Rapid-bubble-growth instability at the onset of microchannel flow boiling

Abstract

The rapid-bubble-growth instability at the onset of flow boiling was studied experimentally in a single, 500 pm-diameter borosilicate glass microchannel subjected to a uniform heat flux of 41.9 kW/m2. Fluid was driven through the microchannel in an open-loop test facility by maintaining a constant pressure difference between a pressurized upstream reservoir and the ambient. Simultaneous measurement of inlet and outlet fluid temperatures and pressures, mass flux, and wall temperature, synchronized with high-speed flow visualization, enabled the underlying mechanisms of the rapid-bubble-growth instability to be quantified. At the onset of boiling, a large liquid superheat (10.3 °C) relative to the outlet saturation temperature causes the nucleated vapor bubble to grow rapidly, expanding in both upstream and downstream directions. This explosive bubble growth results in significant flow reversal and generates a large pressure gradient within the microchannel immediately following boiling incipience. The onset of flow boiling significantly reduced the temperature of the fluid and of the heated wall relative to single-phase flow conditions.