Effect of Temperature Gradient on Marangoni Condensation Heat Transfer for Binary Mixture Vapor

摘要

This article experimentally studied the effects of macroscopic temperature gradients on Marangoni condensation for water-ethanol mixture vapor. A specially constructed brass block was applied in this experiment, and the maximal temperature difference of the condensing surface on which pure steam condensed was 11 °C when the block was cooled by the normal temperature water. As a result of measurement, the condensation characteristic curves over a vapor velocity of 0.3m/s at five vapor concentration of ethanol mass fractions of (0, 0.01, 0.03, 0.22, 0.37) were reported in a previous paper [10]. In this paper, the experimental results of three representative characteristic curves with ethanol concentrations (0, 0.03, 0.22) were compared with the existing results which focus on the concentration gradient effects only, the differences between them arc analyzed and theoretically explained. The temperature difference between two ends drives the surface tension difference of condensing liquid at thin and thick ends; it also drives different intensity of Marangoni convection on condensing surface of thin and thick ends, so condensing coefficients for thin and thick ends are different even under the same surface subcooling temperature. The maximum of heat flux on thin end of testing block was about 1400 kW/m~2 and that on thick end was about 500 kW/m~2, when c=0.22, the maximal coefficient on thin end of testing block was about 87.4kW/m~2K, that of thick end was about 40 kW/m~2K. By contrasted with corresponding published experimental results, this experiment combined temperature and concentration gradients can create the Marangoni dropwise condensation under a low surface subcooling temperature (5K), and obtain a higher heat transfer coefficient than the published experimental results. It means temperature and concentration gradients can improve the intensity of Marangoni convection and enlarge the ranges of surface subcooling for Marangoni condensation.