Visualization and numerical simulations of condensing flow in small diameter channels

The understanding of two-phase flow mechanisms during condensation inside small diameter channels is fundamental to design compact condensers. When dealing with small geometries, experimental investigation can be invasive and the measurement of heat transfer coefficients with low uncertainties becomes difficult. For these reasons, numerical simulations by means of the volume of fluid method are an interesting tool to study two-phase flows and they can also be used in support to experimental investigation. In the present work, first steady-state numerical simulations of R134a condensation inside horizontal channels are presented: the results are used to analyse the effect of the diameter (1 mm and 3.4 mm) on the two-phase flow and heat transfer. Since waves occur at the vapour-liquid interface and they cannot be modelled under the hypothesis of steady-state operating conditions, transient simulations (2-D axisymmetric) have also been performed to investigate the influence of waves on the condensation process. The two-phase flow has also been experimentally investigated and visualizations are compared to the numerical results.