It is necessary to evaluate the temperature of a valve when a specification of a valve is decided in a branch pipe connected to a main loop with a constant flow. The evaluation methods have been established in the case of the horizontal branch pipe below the main loop, and the method is based on the stable thermal stratification. However, a thermal stratification behavior is unstable in the horizontal branch pipe above the main loop because the cold fluid due to a heat removal in the branch pipe is higher than the hot fluid carried into the branch pipe by a cavity flow. The natural circulation is caused by the temperature and height difference between the main loop and the branch pipe. Since the interaction between the cavity flow and the natural circulation affects the natural circulation, it is not clear whether the developed method is applied to the phenomenon or not. Therefore, the experiments were carried out in order to understand the effect of the cavity flow on the natural circulation. From the results, the behavior of the natural circulation depends on the intersity of the cavity flow. In addition, the applicability of the CFD to the phenomenon under the cavity flow was verified only for the weak cavity flow such as 0.07 m/s of the average fluid velocity in the main loop.
