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Abstract: 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. [Copyright &y& Elsevier]

• Raj et al.'s gravity scaling parameters can't be applicable to the controlled wall heat flux condition. • Microgravity boiling mechanisms on chip S 2 × 2 transform from the SDB to BDB regime. • m BDB for controlled wall temperature condition is larger than that for controlled wall heat flux condition. • m BDB for controlled wall heat flux boiling is affected by both T * and L h / L c. • Gravity scaling parameters for controlled wall heat flux boiling were developed. With the ability of dissipating large amount of latent heat in nearly isothermal process, pool boiling heat transfer is an efficient solution for cooling problem of different sized electronic devices in different gravities. For the practical heat dissipation of electronic device usually occurs at controlled wall heat flux condition, it is essential to reveal pool boiling heat transfer characteristics at different sizes and gravities under controlled wall heat flux condition. In this paper, boiling heat transfer mechanisms on two sized smooth silicon chips (chip S 1 × 1 and chip S 2 × 2) in ground gravity and microgravity were studied. It was found that the gravity scaling parameters developed under controlled wall temperature condition cannot be applicable to the controlled wall heat flux condition. Under controlled wall heat flux condition, the microgravity boiling heat transfer performance on chip S 1 × 1 is in surface tension dominated boiling (SDB) regime in the whole heat flux range. While for chip S 2 × 2, it is in SDB regime when heat flux is smaller than 6.44 W·cm−2, but enters into buoyancy dominated boiling (BDB) regime when heat flux is larger than 6.44 W·cm−2. The critical point of the two boiling mechanisms occurs when the threshold gravitational acceleration equals to the experimental microgravity. In BDB regime, the power law coefficient reflecting the gravity effect on nucleate boiling heat transfer performance (m BDB) for controlled wall temperature condition is larger than that for controlled wall heat flux condition, indicating a greater impact of gravity level under controlled wall temperature condition. Moreover, m BDB for controlled wall temperature condition is affected only by non-dimensional wall temperature T *. While for controlled wall heat flux condition, m BDB increases with increasing T * but decreases with increase in heater size, and heater size has a greater influence on m BDB. Based on the available data, the gravity scaling parameters for controlled wall heat flux boiling were developed, which can accurately predict the microgravity boiling heat transfer performance on two sized chips. [ABSTRACT FROM AUTHOR]

Abstract: The effect of surface roughness on nucleate boiling heat transfer is not clearly understood. This study is devised to conduct detailed heat transfer and bubble measurements during boiling on a heater surface with controlled roughness. This first of two companion papers discusses details of the fabrication, construction, and operation of the experimental facility. Test pieces are fabricated from 50.8mm×50.8mm×3.18mm borosilicate glass squares that are roughened by abrading with diamond compound with particles of known size and then annealed in order to control small-scale roughness features. Heater/sensor devices are fabricated by depositing indium tin oxide (ITO), a transparent electrically conductive material, on top of the roughened glass surfaces. A method for calibrating the ITO devices for use as temperature sensors is developed. Experimental boiling curves are reported for seven surfaces of different roughness tested in the perfluorinated fluorocarbon liquid, FC-72. The measured receding contact angle for FC-72 on the ITO-coated surfaces is reported. The wall superheat during saturated pool boiling at atmospheric pressure was found not to vary consistently with surface average roughness (Ra ) values. Qualitative differences between smooth and rough surface boiling visualizations obtained simultaneously from the side and from below the heaters are discussed. [Copyright &y& Elsevier]

Abstract: This paper deals with the experimental evaluation of spray cooling of a horizontal upward-facing 25-mm diameter heater onto which a 5-mm high copper foam disk of same diameter was brazed. The copper foam had a porosity of 90% and a pore density of 10 PPI (pores per inch). A commercially available single-orifice pressure swirl atomizer was used to produce a spray with mass flow rates of 3.0 and 5.0kg/h for system (saturation) pressures of 500 and 600kPa. R-134a was the working fluid. An inlet sub-cooling degree of 5°C was used in all tests. Data are presented in the form of boiling curves and as plots of heat transfer coefficient as a function of the heat flux. Results are compared with those obtained with a plain surface and enhancement factors as high as 1.39 (for a 22W/cm2 heat flux) were observed. A critical heat flux of 30W/cm2 was achieved for the 3.0kg/h mass flow rate for both plain and metal foam surfaces. [Copyright &y& Elsevier]