Study of the Thermal and Fluid Dynamical Effects on the Heat Removal in the Vicinity of Spacers for Supporting a Fuel Rod in a Nuclear Reactor

为了弄清沸水堆燃料棒上液膜破裂的流体动力学机理，本文进行了以下两方面的基础性研究.本文研究了表面局部加热的水平薄水层的热毛细对流对破裂的影响。我们用示踪法进行了直观观察，并提出了一个分析模型。当层流厚度大于2 mm时，实验和计算结果都表明层流中产生了涡，表面形貌发生了瞬态变化。另一方面，如果层厚度远小于2 mm，则液体不能流入局部加热表面正下方的位置，导致该位置处的层变薄。这一现象仅通过计算得到证实，具有促进膜破裂的作用.为了模拟沸水堆中支撑燃料棒的隔板，在水平和倾斜矩形截面管道中的空气-水分层流中设置了平板型障碍物，实验研究了平板对气膜破裂的影响。结果如下：平板本身并不促进液膜破裂，但平板下侧连接的凸起对其有很强的影响。在气流速度为60 - 70 m/s的情况下，由于前缘向下，在投影前方发生薄膜击穿，这是由于水流的冲刷以及在倾斜管道中向上流动的情况下重力对水的排斥。在较低的气体速度下，它也发生在投影的下游。通过在突出物前面设置孔，延迟了干斑形成的发生。

In order to make clear the fluid dynamical mechanisms causing the liquid film breakdown on the fuel rod in BWR, the following two kinds of fundamental investigation were made.1. The effect of the thermocapillary convection on the breakdown of a horizontal thin water layer, with its surface locally heated, was examined. We made a visual observation by using the tracer method and proposed an analytical model. In the case that the thickness of the layer is larger than 2 mm, both the observed and calculated results showed that vortices are induced in the layer and the surface profile changes transiently. On the other hand if the layer thickness is much less than 2 mm, the liquid cannot flow into the position just below the locally heated surface, causing the thinning of the layer at that position. This phenomenon, which was shown by calculation only, has the effect of promoting the film breakdown.2. To simulate a spacer supporting a fuel rod in BWR, a flat-plate-type obstacle was set in an air-water stratified flow in the horizontal and inclined ducts with rectangular cross section and the effects of the flat plate on the film breakdown was experimentally investigated. The results are as follows : The plate itself does not promote the liquid film breakdown but the projection connected on the lower side of the plate has a strong effect on it. In the case of the gas flow velocity of 60 - 70 m/s, film breakdown occurred in front of the projection due to the leading edge down-wash of the stream and due also to the rejection of water by gravitational force in the case of the upward flow in the inclined duct. It also occurred just downstream of the projection in the case of the lower gas velocity. By setting a hole in front of the projection the occurrence of the dry patch formation was delayed.