Physics of the Microchannel Flow Boiling Process and Comparison With the Existing Theories

Abstract

In this study, six benchmark experiments are conducted on bubbles at different growth stages to evaluate the assumptions of the existing microchannel flow boiling heat transfer models/hypothesis. The results show that the bubble ebullition process triggers a spike in the local surface heat flux due to the thin film evaporation and transient conduction heat transfer mechanisms. This enhancement in the surface heat flux is limited to a very small area at the bubble–surface contact region at the nucleation site limiting the overall heat transfer contribution of the bubble ebullition process. The contribution of these two mechanisms of heat transfer increases as the bubble–surface contact area becomes larger. As the bubbles length increases, the time period of activation of the microlayer evaporation mechanism substantially increases while that of the transient conduction mechanism remains relatively unchanged. When the microchannel is mostly occupied by bubbles, the thin film evaporation mechanism becomes the dominant heat transfer mode. The results clearly indicate that single-phase heat transfer mechanism active at surface regions not covered by bubbles is governed by the laminar flow theory (for the test conditions presented here). In essence, a measureable enhancement effect in the liquid phase due to bubbles growth and flow has not been observed. A comparison with the existing microchannel flow boiling models suggests that the three-zone flow boiling model can qualitatively describe the heat transfer events observed in this experiment but fails to accurately predict the magnitude of the heat transfer mechanisms.