【作者】 刘红；

【导师】 解茂昭；

【作者基本信息】 大连理工大学 ， 工程热物理， 2006， 博士

【摘要】 闭孔泡沫金属是一种内部结构含有大量孔隙的新型功能材料。它具有独特的结构和性能,工业中有着重要的应用和广阔的发展前景。制备泡沫金属的方法很多,其中吹气发泡法因为设备简单、成本低、可以连续生产等特点,更适用于规模化生产。在制备泡沫金属过程中,如何控制气泡的尺寸及其均匀度是该项工艺的核心问题。由于影响泡沫尺寸及其分布的因素众多,为了寻求能够对生产过程和产品性能进行科学控制的有效手段,需要对泡沫形成过程中的各个影响因素进行全面、系统的研究。本文采用实验研究和数值模拟相结合的方法对泡沫铝发泡特性及发泡过程中气泡一熔体两相流动过程进行了系统的研究。主要研究内容包括以下几个方面: 在自制井式炉内进行吹气发泡法制备泡沫铝的实况工艺实验。研究不同工艺参数对泡沫铝成品结构和性能的影响。并以泡沫铝的空气发泡过程为研究对象,依据相似原理进行水模拟(聚乙烯醇水溶液)实验,实验采用静态和动态两种形式。静态实验模拟在无涡流场状态下进行,通过改变入射压缩空气的压力与流量、聚乙烯醇水溶液的黏度以及通气孔的直径,对入射空气在聚乙烯醇水溶液中形成气泡的过程以及气泡在水溶液中的运动和变化规律进行研究。动态实验模拟是在强旋涡流场条件下,主要考虑搅拌速度、发泡室尺寸以及搅拌轴位置等因素对铝泡沫尺寸和分布的影响,探讨气泡在涡流场中的行为和变化规律。 气泡直径是影响泡沫铝性能的非常重要的因素。本文应用动力学平衡半经验关系式,分析气体通过锐孔在静止黏性液体中形成气泡的过程,预测气泡直径,确定影响气泡尺寸的主要因素。计算结果表明,在一定物性及一定锐孔直径的情况下,流量是影响气泡大小的重要因素。随着流量的增加,气泡直径增大。相同气体流量下,表面张力系数越大,锐孔直径越大,得到的气泡直径越大。液体密度的影响表现得较为复杂,当流量较小时,气泡体积随液体密度的增大而减小;当流量增加时,液体密度的影响便减小。 应用双流体模型及两相κ-ε湍流模型分别描述气体在静止液态金属中单孔射流及多孔射流两相流动,在总结和分析文献中气泡—液体双流体模型的基础上,提出虚拟入口的方法以简化孔口气泡形成过程,对气液相问作用力模型及湍流模型进行改进,通过与实验数据的比较,表明气液相间作用力修正对气液体系流体动力学行为有重要影响,改进的模型具有较好的预测能力。考察了液相密度、液相黏度、气泡直径、熔池尺寸、通气孔的数目及位置对气含率分布、气泡上升速度和液速的影响。单孔条件下两相流中液相物性会影响更多还原

【Abstract】 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.更多还原