Study On Cavitation Acoustic Response Characteristics And Cavitation Suppression Method Of Centrifugal Pump

The growth,contraction and collapse of cavitation is a complex dynamic process.The whole process will cause distorted flow in the flow field of hydraulic machinery,which has the characteristics of phase change,highly unsteady and multi-dimensional turbulence,and will destroy the normal energy exchange process of hydraulic machinery.How to effectively monitor the occurrence of cavitation in hydraulic machinery or more effectively suppress cavitation and improve the cavitation performance of hydraulic machinery has always been an urgent problem to be solved in the field of hydraulic machinery.Therefore,it is of great significance to study the cavitation monitoring methods and cavitation suppression of hydraulic machinery.When cavitation occurs in hydraulic machinery,strong noise is its remarkable feature.It can be seen that there must be a close relationship between cavitation flow field and cavitation noise.This paper takes single-stage single suction centrifugal pump as the research object.By means of theoretical analysis,numerical calculation and experimental measurement,the unsteady flow field structure and acoustic characteristics of centrifugal pump in different cavitation stages under multiple flow rates were studied.At the same time,the cavitation suppression method of centrifugal pump is explored,in order to obtain the change law of the acoustic characteristic information of the cavitation noise of centrifugal pump,explore the mapping relationship between the cavitation flow field in the centrifugal pump and the noise,obtain the cavitation monitoring method based on acoustics,and seek the effective improvement method of the cavitation performance of centrifugal pump.The main work and creative achievements are as follows:1.A centrifugal pump cavitation noise test-bed was built,and the experimental research on the characteristics of cavitation induced noise of centrifugal pump was carried out.The fluid noise characteristics and frequency spectrum characteristics of inlet and outlet under different flow rates were revealed.The test shows that:The total sound pressure level of inlet cavitation noise is lower than that of outlet noise in intensity,and the internal noise propagation of centrifugal pump shows strong directionality.With the development of cavitation,the inlet and outlet noise show the opposite development law.The total sound pressure level of inlet noise decreases significantly,while the total sound pressure level of outlet noise increases first and then decreases.According to the NPSHa value at the inflection point where the total sound pressure level at the inlet and outlet changes,the occurrence of cavitation can be predicted earlier.The spectrum characteristics show that the cavitation noise at the inlet and outlet under different flows is mainly concentrated in the medium and low frequency band below 300Hz,mainly in the axial frequency,leaf frequency and their frequency doubling.The impact of cavitation on the inlet noise is greater than that at the outlet.2.Based on the power spectrum and octave analysis method,the variation law of cavitation noise energy at the inlet and outlet of centrifugal pump in different cavitation development stages under different flow rates is analyzed and studied.The test shows that:The cavitation flow field of centrifugal pump has a significant influence on the power spectral density distribution of inlet and outlet noise,and shows a certain variation law.The change law of inlet cavitation noise is the most obvious.With the decrease of NPSHa,the discrete noise power spectrum below 300Hz has a sudden drop.The NPSHa at the sudden drop point corresponding to different flows is different,but they are all greater than the critical NPSHa point determined by the traditional method.It can be seen that the occurrence of cavitation in centrifugal pump can be predicted according to this feature.On the whole,the power spectrum of cavitation noise at the exit test point shows the law that the local broadband noise first strengthens and then weakens,while the discrete noise intensity changes little.The 1/3octave analysis shows that the sound pressure levels of the inlet and outlet noise bands of the centrifugal pump change significantly at different center frequencies with the decrease of npsha at different flow rates.3.Based on the theory of wavelet analysis,the noise reduction and wavelet decomposition of the internal field noise of centrifugal pump are carried out,the frequency information characteristics of different decomposition layers are studied and analyzed,and the relative energy characteristics of the corresponding frequency signals of each decomposition layer are obtained according to the change of the relative energy of wavelet coefficients.The test shows that:The inlet and outlet noise of centrifugal pump has continuous frequency components in each decomposition layer,and the periodicity of the signal is more obvious in the lower decomposition layer.The relative energy values of wavelet coefficients at the inlet and outlet layers are affected by the development of cavitation,and the energy fluctuation range of each layer is large,which can characterize the development degree of cavitation,however,the affected layers of inlet and outlet are different.The relative energy variation of the wavelet coefficients of the inlet noise mainly concentrates on the detail coefficients from d3 to d7 and the approximate coefficients of a7 layer.The specific fluctuation characteristics have a certain relationship with the flow.The change of the wavelet coefficient of the outlet noise relative to the energy is relatively concentrated,which is mainly distributed in the detail coefficient from d5 to d7 and the approximate coefficient of a7 layer.The wavelet time-frequency diagram shows that under the same NPSHa,the frequency intensity of the cavitation noise at the inlet and outlet of the centrifugal pump is stable in the whole sampling time,and there is no random mutation frequency component,indicating that the influence of the cavitation flow field on the cavitation noise of the centrifugal pump under the same NPSHa is relatively stable,and the change of the noise signal spectrum and related parameters can characterize the cavitation state of the centrifugal pump.4.The cavitation flow field of centrifugal pump under the flow rates of 0.8Q_d,1.0Q_d and1.2Q_d was studied and analyzed by numerical calculation method.The results show that:Cavitation has a significant impact on the changes of parameters such as pressure,velocity and vorticity in the internal flow field of centrifugal pump.The change of vorticity in cavitation flow field is closely related to the growth and evolution of cavitation.Affected by the asymmetric structure of the volute,the cavitation presents an asymmetric distribution in each channel,which leads to the change and distribution of parameters such as pressure,velocity and vorticity in each channel.5.Based on the broadband noise method,the sound power generated by the turbulence in the centrifugal pump during the development of cavitation at each flow rate is calculated,and the evolution law of broadband noise sound power in the cavitation flow field is studied and analyzed.The results show that:The change of sound power intensity in the internal flow field of centrifugal pump is closely related to the change of turbulent kinetic energy intensity,and the distribution of turbulent kinetic energy is affected by vorticity and cavitation.6.The pressure pulsation characteristics of the vane and the diaphragm in the centrifugal process are studied respectively.The results show that:The main frequency of pressure pulsation at the inlet,volute and tongue wall is the blade frequency BPF and its multiplier,while the main frequency of pressure pulsation on on the blade wall is the axial frequency APF.With the development of cavitation,the amplitude of pressure fluctuation at the pump inlet decreases sharply,and the main frequency changes from blade frequency BPF to shaft frequency APF,which is the same as the development law of discrete noise in the inlet field in the test.In the process of cavitation,the pressure fluctuation on the working face of the blade is stronger than that on the back,and the points with large pressure fluctuation will change with the expansion of the cavitation area.The development of cavitation has little effect on the pressure fluctuation on the volute and the wall of the volute tongue,which is greatly affected by the flow,and the change of the pressure fluctuation intensity at the blade frequency BPF and its frequency doubling is small.It is the same as the development law of discrete noise in the pump outlet field in the test.The above shows that unsteady pressure pulsation is the main inducement of cavitation discrete noise.7.Based on the theory and method of acoustic analogy,the radiation characteristics of internal and external field noise induced by dipole sound sources of centrifugal pump shell and impeller are studied.The results show that:The development of cavitation has a significant impact on the internal and external noise radiation of centrifugal pump.The noise induced by impeller accounts for the main position of cavitation noise of centrifugal pump.The structural mode of pump body has an important influence on the cavitation noise radiation of centrifugal pump.The radiation distribution of external field noise is affected by the structure of pump body,which has obvious directivity.8.The improvement and improvement of cavitation performance of centrifugal pump are explored and studied.The research shows that:The cavitation performance of centrifugal pump can be improved to a certain extent by adjusting the position of balance hole of centrifugal pump without adding and modifying impeller structure.The improvement degree is related to the opening position of the balance hole.At the same time,the axial force can be further reduced on the original basis after changing the position.