Experimental Study And Numerical Simulation On Bubble-melt Two-phase Flow With Gas Injection And Mechanical Stirring Relevant To The Aluminum Foam Processing

Closed-cell metallic foam is a new type of function material, which has important applications and broad development prospects in industry due to its unique structure and performance. There are many approaches to manufacture cellular metallic materials. From which the gas injection method has special advantages in the respect that metallic foams can be produced continuously and their size is little limited. In this technique, a major issue is how to control the size and uniformity of the cells during the foaming process of molten aluminum. In order to explore approaches through which one can effectively control the manufacture process and the performance of aluminum foams, it is necessary to investigate and understand deeply factors affecting the foaming process. This dissertation is focused on this topic. Hydrodynamic behaviors of the bubble-liquid metal two phase flow and bubble moving and distribution characteristics in the melt were systematically studied both experimentally and numerically, and on this basis some insights into their effects on the foam structure have been gained.Laboratorial foaming experiments were conducted with a graphite crucible in a self-making resistance furnace to examine effects of operating parameters on the foam structure. Furthermore, water simulation experiments based on the analogy principle were performed in the static and dynamic states, respectively. The static state means that the melt in the tank was not stirred, herewith the bubble formation process by gas injecting into the water solution as well as the bubble behaviors in the water solution were recorded by high speed photography. The influences of the air pressure and flow rate, liquid viscosity and hole diameter on the bubble size, gas holdup, bubble rising velocity were systematically studied. Dynamic state experiments were carried out with a rotational impeller. Influences of the impeller speed, crucible size, position of the rotational shaft on the bubble size and its distribution were investigated.Bubble size plays a significant role to the aluminum foam performance. The single bubble formation during the foaming process of molten aluminum was analyzed and the bubble size was estimated under constant flow conditions using a semi-empirical model. The calculated results indicate that the bubble size increases with increasing orifice diameter, the airflow rate, the surface tension, as well as the liquid density at low flow rates. When the gas injection velocity exceeds a critical velocity, the gas injected from the nozzle takes the form of a coherent gas jet.Numerical simulations were performed for the hydrodynamics of a gas-liquid system in the static state in the framework of Eulerian-Eulerian two-fluid formulation coupled with a two-phase k-e turbulence model. Based on an analysis of previous work on the two-fluid model reported in the literature, a virtual gas inlet was suggested and used as the boundary condition to substitute the real orifice. Improvements were made to the sub models of interphase forces and turbulence. The influences of the air pressure and flow rate, liquid viscosity, the number and position as well as the diameter of the injection orifices on gas holdup, bubble rising velocity and liquid flow were discussed.Dynamic state simulations were carried out with mechanical stirring by a rotational impeller, which was placed in three different positions: perpendicular, parallel and inclined to the liquid surface. Two-dimensional, quasi-three dimensional and three-dimensional models were employed separately to simulate the fowl field, the impeller region was explicitly included using a Multiple Reference Frames (MRF) method.In order to clarify the bubble size distribution characteristics in stirred melt flow, a population balance model (PBM) was incorporated into the three-dimensional simulations. Variation in the bubble size due to breakup and coalescence was taken into account. Computational results show that the bubble size increases with increasing gas flow rate and orifice diameter and decreasing liquid viscosity. It also increases with enlarging foaming chamber but decreases with rising rotation speed of the impeller. The bubble size and gas hold-up are dependent also on the location in the flow field. Around the tips of the impeller blades bubbles have the mimimum size. Bubbles with larger size gather in the regions behind the blades due to lower pressure there, resulting in a higher gas hold-up. In other parts of the tank, such as at the bottom, near the walls, and the region above the impeller and near the shaft, bubbles have smaller sizes because in these regions gas holdup is small and many circulations of small scales exist. Finally, at the melt surface there are bigger bubbles in the central area, and bubble size is reduced with decreasing distance of the bubble to the walls.