Effect of Gravity During Condensation of R134a in a Circular Minichannel VOF Simulation of Annular Condensation

A number of steady-state simulations of condensation of R134a inside a 1 mm i.d. circular minichannel at two far different mass flux values are proposed. The VOF method is used to track the vapour-liquid interface. The first simulations are run at G = 100 kg m(-2) s(-1) and G = 800 kg m(-2) s(-1) assuming that the channel displays horizontal orientation. The effects of interfacial shear stress, gravity and surface tension are all taken into account in this case and the results are validated by means of experimental data already available. As a further step, the same simulations have been run under normal gravity conditions but vertical downflow and finally assuming zero-gravity conditions. The condensation process is found to be gravity dominated at low mass flux, and thus very different results are obtained when neglecting gravity at this mass flux. An opposite result is achieved at high mass flux, as expected from the increased relative importance of interfacial shear stress in this case. The present results also allow to verify the influence of the surface tension effect during condensation in the circular cross section minichannel.