Boiling heat transfer and flow regimes in microchannels — a comprehensive understanding

Although flow boiling in microscale passages has received much attention over the last decade, the implementation of microchannel heat sinks operating in the two-phase regime in practical applications has lagged due to the complexity of boiling phenomena at the microscale. This has led to difficulties in predicting the heat transfer rates that can be achieved as a function of the governing parameters. From extensive experimental work and analysis conducted in recent years in the authors´ group, a clear picture has emerged that promises to enable prediction of flow boiling heat transfer over a wide parameter space. Experiments have been conducted to determine the effects of important geometric parameters such as channel width, depth, and cross-sectional area, operating conditions such as mass flux, heat flux and vapor quality, as well as fluid properties, on flow regimes, pressure drops and heat transfer coefficients in microchannels. High-speed flow visualizations have led to a detailed mapping of flow regimes occurring under different conditions. In addition, quantitative criteria for the transition between macro- and micro-scale boiling behavior have been identified. These recent advances towards a comprehensive understanding of flow boiling in microchannels are summarized here.