LAUSR

Abstract It is well proved in the literature that gravity affects in-tube condensation heat transfer at low mass flux. Nevertheless very limited data are taken at low mass flux when changing tube orientation, despite the many practical applications. In this paper, convective condensation inside a 3.4 mm inner diameter tube is investigated in horizontal and vertical downflow using R134a as the working fluid. The experiments are performed at low mass flux, between 50 kg m−2 s−1 and 200 kg m−2 s−1, which are usually the less investigated despite the relevance of gravity force at such low velocities. The condensation heat transfer coefficient in vertical downflow can be as low as half the value in horizontal flow at the same operating conditions, since gravity acts for the thinning of the liquid film in the horizontal tube. In vertical downflow, the heat transfer coefficients show an early effect of turbulence, thus a new transition criterion is here proposed. Criteria for predicting the relevance of channel orientation on the heat transfer coefficient are also assessed.