An overview of liquid-vapor phase change, flow and heat transfer in mini- and micro-channels

Heat management in high thermal-density systems such as CPU chips, nuclear reactors and compact heat exchangers is confronting rising challenges due to ever more miniaturized and intensified processes. While searching for heat transfer enhancement, micro-channel flow boiling is found to be an efficient heat removal approach. However, the understanding of micro-scale phase changing process is still not a closed topic, which impedes wider applications of this technique. In the present review, macro- to micro- transition, vapor dynamics, flow instabilities and varying trends of heat transfer characteristics as well as the proposed correlations with respect to diverse conditions in mini- and micro-channels are summarized. The analyses reveal that the understanding of vapor dynamics in confined space is still far from enough. Monitoring the single bubble growth and geometry evolution will help probe into the flow pattern transition and subsequently the heat transfer mechanisms. Besides, flow instabilities are inevitable during flow boiling. However, there is still a lack of a comprehensive classification criterion for various types of instabilities. Moreover, some most widely cited experimental studies on flow boiling heat transfer characteristics and the heat transfer correlations are surveyed. It is realized that the micro-channels with high aspect ratio cross sectional geometry show promising merits in heat transfer enhancement. Some future research suggestions are made based on the present review.