空气-水蒸气混合气体在换热圆管内外冷凝相变数值研究

Numerical Study on Condensation Behaviors of Air-vapor Mixture within and outside of Heat Exchange Pipes

为提高相变热交换器的传热性能,应用Fluent软件分别对空气-水蒸气混合气体在换热圆管内、外的冷凝换热进行数值模拟。气液两相流采用体积函数法模型,水蒸气在空气中的扩散采用组分运输模型,水蒸气冷凝的相变模型采用Knudsen相变系数模型。分析了冷凝相变流场,研究了空气质量分数对冷凝相变过程的影响。结果表明,水蒸气的冷凝需经过对流区、组分扩散区、液膜阻力区;传热系数随混合气体中空气质量分数的增加而降低,随混合气体流速的增大而升高;当换热圆管内空气质量分数为0.3时,传热系数相比纯蒸汽冷凝降低50%;在相同工作条件下,蒸汽于管内发生冷凝的传热系数较于管外低68.4%。

The Fluent software was applied to simulate the condensation behaviors of air-vapor mixture inside and outside of heat exchanger tube to improve the heat transfer performance of the phase change heat exchanger. The gas-liquid two phase flow was described by volume of fluid method, and the diffusion of vapor in air was described by species transport method. The condensation was described by Knudsen phase change coefficient model. The condensation phase-change filed has been analyzed, and the effect of air mass fraction on condensation process was studied. As a result, vapor has to pass through the convection zone, diffusion zone and film resistance zone to reach the phase interface for condensation. The heat transfer coefficient decreases with the increasing of air mass fraction and increases with the increasing of velocity. When the condensation occurs inside heat exchange tubes and air mass fraction of the air-vapor mixture is 0.3, the heat transfer coefficient is 50% lower than that of pure vapor. The heat transfer coefficient of in-tube condensation is 68.4% lower than that of out-tube condensation under the same working condition.