椭圆管外含不凝结气体自然对流凝结换热性能研究

Study on Condensation Heat Transfer Performance of Natural Convection Outside Elliptical Tube Containing Non-Condensable Gas

特殊的换热管型有利于加快气液膜排泄及分离,使换热器的换热性能提高,达到节能的目的。在自然对流的情况下,基于双膜理论和边界层理论,在考虑气液界面热阻的情况下,建立了椭圆管外气、液膜厚度及传热系数的数学模型,利用C++软件进行编程得到不同初始参数、曲率下的气、液膜厚度、凝液量、气、液膜热阻、气液界面热阻和传热系数沿管壁的分布规律。研究结果表明:其他条件不变,传热系数随管壁温度的增大而增大,随混合气压力的增大而减小,随不凝结气含量的增大而减小;对于当量直径相同的椭圆管,其他条件不变,曲率越大,越易发生液膜分离,传热系数越大。管外存在不凝结气体时,气膜热阻>液膜热阻》气液界面热阻。计算结果与文献基本相符,能够较好的反映实际,为强化换热提供一定理论指导。

The special heat transfer tube type is beneficial to accelerate the excretion and separation of gas and liquid membrane,which improves the heat transfer performance of the heat exchanger,thus saving energy.In the case of natural convection,this paper established a mathematic model of gas film,liquid film thickness and heat transfer coefficient of elliptical tube based on double film theory and boundary layer theory with consideration of the vapor-liquid interfacial thermal resistance.The distributions of gas film,liquid film thickness and heat transfer coefficient along the tube wall under different initial parameters and curvatures were obtained.The results indicate that under same conditions,the heat transfer coefficient increases with the increase of the wall temperature,while it decreases with the increase of the mixed gas pressure and the non-condensable gas content.For the elliptical tubes with same equivalent diameter,when other conditions remain unchanged,the larger the curvature is,the easier the liquid film separation occurs,and the greater the heat transfer coefficient becomes.When there is non-condensable gas outside the tube,interface thermal resistance of gas phase thermal resistance>liquid phase thermal resistance>>gas-liquid interface thermal resistance.The results obtained by establishing the model were in good consistent with the existing results,which can reflect the actual situation and provide theoretical guidance for heat transfer enhancement.