Investigation On Turbulence Penetration Phenomenon In A Stagnant Pipe Connected To A Main Pipe

A large number of branch pipes are connected to the main pipe in the nuclear power plant.High-temperature fluid flowing through the main pipe will initiate a cavity flow in the branch pipe.As the phenomenon involves many influencing factors,and the flow in the branch pipe is very complicated,there is no general explanation on the initiation mechanism of cavity flow at present.In this paper,cavity flow in a stagnant pipe connected to a main pipe was studied through experimental research methods.A visualization experimental bench was constructed for experimental research.Studies were conducted on factors of affecting the penetration length,such as the branch pipe diameter,branch pipe layout,main pipe fluid flow rate,temperature difference between the main and branch pipe.Numerical simulation of the selected experimental condition was used to explain the mechanism of temperature fluctuation in the turbulence penetration phenomenon caused by spiral flow.For the case there was no temperature difference between the main pipe and the branch pipe,the experimental correlation of the penetration length and Reynolds number for a main pipe with rectangular cross-section channel was obtained and compared with other research.When there was a temperature difference between the main and branch pipes,the temperature difference would impede the penetration,and the greater the temperature differenced,the more the penetration length decreased.The kinematic viscosity ratio of the main and branch fluid vm/vb was introduced to vb characterize the effect of temperature difference on the penetration length in the experimental correlation.The experimental results show that changing the angle of the main and branch pipe from 90° to 45° in the flow direction did not significantly change the turbulence penetration length.When the main pipe was replaced with a round pipe,the experimental results showed that the penetration length with a diameter ratio of 4.5 was no different from that of the main pipe with a rectangular cross-section channel.When the branch pipe was connected horizontally and vertically upward to the main pipe,the penetration phenomenon in the branch pipe were different from that in the branch pipe connected vertically downward.For the case there was a temperature difference between the fluids of the main and branch pipes,when the flow rate of the main pipe was fixed,the temperature of all fluids in the branch pipe would be consistent with the main pipe eventually.No thermal stratification would be formed.The temperature of the pipe wall would not fluctuate.When the branch pipe connected to the main pipe was with an elbow,the elbow of the branch pipe had a relatively obvious weakening effect on the growth of the penetration length,and there were obvious temperature fluctuations at the elbowLarge Eddy Simulation or LES model was used to conduct our simulation.The simulation results were in good agreement with the experimental results.The simulation showed that at the beginning of the penetration development,the entire branch flow field was dominated by the cavity flow,and in the final stage of the development,there would be periodic large spiral flow.The penetration length was controlled by the spiral flow,the direction and rotation direction of the spiral flow were random in all modes,so there were irregular up and down oscillations.At the same time,the spiral flow would also cause temperature fluctuations.