管内液膜流动失稳现象的直接数值模拟

Instabilities analysis of liquid film in tubes based on direct numerical simulation

为分析液膜流动时垂直液膜的重力加速度分量和表面张力对扰动波振幅的影响,通过FTM模拟了液膜在重力作用下流动失稳过程。结果表明:重力分量和表面张力系数越大,扰动振幅衰减得越快;并且表面张力系数越小,扰动振幅波动的频率越高。此外还分析了促进液膜流动失稳的两个因素,即密度比和雷诺数Re。当增大密度比时,液膜会出现两种极端的变化:一种是液膜不再完全覆盖底面,会形成锲状液膜,随着速度变大和振幅脉动,当锲状液膜前锋出现新的扰动波时,新的扰动会与初始扰动发生干涉产生二次波动;另一种是增加密度比时,液膜将会在剪切力的作用下产生飞脱的现象,在Re数较大时,液膜流动失稳会在剪切力的作用下产生K-H(开尔文-赫姆霍兹)不稳定性,且初始扰动对形成开尔文-赫姆霍兹波浪具有诱导作用。

In order to analyze the relationship of the amplitude of disturbance wave and suppressive factors-gravity longitudinal component vertical to the liquid film and surface tension when the liquid film is flowing,the flow instability process of liquid film under the action of gravity is simulated by means of FTM (Front Tracking Method).The larger the gravity longitudinal component and the surface tension coefficient are,the faster the speed of the attenuation of the disturbance wave amplitude is.Meanwhile,the surface tension coefficient decrease could make the undulant frequency of the amplitude of disturbance wave higher.Besides,two positive factors (density ratio and Reynolds number)are also analyzed,which can promote the instability of the liquid film flow.By increasing the density ratio,the liquid film would come out two extreme changes:one is the liquid film became wedge-shaped liquid film that is no longer covering the bottom of tube completely.With the speed and amplitude strongly pulsating and the forward of wedge-shaped liquid film,a new disturbance wave arise,the new and the initial one would take place the interference of waves.The other one is the phenomenon of liquid film flying off under the force of shear when the density ratio is further increased.The liquid film flow instability would move towards the contrary direction,that is to say,the interface would produce K-H (Kelvin-Helmholtz)instability under shear force,and the initial perturbation had an induction on the formation of K-H wave.