Experimental And Numerical Investigation Of Thermal Stratification Within The Water Tank

IVR-ERVC (In-Vessel Retention-External Reactor Vessel Cooling) strategy is proposed to be adopted as one of the severe accident mitigation measures of China's advanced third-generation PWR technology. But there are still a lot of uncertainties related to IVR-ERVC system performance due mainly to the lack of operational and experimental data. It is necessary to evaluate ERVC system characteristic such as cooling mode, flow and heat transfer characteristic of two-phase flow natural circulation. Passive system is more sensitive to parameters, so it's necessary to enhance the reliability of the ERVC system with studying local thermal stratification effects of the system on the natural circulation. For this purpose, this paper analyses the two-phase flow natural circulation and local thermal stratification effects in IVR-ERVC, based on experiments, simulation tools such as RELAP5 code and FLUENT code.Firstly, a REPEC experiment setup is simply introduced. Using one-dimensional experiment setup with the height direction of 1:1 ratio, heating experiments simulate the process of ERVC by externally heated. A detailed study on flow and heat transfer of ERVC and dynamic characteristics of this system is carried out through changing the size and distribution of heat flux, the structure and size of external flow and the inlet sub-cooling.Secondly, a dynamic numerical simulation is carried out for this experimental loop using RELAP5 code. Applying RELAP5/MOD3 system analysis code, flow and heat transfer characteristic of two-phase flow natural circulation of IVR-ERVC system has been analyzed in case of heating condition. The RELAP5 results generally correspond with the experimental cases. With the increase of heat flux, inlet area and outlet area, the water circulation mass flow rate will increase. But the increase of water circulation mass flow rate is limited when the degree of sub-cooling increase. The system appears two-phase flow instability clearly on the condition of nearly saturated boiling.Finally, the experimental and numerical study of local thermal stratification has been conducted. It is a complicated liquid process in the up-tank. The sensitivity analysis which includes the number of gridding, the turbulence models, the time steps, the inlet geometry and the initial velocity has been conducted. The temperature results by computing and experimental means are compared to get that the temperature difference between them is 5℃,and the computing model and results can be accepted. This article simulates the formation of thermal stratification by using FLUENT 6 code. The code simulations are focused on the effect of initial tank temperature and injection flow rate on thermal stratification process. The distribution of temperature field was depicted. The appearance and cause of thermal stratification phenomenon were analyzed. At the same time, the dimensionless equations for three-dimensional, transient turbulence flow are established and the thermal stratification inside the tank is theoretically analyzed using similarity criterion.The numerical tools and analysis method in this thesis can be applied to future activities in the numerical simulation of ERVC experiment loop for China's advanced third-generation PWR technology. The results achieved provide valuable information for future R&D.