| The rod-type fuel assembly is the core of current commercial pressurized water reactor,and its performance directly affect the reactor safety.Under the influence of ocean conditions or the loss of flow accident,flow fluctuation occurs in reactor circuit.Flow fluctuations cause transient fluctuations of the resistance characteristics,flow field structure,and heat transfer characteristics in fuel assemblies,which threatens the reactive control of nuclear reactor.Therefore,it has important fundamental research value in studies of flow and heat transfer behaviors in fuel assembly under flow fluctuations.In this paper,theoretical analysis,experimental measurements and numerical simulations are used to study the frictional resistance characteristics of rod bundle channels,local resistances of spacer grids,space-time evolution of flow fields,and heat transfer characteristics in fuel assembly under flow fluctuation conditions.The flow fluctuation will cause the flow characteristics in the fuel assembly to deviate from those in steady flows,thus the additional contribution introduced by the flow fluctuation is very important to analyze the transient thermal-hydraulic behaviors of the fuel assembly.Theoretical analysis and experimental research on the transient resistance characteristics of the fuel assembly under flow fluctuations were carried out.Based on the flow resistance characteristics of the fuel assembly under steady flow,the influence of different flow fluctuation parameters on the transient resistance characteristics is obtained.Combined with the instantaneous flow field distribution and turbulence characteristics,it was found that the acceleration and deceleration flow changed the frictional shear stress and turbulent shear stress between the flow layers,and the dominant factors affecting the deviation of resistance pressure drop between the unsteady flows and the steady flows are determined,the mechanism of action of the acceleration and deceleration flow on the pressure drop is revealed.Research on the characteristics of flow transition in the rod channel under flow fluctuations shows that the flow transition is significantly affected by the flow fluctuation,the laminar-turbulent transition is delayed due to the accelerated flow,and the turbulent-laminar transition occurs earlier due to the decelerated flow.The phase difference in fuel assembly under pulsating flow is systematically studied,and the effect of spacer grid on pulsating flow phase difference is explained.The effects of flow pulsation parameters(pulsation frequency,pulsation amplitude,and time-average Reynolds number)on the frictional resistance and the local resistance are analyzed.An empirical formula for predicting resistance characteristics of fuel assembly in the range of experimental parameters is proposed.Based on the continuous particle image velocimetry,combined with a telecentric lens and a pulse controller,a long-time continuous measurement of high spatio-temporal resolution for the complex flow field in the fuel assembly is achieved,and the spatial flow field in the fuel assembly under flow fluctuations is obtained.The spatial evolution of flow field in rod bundles and downstream of the spacer grid,such as velocity distribution,turbulence intensity,and Reynolds stress,are analyzed.Based on the flow field distribution in steady flows,the contribution of flow fluctuations to the spatial flow field distribution and analyzed.The flow fields show that the accelerating flow increases the velocity gradient and axial turbulence intensity between flow layers,and suppresses the lateral velocity and turbulence intensity;the decelerating flow weakens the velocity gradient between flow layers,and increases the lateral velocity and turbulence intensity.According to the flow field in transitional regime,the mechanism of flow transition in acceleration flow and deceleration flow is revealed.Due to the mixing performance of the spacer grid,the impact of flow fluctuation on the instantaneous flow field downstream of the spacer grid is weakened.The influence of flow pulsation parameters on the time-averaged flow field in rod bundles and downstream of the spacer grid was analyzed,and the mechanism of action acceleration flow and deceleration flow for Flow resistance characteristic in the fuel assembly is revealed.The flow fluctuation causes the flow resistance and flow field distribution in the fuel assembly to deviate from the steady flow,which causes the heat transfer characteristics in the fuel assembly varying.Using the harmonic sinusoidal pressure gradient as the boundary condition,the theoretical solution of the pulsating laminar flow velocity and temperature distribution is solved by the separation variable method,and the fluctuation characteristics of the velocities and temperature in different pulsating flows are obtained.An experimental study on the heat transfer characteristics of fuel assemblies under flow fluctuations was carried out,and the fluctuation characteristics of wall temperature,heat flow density,and heat transfer coefficient at different positions in the fuel assembly under different flow fluctuation parameters were obtained.According to the instantaneous flow field distribution in the fuel assembly,it revealed that fluctuation mechanism of heat transfer parameters under flow fluctuations.The research on the transient heat transfer characteristics of the spacer grid under flow fluctuations has been carried out.The research shows that the flow fluctuation conditions have a significant impact on instantaneous heat transfer characteristics of the fuel assembly at low Reynolds numbers.Accelerated flows result in the instantaneous heat transfer performance of the spacer grid being greater than that in steady flows,and decelerated flow causes the instantaneous heat transfer coefficient to be less than that in steady flows.In turbulent flows,the flow fluctuation has a weak influence on the heat transfer characteristics. |
PDF Full Text Request