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。浮区晶体生长是微重力条件下生产高质量半导体和氧化物的一种无容器生长方法。在该方法中，随着受热区和冷棒之间温差的增大或马兰戈尼数Ma的增大，向三维振荡热毛细对流转变。众所周知，即使在空间环境中，振荡对流也会引起晶体结构的有害缺陷。为了建立高质量的材料加工工艺，对流的控制非常重要。作为基础研究，本文采用半区模型（HZ）对热毛细对流进行了简化。对HZ液桥内的过渡和连续流动结构进行了大量的研究。只有少数工作涉及对流的控制。本研究的目的是在大范围内抑制HZ液桥中的三维振荡热毛细对流。控制是通过电加热器局部改变自由表面温度来实现的。利用局部表面温度信号，通过简单的对消算法计算控制输出。为了制定控制方案，采用三维粒子跟踪测速仪对振荡对流进行了测量。实验表明，在较宽的毫安范围内，振荡得到了明显的衰减。在过渡临界点附近，振荡对流被完全抑制。该控制方案实现了温度振荡的显著降低，也远远超出了临界点。

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.