Experimental study on Marangoni heat transfer characteristics of ethanol-water mixed vapor in vertical tubes of different materials

Condensation heat transfer is a critical phase-change heat transfer process in fields such as energy and power, chemical separation, refrigeration and cryogenics, electronics thermal management, and new energy equipment. Its efficiency directly determines the system energy consumption, equipment compactness, and operational reliability. In this study, an experimental apparatus is designed and constructed to investigate condensation heat transfer of ethanol-water vapor mixtures inside a vertical tube. The tube has an inner diameter of 26 mm, and two tube materials, stainless steel and copper, are employed respectively. Flow visualization is also conducted. The experimental conditions include ethanol vapor concentrations of 0%, 0.5%, 1%, 2% and 5%, vapor mass fluxes of 3.92 kg/(m 2 ·s), 9.92 kg/(m 2 ·s) and 17.65 kg/(m 2 ·s), inlet vapor qualities of 1, 0.8 and 0.6, and system pressures of 84.5 kPa and 47.6 kPa. Visualization observations reveal that pure water vapor condensation exhibits filmwise condensation, while ethanol-water vapor mixtures at all tested concentrations exhibit pseudo-dropwise condensation. As the ethanol concentration increased, the condensation patterns inside both tube materials exhibited a general evolution from filmwise condensation to pseudo-dropwise condensation with rivulets, and finally to pseudo-dropwise condensation. As the subcooling degree decreased, the condensation mode transitioned from pseudo-dropwise condensation with rivulets to filmwise condensation. Based on the experimental results, the maximum condensation heat transfer coefficient for both tube materials at different ethanol concentrations is found to be correlated with the vapor mass flux. At low and moderate mass fluxes, the highest heat transfer coefficient is achieved at an ethanol concentration of 1%, whereas at a high mass flux, the maximum value occurs at 5%. Meanwhile, the condensation heat transfer coefficients for both tube materials decrease along the vapor flow direction, and decrease with increasing surface subcooling, decreasing inlet vapor quality and decreasing system pressure. However, owing to the substantially different thermal conductivities of the two materials, the condensation heat transfer coefficient of the ethanol-water vapor mixtures for the copper tube is significantly larger than that for the stainless steel tube under identical experimental conditions, by a factor ranging from 2.4 to 6.1. For pure water vapor and the mixture with an ethanol concentration of 5%, the condensation heat transfer coefficients for both tube materials increase with increasing vapor mass flux. In contrast, at low to moderate ethanol concentrations ranging from 0.5% to 2%, the heat transfer coefficient decreases with increasing vapor mass flux. Based on the experimental results, the condensation heat transfer mechanisms of ethanol-water vapor mixtures in tubes of different materials are elucidated. These findings provide essential experimental evidence and theoretical support for achieving high-efficiency and low-cost optimization of industrial condensation equipment.