Improvement of Two-Phase Flow Simulation in a Thermosyphon Using a New Saturation Temperature Model

Abstract This study presents a mean saturation temperature model for improving the numerical simulation of thermosyphons (wickless heat pipes) using the open-source CFD framework OpenFOAM. Conventional phase change simulations typically assume a constant saturation temperature; however, this assumption is inadequate during transient operation, particularly during startup, when internal pressure and temperature evolve dynamically. The proposed model derives the saturation temperature as a weighted function of the mean temperatures at the evaporator and condenser sections, based on equilibrium conditions of the Lee phase change model. The solver incorporates the Volume of Fluid (VOF) method for interface tracking, the PIMPLE algorithm for pressure-velocity coupling, and conjugate heat transfer between the fluid and solid wall. The model was validated against published experimental data and prior numerical results across multiple heat flux conditions and geometries. The predicted evaporator wall temperatures show improved accuracy over the constant-saturation-temperature approach, with errors below 6% in all cases. The simulations further reveal a relationship between filling ratio and time to steady operation: lower filling ratios require longer steady-state times. Overall, the proposed OpenFOAM-based solver offers a reliable and accessible tool for simulating phase-change heat transfer in thermosyphon systems.