Two-phase flow in compressed copper foam with R134a for high heat flux thermal management: Effects of foam compression ratio and refrigerant operating conditions on thermohydraulic performance

Abstract

Most previous research in metallic open-cell foams has focused on single-phase cooling with air or with water. This work examines the thermohydraulic performance of open-cell copper foam with R134-a refrigerant in two-phase flow boiling conditions at heat fluxes well above those examined in majority of previous studies. Detailed experimental results are presented for the thermal performance and pressure drop of copper foam media, in geometric configurations similar to that which may be utilized in a two-phase cold plate. Key to the present study is an investigation of the effects of compression of the foam on its thermohydraulic behavior and critical heat flux (CHF) during flow boiling. The test coupons consist of foam samples, each with a footprint of mathbf{25}.mathbf{4} mathbf{mm} times mathbf{25}.mathbf{4} mathbf{mm} and a height of 2.54 mm, which are soldered to a copper base plate. The foam samples include a baseline uncompressed sample, and two other samples with compression ratios (CR) of 2X and 4X, with porosities of 0.91, 0.82 and 0.62, respectively, and each with a pore size of 40 PPI. The test coupons are mounted in an engineered flow fixture that allows refrigerant flow to enter in an inlet manifold, then through the foam, before flowing to an exit manifold. Experimental tests were performed for heat fluxes from 1.4 to 175 W/cm2, with mass fluxes ranging from 125 to 250 kg/m2s and inlet saturation temperatures of 30 to 40 °C, while varying the inlet subcooling from 0 to 20 °C. Results show that thermal resistance, optimum exit vapor quality, CHF, and pressure drop for flow boiling in compressed metal foams depend strongly on foam compression ratio, inlet subcooling and saturation temperature.