Study of the vapor-liquid interface during annular flow in a minichannel: OpenFOAM numerical simulations vs optical measurements

Annular flow is one of the most frequently encountered two-phase flow regimes during condensation inside small diameter channels. Since the main heat transfer resistance is associated to the liquid annulus, the knowledge of the vapour-liquid interface evolution in time and space is fundamental for the study of in-tube condensation. In this work, a new approach for the simulation of vapour-liquid annular flow is proposed. The Volume of Fluid (VoF) method is employed to compute the vapour-liquid interface and OpenFOAM opensource code is used to perform transient numerical simulations with refrigerant R245fa flowing inside a 3.4 mm circular channel at 40 °C saturation temperature, mass velocity G = 100 kg m-2 s-1 and different values of vapour quality. The numerical setup consists of 2D axisymmetric domain, downflow configuration, turbulence modelling in both phases and adaptive mesh refinement performed in correspondence of the liquid/vapour interface. Simulations are used to predict the instantaneous and averaged values of the liquid film thickness. Numerical results are compared against optical measurements performed during in-tube condensation inside a vertical test section equipped with a glass window.