Both numerical and experimental studies have previously been carried out to investigate the heat transfer performance of the two-phase closed thermosyphon (TPCT). This paper investigates the performance of a commercially available computational fluid dynamics (CFD) solver (Ansys FLUENT) to predict the complex flow behavior of TPCTs, with special focus on modeling of the mass transfer phase change process. The present study uses four different sets of mass transfer coefficients for condensation and evaporation within a previously documented phase change model to determine their impact on the simulation results. The mass transfer coefficients effectively control the rate of transfer from liquid to vapor phase during evaporation and vice versa during condensation. The choice of coefficients is assumed to represent a balance between numerical accuracy and stability. A baseline simulation is performed for which both the evaporation and condensation coefficients are equal and set to default values. Three additional simulations vary the magnitude of the coefficients and adopt relative values based on density ratio following a recommended method that has been previously found to be effective for these simulations. Initial results show that the case with the highest coefficient of evaporation and coefficient for condensation based on the density ratio is in good agreement with available experimental data of overall thermal resistance of the TPCT., with predictive capability degrading as the values of the coefficients are reduced. Additionally, the 3D CFD models implemented in this study appear to successfully predict the phase change process and vital flow behavior inside the TPCTs, at least in a qualitative sense.