Two-Phase Operation of Microchannel Heat Sinks

Boiling in microchannel heat sinks is extremely attractive for high-performance electronics cooling due to the high heat transfer rates that can be achieved. Since the fluid latent heat of vaporization is exploited, lower flow rates are needed in microchannels undergoing boiling than with single-phase liquid to achieve the same amount of cooling, resulting in lower pumping powers and more efficient and compact systems. The very high heat transfer rates achievable are coupled with the ability to keep the wall temperature relatively uniform, contributing to more reliable operation and longer lifetime for the electronics being cooled. Since the heat transfer coefficient increases with increasing heat flux over typical ranges, two-phase microchannel heat sinks also have the intrinsic ability to accommodate the thermal management of hot spots on the electronics. In this chapter, fundamentals of boiling in microchannel heat sinks are discussed along with the main distinctions from boiling in conventional-sized channels. Conditions under which the transition from macroscale to microscale boiling occurs are evaluated, and transition criteria are developed to identify the range of operating and geometric conditions that define microchannel heat sinks. Flow regime maps for microchannel boiling that can facilitate systematic design of microchannel coolers are presented.