| In a sodium cooled fast reactor,the uneven distribution of core power and the different mass flow rates of subchannel coolant result in a non-uniform temperature distribution of liquid sodium flowing out of the core.When different temperatures of coolant are mixed at the core outlet,where temperature fluctuations are caused by convective turbulent mixing between the hot fluid and the cold fluid,this may result in periodic thermal stress and high cycle thermal fatigue of the central column exposed in the upper plenum.The phenomenon is called thermal striping,which may cause the crack initiation and propagation in the structure.Due to the large temperature variation of liquid sodium and the high thermal conductivity,the effect of temperature fluctuations in sodium cooled fast reactors is severer than in the light water reactor.Therefore,in order to ensure the safe operation of the nuclear power plant to reduce the thermal fatigue of the central column,it is very important to study the thermal striping phenomenon of the upper plenum of the FBR core outlet.Based on the JAEA PLAJEST experiment,a parallel triple jet geometry model is used for numerical simulation to study the fluid and structural temperature fluctuation characteristics.Scaled by 1:1 according to the experimental setup and conditions,the computational domain is half domain as a result of symmetry.The temperature difference of the hot and cold coolant is about 40℃.The velocity boundary conditions are divided into two types,which are isovelocity and non-isovelocity of hot and cold coolant.The WMLES turbulence model is chosen to make calculation possible for the wall bounded flow under high Re number,and to reduce the stringent and number-dependent grid resolution requirements of classical wall-resolved LES.Coupled Wall method in FLUENT is used for conjugate heat transfer between the fluid and the structure regions.And the grid space discretization and time discretization scheme are set up properly.Also setting the monitoring values is essential to ensure that the calculations results are converged.The dimensionless average temperature and the root-mean-square temperature distribution at the measuring lines on the symmetry plane and the near wall plane are compared with the PLAJEST experimental data.The simulation results show a fairly good agreement with the experimental results,verifying the correctness and feasibility of the WMLES turbulence in such simulation.Referring to the real fuel assembly bullet head design and seven-hole outlet arrangement,the simplified model of the CEFR assembly head was designed,and a more realistic core outlet geometry model was established to study the thermal striping phenomenon in the CEFR core outlet region.According to the CEFR mass flow rates of subchannel coolant and outlet temperature distribution of different subchannel in equilibrium state,two sets of typical temperature and velocity boundary conditions are selected.The temperature difference between simulated cold and hot fluid is about 20℃,and the Reynolds numbers are from 0.358 million to 0.393 million.The simulation was performed using a WMLES turbulence model with reasonable numerical settings.The rationality of installation of thermocouples for temperature measurement in geometric center at the height of y=100mm is verified.It is recommended that the temperature sampling time of the monitoring point be 1s.Analysis of time domain and frequency domain characteristics near the center column shows that the temperature fluctuations in the wall direction exhibit attenuation characteristics.The amplitude in the solid domain is less than 1℃,and the amplitude of the larger amplitude is concentrated in 10 Hz. |
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