Effect of inclination during condensation inside a square cross section minichannel

Studies on the condensation process inside minichannels are rather limited in the literature and the effect of channel inclination during condensation is not investigated at all. Some studies have been performed in inclined smooth tubes with larger diameters, where it has been shown that the heat transfer coefficient is strongly affected by the liquid and vapor distribution. But minichannels display a different behavior because the relative importance of shear stress, gravity and surface tension changes and the action of these forces depends on operating conditions and orientation. In the present study, two HFC refrigerants have been tested during condensation inside a single square cross section minichannel when varying the channel orientation. Local heat transfer coefficients of R134a and R32 are measured in horizontal (0), downflow and upflow configurations at varying inclination angles from 15 to 90. The channel has a square cross section with a hydraulic diameter equal to 1.23 mm. Tests have been performed at mass velocity ranging between 100 and 390 kg m-2 s-1. The present test results suggest an effect of inclination in downflow at the lowest values of mass flux. By applying the Buckingham theorem, a criterion has been developed to predict at which mass velocity the channel inclination starts to affect the condensation heat transfer.