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

本报告中所指的翻转是指两层液体的快速混合，最初由于密度差异而稳定地分层在容器中。通过管壁向液层的热入侵在两个液层中都会引起浮力对流。液层内部和两层之间的后续传热传质，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。实验和数值模拟研究了两层流体从浮力对流开始到侧滚开始的过程。在实验中，用阴影技术显示了两液层间界面的轮廓和运动以及流型。同时测量了温度和密度分布的时间变化。通过测量示踪粒子在激光照射下的位移，得到了容器竖壁附近的流速分布。实验结果阐明了导致两种液体完全混合的过程。当密度差变小时，沿壁面上升的下部液体中的边界层流动深入上层，最终到达自由面。从这一刻起，整个流体的快速混合发生了。两种液体之间的边界随着混合的进行而下降，变得模糊，最后消失。使用大型计算机进行的数值模拟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.