Feedback Control of Nonlinear Thermocapillary Convection in a Half-zone Liquid Bridge

在微重力条件下，浮力驱动的对流几乎可以忽略不计。浮区法晶体生长是一种在微重力条件下无容器生长高质量半导体和氧化物的方法。在这种方法中，过渡到三维振荡热毛细对流发生与加热区域和冷棒之间的温差增加或与增加Marangoni数Ma。众所周知，即使在空间环境中，振荡对流也会导致晶体结构中的有害缺陷。对流的控制对于建立高质量的材料加工工艺具有重要意义，作为基础研究，本文采用半区模型（HZ）对FZ中的热毛细对流进行了简化。本文对HZ液桥中的过渡流和连续流结构进行了大量的研究。只有少数的工作已经关注的对流的控制。本研究的目的是抑制三维振荡热毛细对流在HZ液桥在一个广泛的Ma范围。控制是通过修改自由表面温度局部与电加热器。利用局部表面温度信号，通过简单抵消算法计算控制输出。为了开发控制方案，采用三维粒子跟踪测速仪对振荡对流进行了测量，实验结果表明，在很宽的Ma范围内，振荡对流都有明显的衰减。振荡对流的完全抑制实现了接近临界点的转变。该控制方案实现了显着减少的温度振荡，也远远超过了临界点。

In the microgravity condition, a buoyancy-driven convection becomes almost negligible. The floating-zone(FZ) crystal growth is a containerless method to produce high-quality semiconductors and oxides under the microgravity. In this method, a transition to three-dimensional oscillatory thermocapillary convection takes place with increasing temperature difference between the heated area and cold rods or with increasing Marangoni number Ma. It is widely known that the oscillatory convection causes detrimental defects in the crystal structure even in the space environment. Control of the convection is of great importance in order to establish high-quality material processing techniques.As the fundamental research, a half-zone model(HZ) has been employed to simplify the thermocapillary convection in FZ. A large number of research were conducted with respect to the transition and successive flow structures in the HZ liquid bridge. Only a few works have been concerned with a control of the convection.The present study aims to suppress the three-dimensional oscillatory thermocapillary convection in the HZ liquid bridge in a wide range of Ma. The control was applied by modifying the free surface temperature locally with the electric heaters. The control output was calculated by the simple cancellation algorithm using local surface temperature signals. In order to develop control schemes, the oscillatory convection was measured by the three-dimensional particle tracking velocimetry.Through the experiments, significant attenuation of the oscillation was obtained for a wide range of Ma. The complete suppression of the oscillatory convection was achieved close to the critical point of the transition. The control scheme achieved a significant reduction of the temperature oscillation, also far beyond the critical point.