竖直热壁冷却下降液膜瞬变传热及相界面波动

Transient Heat Transfer and Phase Interface Wave of Falling Liquid Film When Cooling Vertical Heated Surface

采用数值方法对恒热流密度竖直热壁表面自由下降液膜冷却流动过程进行瞬态研究,建立了简化为二维问题的物理模型和数学模型,选用RNG k-ε模型求解汽液两相湍流流动,基于VOF多相流模型结合自编程用户定义函数求解相变过程中的传热传质。得到了不同因素影响下,冷却降膜厚度在热壁不同位置的分布规律。结果显示,冷却降膜的流动过程中,其表面存在不同程度及类型的波动。热扰动会降低液膜相界面流动稳定性,提高液膜流量能够抑制液膜表面波动,两种影响因素对于液膜的流动稳定性是相互抑制的。核态沸腾发生时,液膜内汽泡的运动、变化会对液膜相界面波动产生显著影响。瞬态计算结果显示,不同工况下,降膜冷却传热过程中表面传热系数具有相同的变化趋势。液膜的热稳定性受到入口液流量及壁面热流密度的耦合影响。根据汽液两相流动及传热传质特性,分析了发生以上现象的原因。

Transient numerical analysis on flowing process of free-falling liquid film when cooling vertical heated surface with constant heat flux are performed. The numerical model and physical model are developed for simplified two-dimensional flow. Employing RNG k-6 to calculate vapor liquid two phase turbulent flow. The multiphase model volume of fluid （VOF） is used to predict heat and mass transfer combining with user defined function. Distribution law for thickness of liquid film along heated surface under different cases are obtained. Numerical results indicate that phase interface of liquid film waves in different degrees and different styles during flow cooling. Stability of liquid film phase interface becomes weak due to thermal disturbance, however, it can be improved by increasing inlet mass flux. These two factors are reciprocal inhibition with regard to flow stability of liquid film. Movement and transformation of bubbles in liquid film affect wave of liquid film phase interface significantly during nucleate boilingl On the basis of transient numerical results, variation trends of surface heat transfer coefficient are same for different cases. Thermal stability of liquid film are coupling effected by flow disturbance and thermal disturbance. Basing on gas-liquid two phase flow and heat transfer characteristics, reasons of phenomenon appeared is analyzed.