Experimental Investigation of R134a Flow Boiling in Copper Foam Evaporators for High Heat Flux Electronics Cooling

Abstract

Stochastic cellular structured materials have been previously studied as enhanced surfaces for heat sinks used in cooling of modern electronics. Open-cell metallic foam has been shown to be an effective medium for gas-cooled and liquid-cooled heat sinks. Numerous studies exist for metal-foam cold plates using single phase water but there are few studies pertinent to two-phase evaporators. Because of the latent heat of vaporization and higher heat transfer coefficients, flow boiling is more efficient for cooling of high heat fluxes, as compared to single-phase flow. This paper presents an experimental study on the thermohydraulic performance of compressed and uncompressed copper foam evaporators using R134a refrigerant. The foam samples had the same starting pore size of 40 PPI and porosities of 0.62–0.91, with a heated footprint area of 25.4 × 25.4 mm and a height of 2.5 mm. Experiments were conducted for heat flux ranging from 7 to 174 W/cm2, with mass flux varying from 150 to 375 kg/m2s at fixed inlet saturation temperatures of 31 to 33 °C. Compressing the foam by up to 4X resulted in proportionally smaller effective hydraulic diameter, higher surface area per unit volume, higher metal volume fraction, and higher bulk thermal conductivity. The compressed foam results demonstrated up to three-times lower unit thermal resistance and improved critical heat flux. The apparent heat transfer coefficient in the tested compressed 4X foam evaporator maximized at exit vapor qualities of about 70 to 75%, and the pressure drop increased linearly with exit quality.