Effect of Embedding Cavities into Reduced Graphene Oxide-coated Micropillar on Boiling Heat Transfer Enhancement

As cooling capacity demands for systems of high heat flux, i.e. data storage center, nuclear fusion reactor, and computer chip cooling, continuously grows, boiling heat transfer with phase change could be a favorable method since it utilizes latent heat, which allows smaller superheat of system. Critical heat flux (CHF), which indicates the maximum cooling capacity can be obtained by fully developed nucleate boiling, is determined by vapor film formation on the heated surface. If the vapor film covers the entire heated surface, an abrupt increase in wall temperature is observed as the thermal conductivity of the vapor film is a few orders of magnitude smaller than that of liquid-phase working fluid. To delay the formation of vapor film, bubble coalescence should be prevented to supply sufficient, fresh working fluid. The merging of adjacent bubbles can be suppressed by controlling bubble characteristics. Nanoparticle coating with graphene/graphene-oxide could generate smaller bubbles compared to the plain surface, by offering a myriad of micro-sized cavities which promote bubble nucleation. However, bubble coalescence still occurs with high heat flux conditions. To ensure liquid paths with high thermal load, the location of bubbles should be considered. Here, we propose reduced graphene oxide (rGO)-coated patterned surfaces with micropillar array and microcavities to pattern nucleation sites, thus, to suppress bubble coalescence observed in our previous work, rGO coated micropillar. The rGO-coated nucleation pattern surfaces were prepared by stepwise methods of deep reactive ion etching (DRIE) and nanofluid boiling with 0.0005 wt% rGO solution. Micropillars are made to be 20 um in height, 4 um in diameter, and 20 um in pitch, respectively, and square cavities’ width is 200 um, and 1 mm in pitch, respectively. Fabricated surfaces of micropillar only, cavity embedded surface were evaluated by deionized (DI) water pool boiling experiment. The experimental conditions are the saturated temperature of DI water at atmospheric pressure. Experimental surfaces of Plain, rGO-coated micropillar, rGO-coated cavity embedded surface recorded 89 W/cm2, 224 W/cm2, and 261 W/cm2 in CHF and 20.4 kW/m2K, 74.3 kW/m2K, and 85.8 kW/m2K in maximum heat transfer coefficient, respectively. CHF and HTC records confirm that the nucleation pattern surface exhibits superior heat transfer performance to the rGO-coated micropillar surface. Bubble visualization results show that rGO-coated nucleation pattern surfaces have separated bubble departure with the presence of cavities, while bubbles merge laterally with the high heat flux on the rGO-coated micropillar surface. Thus, the CHF and HTC are heightened for the cavity-embedded case owing to the suppressed bubble merger on the heated surface. Bubble images which are taken at 50 W/cm2 heat flux show liquid paths between the cavity pattern on the cavity embedded surface. This means the working fluid could imbibe through the microstructures; thus, bubble coalescence can be effectively suppressed, delaying the CHF. The notable improvement on CHF and HTC in the rGO-coated nucleation pattern surfaces indicates cavity embedment for bubble merger resistance can be a powerful design for nanoparticle-coated surfaces, allowing 320% increased HTC compared to the plain surfaces. This study will be helpful for designing nanoparticle-coated surfaces for enhanced boiling heat transfer on cooling systems for high heat flux devices like heat exchangers and fusion reactors.