Numerical Simulation On The Flow Heat Transfer Characteristics Of Mist Flow In The Complex Fuel Assembly

After the Three Mile Island accident,the Chernobyl accident and the Fukushima nuclear power accident,the importance of the safety of nuclear power is more concerned by the designers,the operators and the public.When the nuclear reactor breaks down and causes the reactor core to be uncovered,the emergency core cooling system inject water into the reactor during the cooling process,a large amount of water droplets will be filled in the upper part of the reactor core,which provides the precursory cooling for the hottest area of the reactor core.In this paper,the numerical simulation is carried out to study the the heat transfer process of mist flow in the reactor.In the numerical study,the Mixture model and the RNG k-? model are selected to carry out the study.The DO model is used to take into account the influence of high temperature radiation heat transfer.In particular,a theoretical analysis is carried out on the mechanism of phase change heat transfer in the main superheated steam region and the random collision with the heating wall.By introducing the boiling time constant and the evaporation area factor of the wall in the droplet flow,the mechanism model of the droplet evaporation phase change model is established.The droplet evaporation phase change model is embedded into Fluent through UDF code to calculate the flow and heat transfer of droplet flow in the complex fuel assembly.Meanwhile,in the numerical simulation,the physical parameters of vapor such as the density,the heat capacity,the thermal conductivity and the viscosity that changed with temperature are fully considered through the UDF code.In this study,the numerical calculution is verified by the experiments results of mist flow in the circular tubeFor the complex fuel assemblies,the fuel assembly with wrapped wire and the fuel assembly with spacer grid are studied in the paper.This paper analyze the difference of heat transfer in mist flow between the the fuel assembly with wrapped wire and the fuel assembly without wrapped wire,moreover the influence of the wrapped wire on the heat transfer of mist flow is studied.The results show that the inner flow temperature of the fuel assembly with wrapped wire is lower than that of without wrapped wire,and the flow velocity is higher than that in the fuel assembly without wrapped wire.At the same time,the fuel assembly with wrapped wire has a stronger heat transfer capacity.In this paper,the heat transfer characteristics of the fuel assembly with spacer grid under mist flow condition are also studied.Due to the lateral mixing effect of the mixing wing,the lateral velocity increases and the heat transfer on the cross section are strengthened by the secondary flow.As the fluid flows downstream along the channel,the mixing effect is gradually decay.The heat transfer characteristics of mist flow in the complex fuel assemblies are studied under different working conditions.The different operation pressures and different vapor fluxes are concerned.According to the simulation result,the pressure distribution,temperature distribution,velocity distribution and phase distribution in the complex fuel assembly are analyzed in detail.