Heat transfer mechanisms during refrigerant condensation when varying inclination

Capacity control in chillers and heat pumps is done more and more by adopting variable speed compressors. In such a case, at partial loads, the refrigerant flow rate running in the equipment is reduced and this is a strong reason for studying convective condensation at low mass flux. Here, the mechanisms of condensation inside a tube with an internal diameter equal to 3.4 mm are investigated. The test section allows to perform flow pattern visualizations, measurements of the heat transfer coefficient and of the liquid film thickness. Experiments have been conducted with R134a and R245fa in horizontal and vertical downward flow at mass velocity equal to 50 and 100 kg m-2 s-1 at 40°C saturation temperature. The visualized flow patterns provide fundamental information to understand the effect of channel inclination. The liquid film thickness during vertical downflow condensation has been measured with a confocal sensor.