Impact of the Lee model coefficient on 3D CFD simulations of microgravity annular flow condensation with two-phase inlet conditions

This study investigates the sensitivity of three-dimensional Volume of Fluid (VOF) simulations of microgravity annular FC-72 condensation to the Lee phase-change relaxation coefficient. The computations reproduce parabolic-flight experiments for condensation in a circular tube of 7.12 mm inner diameter and 791.72 mm cooled length, using two-phase inlet conditions and experimentally derived axial wall heat-flux boundary conditions. Three microgravity operating cases are considered, spanning FC-72 mass fluxes of 129.0, 177.8, and 282.0 kg m −2 s −1 with inlet qualities of 0.89, 0.89, and 0.65, respectively. The Lee coefficient is varied over r i = 1 0 3 , 1 0 4 , and 5 × 1 0 4 s − 1 in order to examine its effect on interfacial thermal equilibrium, phase distribution, wall temperature, and local heat transfer coefficient. The results show that low r i leads to insufficient interfacial relaxation, broader thermal nonequilibrium, and underprediction of condensation heat transfer, whereas excessively large r i drives the interface strongly toward saturation but overpredicts the heat transfer level. Among the values tested, r i = 1 0 4 s − 1 provides the best agreement with experiment in terms of the condensation-section averaged heat transfer coefficient, with deviations of approximately 2%, 4%, and 1% for Cases 1–3, respectively. This agreement is for section-averaged quantities, while the local axial profiles still exhibit noticeable discrepancies in wall temperature and heat transfer coefficient trends. Using this selected coefficient, the fully three-dimensional simulations are also compared with two-dimensional axisymmetric results. The 3D framework substantially improves agreement with experiments and removes key deficiencies of the axisymmetric approximation, including artificial liquid accumulation near the tube centerline and unrealistically low wall-temperature predictions. The results demonstrate that accurate prediction of microgravity annular condensation in circular tubes with two-phase inlet conditions requires not only an appropriate phase-change relaxation coefficient, but also a three-dimensional representation of interfacial distribution. The framework developed here provides an improved basis for predicting averaged condensation heat-transfer behavior in two-phase space thermal management systems, while the remaining local discrepancies highlight the need for further refinement of the axial distribution of wall temperature and local heat transfer.