Research On Cavitation And Pressure Pulsation Of A High Specific Speed Centrifugal Pump

As a kind of general machinery,centrifugal pump plays an important role in the national economy and production.Cavitation is an important parameter that affects the performance and operation stability of centrifugal pump.At the beginning of cavitation,the performance of centrifugal pump declines.In the process of continuous cavitation,vibration and noise will appear in the running process of the pump body,which will affect the safety of operation.Serious cavitation will lead to a steep performance decline,because bubble has been filled with the whole cavity and pipeline during this time.Among them,the narrow and long characteristics of the flow passage and blade structure of the centrifugal pump with high specific speed make the performance change of cavitation at the initial stage more sluggish than that of the conventional speed,moreover it becomes more difficult to judge cavitation.At present,the traditional determination method is based on the proportion of head decline,but the hysteresis of this method is difficult to avoid.Therefore,it is very important to put forward an effective cavitation monitoring method to prevent the cavitation of fluid machinery and ensure the stable operation state.This article taked the centrifugal pump with high specific speed as the research object,and combined the two research methods of experiment and numerical simulation to compare the accuracy of different primary cavitation determination methods for the flow field characteristics and pressure pulsation in different cavitation stages of the centrifugal pump with high specific speed.The main research contents and conclusions are as follows:1.Compared and analyzed different turbulence and cavitation models.Based onIV the standard k-ε turbulence model and Kunz cavitation model,the flow field in the cavitation of centrifugal pumps at different cavitation stages was analyzed,and the time-frequency characteristics of different monitoring points along with the development of cavitation,as well as the pump body vibration were predicted.We found that CFD numerical simulation can accurately predict the performance of centrifugal pump.In the process from uncavitation to cavitation development,the vortex core at the impeller and outlet of centrifugal pump increases obviously.The low pressure area is spreading radially from the inlet of the impeller to the outlet.The amplitude of the pressure pulsation monitoring points of the centrifugal pump also decreased significantly,and the amplitude of the pressure pulsation at the tongue was the largest.2.Based on the traditional cavitation judgment,high-speed photography was conducted to divide the development level of cavitation.When the cavitation coefficient drops to 0.478,a small number of cavitation highlights can be seen in the impeller runner.We define this point as the initial cavitation point.According to the cavitation number defined by high-speed photography,the cavitation number 0.478 is greater than the initial cavitation number 0.425 defined by traditional energy method,it indicating that when cavitation begins to appear in the centrifugal pump,the flow-head characteristic of the centrifugal pump is not affected by cavitation.Therefore,the accuracy of cavitation initiation using high-speed photography is higher than the traditional method.3.The cavitation test and high-speed photography were carried out on the closed test platform while the outlet pressure pulsation was simultaneously collected.The Fourier transform analysis showed that cavitation signals were mainly concentrated in the range of 0~1000Hz,and the pulsation amplitude when cavitation occurred was significantly reduced compared with that before cavitation,and the high-frequency components were also increased.The sensitivity of five determination methods under rated working condition was compared,The order of critical cavitation number is wavelet energy method(σ=0.486),pulse method(σ=0.483),high speed photography method(σ=0.478),numerical simulation(σ=0.425),traditional determination method(σ=0.423).4.The advantages and disadvantages of FFT and wavelet decomposition applications are compared,and the implementation process of wavelet analysis is introduced.The data is analyzed by wavelet analysis from threshold selection,signal reconstruction and denoising.We will collect different cavitation degree of export of pulsating six layers of wavelet decomposition,the data will be divided into different frequency in the frequency domain,and solve the characteristics of each layer of energy.It found that with the emergence of cavitation,different frequency amplitude will have significant change,the corresponding energy eigenvalues have Ea6,Ed5 and Ed6 corresponding changes of energy.The characteristic energy values of Ea6,Ed5 and Ed6 at rated working conditions increased,Ea6 increased by 38.7%,Ed5 increased by 14.2% and Ed6 increased by 58.1%.Other flow patterns were similar.Therefore,it is feasible to judge the cavitation initiation according to the energy variation amplitude of wavelet analysis.5.Based on the primary judgment results of high-speed photography cavitation,the accuracy of five different primary judgment methods,such as traditional test methods,high-speed photography methods,pulsation test methods,numerical simulation and wavelet analysis,are compared in the paper,and the corresponding methods are found The relative error value and cavitation coefficient are different.The numerical simulation method and the traditional method have similar errors,which is the largest error among the five determination methods,and the relative errors of the remaining three methods are within 5%.At the same time,combined with the comparison of the initial cavitation points of the five methods,wavelet analysis is determined as the analysis and processing method of cavitation signal data.Wavelet analysis based on cavitation pressure pulsation data can make an early prediction for cavitation diagnosis of high specific speed centrifugal pumps.