"A Study on Instability Phenomena in Liquid Metal Natural Convection under a Magnetic Field."

在磁约束聚变核反应堆包层材料中使用液态金属，在包层设计中具有很高的换热特性和氚增殖能力。然而，由于液态金属的高导电性，包层内的MHD压降很高，当包层由单相液态金属流冷却时，需要极高的泵功率。为了避免高泵浦功率，人们提出了许多关于包层的概念设计。磁场作用下液态金属的热工水力有许多尚未解决的问题，特别是由于液态金属对光线是不透明的，其流动特性一直没有得到明确的认识。一般认为，外加磁场会抑制换热，而流动则受到洛伦兹力的抑制。但也有少数文献报道，在自然环境和…环境中，温度波动较大，传热增强。外加磁场产生更多的强迫对流。由于流动显示的困难，这些现象尚未被阐明。本研究的目的是通过中子照相技术显示流动，以阐明通常尚未被很好理解的换热强化机理。中子照相技术适合于用示踪剂和染料注入的方法来显示液态金属流动。本文对自然对流和磁场作用下的液态金属流动进行了可视化，得到了以下结果：1)示踪剂和染料注入方法显示了铅铋共晶的流动；2)显示了水平磁场作用下容器内铅铋气两相流动的可视化，并进行了空泡率和流场的测量。高达2500高斯的外加磁场对空泡率分布影响不大，但对流动有抑制作用。3)在图像处理中，当示踪剂尺寸接近像元尺寸时，可以通过示踪剂方法获得容器中铅铋中的流动矢量场。较少

A use of liquid metals as the blanket material in a magnetic confined fusion nuclear reactor has many advantages on the high heat transfer characteristics and the tritium breeding in the blanket designs. However, the MHD pressure drop in the blanket is high due to the high electrical conductivity of the liquid metals and extremely high pump power is required when the blanket is cooled by a sigle phase liquid metal flow. To avoid the high pump power, many conceptual designs on the blanket have been proposed. There are many problems which have not been solved on thermal hydraulcs of liquid metals under magnetic fields, especially flow characteristics have not been clarified since liquid metals are opaque to optical rays. It has been generally reported that the heat transfer is suppressed by the applied magnetic fields as the flow is suppressed by Lorents force. While, a few papers reported that large temperature fluctuation was occured and the heat transfer was augmentated in natural and … More forced convections by the applied magnetic field. These phenomena have not been clarified yet since the flow visualization is difficult.This study was started to clarify the heat transfer augmentation mechanism, which has not been well understood generally, by visualizing the flow using neutron radiography. Neutron radiography is suitable to visualize the liquid metal flows by a tracer and a dye-injection method.In this report, the liquid metal flow visualizations in natural convection and under a magnetic field were carried out and following results were obtained.1) It was shown that the flow of lead-bismuth eutectics was visualized by a tracer and a dye-injection method.2) Lead-bismuth-gas two-phase flow in a container under a horizontal magnetic field was visualized and the void fraction and the flow field measurements were carried out. The applied magnetic field up to 2500 gauss affected the void fraction distribution a little but suppressed the flow. It was shown that flow phenomena in a liquid metal in the presence of a magnetic field was visualized bu nentron radiography.3) Flow vector fields in lead-bismuth in a container could be obtained by a tracer method using a correlation method when the tracer sizes was closed to the image element size in the image processing. Less