Study And Optimization On Flow-Acoustic Coupling Characteristics In Closed Side Branches

The flow medium in the nuclear power system usually is high temperature and pressure.To ensure the operation safety,the overpressure operation of pipelines should be avoided.Therefore,the pressure relief pipeline with a closed side branch pipe structure should be installed at a specific position of the main pipeline.In some specific load operation conditions,the abnormal vibration in the closed side branch pipe often occurs,which causes the high cycle fatigue failure of the connecting parts.Considering the safety of nuclear reactors,it is significant to explore the causes,generation mechanism and occurrence law of abnormal vibration of closed bypass pipelines.In this paper,the experimental and numerical method is used to study the flow field distribution,acoustic transmission and response characteristics of the closed side branch pipeline.Numerical method is used to explore the mechanism of the abnormal vibration of the closed side branches,and to propose the optimization scheme.The main content is as follows:(1)Characteristic analysis of flow field excitation source in closed side branches.Firstly,build the principle bench and model of the closed bypass,and verify the feasibility of the numerical method by comparing the experimental and numerical results.Then,the actual model of the pipeline is established by referring to the relevant parameters of the principle model,and the transient flow field is calculated.The flow and pressure fluctuation characteristics are monitored to explore the mechanism and influencing factors of vortex generation,development and shedding at the junction of the main pipe and branch pipe.Finally,the monitoring data are processed from the time domain and frequency domain,respectively.The regular vortices are formed at the front edge,develop and break up towards the trailing edge.It can be seen that the fluctuating frequency is related with the flow rate and has no direct correlation with the fluid temperature.The Strouhal number is invariable at different flow rates.(2)Acoustic transmission characteristics and response analysis in closed side branches.Firstly,the acoustic transmission characteristics under different temperatures and different states of the valve are analyzed to obtain its inherent acoustic frequency features.Then,the pressure spectrum of the junction is extracted as the acoustic excitation source,and the acoustic response characteristics of the closed side branch pipeline are calculated and analyzed.Finally,the flow field excitation source characteristics and sound field response characteristics are analyzed comprehensively.The acoustic cavity of the side branch pipe has an amplification effect on the pressure pulsation of about 21 Hz from the flow field at a specific temperature and a specific valve state,and the abnormal vibration is caused by the flow induced acoustic resonance.(3)Optimization schemes based on flow and acoustic field.The vortex shedding process is changed by increasing the branch diameter and the trailing edge chamfer,so as to optimize the fluid feedback and reduce the vortex shedding frequency.The acoustic field is optimized by changing the pressure wave transmission process.The acoustic characteristic frequency is reduced by changing the effective connection length of the branch pipe,thus unlocking the flow frequency and the acoustic frequency in the pipe.