Studies on Boiling Two-phase Flow and Burnout Phenomena in Narrow Channel Simulated Subassembly of High Conversion Reactor

具有高密度六方晶格的高转换轻水堆由于其铀燃料利用率高和混合氧化物燃料的有效燃烧而有望成为下一代反应堆之一。反应堆的重要考虑因素之一可能是在正常和非正常条件下从堆芯散热，这是由于堆芯中的水存量低，冷却剂通道又长又窄。当加热通道的水力直径减小时，由于每单位流体体积的功率输入速率增加，流体流动强烈地受功率输入的影响。本文采用氟里昂-113热模拟实验装置，在0.3MPa压力下，对（a）带加热棒的环形窄通道和（B）带螺旋丝的七棒窄通道内的两相流动沸腾传热、临界热流密度和压降进行了研究，主要结果如下：（1）窄通道内流型的转变，既有气泡向泡沫的转变，也有泡沫向环状流的转变，导致窄通道内的品质较低。2)窄缝通道内的沸腾换热系数比普通通道内的沸腾换热系数大，这是由于沸腾引起的流速变化以及螺旋丝强化湍流混合和横流的作用所致。3)窄通道中的临界热通量比Katto从正常尺寸通道中的数据获得的相关性减少约20%。而在七子组件通道中，由于横流的增强，临界通量变为与关联式相同的值。4)采用两相流漂移通量模型和新引入的修正摩擦损失系数和修正过冷区质量模型对总压降进行了较好的估算。

High conversion LWRs with a high density hexagonal lattice are expected as one of next generation reactors because of their high utilization of uranium fuel and effective burning of MOX fuel. One of important considerations for the reactors may be heat removal from the core in both normal and off-normal conditions due to low water inventory in the core with long narrow coolant channels. As hydraulic diameter of the heated channels decrease, the fluid flow is strongly influenced by the power input due to increase of power input rate per unit fluid volume. In this study,boiling heat transfer,critical heat flux and pressure drop for two-phase flow both (a)in a narrow annula channel with a heated rod and (b)in a narrow subassembly channel with seven rods with spiral wires were investigated by using Freon-113 thermal loop at 0.3 MPa.The main results are : 1)Transitions of flow patterns both from the bubble to the froth and from the froth to the annular cause in lower quality regions in the narrow channels compared with these in the normal cahnnel. 2)Boiling heat transfer coefficients in the narrow channels become larger than those in the normal channel,because of flow velocity change induced by the boiling and enhancement of turbulent mixint and cross flow by spiral wires. 3)Critical heat fluxes in the narrow channel decrease about 20 percent less that Katto's correlation obtained from the data in the normal size channel. However,in the seven subassembly channel,the critical flux become the same values as the correlation,because of the enhancement by the cross flow. 4)The total pressure drop is well estimated by a drift flux model of two-phase flow with modified friction loss multiplier and modified quality model in the subcooled flow regin,which are newly introduced.