Research On Natural Convection And Thermal Stratification Of In-containment Refueling Water Storage Tank

Passive Residual Heat Removal Heat Exchanger(PRHR HX)and In Containment Refueling Water Storage Tank(IRWST)are responsible for transferring the residual heat to the outside environment safely and reliably,preventing the core from melting dpwn,so that the safety of the reactor can be ensured.The residual heat will be transferred through the C-shape heat exchanger to the fluid in the IRWST.The flow distribution and thermal stratification have a significant impact on the natural convection due to the density difference and buoyancy in IRWST.In this paper,taking the CAP1400 reactor design as the prototype,Particle Image Velocimetry(PIV)is employed to study the natural convection characteristics.Experiment results show that:at the primary period,the ascending flow generated at the lower-horizontal section of C-shape tube can reach the tank level,and then moves to the unheated area;the cold fluid flows down the inner wall of the tank,so that a circulation can be achieved.As the heating process is continuing,the height that the ascending flow can reach becomes lower and lower,finally it stops at the middle of C-tube,and the natural conviction is mainly in the lower area of C-tube.At the height of lower-horizontal section of C-tube,the fluid flows mainly in the horizontal plane,causing the fluid temperature at the bottom of the tank to remain at the initial temperature.There is a significant thermal stratification phenomenon at this height.Thermal stratification hinders the flow in the vertical direction,seriously affecting the convective heat transfer.On the other hand,fluid convection in the horizontal plane exacerbates the thermal stratification phenomenon.In this paper,the thermal stratification degree of IRWST was evaluated by thermal stratification number Str.Thermal stratification hinders the flow of fluid in vertical direction and seriously affects convection heat transfer.The circulation of fluid in horizontal direction aggravates thermal stratification.According to the internal temperature stratification of IRWST,the water tank can be divided into three parts along the vertical direction.The upper thermal fluid accumulation is called high temperature zone.The upwelling flow in the middle is strongly disturbed and the temperature gradient is uniform.Weak lower natural convection,horizontal section under the heat tube bundle area,heat transfer fluid can only rely on thermal conductivity and thermal fluid,parts and even temperature remains as the initial temperature,referred to as the "dead zone" by convection.In addition,the variable heating power can be regarded as a quasi-static process between two constant heating power when studying the changing trend of flow field and thermal stratification.