Evaporative Cooling Performance of a Brazed Microporous Coating on an Aluminum Surface

Abstract

The effect of an aluminum microporous coating on evaporative cooling performance of water was studied. The coating consisted of aluminum powder brazed to a flat aluminum substrate. Coatings of average thicknesses of 175 μm, 270 μm, 900 μm, and of average aluminum particle sizes of 27 μm, 70 μm and 114 μm were tested. A hot water treatment of the coating insured repeatability of thermal performance. Wickability was measured and compared to Washburn’s equation for capillary flow. Evaporative cooling tests were then performed on coated and plain surfaces. The test surfaces were uniformly heated while kept vertically dipped in an isothermal pool of water. All microporous coatings increased evaporative heat transfer relative to that of the plain surface by their ability to wick fluid to the entire heated area. With particle size increase from 27 μm to 70 μm both the wickability and heat transfer were significantly increased, but no further significant gains in either were obtained with increase to 114 μm. The maximum heat transfer coefficient was increased ∼11 times and the temperature limit (dry-out) heat flux increased ∼10 times with the two larger particle sizes (70 μm and 114 μm). The heat transfer coefficient and dry-out heat flux similarly increased with thickness increase.