Study On Transient Hyduralic Impact Characteristics Of Axial Flow Pump During Accident Shutdown

Axial-flow pump is the key equipment of large pump stations,nuclear power plants and other hydraulic systems.Sudden fast shutdown of Axial-flow pump caused by human activities or system failure is called accident shutdown,which is one of the important factors threatening system safety and causing equipment damage.When the Axial-flow pump operates stably,the axial hydraulic load of the impeller is balanced by the thrust bearing,and the radial load is self balanced by the central symmetric structure of the impeller;The amplitude of medium pressure pulsation is relatively low and the flow state is stable.In the accident shutdown process,the rotating speed decreases rapidly and the hydraulic parameters change instantaneously,leading to system instability;The working state and internal pressure distribution of the impeller suddenly change,resulting in a varying axial force;The sudden increase in pressure load of the medium can impact static structures such as guide vanes,and in severe cases,cause structural damage.Based on the study of the transient changes of the performance and hydrodynamic characteristics of the Axial-flow pump during the accident shutdown process,the transient hydraulic impact characteristics were studied,revealing the relationship between the transient hydraulic parameters and the transient hydraulic impact characteristics,then obtaining the transient characteristics of the axial force of the impeller and the pressure impact on the structure.The main work is as follows:(1)The shutdown experiment of Axial-flow pump was carried out to monitor the transient changes of Axial-flow pump performance and hydrodynamic characteristics during shutdown,and further obtained the transient characteristics of impeller axial force,enriching the data of excitation source;On the other hand,a whole-fluid-domain numerical model for controlling speed changes and synchronously solving the inertial flow in the loop has been established,obtaining visualized internal flow and pressure distribution characteristics of the flow passage components.Furthermore,based on the experimental results,the reliability of the numerical model was verified.(2)Based on the experiments and numerical simulation,the rotating speed,flow rate and their change rate would affect the performance transient characteristics of Axial-flow pump by the shutdown process of different laws.Then,based on the momentum-moment theorem,the instantaneous head of Axial-flow pump during shutdown was analyzed.When the impeller rotating speed drops rapidly,the resistance of the blade to the fluid was the main reason for the significant drop of the instantaneous head.The difference between the instantaneous head during shutdown and the corresponding quasi-steady state value,which is called the additional head,was used to characterize the significance of the transient effect.During high-speed shutdown,the performance of Axial-flow pump changed rapidly,and each parameter deviated significantly from the quasi-steady state value.(3)The transient study on the four quadrant working condition,internal flow characteristics and internal pressure distribution characteristics during accident shutdown showed that during high-speed shutdown such as shaft seizure accident,the change of Axial-flow pump working condition from pump working condition to turbine working condition would lead to a dramatic change of pressure distribution characteristics at the front and back of blades,which was the reason for the change of axial force amplitude and direction.Furthermore,a segmented method for calculating axial force during shutdown was proposed,and the transient characteristics of axial force on the impeller were studied.The additional axial force is used to characterize the hydraulic impact strength of the impeller.During a shaft seizure accident shutdown,the direction of the axial force changed,and the amplitude change reached 175.78%,threatening the structural reliability of the rotor system.(4)Study on the transient characteristics of fluid medium pressure load during accident shutdown showed that the energy of the fluid medium in the flow passage components varies with rotating speed and flow rate,which was the reason for the different pressure impact strengths at different positions and times.Furthermore,a Hydraulic Impact Factor calculation method based on wavelet packet method and information entropy analysis was proposed,and the spatiotemporal distribution characteristics of impact intensity were studied.Through comparative analysis with instantaneous performance,it was found that the change trend of additional axial force and Hydraulic Impact Factor was consistent with additional head,reaching the extreme value synchronously,thus revealing the correlation between transient effect and transient hydraulic impact characteristics,which supplemented the research on transient characteristics of Axial-flow pump.In this paper,experiments and numerical simulation studies were carried out to clarify the transient change laws of performance parameters,hydrodynamic parameters and hydraulic impact characteristics of Axial-flow pump during accident shutdown,reveal the transient mechanism of hydraulic impact load,carry out study on the impeller axial force and Hydraulic Impact Factor transient characteristics,and establish the correlation between transient effect and transient hydraulic impact characteristics.The results could provide theoretical support for the prediction of hydraulic parameters during accident shutdown and the reliability design of Axial-flow pump.