Numerical Simulation Of The Geometry And Moving Behavior Of Bubbles In Molten Metal

Closed-cell aluminum foam is a new-type function material, which has important applications and broad development prospects in industry because of its unique structure and performance. Aluminum foam is a composite material consisting of gaseous and solid phases, which exhibits a porosity of 70~90%, thus much attention should be paid to the control of the size and uniformity of the cells during the foaming process of molten aluminum. One of the important effect factors in the aluminum foam moulding technology is the geometry and moving behavior of a single bubble in the molten aluminum. It plays an important role in the formation of the final size and shape of the cells, which in turn affects considerably the structure and performance of aluminum foam.In this paper, at first, the research histories and current development in the bubble-liquid two-phase flows is summarized and reviewed systemically. On this basis, single bubble formation during the foaming process of molten aluminum is analyzed and the bubble size is estimated. The calculated results indicate that the radius of the bubbling bubble will increase when the radius of the orifice is increasing, or the surface tension is increasing, as well as the liquid density is decreasing at low flow rate. When the gas velocity exceeds a critical velocity, the gas will be injected from the nozzle as a coherent gas jet. For a high velocity stirrer system, the bubble radius will decrease when rotate speed is increased or the flow rate is reduced.Numerical simulation is performed on the deformational bubble shapes and moving behavior of gas bubbles in molten aluminum by a volume tracking technique based on the volume-of-fluid (VOF) method to study the flow mechanism of a bubble in the molten metal and its affecting factors. Based on the numerical computations, the deformation and moving behavior of a single bubble and the coalescence of two bubbles in a still flow field are analyzed and predicted. The simulation results indicate that the bubble in molten metal takes less deformation as the Eo number is decreased and it rises corkscrewly. The shorter the width of container is, the smaller the bubble is and the stronger the bubble swings during its rising. Two bubbles may coagulate into a big bubble under some conditions. Good agreement between the numerical simulation and experimental results for bubble deformation and movement indicates that the calculation results and conclusions are reliable.Finally, three-dimensional moving behavior of the bubble is simulated approximately by the multiple reference frame (MRF) and a quasi-three-dimensional model which means to compute the flow field in the lateral and long direction sections separately by the two-dimensional model. The bubble is broke up by high swirl velocity vanes. In the lateral direction section, the bubble is sucked into the vane vortex due to the suction. In the long direction section, it will swing irregularly along the stream vortex affected by the buoyancy.This paper has finished successfully a prophase research on single bubble formation and the flow mechanism in the molten aluminum, which is the initial stage of the aluminum foam bubbling characteristic research. The results and conclusions of the thesis can be referenced for more comprehensive and further research.