Phase-Change Heat Transfer in Microsystems

Abstract

Recent work on miscroscale phase-change heat transfer, including flow boiling and flow condensation in microchannnels (with applications to microchannel heat sinks and microheat exchangers) as well as bubble growth and collapse on microheaters under pulse heating (with applications to micropumps and thermal inkjet printerheads), is reviewed. It has been found that isolated bubbles, confined elongated bubbles, annular flow, and mist flow can exist in microchannels during flow boiling. Stable and unstable flow boiling modes may occur in microchannels, depending on the heat to mass flux ratio and inlet subcooling of the liquid. Heat transfer and pressure drop data in flow boiling in microchannels are shown to deviate greatly from correlations for flow boiling in macrochannels. For flow condensation in microchannels, mist flow, annular flow, injection flow, plug-slug flow, and bubbly flows can exist in the microchannels, depending on mass flux and quality. Effects of the dimensionless condensation heat flux and the Reynolds number of saturated steam on transition from annular two-phase flow to slug/plug flow during condensation in microchannels are discussed. Heat transfer and pressured drop data in condensation flow in microchannels, at low mass flux are shown to be higher and lower than those predicted by correlations for condensation flow in macrochannels, respectively. Effects of pulse heating width and heater size on microbubble growth and collapse and its nucleation temperature on a microheater under pulse heating are summarized.