Effect of Filling Ratio on Two-Phase Closed Thermosiphon Heat Transfer under Constant Confinement and Weber Number

A 2D numerical study was conducted using STAR-CCM+ to evaluate the influence of filling ratio (FR) impacts heat transfer in a two-phase closed thermosiphon system. The investigation examined five different FRs ranging from 20% to 120%, under constant confinement number (Co), vapor Weber number (We<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</inf>), and input and output heat of 172.87 W (12,512.3 W/m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>). The Volume of Fluid (VOF) method coupled with the Lee model was applied to capture the phase change processes. Results demonstrated that increasing the FR improves thermal conductance by reducing the temperature difference between the evaporator and condenser, with quasi-steady state reached more rapidly at higher FRs. The study revealed that higher FRs enhance water entrainment and expand the wetted wall region (water fluctuation level), thereby increasing the contribution of sensible heat transfer and leading to improved heat flux and wall heat transfer coefficients. The heat transfer coefficient (HTC) classified into three primary components: the latent heat transfer associated with phase change during evaporation, the sensible heat transfer of the liquid phase during heating and cooling, and the sensible heat transfer of the vapor phase. Peak local heat transfer coefficients exceeding 1.5 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> W/ m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>. K were observed in the evaporation zone. These insights highlight the critical role of FR in optimizing thermosiphon performance for thermal management applications.