Development of modeling framework for high-temperature heat pipes in micro reactors

Heat pipes as well as two-phase thermosyphons are highly efficient heat transfer devices utilizing continuous evaporation and condensation of a proper working fluid for two-phase heat transport in a closed system. Because of nearly isothermal and fully passive phase-change heat transfer mechanism, heat pipes and thermosyphons have found many applications in space technologies and other energy systems. High temperature heat pipes could potentially be used as a heat removal device from the primary system and be coupled with power conversion system or process heat application. In this study, a comprehensive one dimensional three-field model has been developed for analysis of high-temperature heat pipes in normal operation conditions and transients. The conservation or field equations of mass, momentum, and energy were presented for the liquid film, vapor, and droplet. In addition, constitutive equations or correlations were reviewed thoroughly for the closure of the three-field equations. Specific constitutive equations regarding interfacial mass and heat transfer at two interfaces, namely film-gas interface and gas-droplet interface, were reviewed for droplet entrainment and deposition rates as well as film and droplet evaporation rates. Additionally, mechanistic correlations of annular flow film thickness were recommended for the modeling of the thermosyphons without a wick as a critical constitutive correlation. Furthermore, experimental data needs from new experiments using a prototype working fluid for the model validation of high-temperature heat pipes in micro reactors were recommended for the future research.