Transient numerical simulations of condensation in a minichannel compared to flow visualizations

Numerical simulations of vapour-liquid flow by means of the Volume of Fluid (VOF) method are more and more considered an interesting tool to study two-phase flow with and without mass transfer. When dealing with small geometries, experimental investigation can be invasive, affecting the phenomenon itself but it is 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 mass velocities around and below 200 kg m-2 s-1 by means of VOF simulations and flow visualizations. From the flow visualizations performed during downflow condensation of R134a inside a 3.4 mm diameter channel, it can be seen that at 100 kg m-2 s-1 mass velocity, the liquid interface is irregular with the presence of waves. Clearly, steady state simulations cannot account for the presence of waves at the vapour-liquid interface. For this reason, time-dependent numerical simulations have been performed. For computational time saving, a 2-D axisymmetric domain has been considered and the results of transient simulations have been compared with flow visualizations of vertical downflow. The present simulations allow to evaluate the effect of waves on the heat transfer coefficient. The influence of mass flux on the occurrence of interfacial waves has also been investigated.