A review of liquid metal high temperature heat pipes: Theoretical model, design, and application

Abstract

Adopting liquid metals as the working fluid, high temperature heat pipes (HTHPs) can operate at a high temperature above 750 K and have extremely high heat transfer capacity, while the most significant weakness of HTHPs is frozen startup and heat transfer limits that lead to the failure of HTHPs. Previous studies on the theoretical models, design-optimization methods, and applications are reviewed and discussed critically. The operation stage of HTHPs is categorized into transient operation stage (lumped model, network thermodynamic model, and heat and flow model), frozen startup (fusion and vapor transition), heat transfer limits and failure. Basic assumptions and limitations of heat pipe models are outlined, and theoretical comparisons are drawn with respect to the applications and results. A review of the design procedure and optimization methods with constraints in HTHPs is presented, indicating that a design-optimization method involving the various wick structures, frozen startup process, and associated thermal performance variables has not yet been established. This review establishes that HTHPs offer viable potential and spread value for optimization and integration into nuclear reactor systems, space exploration, and renewable energy systems, while challenges for the HTHPs lie on (1) heat pipe models need to be improved to consider practical factors including volumetric filling ratio, composite wick, etc.; (2) heat transfer limits and failure mechanism and models verification; (3) incorporation of heat pipe models into optimization algorithm with acceptable efficiency; (4) heat transfer power enhancement and special-shaped HTHPs in potential applications.