Numerical Simulation Of Turbulence Penetration Phenomenon In A Stagnant Branch Pipe

A large number of T-junction are laid out in a nuclear power plant.When hot fluid from the main pipeline flows through a stagnant branch pipe,which is closed at the bottom and filled with cold fluid,the wall of the branch pipe generates thermal stress due to periodic temperature fluctuations.This phenomenon is called turbulent penetration phenomenon.Turbulence penetration can induce small cracks on the surface of the pipe,leading to thermal fatigue failure and,at a later stage,even penetrating cracks in the pipe,causing coolant leakage,which can pose a significant risk to the safe operation of a nuclear power plant.Due to the many influencing factors involved in the phenomenon of turbulence penetration,the flow in the branch pipe is very complex,and the current research on the development process,factors and mechanism analysis of turbulence penetration is not sufficient,and no effective thermal fatigue mitigation measures have been established.Therefore,the study of turbulence penetration phenomenon is of Important meaning for the safe operation of nuclear power plants.In this paper,a physical model of turbulence penetration in a stagnant branch pipe is constructed and the development process of turbulence penetration,the factors affecting the length of turbulence penetration,the temperature fluctuation characteristics of the themal stratification in the branch pipe and the effect ofvortex breaker on the temperature fluctuation characteristics of the thermal stratification are researched by means of Large Eddy Simulation.The results of the study help to prevent thermal fatigue failure of the pipe wall,optimizing the pipe design,avoiding large accidents and provide guidance for the safe operation of nuclear power plants.The specific research contents and conclusions are as follows.The temperature field and flow fields of turbulence penetration in straight and bent branch pipes were obtained through Large Eddy Simulation and the infiltration development process were described and analysed.The factors affecting the turbulence penetration length and the fluctuating temperature characteristics of the thermal stratification were researched.It is found that the turbulence penetration length develops rapidly at the beginning and then tends to level off as time increases,and the relationship between infiltration length and time roughly conforms to the law of logarithmic functions.The greater the temperature difference between the main and branch pipes,the smaller the turbulence penetration length in the branch pipes,and the increasing temperature difference has a suppressive effect on the turbulence penetration;the bent branch pipes can suppress the turbulence penetration length more than the straight branch pipes;the smaller the pipe diameter ratio,the larger the turbulence penetration length.The effect of temperature difference on the temperature fluctuation characteristics of straight and bent branches.The effect of vortex breaker on the temperature fluctuation characteristics of turbulence penetration thermal stratification was researched.It was found that vortex breaker accelerate the development of turbulence penetration,that vortex breaker significantly suppress the temperature fluctuations of turbulence penetration,and that the installation location and cross-sectional area of vortex breaker also affect the temperature fluctuations of the thermal stratification.The closer the installation position to the mains pipe and the larger the cross-sectional area,the more remarkable the suppression of temperature fluctuations.