Materials and Interface Challenges in High-Vapor-Quality Two-Phase Flow Boiling Research

High-vapor-quality flow boiling on structured surfaces has the potential to enhance heat transfer performance and reduce temperature/pressure instability compared to conventional lower quality flow boiling on smooth surfaces. In this article, we discuss recent research progress on structured surface-enhanced flow boiling. We discuss lessons learned and focus on the challenges remaining. Although some degree of mechanistic understanding of the effect of surface structures on the flow boiling process has been gained, many important challenges remain to enable real utilization. Primarily, a need exists for a greater fundamental understanding of the processes if design tools are to be developed capable of guiding engineers to select and implement enhancements. The challenges discussed stem from a community-wide workshop focusing on high-exit-quality flow boiling, held in June 2020. Four key challenges were identified: 1) the need for better understanding of the mechanisms governing surface-structure-enhanced flow boiling and their effect on heat transfer coefficient, critical heat flux, pressure drop, and flow stability; 2) the need for rigorous quantification of surface durability and manufacturing scalability; 3) the need to understand effects of using working fluids other than water including refrigerants, supercritical fluids, and other dielectric fluids; and 4) the need to establish thermal resistance limits dependent on liquid film thickness. We end this article by providing conclusions detailing where we believe that the community should direct both fundamental and applied efforts in order to solve the identified challenges, which limits the implementation of high-vapor-quality flow boiling on surface structures.