Optimizing the performance of microchannel heat sinks: Effects of trapezoidal cover plate on flow boiling heat transfer and stability

Two-phase cooling in microchannels is an efficient thermal management technology. Compared to conventional enclosed microchannels, open microchannels offer advantages such as improved flow uniformity and reduced pressure drop. However, dry-out tends to occur downstream in open microchannels, leading to deteriorated heat transfer performance and increased flow instability. In this work, a trapezoidal cover plate was proposed to improve the two-phase heat transfer performance of open microchannels. Five microchannel heat sinks with different cover plate configurations were tested using HFE-7100 and analyzed for heat transfer performance, flow pattern, pressure drop, and flow instability. The experimental results indicated the Type-II stratified flow and local wall dry-out in the downstream region of the microchannels were the primary causes of flow reversal and the triggering of critical heat fluxes (CHF) under high heat fluxes. The increasing flow velocity downstream in the trapezoidal cover plate promoted bubble departure, significantly delaying the development of flow patterns. It also postponed the transition from Type-I stratified flow to Type-II stratified flow, enhancing the rewetting capability of the downstream microchannel wall. The trapezoidal cover plate configuration significantly improved the heat dissipation capability of the open microchannel heat sinks. For a mass flux of 906 kg/(m²s), a heat flux as high as 313.9 W/cm² and heat transfer coefficient (HTC) reaching 40.8 kW/(m²K) were achieved. For a mass flux of 679.5 kg/(m²s), compared to the other four configurations, the CHF was increased by 27.6 % to 170.2 % and the HTC was improved by 20.0 % to 66.8 %. Furthermore, the trapezoidal cover plate effectively suppressed flow fluctuations. The findings provide valuable insights for the design and optimization of open microchannel heat sinks in two-phase heat transfer systems.