Thermal Mixing Characteristics And Temperature Fluctuation Mechanism In T-junction

T-junctions with various sizes are widely used in the Nuclear Power Plant(NPP)pipeline system.In recent years,thermal fatigue cracks have appeared on the wall of tee pipes of several nuclear power plants.The pipe even breaks causing coolant leakage.Thermal fatigue seriously affects the safe operation of nuclear power plants.This is because the mixing of fluid with different temperatures in the T-junction will lead to wall temperature fluctuation.Therefore,it is of great theoretical significance to study the mixing law and the mechanism of temperature fluctuation in the T-junction for the thermal fatigue evaluation of the pipe for nuclear power generation service.It also has significant practical value to ensure the safe operation of the NPPs.In this paper,a visual experimental study on the mixing process of hot and cold fluid in a T-junction with different structures and conditions is carried out.We found that there are three mixing modes:impinging jet,deflection jet and wall jet.The mixing mode characteristics are determined by the momentum ratio of the main and branch fluids.When the momentum ratio is less than 0.35,the mode is the impinging jet mode,whether it is in a 90°or a 45° T-junction.When the momentum ratio is between 0.35 and 1.55,it is the deflection jet mode.When the momentum ratio is greater than 1.55,it is the wall jet mode.In this paper,the average temperature distribution,uniformity index,intensity and frequency of temperature fluctuation near the wall in each mixing mode are analyzed.It is found that the temperature near the wall fluctuates sharply in the mode of wall jet and impinging jet.And there is an obvious dominant frequency.The risk of thermal fatigue failure is high.In this paper,the effect of buoyancy on the mixing of hot and cold fluids is studied by changing the relative positions of the branch pipe and the main pipe in the direction of gravity and the temperature difference between the main pipe and the branch pipe fluids.It is found that the effect of buoyancy is not significant when the Richardson Number is less than 0.1.When the Richardson Number is larger than 0.1,the effect of buoyancy on the mixing of hot and cold fluids is significant.In this paper,the Large Eddy Simulation(LES)method is used to simulate the mixing process of hot and cold fluid in a T-junction.The LES model suitable for T-junction was determined by the deviation of temperature distribution and temperature fluctuation intensity between the results of the simulation and the experimental data.The Smagorinsky constant,which has an important effect on the near-wall flow in the sub-grid stress model,is also determined.Based on this,LES simulations are carried out for the mixing modes of wall jet and impinging jet which have severe temperature fluctuations near the wall.The large-scale vortex structures which influence the temperature fluctuation of the near-wall surface in the two mixing modes are identified,and the influence of their behavior on the temperature fluctuation is studied.The large-scale vortex structures affecting the near-wall temperature fluctuation in the two mixing modes are identified.The effects of their behaviors on temperature fluctuation are also studied.In the wall jet mode,there is a significant stratification of the hot and cold fluids.The periodic occurrence of the shear vortex at the stratification interface and the spanwise oscillation of the hairpin vortex in the wake region are the main mechanisms of temperature fluctuation of the interface.In the impinging jet mode,the impact of the windward vortexes on the main pipe wall and the swing of the wake vortexes are the main mechanisms causing the temperature fluctuation near the wall.The main frequency corresponding to the energy range of near-wall temperature fluctuation in the two mixing modes is directly related to the behavior of large-scale vortices.In this paper,the mixing process of hot and cold fluid in the T-junctions with different structures is studied by LES.The behavior of large-scale vortex structures in T-junctions with different incident angles and different pipe diameter ratios was analyzed under the mixed modes of wall jet and impinging jet.In the mixed mode of wall jet,the formation and movement of the shear vortex are related to the incident angle of the branch pipe,while the oscillation frequency of the hairpin vortex is independent of the incident angle.The dimensionless frequencies of occurrence of the shear at the stratification interface and the oscillation of the hairpin vortex in the direction of expansion increase with the increase of pipe diameter ratio.In the impinging jet mode,the formation and movement of the upwind vortex are related to the incident angle.The oscillation frequency of wake vortexes is independent of the incident angle.The oscillation frequency of wake vortexes increases with the increase of pipe diameter ratio.The generation frequency of windward vortexes does not change with the change in pipe diameter ratio.The behavior of these large-scale vortex structures will cause the frequency of temperature fluctuations near the wall to change to the corresponding frequency.