Mechanically pumped two-phase loops are strong candidates for future spacecraft thermal control, but condenser performance under long-duration microgravity is not yet well quantified. This work reports condensation heat-transfer measurements for n-perfluorohexane in a tube-in-tube annulus operated onboard the International Space Station. nPFH flows in a 7.24 mm inner tube with water coolant in a 12.70 mm annulus over a 574.5 mm active condensation length, with a water mass velocity of G w = 226 . 79 ± 0 . 04 kg m −2 s −1 . A 36-point matrix combines six nPFH mass velocities ( G nPFH ≈ 60 –182 kg m −2 s −1 ) with two inlet categories: saturated two-phase inlets with x e,in ≈ 0 . 29 , 0.67, and 0.96, and superheated vapor inlets with x e,in ≈ 1 . 07 , 1.12, and 1.16. Axial wall and coolant temperatures are fitted with smooth profiles to reconstruct local heat flux, inner-wall temperature, thermodynamic equilibrium quality, and heat transfer coefficient. The saturated two-phase length L tp increases strongly with G nPFH and is extended by higher inlet quality, ranging from roughly 90 to 550 mm and approaching the full test-section length at the highest mass velocities, where L tp is truncated by the 574.5 mm active length. In both inlet modes, the largest local heat transfer coefficients occur near the upstream end of the saturated region, then decay monotonically as quality decreases, while the downstream subcooled segment exhibits much lower and weakly varying coefficients. When reorganized in terms of local thermodynamic equilibrium quality, the saturated-region data show that, at fixed G nPFH , superheated and saturated inlets follow common h tp ( x e ) and q tp ( x e ) trends, indicating that, for a given mass velocity, local condensation performance is controlled mainly by local quality rather than inlet quality. Seven Earth-gravity correlations are assessed against the two-phase heat transfer coefficients. The correlations of Boyko and Kruzhilin, Kim and Mudawar, and Cavallini and Zecchin reproduce the ISS data with mean absolute percentage errors of about 6%–8%, whereas Akers et al. and Wang et al. exhibit larger positive biases and Shah and Dobson–Chato yield intermediate errors. The results provide a well-characterized microgravity dataset for an annular condenser relevant to spacecraft thermal control and identify which existing condensation models offer the most reliable starting point for mechanically pumped two-phase loop design in low gravity. • Long-duration ISS experiments quantify n-perfluorohexane condensation in microgravity. • Saturated two-phase length increases with nPFH mass velocity and inlet quality. • Local condensation HTC peaks near the upstream saturated region and decreases downstream. • At fixed mass velocity, local quality governs condensation more strongly than inlet condition. • Boyko–Kruzhilin, Kim–Mudawar, and Cavallini–Zecchin provide the closest predictions.
Golezani et al. (Thu,) studied this question.