Experimental study of flow boiling characteristics of open microchannels with elliptical cavities and elliptical ribs

Open microchannel heat sinks (O-MCHS) stand out among many thermal management technologies due to their extra open gaps that reduce pressure drop and mitigate flow instability. Recently, there has been a growing trend of incorporating microstructures onto microchannels in order to enhance the hydrothermal performance of heat sinks. However, the heat transfer characteristics and bubble behavior of O-MCHS with microstructures are still rarely studied. In this study, open copper-based microchannel heat sinks with elliptical cavities and elliptical central ribs (O-EC-ECR) and elliptical cavities and elliptical side ribs (O-EC-ESR) were designed and manufactured; the flow patterns, heat transfer performance and pressure drop were systematically studied and compared with those of rectangular open microchannel heat sink (R O-MCHS). The working fluid was degassed DI water and the experiments were performed with the inlet subcooling of 75 degrees C at efficient heat fluxes of 668.00-2416.16 kW/m2 under various mass fluxes ranging from 421 to 801 kg/m2.s. The experimental results demonstrated that the O-EC-ECR exhibited the best two-phase heat transfer performance among three structures at all mass fluxes. The average two-phase heat transfer coefficient of O-EC-ECR was increased by 12.2 % compared to R O-MCHS at the mass flux of 801 kg/m2.s. The average wall temperature of O-EC-ECR was found to be 10.1 degrees C lower than that of R-O-MCHS and 6.7 degrees C lower than that of O-EC-ESR at the mass flux of 801 kg/m2.s. This should be attributed to the dramatically increased convective heat transfer area as well as the separating and breaking effect of the central ribs on the bubbles, which was manifested by the increase in heat transfer coefficient and critical heat flux (CHF). In addition, the central ribs improved the flow boiling stability by effectively suppressing the generation of confined bubbles and the formation of slugs. The arrangement of the side ribs and cavities made bubbles easier to condense, which facilitated the mixing of hot and cold fluid. At low to medium mass fluxes, the O-EC-ESR only showed superior heat transfer performance during the bubbly flow dominated nucleate boiling regime. Whereas high mass flux can improve the boiling heat transfer performance; the average heat transfer coefficient of O-EC-ESR was increased by 6.1 % compared to R O-MCHS. The average pressure drop of O-EC-ESR was decreased by 21.8 % compared to R O-MCHS and 25.6 % compared to O-EC-ECR at the mass flux of 801 kg/m2.s.