Thin Film Evaporation through Engineered Copper Micropillar Arrays
This work explored and characterized the evaporator section of a fully engineered heat pipe, a proposed semi-passive cooling solution for the thermal management at high heat fluxes, with the use of thin film evaporation. High heat loads present a problem in electronics, circuitry, space, and defense applications which are the main target areas for this cooling solution. Conventionally manufactured flat samples made of copper micropillar arrays were tested to determine their heat removal capabilities for high heat flux applications. The samples were manufactured with 400 μm micropillar hydraulic diameter, various micropillar spacings ranging from 500-1100 μm, with 500 μm and 600 μm micropillar height. These known micropillar dimensions were used to adapt a theoretical model which is used to predict the capillary-limited dry out heat flux. This model considered fluid properties, material properties of the manufactured sample, and micropillar dimensions. The sample’s ability for fluid transport and the measure of void spaces throughout this micropillar array were determined for each tested sample based on its micropillar dimensions. Models for these samples were adapted from literature and a relation between micropillar dimensions and theoretically predicted capillary dry-out heat flux was made. Micropillar dimensions are to be optimized to maximize heat removal capacities for these manufactured surfaces for exposure to high temperature and extreme heat fluxes. Higher dry-out heat flux is desired for maximum cooling. An experimental set up was designed and assembled to test these manufactured test samples using the phase change cooling technique of thin film evaporation with deionized water as the working fluid. This setup was designed to precisely control the flow of deionized water as it enters the manufactured test samples to ensure wicking of the fluid onto the micropillar arrays by capillary action. Pressure and temperature readings were closely monitored to ensure saturation conditions were present as the tested sample was gradually heated up until dry out is observed. Temperature readings throughout the evaporation assembly were recorded and used to determine the heat flux of the test sample throughout the test. Calculated heat flux was plotted as a function of the temperature difference between the test sample and the saturation temperature. Various flow rates were tested to determine the point where heat flux becomes independent of flow rate, assuring that dry-out occurred due to high surface temperature and not lack of fluid capillary fluid supply. Manufactured samples made of copper were tested at up to 50 mL/min volumetric flow rate to determine the maximum heat flux they can withstand before dry-out is observed. Samples with the smallest micropillar spacings of 500 μm had the lowest heat flux value of 64.17 W/cm2 while the manufactured sample having 800μm spacing dissipated 92.2 W/cm2, the most of all samples, showing the most optimized micropillar dimensions of the tested samples. Experimental results were compared with the theoretical model. Experimental results showed good agreement with expected values and showed similar trend as the predicted plot.
Amador, Alejandro, "Thin Film Evaporation through Engineered Copper Micropillar Arrays" (2022). ETD Collection for University of Texas, El Paso. AAI29324086.