Numerical simulation of the transient behaviors in an open natural circulation system with a large scale

A code based on the homogeneous equilibrium model for two-phase flow was developed to simulate the transient behaviors of the open natural circulation system with a large tank as the heat sink. Detailed mechanism analyses with regard to different behaviors are conducted, and then some sensitivity studies of the systematic parameters are performed. The results show that the natural circulation system may experience several difference types of the operating periods. The response time of the system from the standing-by state to starting up is quite short and less than 200 s. In the single phase flow period, the upper tank has great influence on the trend of the flow rate evolution because of the stratification of the water in it. During the period of transition process from the single-phase to two-phase flow, flashing and flashing-induced oscillation may take place in the adiabatic section. With the increase of the temperature of fluid entering into the heat exchanger, the flow oscillation is weakened and then disappears gradually. When the temperature of fluid in the tank reaches the saturation temperature under atmospheric pressure and remains unchanged, the approximate stable two-phase flow and flashing are achieved. It can be seen from the sensitivity studies that increasing the heat transfer coefficient outside the heat transfer tube and the temperature in the containment can enhance the flow of the system both in single-phase and two-phase conditions. Increasing the height of the adiabatic section can only accelerate the single-phase flow rate, but has little influence on the two-phase flow and the flashing region during the long-term operation condition.