Key Factors Affecting the Boiling Heat Transfer Coefficient of Fc-72 in Saturated Pool Boiling Using Lotus-Type Porous Copper

Boiling immersion cooling using dielectric liquids, such as FC-72, has gained significant interest in semiconductor device applications owing to its low power consumption. However, the heat flux of electrical devices, such as high-performance computer chips, will reach 100 W/cm2­ with miniaturization and a larger operating current, approximately seven times higher than the critical heat flux (CHF) of saturated FC-72 at atmospheric pressure. Therefore, this study proposes a lotus-type porous copper (lotus copper) with a unidirectional pore structure to cause the “breathing phenomenon” and demonstrated CHF of 110 W/cm2 above the target value. We focused on reducing the equipment temperature, that is, improving the heat transfer coefficient (HTC), which is as important as the CHF. First, the effects of the structure of the grooved heat-transfer surface on the HTC were evaluated without joining the lotus copper. The experimental results indicated that incipient boiling mainly occurred on the top surfaces of the grooved structure, suggesting the top surface area size is the main factor determining HTC. Based on these results, two types of grooved structures were selected to evaluate the effectiveness of the lotus copper. Moreover, the suitable pore size of lotus copper for improving HTC depends on the heat flux conditions. Lotus copper with a large pore diameter is superior to that with a small pore diameter because of the ease in discharging the generated vapor even when the amount of vapor increases in the high heat flux regime, maintaining the breathing phenomenon. In contrast, small-pore lotus copper is suitable for improving HTC under low heat flux conditions. The visualization results show that incipient boiling occurs at the interface between the lotus copper and top surface of the groove. Therefore, HTC is enhanced by small-pore lotus copper because the number of bubble nucleation sites is increased.