High-resolution Measurement For Flow Characteristics In Rod Bundle Fuel Assembly And Sub-channel Mixing Model Development

In the fuel assembly of pressurized water reactor(PWR),mixing vanes on spacer grid enhance heat transfer from fuel rods to coolant and mass and energy exchange between sub-channels,increasing the pressure drop in fuel assembly.So the secondary flow intensity,the turbulent intensity in subchannels,cross flow and turbulent mixing between sub-channels are important parameters to quantify the performance of spacer grid and mixing vanes.In sub-channel analysis,it is important and necessary to precisely predict the cross-flow mixing,turbulent mixing and pressure drop.However,the computational fluid dynamics(CFD)simulation cannot precisely predict the flow and sub-channel mixing downstream the spacer grid.And the rod bundle experiments cannot give high-precision and high-resolution experimental data of lateral flow and sub-channel pressure drop for now.In this paper,to solve the challenges in flow field and pressure drop measurements,the author researches the methods of fine full-field visualization measurements,develops sliding pressure drop measurements in rod bundle.The dissertation includes following contents.Through evaluating multitudinous methods of matched index of refraction(MIR),Fluorinated Ethylene Propylene(FEP)and water is chosen as MIR materials for rod bundle experiments.By a small-scale 5x5 rod bundle test with spacer grid,the particle image velocimetry(PIV)with assistance of FEP and water is verified for application of full-field measurement in rod bundle.The author conducts systemic-overall error analysis of MIR assisted PIV,presents the best test method of two-dimensional(2D)PIV for rod bundle experiments,deduces the uncertainty propagation from error of instantaneous velocity to the turbulent statistics and quantifies the uncertainty of the turbulent statistics.With application of the 2D PIV best test method,the threedimensional(3D)velocity field in 5x5 rod bundle with split-type mixing vane spacer grid is measured.Based on the high-precision and highresolution experimental database,the influence of mixing vanes to secondary flow,axial vorticity and turbulence is discussed.And the subchannel mixing and Reynolds number effect are analyzed.A new sliding pressure drop measurement system is developed.And the axial pressure drop upstream and downstream of the spacer grid and lateral pressure drop downstream of the spacer grid are measured successfully.Based on the highprecision pressure drop database,the author builds up the friction resistance coefficient model,the spacer grid resistance coefficient model and lateral resistance coefficient model.Here are several important conclusions.The 2D PIV best test method minimizes the uncertainty of turbulent statistics lower than 1% of the bulk velocity.The influence of split-type mixing vanes to the flow downstream of the spacer grid can be divided into three regions,including the lateral flow increasing region from the root of mixing vanes to the end of mixing vanes,the lateral flow structure transition region near the space grid and the lateral flow slowly damping region.In the lateral flow increasing region,the axial flow converts into lateral flow because of the guidance effect of the mixing vanes.The lateral flow goes into neighboring sub-channels through gaps.And the axial flow and lateral flow blend intensely,generating strong turbulence along axial and lateral flow.So the axial velocity decreases sharply,while the lateral velocity increases quickly.And both the axial and lateral turbulence intensity and longitudinal and lateral Reynolds shear stress increase sharply.In the lateral flow structure transition region,the strong cross flow forms shear flow in sub-channels,generating a pair of strong vortices in each sub-channel.The kinetic energy and strength of the pair of vortices decreases quickly because of the shear stress between them.With the area of vortices increasing,the pair of vortices merge into one large vortex covering the whole sub-channel.Finally,the lateral flow structure changes from two vortices into one vortex in sub-channel.The axial velocity in the centers of sub-channels decreases fast,while the axial velocity in the gaps increases fast.And the lateral velocity increases to the maximum value under the inertia effect and then decreases quickly.The axial and lateral turbulence intensity and the longitudinal and lateral Reynolds shear stress decreases quickly.With lateral flow structure transition,the distribution pattern of lateral Reynolds shear stress shifts from high lateral Reynolds shear stress in the gaps and centers of sub-channels to high lateral Reynolds shear stress near the two diagonal lines of sub-channels.And the distribution pattern of lateral turbulence intensity changes from the high turbulence intensity near gaps and low turbulence intensity in centers of sub-channels to the chess-board pattern with low turbulence intensity near gaps and high turbulence intensity in centers of sub-channels.In the lateral flow slowly damping region,the single vortex dissipates slowly.The axial velocity in sub-channel centers increases gradually,while the axial velocity in gaps decreases slowly.The lateral velocity,the axial turbulence intensity and lateral turbulence intensity follow the exponentially damping law.The longitudinal and lateral Reynolds shear stress decrease.Based on the analysis of the secondary flow intensity and lateral turbulent intensity in sub-channels and between sub-channels,the secondary flow intensity model,the lateral turbulent intensity model,the cross-flow mixing model and turbulent mixing model are presented.The secondary flow intensity and lateral turbulent intensity in sub-channels decay in exponential law.The cross-flow mixing factor between sub-channels increases near the space grid,then decays in exponential law.The turbulent mixing decreases quickly first,and then slightly increases near the spacer grid.Finally,it decays in exponential law.The friction resistance coefficient,the spacer grid resistance coefficient and lateral resistance coefficient decrease with Reynolds number increasing and it also decreases with the Reynolds number increasing.And the lateral resistance coefficient follows exponentially decaying function downstream of the spacer grid.And the 2D PIV measurement method with the MIR technology and the error analysis method can be a guideline for the future flow field measurements in rod bundle.The sliding pressure drop system is a robust and reliable method for the axial and lateral pressure drop measurements.The sub-channel mixing models and resistance models can be applied in sub-channel analysis.