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液膜破裂的流体力学机理，开展了以下两方面的基础性研究工作。研究了热毛细对流对表面局部受热的水平薄水层破裂的影响。利用示踪法进行了可视化观测，并提出了分析模型。当膜层厚度大于2 mm时，观测和计算结果都表明，在膜层内产生了涡旋，表面轮廓发生了瞬时变化。另一方面，如果层厚远小于2 mm，液体就不能流入局部加热表面正下方的位置，导致该位置的层变薄。这一现象仅仅是通过计算而表现出来的，具有促进电影分解的作用。为模拟沸水堆燃料棒支承隔板，在水平和倾斜矩形截面管道内的气水分层流中设置平板型障碍物，实验研究了平板对气膜破裂的影响。结果表明：平板本身并不促进液膜破裂，但平板下方连接的凸起对液膜破裂有很大的影响。在气流速度为60-70m/S的情况下，由于水流的前缘向下冲刷，以及在倾斜管道中向上流动的情况下，由于重力的拒水作用，在投影前发生了膜破裂。在气速较低的情况下，它也发生在投影的下游。通过在凸起前面设置一个洞，推迟了干燥斑块的形成。

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.