Numerical Study Of Two Phase Flow And Heat Transfer In PWR Rod Bundle Channel

In order to improve the flow and heat transfer of coolant and improve the thermal and hydraulic performance,small components such as mixing vanes,springs and dimples are usually installed on the spacer grid.However,the existence of the spacer grid will change the original flow direction of the fluid and narrow the channel area,resulting in pressure loss and reducing the thermal efficiency of the reactor.In order to improve the core outlet temperature and thermal efficiency,a small amount of subcooled boiling are also allowed in the hotter channels of PWR.In addition,in case of LOCA,the coolant flow is greatly reduced,which will lead to local heat transfer deterioration and boiling critical phenomenon,which has a serious impact on reactor safety.Therefore,subcooled boiling and boiling critical phenomena in rod bundle channels are also worthy of further study.Compared with the experimental research,the numerical model by CFD method has the advantages of lower cost,shorter time and easier expansion.With the continuous improvement of the reliability of CFD method,CFD method can be deeply applied to the research of reactor thermal and hydraulic design and operation,and also provide a theoretical basis for CFD method to replace some experimental processes in the future.Firstly,based on the Euler-Euler two fluid model,the geometric modeling and CFD reliability verification of the rod bundle channel subchannel provided in the PSBT benchmark are carried out.According to the published experimental data of subcooled boiling in 3×3 rod bundle channel compare the applicability of two-phase flow turbulence model in rod bundle channel.Then,a 5×5 rod bundle channel model with mixing vanes spacer grid is established by Solid Works.For the geometric model of rod bundle channel,the fluid domain is meshed by means of mixed meshes in ICEM CFD,and the most appropriate number of meshes is selected by adjusting the global size factor.Finally,the subcooled boiling and boiling critical in the rod bundle channel are numerically simulated by ANSYS Fluent.Under the condition of subcooled boiling,different bending angles of the mixing vanes are set,and the performance of the mixing vanes is evaluated by analyzing the upstream and downstream flow field and temperature field of the spacer grid.Under the condition of appropriately increasing the void fraction,the boiling critical phenomenon in the rod bundle is simulated by CFD method,and the critical heat flux is predicted by boiling curve and critical void fraction.The effects of different bending angles of mixing vanes on the critical heat flux at higher void fraction are compared.The numerical simulation results show that the reliability of the calculated values obtained in the subchannel by CFD method is high,and CFD method can better simulate the two-phase flow in the rod bundle channel;RNG-k-ε turbulent model considering the effects of turbulent Reynolds number and swirl,this model is more suitable for the rod bundle channel in this paper;through the comparative analysis of the thermal and hydraulic parameters of the upstream and downstream of the spacer grid,it is considered that the performance of the mixing vanes are better when the bending angle are 23° and 28°,and it is found that the action range of this spacer grid to the upstream is about 3Dh and to the downstream is about 10Dh-15Dh;the critical heat flux is predicted by boiling curve and critical void fraction.It is found that the predicted value obtained by critical void fraction method is smaller than that obtained by boiling curve method.By comparing the critical heat flux when the bending angles of the mixing vanes are 23° and 28°,it is found that the bending angle of 28° can effectively improve the critical heat flux and has little effect on the pressure drop.Therefore,this paper considers that the optimal bending angle is 28°.