Experiment investigation on flow boiling heat transfer in a bidirectional counter-flow microchannel heat sink

Abstract

Improving the critical heat flux (CHF) and avoiding dry out downstream are the keys to enhance the two-phase cooling performance of microchannel heat sinks. In this work, a novel type of bidirectional counter-flow (BCF) microchannels with 600 μm depth and 300μm width, giving a hydraulic diameter of 400μm, is fabricated in an oxygen-free copper base. Flow boiling experiments are conducted in bidirectional counter-flow microchannels and conventional unidirectional parallel-flow (UPF) microchannels, with the deionized water being used as the working fluid. Cases under six mass fluxes ranging from 118 kg/m2·s to 370 kg/m2·s and three inlet subcoolings of 30 ℃, 50 ℃, and 70 ℃ are tested. Two-phase flow behaviors in microchannels are simultaneously captured by a high-speed camera. It is found that, unlike conventional UPF microchannels, the location for the onset of nucleate boiling (ONB) in BCF microchannels is moved from downstream to the middle region. The downstream dry-out, which is usually occurred in UPF microchannels, can be well avoided in BCF microchannels due to the substantial feeding of liquid in adjacent microchannels. The CHF in BCF microchannels can be increased by 33.8∼57.2% compared with the conventional UPF microchannels. Under the combined effect of expanded nucleate boiling region and shortened subcooling region, the average heat transfer coefficient (HTC) in BCF microchannels can be increased by 36.6%∼56.7%. More interestingly, the improvement of heat transfer performance is found to be accompanied by a significant reduction in pressure drop for BCF microchannels in all the experiment cases. In addition, two-phase instabilities in terms of wall temperature and pressure drop oscillations can be significantly suppressed by using BCF microchannels instead of the conventional UPF microchannels. This study proposes a more efficient microchannel heat sink design scheme to deal with the microelectronic cooling challenges.