Experimental Visualization Measurement Of Flow Field In Rod Bundle With Mixing Vane Grids

The spacer grids in the rod bundle fuel assembly of Pressurized Water Reactor can maintain the circumferential position of rods.The mixing vanes on grids will strengthen the mass and energy mixing between adjacent subchannels downstream the grids,thus enhancing the heat transfer efficiency between the fuel rods and the coolant,and improving the thermal-hydraulic performance of rod fuel assembly.Computational Fluid Dynamics(CFD)is becoming an important tool for the design of spacer grids and for the analysis of rod bundle flow field.However,experiment data of high spatial resolution and quality is needed to ensure the exactness of CFD computation results.To provide high spatial resolution data of flow field downstream the grid,two kinds of visual experimental methods——Laser Doppler Velocimetry(LDV)and Particle Image Velocimetry(PIV),are applied to investigate the axial and lateral flow field at 5 axial locations of 3 to 20hydraulic diameter downstream the grid in 5×5 rod bundle with two split-type mixing vane grids,respectively.Axial mean velocity and RMS velocity downstream the two grids with same dimension is obtained using LDV.Comparison shows the difference level is same with repeatability error as for the axial mean velocity and RMS velocity downstream the two grids,which indicates that upstream flow field induces little influence on the downstream flow field.Data from LDV measuring indicates that axial velocity peak happens in the center of different subchannels while axial velocity in rod gap achieves minimum value.Axial RMS velocity show that axial turbulent mixing in the subchannels center is greater than that in edge and corner subchannel,and axial turbulent intensity becomes weaker on the further cross section downstream the mixing vane grid.The rod array structure and the specific decaying feature of lateral flow make it much more difficult when measuring the lateral flow field.With the aid of optical technique of refractive index matching,PIV is used to get the whole flow field with 16 inner subchannels.In order to improve accuracy of PIV measuring,a best practical guideline based on the hypothesis of Gaussian distribution of instantaneous velocity comes into being when optimizing some key parameters,such as cross correlation algorithm,rule of the excluding of abnormal data,time interval between the pulse laser and sampling image number and so on.Lateral velocity vector field,lateral RMS velocity field,lateral turbulent intensity and axial voracity field at different cross section downstream the mixing vane grid are obtained in the PIV measurement based on the rules.Lateral velocity vector field shows there alternately exist clockwise flow and counterclockwise flow around the rod when the fluid flows through the mixing vane grids.At the same time,the mixing vanes induce strong lateral flow between subchannels while velocity in the center of subchannels is relatively low.Measured velocity fluctuation of velocity components U and V both achieves local maximum value in the vicinity of the subchannel center and achieves local minimum value in rod gap where lateral flow is strong enough.Vortices exits in pairs in the subchannel at the location of Z=3D_h downstream the grid because of velocity shear action,but they will quickly decay downstream the grid.A comprehensive uncertainty analysis of PIV measuring shows system error mainly influences the mean velocity while random error mainly influences the velocity fluctuation.