【作者】 全晓军；

【导师】 彭岚；

【作者基本信息】 重庆大学 ， 热能工程， 2004， 硕士

【摘要】 Czochralski方法是晶体材料制备中广泛应用的一种方式,在微重力条件下,表面张力梯度引起的对流成为影响材料品质的突出因素,因而成为重要的研究课题。为削弱热毛细对流,近年来发展起来的液封Czochralski方法(Liquid Encapsulant Czochralski,或称LEC)在抑制热毛细对流方面颇具魅力。本文主要研究环型液池内不相溶混的两层流体的热毛细对流,建立了不相溶混的两层流体热毛细对流的物理模型和数学模型,采用有限容积法分别对微重力和常重力条件下具有液封的熔体内热毛细对流进行了数值模拟,得到了双层流体主流区的温度场、流场分布和无量纲参数A、B、Ma、Pr以及上、下层流体物性参数比对热毛细对流的影响规律,证实了液封技术能够削弱熔体主流区的热毛细对流,从而提高晶体生长质量,并找出了影响因素及作用规律,同时发现双层流体的流态比单层流体要复杂得多,影响其流态变化的因素也大大增加。结果表明:在微重力条件下,随着液封层流体厚度的减薄,上、下层流体的热毛细对流均被削弱,液封层厚度越薄,对熔体热毛细对流的抑制越强;随着液封流体与熔体之动力粘性系数比??的增大,液封层流体粘性扩散能力增强,使得熔体的热毛细对流显著削弱,说明动力粘性系数对流态的影响较大;当自由表面张力温度系数与液—液界面张力温度系数之比??为负值时,随着??绝对值的增大,熔体层流函数显著增大,等温线非线性分布增加,表明表面张力温度系数对流态的影响非常大;增大熔体的Pr数,熔体热毛细对流减弱;熔体的Ma数越大,热毛细对流越强。选择合理的具有恰当物性参数的液封流体,可以有效抑制熔体主流区内的热毛细对流。在常重力条件下,当熔体的Gr数大于104时,浮力对流才发挥作用。在本文模型中,浮力对流促进了液封层的流动,从而加大了对熔体热毛细对流的抑制作用。更多还原

【Abstract】 Czochralski method is widely used in producing crystals. The thermocapillary convection induced by interface tension gradient becomes a prominent factor influencing the quality of materials under microgravity condition. The liquid encapsulation Czochralski (LEC) technology of crystal growth plays a very important role in reducing thermocapillary convection. The physical model of two immiscible liquid encapsulation technology is researched in this paper. The results obtained are directly used to crystal growth procedure in the production of pure and high quality crystals.The physical and mathematical models of thermocapillary convection in two immiscible liquid layers are established. The distributions of temperature and velocity in liquid layers are then obtained and effects of a series of non-dimensional parameters on the thermocapillary convection are analyzed. It is verified that liquid encapsulation can reduce thermocapillary convection in the LEC, and the flow profile of two liquid layers is much more complex than that of single liquid layer.In results, under microgravity condition, when the thickness of liquid encapsulation decreases, thermocapillary convection of the fluid in the up and down layers becomes weaker. The augmentation of kinetic viscosity ratio ?* of the liquid encapsulation to molten liquid makes thermocapillary convection of molten liquid be weaken a lot, and it is found that kinetic viscosity has an extremely remarkable effect on the flow profile. For ??<0, the nonlinear distribution of temperature and flow strength become much stronger along with the increase of the absolute of ratio ???of the temperature coefficient of free interface tension to liquid-liquid interface tension. The thermocapillary convection in the down liquid layer appears to get weaker along with the augmentation of Pr. When Ma increases, thermocapillary convection in the two liquid layers gets stronger.The thermocapillary convection in molten liquid can be much more weakened by selecting proper liquid encapsulation with reasonable physical parameters. In earthgravity environment, Gr>104, it is impossible to neglect the impact of buoyancy convection on flow and heat transfer. The convection in molten liquid can be much more weakened by buoyancy convection increasing the flow of liquid encapsulation in this model.更多还原