弱熔体对流对定向凝固中棒状共晶生长的影响

利用渐近方法求出在弱对流熔体中定向凝固棒状共晶生长的浓度场的渐近解,研究了弱熔体对流对定向凝固中棒状共晶生长的影响.结果表明,弱熔体对流对定向凝固中棒状共晶生长有显著的作用;平均界面过冷度不仅与棒状共晶的棒间距、生长速度有关,还与流动强度有关;当生长速度一定时,随着流动强度增大,棒状共晶的平均界面过冷度减小.利用最小过冷原则,获得棒间距与生长速度和流动强度的关系.结果表明,当生长速度比较小时,随着流动强度增大,棒状共晶的棒间距增大;当生长速度比较大时,随着流动强度增大,棒状共晶的棒间距变化减弱;棒状共晶的生长速度越小,流动对棒状共晶生长的影响越大.利用本文的解析结果计算在对流条件下Al-Cu共晶的棒间距,结果显示随着转速增大或径向距离增大,共晶的间距增大,这与Junze等的实验结果相符合.     

对流扩散方程的涡区分离解法

对流扩散方程是流体计算中一个基本方程,常用的数值方法导至解一个高阶的代数方程组,要求较大的存贮量和较长的计算时间。本文提出一种涡区分离解法,它利用对流扩散方程的迎风性质,把涡区从对流支配区分离出来,仅在各个涡区建立代数方程组并求解。而在对流支配区,则充分利用其抛物性,只需采用显式格式进行计算。由于在各涡区建立的这些方程组阶数和带宽都较小,因此要求存贮量较小,计算速度较快。对于雷诺数较大,涡区范围较小的问题,该方法特别有效。     

数值分析环形池硅熔体旋转-热毛细对流及其稳定性

数值模拟了内半径20 mm、外半径40 mm、深5 mm环形池内硅熔体在旋转和热毛细力共同驱动下的热对流,通过线性稳定性分析确定了旋转-热毛细对流失稳的临界Marangoni数等临界条件。研究结果表明,液池低速旋转会降低轴对称热毛细对流的稳定性,而较高速度的旋转能增强热毛细对流的稳定性。临界条件下旋转-热毛细对流耗散结构波纹的传播方向与液池的旋转方向相同,临界周向波数随旋转速度的增加而增加。在较大的旋转速度下,液池底部出现涡胞,底部涡胞对热毛细对流的稳定性具有削弱作用。     

细蛇形管内超临界压力CO_2层流对流换热数值研究

本文对超临界压力CO_2在微细蛇形管内层流对流换热开展了数值模拟研究。研究的蛇形管内径0.5 mm,弯曲半径2 mm,入口雷诺数200~500,压力9 MPa。分析了变物性、浮升力和离心力的影响,对特征截面温度与速度分布进行了详细分析。结果表明:向上流动时,截面温度和速度呈对称分布,水平流动时,对称性消失;截面上两对涡的分布在向上和水平流动时呈现不同特性,换热强化和减弱区域不同。     

微重力下温差对液桥自由界面变形的影响

浮区法是一种无坩埚容器接触、生长高质量单晶的重要晶体生长技术。微重力环境下,热毛细对流成为熔体中的主要对流,因此,研究浮区法晶体生长中热毛细对流的不稳定性具有重要意义。本文利用VOF自由表面追踪模型来捕捉自由界面的运动,数值研究了微重力下动态自由界面液桥内热毛细对流特性及温差(△T)对自由界面变形的影响。结果显示,在较强的温差作用下,整个熔区出现三维不稳性的热毛细对流振荡,并且随着温差的增加,振荡强度增强,振荡波数也随之增加;自由界面呈现出冷、热壁面膨胀而中间收缩的"瓶颈"状分布。自由界面变形率随温差先增长较快,后增长较慢。     

铸件在凝固过程中的传热和自然对流计算

论文中以流函数、涡度和温度为基本变量,建立了铸件凝固时的传热和自然对流的数学模型。控制方程采用半隐式有限差分在固定网格上求解。在模型建立过程中,着重讨论了潜热的处理和固液相区(界面)速度条件。对于潜热处理,采用了基于合金平衡相图的比热焓法;对于固液相区(界面)速度条件,提出了一种新的、基于固相率进行修正的处理方法。另外还探讨了将本文提出的解法应用于复杂几何条件的可能性。最后,通过对一个典型例子进行计算,对自然对流对铸件凝固的影响作了深入的讨论。     

水平磁场对双层流体热毛细对流的影响

三维数值研究了零重力时水平温度梯度作用下,B2O3封闭液与InP熔液组成的双层流体系统在水平磁场作用下的热毛细对流.结果显示,当磁场强度较小时,上层流体中对流涡的结构变化其微,而下层流体中的逆对流涡得到了抑制;随着磁场的增强,温度场分布逐渐趋于均匀,涡量强度逐渐减小,表明热毛细对流强度逐步地得到削弱.当磁场增强列Bx=0.4 T时,上层流体中的对流涡较均匀地充满上层流体区,而下层流体中对流涡紧贴着交界面.     

多组元化合物晶体生长过程中的双扩散对流现象

针对多组元化合物晶体的Bridgman生长过程,通过对液相区流场和浓度场的数值模拟,研究生长过程中容器内部双扩散对流与组分分布的情况.首先对比了三段场和梯度场炉壁温度设计情况下液相区的流体双扩散对流以及组分分布情况.在此基础上,针对梯度场炉壁温度条件,分析了不同参数的影响.结果表明:在溶质格拉晓夫数较大时,它的增大能够明显地削弱液相区的流体流动,从而使得界面附近的组分分布也产生变化;此外,拉晶速度的增大也能够使得液相区的双扩散对流受到抑制.     

超临界压力下正癸烷在微细圆管内对流换热实验研究

本文对超临界压力下正癸烷在内径为0.95 mm和2 mm竖直微细圆管内对流换热进行了实验研究.入口压力p_(in)=3 MPa和入口雷诺数Re_(in)=4000时,分析了管径、变物性、浮升力和加速对对流换热的影响.结果表明:在所研究的工况范围内,对于0.95 mm内径圆管,浮升力和加速对换热的影响可忽略,对流换热主要受变物性的影响;而对于2mm内径圆管,在高热流密度时,浮升力对正癸烷的对流换热影响很大,向上流动时引起换热恶化,向下流动时引起换热强化,加速对流动换热的影响可忽略.     

分离结晶过程中熔体热毛细对流的转变

为了了解微重力条件下新型分离结晶生长过程中熔体热毛细对流的基本特征,利用有限差分法进行了数值模拟,熔体深径比A取1和2,自由界面无因次宽度B分别取0.05、0.075和0.1.当熔体上表面为自由表面时,得到了分离结晶Bridgman生长过程中熔体热毛细对流的流函数和温度分布.计算结果表明:当Ma数较小时,在上下两个自由表面的表面张力的驱动下,熔体内部产生了两个流动方向相反的流胞,流动为稳态流动,随着Ma数的增加,上下自由表面速度增大,温度分布的非线性增加;当Ma数超过某一临界值后,流动将转化为非稳态流动;与熔体上表面为固壁时相比,A=1时的临界Ma数减小,而A=2时的临界Ma数增大;流动失稳的物理机制是流速的变化和阻力的变化之间存在滞后.     

圆柱腔体内水凝固过程热对流实验研究

本文通过匹配折射率和标定技术,应用PIV测量了圆柱腔体内水在底部冷却过程中热对流的变化规律．实验发现了主流场的涡结构;另外在底部拐角处出现新的涡,并且不断向上推移;冷却后期整个液相区出现流动反转现象,靠近壁面处水向上流动,中心区域水向下流动。因为存在着复杂的热交换和密度变化,水在凝固过程中存在流动结构失稳,使得液相区内发生强烈的热对流．最后流动强度减弱,重新形成稳定结构。结合泰勒不稳定性分析,本文对这一过程给出了合理解释．     

稳定自然对流下的温度梯度液相外延

介绍了稳定自然对流下的温度梯度液相外延,稳定自然对流由加在溶液上的水平温差产生,用简化模式计算了该生长的生长速率,计算给出:该生长的生长速率与水平温差的平方根成比例;具体对Ga_0.85Al_0.15AS生长,在本工作给定的条件下,生长速率为稳态扩散理论预示生长速率的1124倍,在大部份生长面积内,外延层的厚度变化小于平均厚度的±10％,设计了稳定自然对流下的温度梯度液相外延装置,用该装置生长了厚Ga_1-xAl_xAs层,实 关键词：     

远场来流对过冷熔体中球状晶体生长的影响

利用渐近分析方法研究了远场来流引起的对流对过冷熔体中球状晶体的生长形态的影响.结果表明,由远场来流导致的对流使得正在生长的球状晶体的界面在向着来流的前部朝来流方向相反的方向生长, 并且提高了朝来流的相反方向的生长速度, 在背风方向衰减；正在衰减的球状晶体的界面在向着来流的前部加速衰减, 在背风方向减缓衰减. 关键词： 球状晶体 远场来流 对流 界画形态     

Preliminary study of two immiscible liquid layers subjected to a horizontal temperature gradient

Marangoni convection, driven by interfacial instability due to a surface tension gradient, presents a significant problem in the crystal growth process. To achieve better materials processing, it is necessary to suppress and control this convection, especially in crystal growth using Liquid Encapsulated Czochralski techniques in which the melt is encapsulated in an immiscible medium. Marangoni convection can occur at the liquid-liquid interface and at the gas-liquid free surface. Buoyancy driven convection can also affect and complicate the flow. The present report studied Marangoni convection in a two-liquid layer system in an open and enclosed cavity. Flow in the cavity was subjected to a horizontal temperature gradient. Interactive flow near the liquid-liquid interface was measured by the Particle Image Velocimetry (PIV) technique. The measured flow field is in good agreement with numerical predictions.     

Effect of gravity convection on interface morphology during solidification

An experimental apparatus consisting of a crystal growth room and a crystal growth observation system was developed for the study of the effect of the gravity convection perpendicular to the growth direction on the growth process by use of model alloy succinonitrile (SCN)-5wt%ethanol. It was found that the convection improves the stability of the interface and causes the downstream alternation of the cell growth direction because of the dual effect of the Stokes force and the gravity. The second dendrite arm facing the flow comes into being earlier than that at another side when the interface transforms cell to dendrite. Then the dendrite at the side facing the flow comes into being earlier. The second dendrite arm facing the flow grows faster and is more developed than that at another side. In addition, the primary dendrite arm spacing increases and the dendrite tip radius decreases under the gravity convection. Supported by the National Natural Science Foundation of China (Grant Nos. 50331040 and 60171043)     

微重力下液封对液桥内热毛细对流影响的研究

本文建立了具有液封的液桥（不相溶混的双层同轴液柱）内热毛细对流的物理模型和数学模型。采用涡量－流函数法对微重力条件下具有液封的液桥内热毛细对流进行了数值模拟，得到了双层液柱主流区的温度场和流场，证实了液封能够削弱液桥内热毛细对流，从而提高浮区晶体生长质量，并得到液封厚度对液桥内热毛细对流的影响规律。     

Magnetic field controlled floating-zone single crystal growth of intermetallic compounds

Radio-frequency (RF) floating zone single crystal growth is an important technique for the preparation of single bulk crystals. The advantage of the floating-zone method is the crucible-free growth of single crystals of reactive materials with high melting points. The strong heat diffusion on the surface, as well as the melt convection in the molten zone due to induction heating, often leads to an undesired solid-liquid interface geometry with a concave (towards the solid phase) outer rim. These concave parts aggravate the single crystal growth over the full cross-section. A two-phase stirrer was developed at IFW Dresden in order to avoid the problems connected with these concave parts. It acts as a magnetic field pump and changes the typical double vortex structure to a single roll structure, thus pushing hot melt into the regions where the concave parts may arise. The current in the secondary coil is induced by the primary coil, and the capacitor and the resistance of the secondary circuit are adjusted to get a stable 90 degree phase-shift between the coil currents. Single crystal growth of industrial relevant RuAl and TiAl intermetallic compounds was performed based on the material parameters and using the adjusted two-phase stirrer. Very recently, the magnetic system was applied to the crystal growth of biocompatible TiNb alloys and antiferromagnetic Heusler MnSi compounds.     

Effect of buoyancy-driven convection on steady state dendritic growth in a binary alloy

The effect of buoyancy-driven convection on the steady state dendritic growth in an undercooled binary alloy is studied. For the case of the moderate modified Grashof number, the uniformly valid asymptotic solution in the entire region of space is obtained by means of the matched asymptotic expansion method. The analytical results show that the buoyancy- driven convection has a significant effect on the needle-like interface of dendritic growth. Due to the buoyancy-driven convection, the needle-like interface shape of the crystal is changed. When the Peclet number that is not affected by the buoyant flow is less than a certain critical value, the interface shape of the dendrite becomes thinner as the Grashof number increases; when it is larger than the critical value, the interface shape becomes fatter as the Grashof number increases. In the undercooled binary alloy the morphology number plays an active role in the interface shape and leads to the buoyancy effect that is different from the situation for the pure melt. The smaller the morphology number is, the more significant change the interface shape has. As the Peclet number further increases, the effect of buoyancy on the interface diminishes eventually.     

多孔介质融化相变自然对流数值模拟

根据体积平均理论,建立多孔介质中融化相变过程自然对流模型。数值模拟了以石英砂颗粒为骨架、冰为填充相的二维融化过程。数值结果表明,在融化区上部发生较强烈的自然对流,融化界面与水平线倾角随融化时间越来越大。