Effect of pore number density on nucleate boiling heat transfer performance of aluminum surface with microporous structure

Abstract

Electronic equipment cooling devices need to be installed inexpensively in high-density packaged equipments such as 1U-height servers. As a cooling device, a thermosyphon has attracted attention from the viewpoint of its cooling performance and cooling power saving. For cooling CPUs, thermosyphons especially made of copper with water coolant have been studied so far. From the viewpoint of cost and weight reduction, however, a thermosyphon need to be made of aluminum. And by optimizing the boiling heat transfer surface profile of a thermosyphon, CPUs can be operated stably at a lower operating temperature by reducing the wall superheat. Therefore, in this study, we have made the aluminum boiling heat transfer surface that has a porous structure formed by micro-curl skived fins to form reentrant cavities in order to enhance a boiling heat transfer performance. We verified the effect to enhance the nucleate boiling heat transfer characteristic of fluorine-based refrigerant HFE-7000 on the aluminum structured surface with micro-curl skived fins up to a heat flux 100kW/m² (typical condition of CPU cooling), especially dependence of saturated vapor pressure (0.10MPa, 0.14MPa, 0.18MPa) and number of micro pores (467[1/cm²]～1250[1/cm²]) on the wall superheat and boiling heat transfer coefficient. From these examinations, mainly we concluded the following. (1)When the pore number density of structured surface is 833[1/cm²], the wall superheat reduces to below 1K and the boiling heat transfer coefficient enhances to around 100kW/(m²・K) at saturated vapor pressure 0.14MPa. (2)Basically, the wall superheat decreases as the increase of the saturated vapor pressure, however, in case of the structured surface with skived-fins, especially at low heat flux (under 40kW/m²) and high vapor pressure (0.18MPa), it tends to increase. (3)At the pore number density over 833[1/cm²], this tendency becomes more remarkable and at 1250[1/cm²] an instability of the wall superheat occurs.