Numerical Simulation Study Of Two-Phase Flow For External Reactor Vessel Cooling

In-vessel retention(IVR)utilizing external reactor vessel cooling(ERVC)is a scheme for severe accident management in pressurized water reactor(PWR)which helps to retain the core melt within the reactor pressure vessel(RPV).In IVR-ERVC technology,the lower head of reactor pressure vessel is flooded with water and thus obtained external reactor vessel cooling is then used to retain the structure of reactor pressure vessel.Boiling heat transfer is the heat transfer mechanism in this case that is strongly dependent on critical heat flux.As long as the local heat flux is less than the critical heat flux,the heat transfer will be in nucleate boiling regime and integrity of RPV can be guaranteed.Therefore,CHF puts a limit on the coolability of ERVC.So,the effectiveness of IVR by external reactor vessel cooling strongly depends on the critical heat flux(CHF).The flow area of ERVC is very important for the development of natural circulation flow,for enhancing the CHF and thus improving the IVR technology.In this research,a numerical simulation is carried out on an experimental facility-ULPU-2400 to investigate various flow parameters such as velocity distribution,temperature distribution and pressure losses.The simulation is performed on a CFD code Fluent using Eulerian approach coupled with a user-defined function(UDF).The effect of different gap sizes on the flow and heat transfer performance are discussed.Three gap sizes are studied here,first two having equidistant baffle at 76 mm and 152 mm respectively from heating wall while the third one has varying gap size;with 76 mm at the lowest point of baffle and 152 mm at the highest point of baffle.The velocity distribution,void fraction distribution and heating wall temperature distribution are numerically investigated.The simulation results indicate that the velocity and void fraction for the two-phase flow increase while flowing upward from inlet to outlet.The wall temperature increases as the gap sizes increases and the best heat transfer rate is found in the smallest gap size.The velocity in the smallest gap size is the largest among all three available gap sizes.The outlet velocity profile and the velocity contours indicate that the velocity near the heating wall is higher than the velocity near the adiabatic wall.There exists a pressure gradient from the heating wall to the adiabatic wall because of higher velocity near the heating wall.For some cases.the temperature distributions at specific points and the pressure losses are also compared with the experimental study to validate the simulation model and they are found in good agreement.