Liquid-film thickness, flow pattern, and void fraction of hydrocarbons and their zeotropic mixtures during convective condensation

Abstract

The present study concerns an experimental investigation of the liquid-film thickness, flow patterns, and void fraction during convective condensation of hydrocarbons and their zeotropic mixtures. Experiments were performed for R600a, R290, R1270, R600a/R290 (70/30 molar fraction) and R600a/R1270 (75/25 molar fraction) in a horizontal tube with an internal diameter of 9.43 mm. In the experiments, the saturation temperature was kept at 35°C, mass velocities ranged from 50 to 250 kg/m2s, vapor qualities from 0 to 1, and heat fluxes from 5 to 60 kW/m2. It was noticed that the transition from gravitational to shear-driven flow patterns can be quantitatively characterized by a peak in the newly defined base liquid-film thickness measured at the top region of the channel at a certain vapor quality. At mass velocities higher than 100 kg/m2s, shear-driven flow patterns were dominant, and noticed in a wider range of vapor qualities, presenting a sharp transition from gravitational-driven flow patterns, which is independent of the refrigerant. At lower mass velocities, R290 and R1270 showed a higher vapor quality associated with the transition from gravitational to shear-driven flow patterns than the other fluids. Such behavior was associated with the lower vapor specific volumes of R290 and R1270. Gravitational to shear-driven transition prediction methods were compared against the data from the present study based on the peak of the base liquid-film thickness, showing reasonable results for flow pattern prediction methods developed based on heat transfer data. The void fraction was estimated based on the LFT measurements for annular flows, and the results compared with several prediction methods with two of them providing accurate predictions.