水-酒精混合蒸气在竖直管外凝结特性的研究

Condensation Characteristic of Ethanol-Water Mixtures on External Wall of Vertical Tube

在低于大气压下,通过实验研究了水蒸气和不同酒精含量的水-酒精混合蒸气在竖直铜管外的凝结换热特性,同时记录了凝结状态.实验结果显示:在不同的酒精含量下,随着过冷度的变化,凝结换热系数均呈现出具有峰值的非线性变化特征;随着酒精含量的减少,凝结换热系数显著增大;当酒精的质量分数为1%时,凝结换热系数的峰值是相同条件下水蒸气凝结换热系数的9倍;当酒精的质量分数为50%时,凝结换热系数与相同条件下的水蒸气凝结换热系数相当.分析表明,凝结换热系数的非线性特征是由于凝结液热阻以及扩散热阻的复杂变化所引起的.

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 · （m^2 · K）^-1 at p=47. 36 kPa, u=6 m · s^-1 and wv=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.