Experimental research on heat transfer and flow resistance of in-tube condensation of humid air

The sensible heat transfer coefficient and flow resistance are two essential considerations in calculating vapor condensation in the presence of non-condensable gases. In this study, the effects of interfacial roughness on the sensible heat transfer and flow resistance in the condensation of humid air are investigated. A horizontal annular heat exchanger was used in the study, with the vapor–air mixture Reynolds number ranging from 9000 to 19,000 and specific humidity of up to 28.2 g/kg dry air. The sensible heat transfer coefficient and flow resistance were measured experimentally during the condensation process with constant wall temperature. The results were compared to some conventional correction methods to examine their applicability. It was found that the correction methods for the roughness effect results in an overestimation of the sensible heat transfer coefficient in humid air condensation. The suction effect was also investigated and proved to be negligible during the condensation process. The two-phase pressure drop was found to be approximately 10% higher than that for non-condensing convection flow and can be reasonably predicted by the homogeneous and Lockhart–Martinelli models with slight underestimation.