Oscillatory Thermal Convection and its Three-Dimensional Structure in a Cylindrical Cavity Filled with Liquid Metal.

本文以提拉法生长直拉法晶体生长过程中的坩埚为模型，对圆柱腔内的热对流进行了数值和实验研究。腔体的侧壁和底部保持在均匀的高温。熔体表面以生长单晶的形式与棒体接触，固体/熔体界面保持在均匀的低温下。在低普朗特数(Pr)流体中出现的对流振荡会导致生长的晶体出现不希望看到的结构，称为条纹。数值计算了空腔的长径比Ar=2，半径比Rr=0.3和0.5，Pr=0.025，不同的Grashof数，a)熔体表面无热损失(绝热)且杆不旋转，b)无热损失且杆旋转，c)有热损失且杆旋转。研究结果如下：1.研究结果。当自然对流占主导地位时，由于杆体的旋转而产生的强迫对流，其流动结构由中心轴线附近为下降流、腔壁为上升流的单环滚流构成。环卷是旋转对称的，方位波数为3或4.3。随着旋转速度的增加，出现了方位波数为3的相速度的旋转流动。当Ar=2，RR=0.5时，用镓进行的实验结果如下：1。当自然对流占主导地位时，发生振荡的临界Grashof数随杆的旋转增加而减小。当强迫对流占主导地位时，杆的旋转一般表现出抑制振荡的趋势。

Thermal convection in a cylindrical cavity, as a model of a crucible in Czochralski crystal growth process, has been studied numerically and experimentally. The sidewall and bottom of the cavity are maintained at a uniform high temperature. The melt surface is in contact with a rod as a growing single crystal, and the solid/melt interface is maintained at a uniform low temperature.Convective oscillations, which appear in low Prandtl number (Pr) fluids, cause undesirable structure of growing crystals called as 'striations.' It is the aim of this study to clarify the convective structure.Numerical calculations are carried out for the aspect ratio of the cavity, Ar=2, the radius ratio, Rr=0.3 and 0.5, Pr=0.025, and various Grashof numbers under the conditions, a) without heat loss on the melt surface (adiabatic) and without rotation of the rod, b) without heat loss and with rotation of the rod, and c) with heat loss and with rotation of the rod. The results are as follows.1. When the natural convection dominates in comparison with the forced convection due to rotation of the rod, the flow structure consists of a single toroidal roll with descending flow in the vicinity of the central axis and ascending flow along the cavity wall.2. The toroidal roll is rotationally symmetric with an azimuthal wavenumber of 3 or 4.3. As the rotation rate is increased, a rotating flow appears with phase speed of azimuthal wavenumber of 3.The experiments conducted using Gallium for Ar=2 and Rr=0.5 shows the following results.1. When the natural convection dominates, critical Grashof number, at which oscillation occurs, decreases with increasing rotation of the rod.2. When the forced convection dominates, the rotation of the rod generally shows a tendency to suppress oscillations.