Hydrodynamic Instability of Downward Two-Phase Flow

在轻水堆或先进热反应堆（ATR）的冷管段假设破裂时，反应堆堆芯可能会出现向下的两相流。在这种情况下，ECC水被设计成从其顶部注入，在堆芯中向下流动;其流速受到CCFL -逆流流动限制的限制。因此，本项目的目的是研究向下流动的特性，特别是关注其流动不稳定性;有些可能是由CCFL引起的，导致加热棒干燥。首先，实验装置由两个加热通道组成，每个加热通道由第一提升管、下降管和第二提升管组成。从这个实验中，观察到三种类型的不稳定性：2-nd-riser-dominated动态不稳定性，偏移型不稳定性，导致在一个通道中的阻塞，和另一种类型。第二个实验是用四个加热降液管进行的，观察到两种不稳定类型。对于低流速，蒸汽空隙逆着下降的液体流向上，在开始干燥的顶部积聚。对于高流速，溢流导致底部干涸。对这些现象进行了分析，结果与实验数据吻合较好。

In postulated breaks in cold legs for LWR's or the advanced thermal reactor (ATR), downward two-phase flow may arise in the reactor core. In this case, ECC water is designed to be injected from its top, to flow downward in the core; its flow rate is limited by the CCFL - counter current flow limit. Thus, this project aims to investigate into the characteristics of downward flow especially focusing on its flow instabilities; some might be caused by the CCFL, resulting in the dryout of heating rods.In this study, the follwing two types of experiments are carried out. Firstly, the experimental apparatus is made of the two heater channels, each consists of a 1-st riser, a downcomer and a 2-nd riser. From this experiment, three types of instabilities are observed: a 2-nd-riser-governed dynamic instability, an excursion type instability which leads to a blockage in one channel, and another one type. The second experiments are carried out with four heated downcomers, and two instability types are observed. For low flow rates, steam void goes upward against falling liquid flow, being accumulated at the top where the dryout is initiated. For high flow rates, the flooding causes the dryout at the bottom. Analyses are performed for these phenomena, which results show good agreement with the experimental data.