Numerical Investigations On Turbulence Penetration And Thermal Stress Of Pipe Wall In A Stagnant Branch Pipe

The phenomenon of turbulence penetration is common in the stagnant branch pipe connected to the main pipe in the piping system of nuclear power plant.This would form thermal stratification in the branch pipe,which may cause thermal stress.For the pipeline with turbulence penetration,the thermal fatigue damage is more likely to occur due to the repeated loading of thermal stress under long-term operation conditions,which would cause the coolant leakage accident and bring great hidden danger to the safety of nuclear power plant.Therefore,the study on the turbulence penetration phenomenon and the thermal stress of pipeline have important theoretical significance for the thermal fatigue evaluation of nuclear power plant pipeline.Based on the method of fluid-solid-thermal coupling,the turbulence penetration phenomenon in fluid domain and the thermal stress in solid domain are simulated respectively.For the fluid domain,the numerical simulation of the straight branch pipe is carried out first.The results show that the turbulence penetration will lead to the formation of three parts in the branch pipe:high temperature zone,thermal stratification zone and low temperature zone,and the length of turbulence penetration increases with the increase of the main flow velocity.The results show that the bent pipe structure has an obvious inhibition effect on the development of the turbulence penetration length.Through the analysis of the volume proportion of different temperature intervals in the fluid domain,it is found that the inhibition effect of the bent pipe actually inhibits the development of the fluid in the high temperature zone,but speeds up the extension of the thermal stratification zone in the pipeline.In addition,when the size of the branch pipe was kept the same,the size of the main pipe was changed,and the rectangular main pipe was regarded as the case that the diameter ratio of the pipe was infinite.The results of the turbulence penetration of the pipe diameter ratio of 1,2,5 and infinity were compared.It was found that the smaller the diameter ratio,the larger the turbulence penetration length.Further analysis shows that this is related to the inlet area of branch pipe.The smaller the diameter ratio is,the larger the inlet area of branch pipe will be,and the larger the final turbulence penetration length will be.However,when the diameter ratio increases to 5,both the inlet area of branch pipe and the turbulence penetration length are very close to the result that the diameter ratio is infinite.For the solid domain,the finite element method is used to analyze the thermal stress of the branch pipe where the turbulence penetration occurs.The branch pipe can be divided into two structures:straight pipe and bent pipe.For the straight pipe,two extreme constraint modes are used,to allow the free displacement of the pipe end face and completely constrain the displacement of the pipe end face.It is found that the thermal stress of the pipe is mainly concentrated in the thermal stratification area under the first constraint mode.Under the second constraint mode,there will be thermal stress in the whole pipeline,among which the maximum value is in the thermal stratification area,and the lower the thermal stratification position of branch pipe,the higher the overall level of thermal stress in the pipeline.For bent pipe,the same constraint modes are adopted as that for straight pipe.Thermal stress concentration occurs at the elbow under both constraints,and the thermal stress is larger when the displacement is completely constrained.Different from the straight pipe,the bent pipe has deformation tendency in both horizontal and vertical directions,and the actual deformation in both directions will be coordinated and continuous at the elbow.In addition,the deformation tendency of the bent pipe can be changed by limiting the displacement of the end face in different directions.