Key factors affecting the boiling heat transfer coefficient of FC-72 in saturated pool boiling using lotus-type porous copper

Abstract

Boiling immersion cooling using dielectric liquids like FC-72 has shown promise for managing heat dissipation in semiconductor devices due to its low power consumption. However, the miniaturization and increasing operational currents of high-performance computer chips are driving heat fluxes towards 100 W/cm², exceeding the critical heat flux (CHF) of saturated FC-72 at atmospheric pressure by approximately sevenfold. To address this challenge, this study proposes a lotus-type porous copper (lotus copper) with a unidirectional pore structure designed to induce a "breathing phenomenon" that facilitates efficient vapor removal. This approach achieved a CHF of 110 W/cm², surpassing the target value. Beyond CHF enhancement, this study also focuses on improving the heat transfer coefficient (HTC) to minimize device operating temperatures. Initially, the influence of grooved heat-transfer surface structures on HTC was investigated without lotus copper. Results indicated that incipient boiling predominantly occurred on the top surfaces of the grooves, highlighting the significance of top surface area in determining HTC. Based on this finding, two groove structures were selected to evaluate the effectiveness of lotus copper integration. Furthermore, the study demonstrates that the optimal pore size for maximizing HTC is dependent on the heat flux. Large-pore lotus copper excels at high heat fluxes due to its ability to efficiently expel vapor and maintain the breathing phenomenon. Conversely, small-pore lotus copper enhances HTC under low heat flux conditions by increasing bubble nucleation sites at the interface between the lotus copper and the groove surface.