Study Of Bubble Dynamic Characteristics Based On CLSVOF Model

Abstract:Bubbles exist widely in gas-liquid two-phase flow. The formation and motion process of gas bubbles were important in gas-liquid two-phase flow. Consequently, further investigations regarding to the bubble dynamics could act as guidance for solving many practical engineering problems. The formation, motion and deformation of gas bubbles were studied by numerical simulation using CLSVOF model, the model was based on the equations of surface tension, and the bubble dynamic characteristics in vertical pipeline was further studied. The main works and conclusions include are as follows:(1) CLSVOF model is used to track the interface of two-phase flow, which combined the VOF model and Level Set model. The solving process of this model and the implementation of some key process, such as interface reconstruction and the distance function reinitialization, were given. Through the numerical simulation of the formation and motion of a single bubble in a gravity field, point out that CLSVOF model can make up the disadvantages of VOF model, such as inaccuracy of curvature calculation, and poor interface smoothing. Moreover, it solved the problem of Level Set model that quality is not conserved.(2) The formation process of a single bubble were simulated under different conditions, and the analysis of the effects of orifice flow rate, orifice diameter, contact angle, relative viscosity and relative density on the detachment time and volume of bubbles were obtained. For θe<50°, the bubble volume and detachment time are independent of static contact angle. For θe≥110°, the bubble volume is about treble larger than the volume get when θe<50°. The bubble volume is almost independent of orifice flow rate for v<0.05m/s. For v<0.05m/s, the bubble detachment time changes according to T=0.3471π Q1/5/g3/5.The constant orifice flow rate was increased stepwise until a transition from single periodic (SP) to double periodic (DP) formation was detected. For θe=70°, the bubble volume get when relative density equals to0.25, is4.7times larger than the volume get when relative density equals to1. For relative viscosity increases from0.1to100, the bubble volume increases44.64mm3.(3) The motion path, velocity and deformation of gas bubble were simulated under different conditions. The motion paths of the bubbles with different diameters were obtained, the calculation is in excellent agreement with the experimental results obtained by high-speed photography. The rising velocity of bubble fluctuates around the terminal velocity after the velocity of bubble increases to a certain value. The terminal velocity of the bubble with different diameters that obtained by simulation is in good agreement with the experimental result viscous-dominated regime and inertia-dominated regime, but different in surface-tension-dominated regime. If the surface tension and viscous force are smaller, the bubble will deformed earlier, and the degree of deformation will be greater.(4) The motion and deformation of two identical bubbles moving side by side or in a line in viscous fluid were simulated. The coalescence behavior between two bubbles associates with the form of lift. For the parallel bubbles with different diameter in air-water system, the symbol of lift coefficient changes when the diameter of bubbles is between5mm and6mm. For S≤2.25, two bubbles coalescence. For S>5, they no longer affect each other. For the coaxial bubbles, the wake flow of the above bubble pump the below bubble, which may make the rising velocity of the below bubble larger than the above bubble, eventually lead to two bubbles coalescence. In the coalescence process, the rising velocity of the bubbles placed in these two different ways both decreases at first and then increases. The coalescence time of bubbles is affected by the distance of bubbles, Eo and Re.