| Nuclear safety is the premise and foundation of the development of nuclear energy.The flow rate of the primary coolant in a nuclear reactor will fluctuate during accident conditions or in floating reactors that are affected by inertial forces in the ocean condition.These fluctuations may have a substantial impact on heat transfer by the primary coolant in the reactor.It has important academic and practical application value to reveal the mechanism of transient conditions on the flow and heat transfer in the fuel assembly,and to establish a numerical simulation and prediction model for the thermal characteristics of the nuclear reactor.In current research,based on the steady state measurement results,the flow and heat transfer characteristics of rod bundle under unsteady flow are studied through theoretical analysis,experimental research and numerical simulation.Firstly,the flow and heat transfer characteristics of rod bundle under steady-state conditions are studied.The time-resolved particle image velocimetry(TR-PIV)were used to study the flow field in a large range(0D_h～22D_h)downstream a spacer grid in a 5×5 rod bundle at different Reynolds number(Re=1508～12590).From the TR-PIV velocity vector fields obtained downstream of the mixing vanes,time-averaged,statistical,spectral,and cross-correlation analysis were used for full-field and instantaneous experimental data.The results show that there are periodic vortices when the fluid flows through the cross-arranged mixing vane on the spacer grid,and the vortex shedding frequency will increase with the increase of Reynolds number.These vortexes are gradually propagated downstream along the inner subchannel and dissipated in the gap subchannel due to the effect of viscosity.A single mixing vane will not generate periodic vortices.The cross-correlation analysis shows that when the fluid propagates to the downstream,the coherent structure scale gradually increases and the transport velocity gradually decreases.The experimental results of convective heat transfer in a 3×3 heated rod bundle channel show that the velocity distribution in the bare rod region is more irregular under the action of buoyancy force at low flow velocity,which enhances the heat transfer,while the buoyancy force hardly changes the velocity distribution downstream of the grid.Based on the experimental data,the calculation formula of mixed convection heat transfer of fuel assembly is proposed.Furthermore,the pressure difference sensor and phase-locked PIV technology are used to study the flow field and resistance distribution characteristics of fuel assembly under the pulsating flow.Based on the different pulsating parameters experimental data,the effects of unsteady flow in rod bundle channel on the instantaneous and periodic average resistance factors are studied.The results show that the time average local and frictional pressure drop increases significantly under pulsating conditions,which is proportional to the amplitude and frequency,and inversely proportional to the time averaged Reynolds number.The flow field measurement results show that the velocity distribution in the rod bundle channel is changed by the pulsating flow,and the velocity near the channel and the rod wall increases in the acceleration phase,while the velocity near the center decreases.In addition,the results of unsteady flow on the velocity distribution,turbulence intensity,vortex evolution and other parameters show that the grid mixing suppresses the effect of unsteady flow,and the velocity distribution of acceleration and deceleration phase gradually changes after 5 D_h downstream of the grid.The fluid deceleration increases the vortex size downstream of the grid and strengthens the secondary flow in the subchannel.Then,based on the 3×3 heating rod bundle,the convective heat transfer characteristics in the rod bundle channel under the pulsating flow are studied.The results show that the pulsating flow can enhance the heat transfer,and the effect of pulsating flow on the heat transfer enhancement in the downstream of the grid is better than that in the upstream of the grid.Besides,based on the experimental data,the correlations for predicting the instantaneous and time average heat transfer coefficients of the grid upstream and downstream under pulsating flow are proposed.The temperature field of different phases was measured by LIF measurement technology.The influence mechanism of heat transfer coefficient under the pulsating flow was revealed by micro measurement combined with theoretical analysis.In addition,in order to meet the demand for predicting the thermal and hydraulic characteristics of unsteady fuel assemblies,this paper compares the simulation results of different grid numbers and turbulence models with high-fidelity experimental data,and selects the turbulence model with the best simulation accuracy.The results show that the SST k-ωmodel has good prediction results for the pressure drop of the rod bundle channel and the velocity distribution downstream of the grid,and the anisotropic RSM model has good prediction results for the turbulence intensity.Subsequently,by analyzing the effects of different boundary conditions on the transient simulation results,and carrying out a quantitative evaluation study on the efficiency of non-steady-state numerical calculations,the two non-steady-state time advancement algorithms,NITA and ITA,are compared for the number of iteration steps and single The results show that the pressure inlet and flow outlet boundary conditions can capture the high-frequency pulsating flow backflow phenomenon,and the use of the NITA scheme can greatly improve the calculation efficiency of the pulsating flow under the premise of ensuring accuracy.Finally,the CFD prediction results are verified based on the experimental data in current research.The results show that URNAS can predict the thermal-hydraulic characteristics of the rod bundle under pulsating flow. |
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