Nucleation characteristics and stability considerations during flow boiling in microchannels

Abstract

Flow boiling in microchannels and minichannels has received considerable attention in the past decade. The interest stems from the possibility of achieving the extremely high heat fluxes, over 1 kW/cm 2, needed for future generation electronics cooling application. The attention has been focused on obtaining experimental heat transfer and pressure drop data, but the fundamental aspects of nucleation have been largely overlooked. In the present paper, the local wall superheat and bulk liquid subcooling prevailing in the channel at the onset of nucleation are identified as critical in the flow boiling stability. To understand the role of local conditions on nucleation, the available literature on onset of nucleate boiling is critically reviewed and the relationships between the local bulk subcooling and local wall superheat as a function of nucleation cavity diameters are presented. The unique flow characteristics in microchannels and minichannels are further analyzed and their influence on flow boiling stability is investigated experimentally using visual images generated with a high-speed camera.