Research On Unsteady Flow Characteristics Of Stall In Axial-flow Circulating Pump

Axial-flow cooling circulating pump is one of the core rotating equipment in the nuclear power ship cooling subsystem.The low frequency vibration and noise induced by the complicated unsteady flow in the pumps seriously endanger the safety of the system during the part-load operating conditions,especially in the stall condition.This paper is supported by the China Postdoctoral Science Foundation Project(No.2017M611722)and the Jiangsu Postdoctoral Research Foundation Project(No.1701054B).A certain type of axial-flow circulating model pump used in warship as the research object is adopted in this paper.By using numerical simulation and experimental measurement,the flow characteristics,pressure pulsation and vibration characteristics of the model pump under typical flow conditions are comprehensively researched and analyzed.In this paper,the main research work and conclusions are as follows:1.According to the design parameters,the optimal design of hydraulic components and precise manufacturing of model pump are carried out.The test loop and system are constructed,and the performance parameters,pressure pulsations and vibration signals of model pump are synchronously measured under different flow conditions.The test results show that the flow rate of critical stall and deep stall is 0.63 and 0.42 times of the optimal condition respectively.When the model pump operating flow rate is greater than the flow rate of critical stall,the pressure pulsation signal is obviously periodic,and when operating in the stall condition,the periodicity of the excitation signal is weakened.2.The large eddy simulation method is used to simulate the three-dimensional unsteady flow under typical flow conditions in the model pump.Combined with the flow uniformity index,BVF diagnosis,limit streamline and Q criterion identification,etc.,the variation of flow field in inlet section,the typical vortex structure and relative velocity in the impeller are analyzed.The research shows that the impeller inlet flow is mainly affected by the leakage vortex,and the typical flow structure is the inlet IV shroud backflow,outlet hub backflow and the suction outlet oblique flow.Meanwhile,the strong unstable flow occurs in 0.5 times higher leaf area at critical stall condition.The impeller inlet flow is mainly affected by the leakage vortex and inlet backflow.The typical flow structure is the cross-passage vortex through the impeller channel,its rotation speed is 3/4 of the impeller rotation speed in the deep stall condition.3.Combined with the experimental measurement and LES numerical calculation results,the pressure pulsation signal is processed and analyzed under typical flow conditions.The typical characteristics of the pressure spectra domain signal,the variation of characteristic frequency amplitude and the RMS value indifferent frequency domains with the flow rate are obtained.The research shows that the main characteristic frequency of pressure pulsation is fBPF and 2fBPF.The cross-passage vortex frequency is 0.74 fR under the stall condition(0.58 QN),and the minimum value of the pressure pulsation RMS value in the domain of 0~fR and 0~fBPF corresponds to the optimal condition on the pump inlet section.The pressure pulsation amplitude at 0.63 QN is significantly larger than QN and 0.42 QN on the inlet section of the impeller.The main characteristic frequency is fBPF and its higher harmonics on the center section of the impeller rotation,and the pressure pulsation coefficient at fBPF reaches the minimum value in deep stall condition.The main characteristic frequency is fBPF on the impeller outlet section.The pressure pulsation signal in the guide vane is unevenly distributed along the circumference direction,especially in the inside flow channel of the outlet elbow.4.The LMS vibration test system is used to analyze the vibration acceleration signal under typical flow conditions.Moreover,the main characteristic frequencies of the vibration,the variation of the total vibration energy,and the RMS value of vibration acceleration in different frequency domains directions with the flow rate is revealed.It is convinced that the total vibration energy at the pump inlet,impeller outlet and pump outlet has an linear increasing trend with the decrease of flow rate under non-stall conditions(greater than 0.63QN).The total vibration energy increases first and then decreases with the decrease of flow rate under stall conditions.The vertical vibration has the greatest contribution to the low-frequency vibration level on the pump inlet,the impeller outlet and the vane outlet.In further,the axial vibration on outlet elbow contribution to the low-frequency vibration level is the largest.