocity and the spray droplet density, with the aim of analyze the influence of the geometry of the nozzle on the variations of the film thickness along the spray cooling curve. The experimental procedure and the analytical equations to obtain these spray parameters are presented in Part I paper. The results are presented in Table 1.
3. Experimental methods
Fig. 1. Test rig diagram (a) and detail of the heater (b).
are the same that the ones described in the accompanying paper, Part I, where the heat transfer measurements are presented and analyzed. The pressure drop across the spray nozzle is the parameter used to fix the volumetric flow rate. A pressure transducer was connected upstream of the nozzle to measure the working pressure and in order to measure the saturation condition in the chamber,
Fig. 2. Test rig installation.
Film thickness is defined as the thickness of the liquid film that is formed in the atomization process. However, when phase change appears, the thickness of the film is not referred only to the liquid phase but also to the vapour phase of the bubbles inside it. According to that definition, there will be an increase in the film thickness when two-phase is reached. The sprayed refrigerant film thickness was measured in a zone just outside of the spray cone but over the square heater, as represented schematically in Fig. 3a and shown in the photograph in Fig. 3b. The length of this zone where the film thickness is measured is 3.3 mm. In this picture, a reference target that illustrates how many micrometers correspond to each pixel is placed close to the heater. Due to use of the long distance microscope, the size of the pixels of the images is in between 6 and 9 lm, depending on the test. The film thickness measurements were made in this zone because the high density of the spray prevented us from distinguishing the film from the spray cone in the impact zone. Although the measure zone is outside the spray cone, it was assumed that film thickness analysis in this zone could give a first approximation and help to better understand the heat transfer mechanisms in the spray cooling technique. The film thickness was measured directly from photographs taken with a high speed camera. All the images were taken at 2000 frames per second with 752 376 pixels of resolution. Details of the film thickness measurements and image processing are presented in [14]. The average film thickness uncertainty, for all the tests, was ±11.6%. In Fig. 4, an example of the film thickness boundary (the film edge and the heater edge) can be seen. The heater edge had been previously determined from a picture without spray. As can be observed in Fig. 4, the border of the film is not flat. It has waves, valleys and protuberances that vary over time due to the impingement of the spray and to the renovation movement of the film. For this reason, the film thickness presented is an average over time from five photograph series taken at different times for all tests, where each series takes into account 2000 frames [14]. The local average film thickness over time has been spatially averaged in order to have only one value, which characterizes the film thickness.
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Table 1 R134a spray characteristics. TG 0.4
DP (bar) 1.05 2.05 3.10 a b c
TG 0.7 Q (l/min) 0.15 0.21 0.25
a
d32 (lm) 120.8 106.7 74.4
V (m/s)
b
13.1 18.3 22.5
N_ (drops/s)c 6
2.7 10 5.4 106 1.9 107
DP (bar)
Q (l/min)
d32 (lm)a
V (m/s)b
N_ (drops/s)c
1.07 2.08 3.00
0.28 0.38 0.49
118.9 97.0 58.2
13.1 18.4 22.1
5.4 106 1.3 107 7.9 107
Data obtained from the GSV measurements. Data obtained from Eq. (1) in paper Part I. Data obtained from Eq. (2) in paper Part I.
4. Results and discussion 4.1. Film thickness measurements Fig. 5 shows the total average film thickness, H, as a function of the heat flux for both nozzles, both surface roughnesses and at different volumetric flow rates. Film thickness in the test with the
Fig. 3. Measurement zone of the film thickness: (a) sketch of the heater base and spray cone impact area; (b) picture of the spray cone impinging over the heater.
Fig. 4. Example of the film thickness measurement.
heater surface of greater roughness (Ra = 0.56 lm) and the maximum flow rate using nozzle TG 0.7 (Fig. 5b) could not be measured because the amount of refrigerant expelled from the heater after the impingement was so high that the video measurements were difficult to obtain with good accuracy. In general, it can be observed that, for a given flow rate, the larger the heat flux, the thicker the film. This is coherent with the increase of the number of vapour bubbles generated inside the film over the heater as the applied heat flux is increased. Similar behaviour is found regarding the flow rate for the cases with a rough heater surface: the larger the flow rate, the thicker the film. In this case, it is expected that the film thickness grows with the amount of refrigerant sprayed. But when the heater has a polished surface this variation of the film thickness with the volumetric flow rate is not so clear. The reason for this could be attributed to friction: the pressure drop due to friction is proportional to the flow rate, but if the friction is negligible that proportionality is reduced. So for cases with a polished heater surface, the thickness of the film is more or less the same independently of the volumetric flow rate. This is true for the three volumetric flow rates for nozzle TG 0.4 and for two of the volumetric flow rates for nozzle TG 0.7. Regarding the influence of the heater surface roughness on film thickness, from the results presented in Fig. 5, a general rule of behaviour cannot be observed. It could be expected that for a rough heater (Ra = 0.56 lm) the film thickness would be higher and so, as it was previously pointed out, the roughness favours the nucleation sites, which implies more bubbles inside the film and therefore a thicker film. This is true for the two higher values of the flow rate for nozzle TG 0.4. However, for the low flow rate, in the nucleate boiling region there is no difference in the film thickness for the two values of the heater surface roughness. The explanation could be that for low flow rates the residency time of the liquid over the heater is longer, and therefore an important heat transfer mechanism is surface evaporation (at the liquid–gas interface of the film). So for low values of the volumetric flow rate, a thicker film is expected over a rough surface (more bubbles due to more nucleation sites), counteracted by the increase in friction, which means lower velocities, more residency time of liquid on the surface and more surface evaporation (resulting in a thinner film). In this paragraph, the discussion is focused on the behaviour of nozzle TG 0.7, in which the film thickness is greater for the smaller surface roughness (Ra = 0.04 lm). On the one hand, for this nozzle the roughness means more nucleation sites (creating a thicker film), but on the other hand, nozzle TG 0.7 has volumetric flow rates that are double those of TG 0.4 (for a given pressure drop in the nozzle). Since the velocity reduction is proportional to the friction coefficient (roughness) and to the flow rate, in this case for the tests with a greater surface roughness there will be an increase in the residency time due to a reduction of the velocity due to friction and probably due to the impact of the droplets. Nozzle TG 0.7 has droplets with a larger d32 (for a given flow rate) than those produced by nozzle TG 0.4, and these larger droplets may have the capacity to slow down the flow over the surface. The
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Fig. 5. The total average (time and space) film thickness curves as function of the heat flux for nozzles TG 0.4 (a) and TG 0.7 (b).
deceleration will have more influence at lower velocities since the droplets have the same inertia independent of the surface roughness. That means that since the velocity with a higher surface roughness (Ra = 0.56 lm) is lower than with a surface with small roughness (Ra = 0.04 lm), the extra velocity reduction due to bigger droplets (higher d32) will be greater. This in the end means more residency time, and more surface evaporation in a way that reduces the film thickness even more than in the case of a surface of smooth roughness. The effect of the impact of bigger droplets is diminished with greater flow rates. This is expected since greater flow rates mean there is a greater amount of liquid on the heater surface and greater inertia of the fluid that flows on the surface. That is a reasonable cause why with higher flow rates, the thickness increases again with a surface of a high roughness value. In this case, the contribution of the increase of nucleation sites may be the most important, while the residency time (and the surface evaporation) is lower and may not be so important. Anyway, this is a simple hypothesis to explain the issue that needs to be validated by measurements or estimations of number bubble nucleation sites, exit film temperature and velocity, three parameters that could influence in the presented explanation.
4.2. Correlation between the heat transfer and the film thickness measurements In order to get a better understanding of the heat transfer mechanisms that take place in the spray cooling technique, the thermal
measurements presented in Part I and the film thickness measurements of this paper are compared in this section. In Figs. 6 and 7 the H (total average film thickness) and the Nu curves are shown together, as functions of the heat flux for three different volumetric flow rates, for the two types of nozzles and for the two values of heater surface roughness. These plots have been made in order to see the effect of the surface roughness on the spray cooling performance and the clear relationship between the Nusselt number and the H variations during the different heat transfer regimes that take place in the spray cooling technique. The Nusselt number is defined according to Eq. (1):
Nu ¼ h L=k
ð1Þ
where L is the heater length, k is the thermal conductivity of the liquid and h is the average heat transfer coefficient which is defined as:
h ¼ q00 =DT
ð2Þ
Figs. 6 and 7 show that the changes in the slope of the Nu and H curves occur at approximately the same values of the applied heat flux. Therefore, the variation in both parameters is related and it is possible to identify different zones, as the heat flux increases. More details about this identification can be found in [14]. Firstly, there is a zone I, which can be seen in all the figures, where the H can be considered constant and the Nusselt number increases slightly as the heat flux increases. As mentioned earlier, in this zone the heat transfer regime is mainly through
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Fig. 6. Nusselt number and H as functions of the heat flux for nozzle TG 0.4 and both roughnesses.
single-phase forced convection, and therefore there is no generation of vapour bubbles inside of the film over the heater surface, though some evaporation in the liquid–vapour interface of the film is expected. As the heat flux increases, the onset of the nucleate boiling regime is reached, comprising zone II. This onset can be inferred from Figs. 6 and 7 by the abrupt increase of the H and the Nu, immediately after zone I. In the nucleate boiling regime, surface roughness plays an important role because it fosters vapour generation, as it was mentioned earlier. Therefore, once the nucleate boiling regime starts, for the rough surface case (Ra = 0.56 lm) three zones (zones II, III and IV) with different nucleate boiling regimes can be distinguished, but for the smooth surface roughness (Ra = 0.04 lm) only one zone (zone II) can be distinguished before reaching the CHF. This distinction can be done according to the changes in the slope of both curves, the Nusselt number and the H, in Figs. 6 and 7.
For the rough surface case, the three zones in the nucleate boiling regime can be described as follows. In zone II, the number of sites over the heater where the vapour nucleation takes place increase as the heat flux increases. As commonly acknowledged, the phase change removes more heat (latent) than the single-phase forced convection (sensible heat), which produces an increase in the Nusselt number. The increase in the number of nucleation sites, and therefore in vapour formation inside the film, makes the H increase as well. Beyond zone II, more and more bubbles are produced as the heat flux increases. Therefore, zone III is where the Nusselt number has reached the maximum value and the H is more or less constant as the heat flux increases. This phenomenon is caused by the fact that the amount of nucleation sites has reached its maximum, that is, it becomes difficult to find more sites to produce more bubbles. Hence, greater bubble production implies an increase of the
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Fig. 7. Nusselt number and H as functions of the heat flux for nozzle TG 0.7 and both roughnesses.
Nusselt number, but more vapour-phase in contact with the surface implies a lower Nusselt number. Finally, the third nucleation zone, zone IV in Figs. 6 and 7, corresponds with the moment in which bubble production is so high that as the heat flux increases so does the H, but unlike the other zones, the Nusselt number decreases. The reduction in the Nusselt number occurs because the high amount of vapour over the heater makes liquid film regeneration more difficult and also because the thermo-physical properties of the vapour-phase are worse than those of the liquid-phase. Finally, the CHF is reached when a vapour film totally covers the heater surface and the sprayed refrigerant is not able to rewet it. This means that the average heat transfer coefficient drops sharply and the average heater temperature increases drastically. In this zone, film thickness increases because of the higher amount of vapour over the heater.
For the case of heater surface with low roughness, the behaviour of both curves, the Nusselt number and the film thickness in the nucleate boiling regime (zone II) is explained as follows. Firstly, nucleation is delayed compared to the rough surface due to the lower number of the nucleation sites. But once nucleation starts, the process is faster and shorter. As explained when analyzing the spray cooling boiling curves, the reason for this behaviour could be that for the polished surface the separation of the vapour bubbles from the heater is faster than for the rough surface. When the surface of the heater has low roughness, the vapour bubbles generated on the nucleation sites can be easily separated from the sites (due to their small depth) by the film’s moving liquid. In this way, a new vapour bubble can be generated in a small period of time at the same nucleation site. This results in the lack of zones III and IV: once the nucleation starts, the transition to the regime of vapour columns
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and jets is immediate and, in the same way, the transition to the formation of the vapour film over the heater is immediate, resulting in a continuous increase of the Nu and the H until the CHF is reached. 5. Conclusions Film thickness measurements were taken using a closed-loop spray cooling set-up with R134a as a working fluid. Variations in the volumetric flow rates, in the values of the heater surface roughness and in the spray droplet characteristics (through the nozzle type) were carried out in order to get a deeper insight into the spray cooling performance. Although, it has been hypothesized some factors as relevant ones (the volumetric flow rate, the relation of surface roughness with friction, the relation of friction and droplet size with the liquid residency time over the surface, the relation of the surface evaporation heat transfer mechanism with the residency time of the liquid over the component), there is not a clear influence of the former parameters on the average total film thickness variations along the spray cooling curve. However, it has been observed that the variations of the film thickness along the spray cooling boiling curve are related to the heat transfer regimes that occur during the process. It was found that there is a relation between the variations of the Nusselt number and of the total average film thickness, H, along the spray cooling boiling curve, and that both variations are correlated with the heat transfer regimes developing along it. For the heater of high roughness (Ra = 0.56 lm), three zones can be defined in the nucleate boiling region of the curve as a function of the heat flux in which both the Nu and the H change in a related way. However, for low surface roughness (Ra = 0.04 lm) only one zone can be defined in the nucleate boiling regime before the CHF. The analysis of the film thickness variations outside the sprayed region and the average Nusselt number (over the whole heater) variations during the spray cooling boiling curve provides a good qualitative explanation of the heat transfer mechanisms that take place over the heater. Due to the found relationship, the measurement of the film thickness in the region where the spray is absent may be used as a tool to distinguish the different regions of the spray cooling boiling curve (over the whole heater), as for example the onset of the nucleation. Acknowledgement The authors wish to acknowledge the financial support of Cátedra Fundación Antonio Aranzábal – University of Navarra.
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