Experimental investigation of flow boiling heat transfer and instabilities in microchannels at high liquid subcooling

Flow boiling in a microchannel effectively removes high heat flux and plays a crucial role in the thermal management of electronic devices. Microchannel dimensional parameters have a significant impact on flow boiling characteristics, but their quantitative correlation with flow boiling behavior remains undetermined. In this work, five microchannels with different dimensional parameters were designed to investigate the effects of geometric parameters on flow boiling characteristics under high liquid subcooling (70 °C). The study identified two primary flow patterns in the microchannel: bubble flow and slug flow. Special bubble behavior was observed. The slug flow expanded rapidly at the exit of the microchannel, and the head shrank rapidly into small bubbles and detached. The remaining bubbles in the channel would be temporarily quiescent. Additionally, increasing the rib width and decreasing the channel width accelerated the transition of flow patterns and prematurely triggered the critical heat flux (CHF). Increasing the number of channels can lead to a reduction in CHF but significantly lower the wall temperature by more than 18 °C. A high heat flux of 280.7 W/cm2 was achieved with a R200-200 superheat not exceeding 13 °C. Increasing the rib width and reducing the number of microchannels can significantly improve the heat transfer coefficient (HTC), with the maximum HTC exceeding 26 kW/m2K. The growth and elongation of the constrained bubbles upstream were the primary reasons for flow reversal. Increasing the rib width, reducing the channel width, and increasing the number of channels resulted in significant fluctuations in wall temperature and pressure drop in the microchannels. This study complements the understanding of the effects of microchannel geometric parameters on heat transfer performance, providing valuable insight for optimizing heat dissipation design.