Excitation Characteristics Induced By Unsteady Flow Within A Centrifugal Pump

Noise and vibration performances are key parameters of centrifugal pumps, and in some particular fields, for instance ships, nuclear reactor, submarine, they are the most important evaluation criteria. Turbulent flow structures within the centrifugal pump are pretty complex accompanied by multi-scale vortices and macroscopical separate flow, wake, rotor-stator interaction, et al. Flow induced hydraulic excitation mechanism is extremely complicated due to multi-flow flied coupling effect. Unsteady flow structures are the fundamental sources of hydraulic force induced noise and vibration. So it is really essential to establish the correlation between unsteady flow structures and flow induced hydraulic forces. It is also of great importance to constructing low noise centrifugal pump design theory and developing active control technique of noise and vibration. This research focuses on unsteady flow structures induced hydraulic forces, and it is funded by National Natural Science Funds of China (Vortex Dynamics and Excitation Mechanism of Confined Turbulent Flow in Pumps, Grant No.51576090), Military Entrusted Project, and the Research and Innovation Project for College Graduates of Jiangsu Province (K.YLX-1036). The main work and obtained results are as following:Numerical simulation method is applied to capture the detailed flow structures within the model pump under different operating conditions. Meanwhile, pressure pulsation signals on the volute casing are extracted. From the point of vortical evolution process, the internal rotor-stator interaction mechanism is first investigated. Emphasis is laid on the distribution of unsteady vortical structure and its evolution process. The shedding vortex from the blade trailing edge and its striking with the volute tongue are analyzed. The interaction effect, moving, deforming, dissipating processes of the vortical structure are detailedly depicted in subsequently. The correlation between pressure pulsation energy and vortical structure is formed. We consider that pressure pulsation energy is determined by the corresponding vorticity strength.The influence of different parameters of the pump on pressure pulsations is detailedly summarized. Based on the point of active control of the shedding vortex strength from the blade trailing edge, the influence of different blade trailing edges on the pump performance, vortex strength and pressure pulsation energy is discussed. From numerical results, it indicates that by modifying the pressure side of the blade outlet, pressure pulsation energy could be reduced about 7% at the blade passing frequency.From combination analysis of cavitation visualization and cavitation induced vibration characteristics, cavitation stages versus cavitation number are first divided into four typical states, the steady attached sheet cavitation, periodically shedding cloud cavitation, compressible bubble cavitation and significant interface between water and vapor state. Cavitation number intervals of different cavitation stages are defined. Besides, the developing and evolution processes of cavitation at different stages are studied.Experimental platforms of pressure pulsation and vibration are established. Amplitudes of discrete components and RMS values in particular frequency band at different operating conditions are analyzed. The responses of pressure and vibration spectra to flow rate are founded. Combined with complex flow structures within the model pump, the complicated relationships between flow rate, flow structure and flow induced hydraulic excitation forces are preliminary established.Cavitation induced pressure pulsations and vibration characteristics are further studied. The evolutions of pressure and vibration energy versus cavitation number are depicted, and it is considered that the varying trend is closely associated with cavitation status within the model pump. Based on cavitation induced vibration characteristics, we define the 1% head drop point as vibration critical point. At that point, cavitation induced vibration energy increases to a local maximum in low frequency band. It indicates that the conventional 3% head drop criterion of cavitation occurring seems insufficient. From the point of cavitation induced vibration, cavitation could be detected in advance compared with the 3% head drop. As a result, the pump could be prevented working under cavitation condition, besides, the potential damages to the safe operating of the pump system and the impeller due to cavitation erosion effect could also be avoided.Through this research, unsteady flow structure induced hydraulic forces behavior is preliminary obtained. This research would contribute to the theory of designing low noise centrifugal pumps and developing active control techniques of flow induced hydraulic forces. It would of some help for the pump designers and establishing basic design theory of low noise centrifugal pumps.