水-酒精混合蒸气在水平圆管横截面换热特性的研究

本文搭建了水平圆管管外凝结传热实验台,以水-酒精混合蒸气为工质,在不同蒸气压力和不同酒精浓度的条件下对水平圆管外凝结换热特性进行了研究,并实现了实验的可视化,分析了不同酒精浓度下造成凝结换热特性差异的原因.实验结果表明：凝结表面不同位置的传热系数不同,表面张力梯度大的位置凝结传热系数较大.在相同流速和浓度条件下,酒精浓度较低时凝结传热系数随蒸气压力的升高而升高,但在高酒精浓度下,凝结液表面张力梯度减小、扩散热阻的影晌增大,蒸气压力对于凝结传热系数的影响并不明显.酒精浓度对凝结传热系数的影响因不同横截面测温点有明显差异,在浓度为2%和5%时传热系数差异较大,而在其他浓度时横截面不同测点的凝结换热效果差异较小.

Research on Condensation Heat Transfer Characteristics of Ethanol-water Vapor on Cross Section of Horizontal Tube

In this article the effects of vapor pressure, vapor concentration on the condensation heat transfer characteristics of ethanol-water mixture vapor on horizontal tube were investigated experimentally.Visual results were showed in this experiment. In order to clarify the heat transfer characteristics of vary ethanol vapor concentrations, heat flux and vapor-to-surface temperature difference were measured. The results demonstrate that the local heat transfer coefficients at different locations on condensing surface were different, and at the local position with greater tension gradient the heat transfer coefficient was higher. On the same condition of vapor concentration and speed, heat transfer coefficients of high pressure are bigger than those of low pressure in the low-ethanol concentration range. But the effect of vapor pressure on the heat transfer coefficients is not obvious in the high –ethanol concentration range. The reason for this is that bigger diffusion resistance and the gradient of the surface tension versus the condensation mass fraction becomes smaller with the increase in vapor concentration. It has been found that different concentration of ethanol in the liquid phase has significant influence on heat transfer coefficients on different cross section thermocouples. The D-value of heat transfer coefficients are greater when ethanol composition is 2% and5%, and lower in other composition at different position in the same circumstance.