Experimental investigation of liquid film thickness and heat transfer during condensation in microgravity

Considering the increasing complexity of future space missions and the growth of heat fluxes to be removed in next generation satellites, thermal control systems will be asked to provide high heat dissipation rates with reduced power input, size and weight. Two-phase systems represent a reliable technical solution to meet all these specifications but their accurate design requires validated tools and a deeper understanding of the effect of gravity on heat transfer. While several experiments and numerical simulations have been conducted to investigate the gravity effect on pool and flow boiling, studies on in-tube condensation under reduced gravity conditions are still limited in the literature. In the present work, the effect of gravity is experimentally investigated during condensation of HFE-7000 inside a 3.38 mm internal diameter channel with mass velocity ranging from 30 kg m−2 s−1 to 50 kg m−2 s−1. Condensation tests were carried out during the 70th ESA (European Space Agency) Parabolic Flight Campaign by simultaneously measuring the liquid film thickness and the heat transfer coefficient inside the channel in both normal gravity and microgravity conditions. The liquid film thickness is determined by coupling a shadowgraph technique with the measurements performed by a chromatic confocal sensor and an interferometer. The reduced gravity condition is responsible for a heat transfer penalization with respect to normal gravity, which is found to be more severe when the mass velocity decreases. The change in the gravity level affects the characteristics of the interfacial waves in terms of frequency, height and velocity. The prediction accuracy of several models for annular flow condensation is assessed against the experimental results taken in microgravity conditions.