Condensation characteristic of ethanol-water mixtures on external wall of vertical tube

The condensation heat transfer of the vapor mixture on a vertical tube was studied experimentally under a wide range of ethanol concentrations. Moreover, the condensation modes were observed. The results indicate that, with the change of the vapor-to-surface temperature difference, the heat flux and the heat transfer coefficients revealed nonlinear characteristics with the peak values for all ethanol concentrations. The heat transfer coefficients were significantly intensified in the low ethanol concentration range. The maximum heat transfer coefficient was 165 kW·(m²·K)^-1 at p=47.36 kPa, μ=6 m·s^-1 and w_v=1%, which was around 9 times as large as that of the steam. This augmentation was caused by the Marangoni effect which yielded drop-wise condensation and a corresponding decrease in the thermal resistance of the condensate. At the low ethanol concentration, the thermal resistance of the mass diffusion layer between the bulk vapor and the surface of the condensate was small. With the increase of ethanol concentration, the condensation heat transfer coefficient decreased obviously. When the ethanol concentration reached 50%, heat transfer coefficients were lower than those of steam.