Comprehensive review of enhancement techniques and mechanisms for flow boiling in micro/mini-channels

Due to the urgent need for heat dissipation of high heat flux devices in many engineering applications to maintain high performance and stable operation, flow boiling in micro/mini-channel heat sinks is an enabling and promising approach to tackling high heat flux cooling issues in emerging cutting-edge technology. Various enhancement techniques for flow boiling in micro/mini-channels have been investigated in recent years. These mainly include various surface modifications, enhanced channel structures and composite enhancement techniques. These techniques are able to improve flow boiling heat transfer coefficient (HTC), critical heat transfer (CHF) and mitigate two phase flow instabilities. The composite enhancement techniques take the advantage of each individual technique for flow boiling to achieve more efficient heat transfer and stable flow. However, there are challenges to optimizing these techniques and understanding the very complex mechanisms involved in these emerging techniques. This paper presents a comprehensive review of the studies of various enhancement techniques for flow boiling and mechanisms in micro/mini-channels having hydraulic diameter of 0.1–3 mm over the past five years. First, the criteria for distinction of micro/mini-channels and classification of flow boiling enhancement techniques are discussed. Then, studies of flow boiling enhancement techniques using surface modification techniques and enhanced structures are critically reviewed. Next, the effects of surface modification techniques and enhanced structures on flow boiling heat transfer, CHF, two phase flow patterns, two phase flow instability and mechanisms in micro/mini-channels are discussed and analyzed. Other enhancement techniques such as using surfactants to enhance flow boiling is also mentioned. Finally, future research needs have been identified and recommended according to the comprehensive review and deep analysis. Systematic research on the physical mechanisms underlying the composite heat transfer enhancement techniques is urgently needed. Optimization design of micro/mini-channel flow boiling heat transfer enhancement techniques is a big challenge and should be focused on in future. In order to understand the mechanisms, the bubble dynamics, bubble nucleation and flow patterns of flow boiling in the composite enhancement techniques in micro/mini-channels should be systematically investigated to understand the mechanisms and optimizing the enhancement techniques. Effort should be made to develop prediction methods for flow boiling heat transfer and CHF in the long run.