Effects of geometry and fluid properties during condensation in minichannels: experiments and simulations

The present paper aims at providing a picture of the condensation process inside micro and minichannels, above all at low mass fluxes, when we can expect more discrepancies with conventional channels. At high mass flux, the condensation in minichannels is shear stress dominated. Therefore, models originally developed for conventional channels can still do a good job in predicting the heat transfer coefficient. Besides, at high mass flux, the effect of the channel shape may not be significant, and this is again related to the importance of the shear stress at the liquid-vapour interface in comparison with the capillary forces. When the mass flow rate decreases, the condensation process in microchannels starts to display differences with the same process in macrochannels. With the purpose of investigating condensation at these operating conditions, new experimental data are here reported and presented together with data already published in the literature. In particular, heat transfer coefficients have been measured during R134a and R1234ze(E) condensation inside circular and square cross section minichannels at mass velocity ranging between 65 and 200 kg m-2 s-1. These new data are compared with those of R32, R245fa, R290, R152a to show the effect of channel shape and fluid properties and to assess the applicability of correlations for macroscale condensation. Beside experiments, three-dimensional VOF simulations of condensation in horizontal 1 mm i.d. minichannels with circular and square cross section are reported.