Numerical Simulation Investigation Of Flow And Heat Transfer In Fuel Assemblies With Mixing Grid

The spacer grid not only supports the fuel rods,but also improves the heat transfer efficiency of the fuel rod.Therefore,it's with great significance to carry out the numerical simulation of the grid.Based on many literatures,the 5×5 fuel bundle channel model with spacer grid is established.The characteristics of flowing and heat transfer in flow flied are simulated by using ANSYS CFX 16.0 software.According to the comparison of simulation results and experimental data,the applicability of three turbulence models to this research is discussed.Secondly,the flowing and heat transfer characteristics in the channel of the fuel assembly are calculated under the actual core operating conditions.Three kinds of grid models with different structures are established to study the effects of mixing vane,spring and rigid on the flow field.At the same time,the two-phase flow simulation in the bundle is carried out by using the MUSIG model which considering both of the fracture and coalescence about bubble.In addition,the influence of the grid on the bubble average diameter distribution and vapor volume fraction distribution is discussed.The results show that when the fluid flows through the grid,the mixing vane can induce the secondary flow and the vortex,which will continue until the distance downstream of the grid,and improve the heat transfer efficiency.The structure of spring,rigid and so on can also cause the cross flow to improve the heat transfer performance.In the calculation of vapor-liquid two-phase flow,the distribution trend of vapor phase is very similar to that of cross flow,which decreases rapidly within a short distance at the exit of the grid,and then rises gradually along the downstream.In the flow channel with mixing vane grid,the vapor accumulation level is much lower than that one without the mixing vane.This phenomenon also occurs on the surface of three typical fuel rods,which shows that the disturbance and cross flow caused by mixing vanes have a great influence on the redistribution of vapor phase and can flatten the flow field enthalpy rise in the field to suppress boiling critical.According to the results of two-phase transient calculation,every structural component of grid has the function of dividing and tearing bubbles.Bubble retention occurs on the back flow surface of the mixing vane when bubble flow passes through,but the vapor volume fraction will gradually decrease along the vane extension direction under the influence of vortex flow.After the bubble group passes through the mixing vane,the average diameter of bubbles in the channel and the volume fraction of vapor phase tend to be smooth gradually.This shows that the turbulences which induced by the mixing vane can inhibit the coalescence of bubbles,promote the separation of the bubbles into smaller ones,prevent the coalescence phenomenon of bubbles in the narrow channels and prevent deterioration of heat transfer.The research results of this paper provide the relevant data basis for further development of spacer grid structure optimization,and have certain reference significance.