Mechanism of flow boiling in microchannels and structure optimization of heat exchangers used in pumped two-phase cooling system for data center cooling

The microchannel flow boiling is one of the most potential cooling methods for high heat flux density devices, but a multiple complex process and some complicated issues limit its wide application. In this chapter, the mechanism of flow boiling, flow patterns, and flow instability in microchannels is reviewed, and the effect of expanding microchannels on flow instability inhibition is further analyzed. Flow boiling stages include single-phase convection, subcooled boiling, saturated boiling, transition boiling, and film boiling. These boiling stages are relevant to different flow patterns, such as bubbly flow, slug flow, elongate flow, and annular flow. However, flow boiling instability in micro/confined channels is more easily triggered than in macrochannels. It reduces the critical heat flux and enhances the system pressure oscillation, as well as the wall temperature oscillation in microchannels to form a vicious circle. Generally, flow boiling instabilities in microchannels have been classified as static and dynamic types. System pressure has a significant effect on the static instability, and a higher system pressure could improve the flow stability in microchannel. The dynamic flow instability is mainly caused by the development of large bubbles in a limited channel. Modifying the microchannel geometry is an efficient way to inhibit the flow boiling instability. Then, the expanding microchannels are introduced to inhibit the flow instability. This chapter will contribute to a better understanding of the complex flow boiling behavior in microchannels and the design of reliable microchannel heat exchangers.