Flow boiling in a relatively large copper heat sink comprised of Tesla microchannels

Abstract

Flow boiling in copper microchannel heat sink is widely used for the cooling of high power electronic modules, particularly the IGBT power electronic modules with large sizes. However, it is challenging to significantly enhance the flow boiling performance of copper microchannel heat sink due to the long-lasting issue of vapor backflow and liquid supply that severely deteriorates flow boiling heat transfer. Also, a high channel length to hydraulic diameter ratio (L/D_h) of a large heat sink is not favorable for efficient two-phase transport, resulting in the early occurrence of boiling crisis. In this work, a relatively large copper heat sink (L × W = 10 cm × 5 cm) comprised of Tesla microchannels characterized with excellent flow diodicity was designed and fabricated. The L/D_h ratio of the as-designed heat sink is about 220, which is much larger than the reported studies. In this new heat sink, the periodic Tesla valve structures in each channel is capable of inhibiting the severe vapor backflow to dramatically enhance the two-phase transport and then delay the dryout of heating surface. To demonstrate the advantages of our design, the flow boiling performances in terms of heat transfer coefficient (HTC), critical heat flux (CHF), and two-phase flow stabilities were experimentally studied and a comprehensive comparison against a heat sink consisted of plain-wall microchannels is presented. Experiments were conducted on DI-water with total inlet flow rate varying from 20 ml·min^-1 to 50 ml·min^-1. The results of this study show that flow boiling performances and two-phase flow stabilities are significantly enhanced owing to the successful suppression of two-phase backflow and efficient two-phase transport. For example, at a flow rate of 50 ml·min^-1, the CHF and HTC of this design in the forward direction are about 30.6 W·cm^-2 and 49.7 kW·m^-2K^-1, respectively, accompanied by significant enhancements of 57.4 % and 106.7 %, respectively, in contrast to the heat sink with plain wall microchannels. Additionally, the standard deviations (STD) of wall temperature and pressure drop of the conventional heat sink are 17.1 and 12.6 times higher than that of this new heat sink. Visualization studies were conducted to elucidate the working mechanism of Tesla valves in regulating vapor backflow.