Numerical Simulation Of Single Bubble Motion Behavior On The Free Surface Of Molten Metal

During the process of manufacturing a new-type of functional materials-aluminum foam by gas injection method, the behavior of bubbles in molten metal is an important factor affecting the product quality. The current related studies in this area have been mainly focused on two aspects:one is the process of bubble foaming, rising, stirring and dispersion in the melt; and the other is the drainage and evolution process from wet foam to dry one. However, about the behavior of bubble on the free surface of molten metal which is on the midway between the two processes, there is still a blank. For making the study of aluminum foam more complete and coherent, in this paper, based on the exiting methods and results about a bubble on free surface numerical simulation was performed on the interaction between a bubble and the free surface of liquid metal by the volume-of-fluid (VOF) method.Firstly, by 2D simulation we analyzed and predicted the effect of relevant factors:such as, surface tension, the liquid aluminum viscosity and the bubble size on the rising and deformation behavior of bubble at the free surface, the film thickness and the break time. Introducing the dimensionless quantity Mo and Eo investigate the trend of break time. To study the interaction between bubbles with free surface of liquid metal we also simulated a new bubble stated at the initial position of the original bubble which has been interacted with free surface for a time. Secondly, by using the adaptive grid method, which greatly reduces the calculation time with the accuracy ensured,3D simulation was carried out to gain more straightforward results than the 2D case. Finally, we made the theoretic formula for calculating the film thickness more suitable for aluminum liquid by modifying the coefficient and obtained the same variation trend of the film thickness with viscosity as the numerical prediction.From the results of simulation and calculation above, we draw the following conclusions on the behavior of bubble on the free surface of molten metal:increasing surface tension makes the rising heights of the bubble and free surface decreased and the break time advance; increasing viscosity slows down the rising velocities of the bubble and free surface, reduces the velocity of film draining, prolongs the draining time, delays the break time and increases the terminal heights of the bubble and free surface; increasing the initial radius of bubble makes the rising velocity and heights of the bubble and free surface increased, slows down the film thinning and slightly delays the break time; at large Mo numers, the break time delays as the Mo increased. At small Mo, the break time is also affected by Eo:at the same Mo, the break time delays as the Eo increased. For the two bubble case, the new bubble makes the rising heights of the original bubble and free surface increased. The new bubble affected by the original bubble has a higher rising height than a single bubble and is stretched, resulting in coalescence of the two bubbles. The bubble induced changes in the pressure and velocity distributions of the free surface propagate from the center to the periphery like a dimple. Near the center of free surface, there is a pressure drop which drives the liquid in the film to drain out of the center.