Oscillatory Thermal Convection of Liquid Metal and its Three-Dimensional Structure.

对液态金属等低Prandtl数(Pr)流体的自然对流进行了三维含时数值模拟。计算了宽高比(Ar)为2和4、宽比(WR)为0.5~4.2的差热侧壁和绝热壁矩形围护结构。研究结果如下：1.研究结果。在(Ar，Wr)=(2，1)，(2，2)和(4，1)的情况下，发生振荡的临界Grashof数(Gr^c)在很大程度上取决于Pr.2。流动结构以胞状结构和纵向涡旋(卷)为特征。随着WR的增加，Gr_c除在WR的较短范围内外，其余均呈下降趋势。Gr_c在那里发生突变。这归因于细胞模式的转变，或卷曲排列的变化。在Pr=0和0.015的情况下，Gr_c处的对流结构一般是单胞的。然而，在Pr=0.025的情况下，当Ar=2时，在Wr[小于或等于]1的范围内观察到振荡的两单元模式，或者当Ar=4时，在Wr[小于或等于]1.4时观察到振荡的两单元模式。对于Wr.5的其他范围，流动结构是单胞的。振荡对流对换热速率有显著影响。当Ar=4时，振荡对平均Nusselt数的影响约为10%。使用镓对Arz2、4和WR=0.5-4进行的实验结果如下：1.会发生可重现的振荡。由温度振荡确定的临界Grashof数与数值结果定性一致。在这两种长径比中，Gr_c都随着WR的增加而减小。Gr_c.3存在一定的滞后性。振荡的周期从大约10秒到1分钟不等。这段时间显示出围绕特定WR的跳跃。当Ar=4时，实验数据与模拟结果吻合较好。

Three-dimensional, time-dependent numerical simulation has been performed on natural convection of low Prandtl number (Pr) fluids such as liquid metals. Calculations are carried out for rectangular enclosures with differentially heated sidewalls and adiabatic walls of aspect ratios (Ar) 2 and 4, and width ratios (Wr) ranging 0.5 to 4.2. The results are as follows.1. In cases (Ar, Wr)=(2, 1), (2, 2) and (4, 1), the critical Grashof number (Gr^c) at which the onset of oscillation occurs largely depends on Pr.2. The flow structure is characterized by cellular pattern and longitudinal vortices (rolls). As Wr is increased, Gr_c decreases except in a short range of Wr. An abrupt change in Gr_c occurs there. This is attributed to transition in the cellular pattern, or to variation in the arrangement of rolls.3. In cases Pr=0 and 0.015, the convective structures at Gr_c are unicellular in general.4. In the case Pr=0.025, however, oscillatory two-cell patterns are observed in the range Wr【less than or equal】1 for Ar=2, or Wr【less than or equal】1.4 for Ar=4. The flow structures are unicellular for other range of Wr.5. The oscillatory convection has significant effect on heat transfer rate. For Ar=4, oscillations make an about 10 percent difference among the average Nusselt numbers.The experiments conducted using Gallium for Arz2, 4, and Wr=0.5-4 show following results.1. Reproducible oscillations occur. The critical Grashof numbers determined from temperature oscillations show qualitative agreement with numerical ones.2. In both aspect ratios, Gr_c decrease as Wr is increased. Some hystereses exist among Gr_c.3. The period of oscillation varies from about 10 seconds to one minute. The period shows a jump around a particular Wr. For Ar=4, experimental data are in good agreement with simulation results.