Investigating the condensation heat transfer inside small diameter channels: an experimental study in different gravity conditions

Condensation heat transfer finds application in many energy systems operating in both terrestrial and space environments (heat pumps, chillers, Rankine cycles, spacecraft thermal control devices). For the development of reliable tools to be employed in heat exchangers design, it is essential to fully understand the heat transfer mechanisms involved during the condensation phase-change phenomenon. This work aims at studying the convective condensation process inside two small diameter channels (3.38 mm and 2.91 mm inner diameter, respectively) and in different gravity conditions. Tests were performed with refrigerant R1233zd(E) which has been proposed as a substitute for R123 and R245fa, considering its low Global Warming Potential (GWP) value and the similar saturation curve. Condensation heat transfer and single-phase pressure drop experiments were conducted inside a 2.91 mm inner diameter test section realized by Additive Manufacturing techniques, designed to evaluate the local heat transfer coefficients. The flow pattern was also visualized through a glass window by means of a high-speed camera and a LED light source. The experimental results, obtained over a mass velocity range of 30 to 300 kg m-2 s-1 at 40 °C, were compared against the predictions from models available in the literature. Moreover, R1233zd(E) condensation heat transfer was investigated during the 84th Parabolic Flight Campaign of the European Space Agency (ESA). The main goal was to understand the effect of gravity level variations (1.8 g, 1 g, and microgravity) on the condensation process. Experiments were performed with mass flux ranging from 30 kg m-2 s-1 to 40 kg m-2 s-1 in a circular cross-section channel with an internal diameter of 3.38 mm. The setup allowed to conduct quasi-local measurements of the heat transfer coefficient during condensation and to characterize the liquid phase distribution inside the channel by means of direct flow visualization. The reduction in the gravity level was found to lower the heat transfer coefficient, with this effect becoming more pronounced as mass velocity decreases. Finally, the experimental results in microgravity were compared with those from the 70th ESA Parabolic Flight Campaign with HFE-7000 as working fluid, as well as with predictions from several annular flow condensation models available in the literature.