Bubble behaviors and flow boiling characteristics in open microchannels with large aspect ratio

Abstract

Large aspect ratio of microchannels enables the corresponding heat sink possess higher heat transfer capability, however, the generated bubbles tend to rapidly extend forward and backward due to the large aspect ratio of the channels, thereafter blocking the channels, deteriorating the heat dissipation and increasing the pressure loss. Open microchannels have been reported to inhibit the two-phase flow instability due to the existence of extra gap for vapor expansion. However, the visualization of bubble dynamics and characterization of flow boiling behaviors of open microchannels with large aspect ratio are limited. In this study, the open microchannel heat sink with large aspect ratio was proposed and flow boiling tests were conducted at two mass fluxes of 153 and 229 kg/m2·s with the effective heat flux ranging of 180.81–2165.15 kW/m2. The inlet temperatures of the test module were set in the range of 5-85 °C. Visualization results showed that the flow patterns can be classified into bubbly flow, combined bubble flow and stratified slug flow. The flow mode tended to transition to stratified flow as the inlet temperature increased, and the combined bubble flow was found to be absent at high inlet subcooling. The size of the nucleated bubbles increased with the increase of inlet temperature during bubbly flow. The length to width ratio, survival time of nucleated bubbles and the bubble dynamic behaviors were respectively discussed. The heat transfer characteristics and pressure drop were strongly dominated by the two-phase flow patterns. The optimum flow boiling heat transfer was achieved when the flow pattern transitioned to combined bubble flow and early stage of stratified slug flow, during which bubble nucleation and thin liquid film evaporation jointly contributed to the heat dissipation. The pressure drop increased with the increase of inlet subcooling because of the greater liquid viscosity, and the sudden spurt of pressure drop was observed due to the occurrence of stratified slug flow. This work provides a preliminary investigation of two-phase flow boiling performance in large aspect ratio open microchannels, which may provide important insights of bubble dynamics or interfacial mechanics in open microchannel heat sinks.