Research On Cavitation Diagnosis And Its Characteristics Of Flow Field And Acoustic Field In Centrifugal Pump

This study is financial supported by the National Natural Science Fund of China(No.51509111,51779106).In order to meet the rapidly developing needs of the national security strategy equipment field,centrifugal pumps as an important energy conversion equipment,are gradually developing in the direction of high speed and high performance.While cavitation is the core issue that limits its further development.Mastering the generation mechanism of cavitation induced vibration and noise in centrifugal pumps,studying the effect of cavitation on the structure of unsteady flow field,proposing a reliable cavitation detection method,and establishing a reasonable cavitation stage evaluation criteria,can not only provide a theoretical basis and technical support for fault diagnosis of hydraulic machinery,but also can greatly increase the reliability of equipment operation and increase the service life.In this paper,the theoretical analysis,experimental and numerical investigation are used to study the variation law of cavitation signal in centrifugal pump and the characteristics of flow field and sound field distribution under cavitation conditions.The aim is to establish a high-sensitivity and high-reliability method for detecting the cavitation of centrifugal pumps,to master the unsteady flow characteristics of centrifugal pumps under cavitation conditions,and to propose a reasonable cavitation noise prediction method.The main research contents of this dissertation are below:1.The mechanism and main forms of cavitation and erosion is explained,the research progress of numerical calculation of cavitation flow field and cavitation acoustic field is introduced,and the research status of judging method of cavitation in the centrifugal pump is summarized.2.Taking a n_s=117.3 centrifugal pump as the research object,based on the visualized closed test bench,the pump performance parameters,the distribution pattern of the bubbles and the vibration and noise signals are collected synchronously,and the variation law is studied.According to the results of high-speed photography experiments,different cavitation detection methods based on the total level values of liquid-borne noise,solid-loaded vibration,and air-borne noise are proposed.The results show that,the highest sensitivity of cavitation detection is based on high-speed photography;the cavitation thresholds based on the total level values of the liquid-borne noise,the solid-borne vibration,and the air-borne noise are 0.5%,0.5%,and 0.7%respectively.Compared to the solid-borne vibration and air-borne noise,the method based on total sound pressure level of liquid-borne noise has high sensitivity and is less affected by the environment.3.On the basis of the traditional cavitation stage division method,taking the relationship between the change of each signal quantity and the development of cavitation into account,a more elaborate cavitation stage division method is proposed.Based on the distribution of cavitation coefficient corresponding to the maximum value of each signal magnitude curve,taking the relationship between cavitation development and energy loss into account,it is found that the cavitation based on extremum point of the solid-borne vibration is more severe comparable to the cavitation extremum point of the liquid-borne noise.The serious cavitation stage and over-serious caviattion stage is divided according to extremum point of the solid-borne vibration.Taking the 3%drop in head as the critical point of cavitation,the developed cavitation stage and the serious cavitation stage is divided.The developed cavitation stage and onset cavitation stage is divided by a 0.5%increase in the total sound pressure level of liquidborne noise.The non-cavitation stage and the onset cavitation stage is divided by the visible cavitation point.4.Due to the restrictive nature of high-speed photography,in order to further improve the accuracy of the cavitation judging method,by comparing the variation of the flow coefficient and the cavitation coefficient on the vibration and noise spectrum,acquired the frequency band for cavitation detection.And the 3?principle is used to study the cavitation threshold values of vibration and noise signals.The method of wide-band sound pressure level of liquid-borne noise in the 2000~3000Hz frequency band increased by 1%,the wide-band vibration acceleration level of solid-borne vibration in the 10~50Hz frequency band increased by 1%,the wide-band vibration acceleration level of solid-borne vibration in the 1000~3000Hz frequency band increased by 1.1%,have higher sensitivity to cavitation.5.By means of an ultra-low specific-speed centrifugal pump with a specific speed n_s=25,further quantificational analysis the change rules of each signal.Different cavitation judging methods are analyzed to verify the accuracy and reliability of the this methods.The rationality of the cavitation stage division method and the accuracy of the cavitation determination method based on the total semaphore values is verified.The cavitation spectrum interval is done statistics verifing the frequency band for cavitation detection,and the reliability of the cavitation thresholds for each frequency band.6.In order to obtain the effect of cavitation on the internal flow field characteristics of the pump,by selecting the appropriate turbulence model and cavitation model,the internal flow characteristics of centrifugal pump at different cavitation stages are simulated.The results show that:With the decrease of the cavitation coefficient,cavitation occurs first in the vicinity of the inlet edge of the suction surface of the blade,and gradually develops to the direction of the impeller outlet and pressure surface along with the blade suction surface.The high flow rate condition is more apt to cavitation than the low flow rate condition;the vortex cavitation is formed due to the inlet reflux,which results in the uneven distribution of the cavitation bubbles in the blades of the impeller.With the decrease of the cavitation coefficient,The main frequency of pressure pulsation gradually changes from blade passing frequency(BPF)to axial passing frequency(APF),the characteristic components at BPF and APF increase,while the high frequency pulsation decreases.The radial force of the impeller decreases,and its distribution regularity deteriorates.7.In order to study the effect of cavitation on the acoustic field distribution of pumps,a cavitation-induced noise calculation model is constructed.For the liquid-borne noise,the far-field and near-field noise distributions in different cavitation stages are calculated based on the boundary element method and the Proudman semi-empirical model method respectively.For air-borne noise,the flow noise and flow-induced noise analysis models under different sound sources are established,and the sound pressure distribution and contribution of different nature noise sources are analyzed.The results show that:After cavitation occurs,the turbulent kinetic energy,turbulent kinetic energy dissipation rate and near-field acoustic power of liquid-borne noise gradually increase,with the decrease of the cavitation coefficient.The sound pressure level at characteristic frequencies of the liquid-borne noise and the air-borne noise shows a law of decreasing first and then increasing.The sound pressure level at outlet monitoring point is larger than the inlet monitoring point.The sound pressure distribution of air-borne noise is different under different acoustic sources.The sound pressure directional distribution of flow-induced noise and flow induced structure noise of air-borne noise under the same acoustic source are not the same.The radiated energy of flow-induced structure noise provides a major contributor to air-borne noise.