Performance Evaluation of a Two-Phase Closed Thermosyphon Having a Converging-Diverging Pipe Body

The present study focuses on improving the heat transfer performance of a two-phase closed thermosyphon (TPCT) by altering the geometry of a typical straight pipe thermosyphon body to have a converging-diverging section. The flow path of the liquid-vapor fluid mixture is augmented with this new design to induce thermal boundary layer mixing thereby enhancing the convection heat transfer within the system during operation. A multiphase numerical simulation model has been developed to simulate the fluid phase change and wall temperature distribution of a two-phase closed thermosyphon. The Lee model is used to calculate mass transfer source terms during the condensation and evaporation phase change processes and the Volume of Fluid (VOF) method is employed to track liquid-vapor interface movement during the simulations. Wall temperature distributions as well as overall thermal resistance values are compared with experimental values available in the literature in order to validate the simulation model for a straight pipe geometry. Two additional model geometries are then used for comparative study where the converging-diverging (CD) section is positioned within the adiabatic section and condenser section respectively. The numerically simulated wall temperature distribution and overall thermal resistance results indicate that the most significant impact can be made when the CD section is positioned in the adiabatic section and condenser section exhibiting reduction of 1.7% and 3.4%, respectively, in overall thermal resistance. Keywords-Thermosyphon; Phase-change; Multiphase flow; Computational fluid dynamics; Numerical simulation;