Evaporative cooling performance of a brazed microporous coating on an aluminum surface

The effect of an aluminum microporous coating on evaporative cooling performance was studied using distilled water as the working fluid. The aluminum microporous coating was fabricated by brazing aluminum particles to an aluminum substrate. Microporous coating thicknesses of 175 microm +/- 20 microm, 270 microm +/- 20 microm, and 900 microm +/- 90 microm, and average aluminum particle sizes of 27 microm, 70 microm and 114 microm were used in a parametric study to determine the optimum aluminum microporous coating. A hot water treatment maximized the wickability of the microporous coating. Wickability was measured by vertical dipping of the coating. Both a mass approach and a height approach were employed in a vertical dipping test and the results were compared to Washburn's equation. Evaporative cooling tests were then performed on both the microporous coated samples and a plain aluminum reference surface. The results of the evaporative testing were analyzed by plotting heat flux versus average temperature difference between the surface and water. Heat transfer coefficients were plotted versus heat flux. The microporous coating increased evaporation heat transfer by its capillary pumping ability to deliver a film of fluid to a large area. When the particle size was increased from 27 microm to 70 microm the wickability of the microporous coating was enhanced. This enhancement in the wickability of the microporous coating increased the heat transfer coefficient by up to 600 % when compared to the plain aluminum reference. However, as the particle size increased from 70 microm to 114 microm no significant further increase in wickability or heat transfer performance was observed. Additionally, as the thickness of the microporous coating was increased, a larger volume of fluid was delivered to the heated surface and the onset of dry-out was delayed to higher heat fluxes. The thickest coating, 900 microm +/- 90 microm (70 microm particle size), increased the dry-out heat flux 16 times relative to the plain aluminum reference.