A study on rollover of two density-stratified layrs of liquids

本报告中提到的翻滚是两层液体的快速混合，这两层液体最初由于密度差而在容器中稳定分层。热侵入通过容器壁进入液体层引起浮力对流在两个液体层。每一液体层内部和两层之间随后的传热传质，以及自由上表面的蒸发，逐渐减小了两种液体之间的密度差。最后，两层的稳定性丧失，导致两种液体快速混合。对浮力对流开始到侧翻开始的顺序过程进行了实验和数值研究。在实验中，采用阴影技术对两液层界面的轮廓、运动和流态进行了可视化。同时测量了温度和密度分布的随时间变化。通过测量激光照射下示踪粒子的位移，得到了容器垂直壁面附近的流速分布。实验结果阐明了两种液体完全混合的过程。当密度差变小时，下层液体沿壁面上升的边界层流动深入上层，最终到达自由表面。从这一刻起，整个流体开始迅速混合。随着混合的进行，两种液体之间的界限下降，变得模糊，最后消失了。用大型计算机进行的数值模拟也显示了完全相同的结果。

The rollover referred to in this report is a rapid mixing of two layrs of liquids which initially were stratified stably in a vessel due to a density difference. Heat invasion through the vessel wall into the liquid layrs induces buoyancy convection in both of the liquid layrs. Subsequent heat and mass transfer inside each of the liquid layrs and between the two layrs, and evaporation at the free upper surface gradually reduces the density difference between the two liquids. Finally the stability of the two layrs is lost, leading to a rapid mixing of the both liquids.The sequential processes from the start of buoyancy convection to the onset of rollover were studied experimentally and numerically. In the experiment, the profile and movement of the interface between the two liquid layrs and the flow patterns were visualized with the shadowgraph technique. The timewise changes in the distributions of temperature and density were measured at the same time. The distribution of flow velocity near the vertical wall of the vessel was obtained by measuring the displacement of tracer particles illuminated with a laser light. The results of experiment elucidated the process leading to the complete mixing of the two liquids. When the density difference became small, the boundary layr flow in the lower liquid rising along the wall penetrated deep into the upper layr and finally reached the free surface. From this moment on the rapid mixing of the entire fluid took place. The boundary between the two liquids descended as the mixing proceeded, became fuzzy, and vanished in the end.Numerical simulation using a large-scale computer has shown quite the same results.