Steady State and Transient Numerical Simulations of Condensation in Small Diameter Channels

In the recent years, numerical simulations by means of the Volume of Fluid (VOF) method have become an interesting tool to study two-phase flow with and without phase change. When dealing with small geometries, experimental investigations can be invasive and affect the phenomenon itself and also very expensive and time demanding. However, validation of the numerical simulations by experiments is still a key point to obtain reliable results. The present paper aims at providing an understanding of the annular condensation process inside minichannels at low mass velocities (G 200 kg m-2 s-1) by means of VOF simulations and experimental heat transfer measurements. Three-dimensional and steady numerical simulations of R134a condensation inside 3.4 mm i.d. circular minichannel have been validated with the experimental data to analyze the influence of liquid film distribution and liquid film flow on the heat transfer coefficient. Furthermore, since from experimental flow visualizations at these operating conditions it is possible to note the presence of waves at vapour-liquid interface at certain mass fluxes, 2-D axisymmetric and unsteady simulations have been performed to study the waves formation during adiabatic flow inside 3.4 mm and 1 mm i.d circular minichannel.