Improved flow boiling performance in high-aspect-ratio interconnected microchannels

Abstract

High aspect ratio microchannels have the advantage of dissipating more heat per base area. However, flow boiling instability of bubble expansion towards upstream and downstream is more easily triggered as soon as boiling starts due to the narrower channel width of high aspect ratio microchannel. To suppress flow boiling instability, in this study, the flow boiling heat transfer performance is experimentally investigated in the high-aspect-ratio (aspect ratio of 2.5) interconnected microchannels (IM) with micro connection slots. Two slots of 20 μm and 40 μm in width, denoted as IM20 and IM40, respectively, are considered. Experiments are conducted at mass fluxes of 446-963 kg/m2•s and effective heat fluxes of 36.2-427.9 W/cm2, using deionized water as the working fluid. The outlet of the channel is maintained at atmospheric pressure with a saturation temperature of 100 °C. Results show that obviously earlier onset of nucleate boiling (ONB) in IM is demonstrated, compared to that in plain-wall microchannel (PM). Meantime, the nucleation bubble is easily captured in IM, whereas almost no bubbly flow is observed in PM. More importantly, significantly stable flow boiling from the incipience of boiling to near the critical heat flux (CHF) point is found in IM. It is observed that regular liquid film redevelopment and annular flow appear alternately among parallel microchannels in IM. By contrast, due to flow maldistribution of PM, various irregular flow patterns appear simultaneously among parallel microchannels. These micro connection slots promote nucleate boiling and fluid mixing of neighboring channels. Eventually, remarkable heat transfer enhancement is achieved in IM without compromising the two-phase pressure drop. The enhanced heat transfer coefficient and critical heat flux of IM are respectively around 25.7%-86.9% and 13.9%-59.1% over the entire range of tested cases. In addition, owing to enhanced fluid mixing and more liquid maintaining in the larger slot, IM40 shows more heat dissipation than IM20. This study demonstrates that the microchannel heat sink with the interconnected configuration with tens of micro-slots can be an effective approach for suppressing flow boiling instability and enhancing flow boiling heat transfer performance.