Subcooled flow boiling in an expanding microgap with a hybrid microstructured surface

Abstract

Flow boiling in microgap is attracting increasingly wide attention in heat transfer community due to its promising application potential for thermal management of electronic devices. In this study, an expanding microgap with a hybrid microstructured surface is proposed. It consists of a roughened inlet region followed by an array of staggered micro triangular pin-fins machined downstream. Subcooled flow boiling tests were conducted using distilled water as the working fluid with the inlet subcooling between 20 °C and 40 °C at different flow rates ranging from 80 to 480 mL/min over a heat flux range of 100.0–651.7 W/cm2. High-speed visualization was performed to explore the flow pattern transitions and the effects of operating conditions on heat transfer and pressure drop characteristics. A new flow pattern was observed in which the bubbles blanket the surface over both roughened upstream surface and the micro pin-fins downstream surface at higher flow rates and higher heat flux conditions. Combination of the increased bubble nucleation site density on the roughened surface and suppression of local dryout from the micro pin-fins due to their capillary effect jointly led to the high heat transfer performance in the bubble blanket pattern. In addition, the expanding configuration provides additional space downstream to facilitate bubble growth and two-phase flow in the direction of expansion with reduced resistance, resulting in a relatively low pressure drop in the present system.