Transforming pool boiling into self-sustained flow boiling through bubble squeezing mechanism in tapered microgaps

Abstract

We demonstrate a mechanism to transform a pool boiling system into a self-sustained flow boiling system through bubble-induced two-phase flow in a tapered microgap over the heater surface. The nucleating bubbles in the gap are squeezed and expand preferentially in the increasing taper direction. The expanding flow cross section decelerates the two-phase flow and converts the momentum loss into pressure buildup. The bubble squeezing and the pressure recovery effects overcome the frictional and acceleration pressure drops to provide a net pumping head. We hypothesize that the resulting self-sustained flow eliminates the need for an external pump and transforms a pool boiling system into a pumpless flow boiling system with significantly higher heat transfer potential. Experiments were carried out with water in 10° and 15° tapered gaps over a 10 mm square heater surface with an inlet gap height of 1.27 mm. The plain heater surface with a 15° tapered gap reached a critical heat flux of 288 W/cm2 with a heat transfer coefficient of 119 kW/m2 °C as compared to 127 W/cm2 and 52.5 kW/m2 °C with only a plain surface. High-speed imaging revealed that the bubbles nucleating at the entrance in the gap are squeezed and expand in the increasing cross section direction. A homogeneous flow model was used for pressure drop analysis in the expanding gap. The results indicate that the pressure recovery mechanism alone cannot sustain the flow and the bubble squeezing action plays a critical role in establishing the self-sustained flow.