Vapor stem bubbles dominate heat transfer enhancement in extremely confined boiling

Abstract

Boiling has long been sought as the heat dissipation mechanism for a wide variety of compact thermal management applications owing to low-resistance heat transport, high heat flux limits, and surface isothermalization. This work aims to elucidate the thermofluidic transport mechanisms of boiling in extremely confined gaps through experimental measure of the temporal evolution of heat fluxes and surface temperatures during deionized water boiling, as well as high-speed visualization of bubble formation. The flow visualizations reveal small residual pockets of vapor, termed ‘stem bubbles’ herein, that remain on the boiling surface through a pinch-off process where vapor escapes through the edges of the confined heated region. These stem bubbles act as seeds for vapor growth in the next phase of the boiling process. These bubbles dictate the boiling performance for extremely confined boiling as defined based on a dimensionless ratio of the gap spacing to capillary length (Bo≤ 0.35 - 0.5). This conclusion is supported by the enhanced thermal response of the surface compared to nucleate boiling. Because activation of nucleation sites is not required for stem bubble boiling, phase change occurs at a reduced surface superheat at a given heat flux compared to nucleate boiling. Criteria for the dimensionless confinement gap spacing are identified to harness this improved heat transfer rate of the stem bubble boiling regime. This new understanding of boiling in extremely confined gaps offers a new direction to design compact two-phase thermal management solutions through using the unique enhancements provided by the vapor stem bubble boiling regime.