Marangoni condensation of steam-ethanol mixtures on a horizontal smooth tube

Abstract Experiments were conducted to measure condensation heat transfer of steam-ethanol mixtures on a horizontal smooth tube. Heat flux and vapor-to-surface temperature difference were accurately measured over a wide range of mass concentration of ethanol (0.001%, 0.005%, 0.01%, 0.025%, 0.05%, 0.1%, 0.5% and 1.0%.) at atmospheric pressure. The present work extends earlier data of Murase et al. (2007) and Ali et al. (2013) to lower mass concentration of ethanol and higher vapor-to-surface temperature difference. Visual observations, the appearance of condensate, show the condensation modes and their transitions of pseudo-dropwise, rivulets and filmwise. Heat flux and heat-transfer coefficient are found to increase, reach to a peak and decrease as vapor-to-surface temperature difference increases for all mass concentrations of ethanol and vapor velocities. The maximum heat-transfer coefficients appear in the range of mass concentration of ethanol from 0.01% to 0.025%. The highest heat-transfer coefficient values are 112.4 kW/m2 K and 93.9 kW/m2 K at ΔT = 3.43 K and 4.46 K, respectively. The maximum enhancement ratio of 5.5 is achieved at vapor-to-surface temperature difference of 4 K and mass concentration of ethanol 0.01%. The results also show that heat-transfer coefficient of filmwise eventually approaches to a value slightly higher than that of pure steam.