Investigations On Flow And Heat Transfer Characteristics Of Plate Fuel Assemblies Under Blockage Conditions

The coolant channels in the plate fuel assemblies are narrow and slender.Once a blockage accident occurs in the rectangular channel,the flow will deviate from the normal operational state along with vortices of various scales and the flow redistribution,which may interfere with the flow transfer characteristics.To that end,the particle image velocimetry(PIV)technology and the laser induced fluorescence(LIF)technology are used to carry out a waterhole-wide,interference-free and visualized measurement research on the blockage scenarios of the plate fuel assembly.Specifically,the distributions of the flow and temperature field under various blockage conditions are obtained through the Proper Orthogonal Decomposition(POD)method,the Power Spectral Density(PSD)method and turbulence characteristics.Meanwhile,the experimental measurement of flow distribution in the parallel plate bundle is carried out,and the influence of machining errors on the flow distribution of each channel is evaluated.The numerical error of each turbulent model in Computational Fluid Dynamics(CFD)is analyzed based on the experimental data.Additionally,the high parameter simulation of the blockage scenarios is performed to comprehensively evaluate the influence of machining errors and various blockage parameters on the flow redistribution,flow heat transfer characteristics and reactor safety.The experimental analysis of the flow and temperature field characteristics under edge and center blockage scenarios in a single rectangular channel shows that the fluid will separate and generate shear flow on both sides of the obstacle,in which the vortex will form,develop,evolve,and die out.The separation of shear layers is the main reason of the flow velocity fluctuation.While the downstream evolution of the vortex causes the spanwise velocity pulsation mainly.As shown by the comprehensive analysis of systematic and local characteristics,the existence of the vortex will degenerate the heat carrying capacity of the fluid and significantly increase the fluid’s temperature with blockage.The temperature field distribution is more sensitive to the blockage ratio rather than Re.According to the POD modal analysis,the distribution and energy characteristics of coherent structures in the flow field at different scales are demonstrated.The vortex structure pertaining to the first four POD modes contains the main energy of the flow field and is the dominant coherent structure of the flow field.The large-scale vortex structure of the first two POD modes is the evolution of the spatial structure with the varying time,and they have similar energy and the same structure,with a certain phase difference.In the third and fourth POD modes,two columns of small-scale vortex structures with opposite directions are formed hidden in the fluid.The trajectory of the small-scale vortex is perturbated and restricted gradually by the wall surface,which results in an interference on the turbulence distribution.As analyzed by the PSD,the vortex shedding frequency is relevant to the width of the obstacle.And with the increase of the Reynolds number,the turbulence in the fluid gradually becomes stronger,and the frequency of the vortex motion gradually increases.As shown by the experimental results of the flow distribution in parallel plate bundle under the non-blocking conditions,the relative difference between the actual flow rate and the average one increases with an increased machining error,resulting from a larger number of channels,as well as its fluctuation range.The relative difference of flow rate in most channels is within the scope of-10% ～ 10% if the machining error belongs to-5% ～ 5%.The flow resistance of edge channels is large due to geometric structure,and the relative difference of the flow rate increases to-23% ～ 18%.Besides,negative machining deviation is more likely to cause a larger flow deviation.The numerical simulations of the flow redistribution of the plate fuel assembly under blockage scenarios show that the effects of edge and center blockage scenarios on flow redistribution are similar.The flow rate in the blocked channel decreases rapidly due to the increase of flow resistance,and the flow in the adjacent second channel increases the most.The relative difference between the flow rate in each channel under blockage and normal scenarios is less affected by the blockage type and position,but more by the blockage ratios.As verified by the turbulence model,the SST k-ω model is more suitable for simulating the flow and heat transfer characteristics of blockage scenarios in the plate fuel assemblies.The velocity error between experiments and simulations is of the range 11.4%～14.3%,the maximum of which appears in the mainstream region.Whereas the temperature error fluctuates within 7.45% ～ 13.9%,the maximum of which appears in the recirculation region.According to the numerical simulation of flow distribution based on the actual machining size of the experimental assembly,the simulation errors are within ±10% mostly,and large positive and negative errors mainly occur in the edge channel.Considering the degree of influence on reactor safety with blockage,among various blocking types,more complex vortex structures are formed in the edge blockage scenarios under the same boundary conditions.As a result,heat is more likely to accumulate in the flow passage,resulting in greater local temperature rise of the fluid,cladding and fuel.Consequently,the risk of edge blockage scenarios is higher than that of the center blockage scenarios.In different blockage positions,the middle blockage will cause the maximum flow redistribution for an equal blockage ratio.Therefore,the local temperature rise of fluid,cladding and fuel is maximum when the central blockage occurs.Although there is little difference in terms of flow redistribution in each channel between the inlet and outlet blockage scenario,and the local temperature rise of each component is of little difference,there is no complex vortex movement in the outlet blockage scenario.Consequently,the flow channel blockage risk is ranked as:middle > inlet > outlet.However,even if the blockage occurs with α = 90% and the flow in the blockage channel is reduced by 85%,the heat can still be effectively transported to ensure the integrity of the fuel assembly.