Experimental Study On Radial Clearance Cavitation Of Rolling Piston Compressor

The radial clearance structure of rolling piston compressor is a venturi structure of "open compact open".During the operation of the compressor,cavitation will occur easily in the oil film of the radial clearance.Cavitation not only causes noise,vibration and cavitation,but also destroys the sealing performance of radial clearance oil film.Therefore,the research on radial clearance cavitation of rolling piston compressor is of great practical significance to optimize compressor performance.In this paper,the type of cavitation bubble in radial clearance is discussed by calculating the oil film pressure distribution and cavitation number in radial clearance.The "audio visual coupling" experimental method of "high-speed photography + acoustic emission technology" is used to observe the cavitation field of compressor.Firstly,the cavitation phenomenon of the self-designed equivalent compressor radial clearance is experimentally studied by using high-speed photography,and the results are obtained At the same time,the intensity of cavitation under each condition is evaluated.Secondly,the acoustic emission signal of the equivalent compressor is collected by using the acoustic emission acquisition technology.After the processing and analysis of the acoustic emission signal,the corresponding signal frequency band of cavitation is obtained,and the cavitation phenomenon is calculated Time and intensity of cavitation under various conditions.Finally,by comparing the experimental results of high-speed photography and acoustic emission acquisition,the relationship between cavitation and the change of acoustic emission characteristic parameters is established.The main contents of this paper include the following aspects:(1)According to the structure and working principle of the rolling piston compressor,the volume change of the two chambers and the pressure change curve of the chamber are calculated.The modified Reynolds equation of two-phase flow is established and solved to obtain the pressure distribution in the oil film of the radial clearance during the operation of the compressor.The results show that the pressure drop begins to appear at about 8 ° before the radial clearance(toward the compression cavity).In the analysis of cavitation number,it is found that the higher the fluid velocity is,the easier cavitation will occur.Considering the maximum speed of experiment is 1300 rpm,based on the oil film pressure distribution,the cavitation number of each position in the radial clearance oil film at 1300 rpm is calculated.The results show that the cavitation number decreases from the high pressure cavity to the low pressure cavity,and it is larger than that commonly used to judge cavitation in fluid machinery.Finally,according to the definition and formation mechanism of cavitation type,it is judged that the cavitation type of equivalent compressor is similar to cavitation.(2)A set of equivalent compressor device is designed,and the cavitation experiment of radial clearance is carried out by using high-speed photography technology.Through the experiment of changing the equivalent compressor speed and the pressure difference between two chambers,the occurrence of cavitation in two factors at different levels was obtained,and the evolution process of cavitation bubble from primary generation to collapse was observed.According to the definition,cavitation bubble was judged as cavitation bubble,and the initial generation angle of cavitation bubble was observed between180 ° and 210 °.When evaluating the cavitation intensity under various conditions,it is found that the pressure difference has a greater impact on cavitation than the speed.In addition,the change process of oil film width is analyzed,and the change trend of oil film width before cavitation is obtained by means of fitting,and the angle of theoretical breakdown of seal oil film is calculated,which is compared with the angle of actual breakdown.The results show that when cavitation occurs,the difference between the calculated breakdown angle and the actual breakdown angle is greater,that is,the cavitation phenomenon plus The failure process of sealing oil film is accelerated.(3)The cavitation monitoring of the equivalent compressor is carried out by using the acoustic emission technology,the cavitation acoustic emission signal is collected,and the obtained signal is denoised by using the wavelet soft threshold denoising method.The results show that there is a 130-170 k Hz high frequency component in the corresponding spectrum of cavitation experimental group,which can be preliminarily judged as the corresponding frequency band of cavitation.Using short-time Fourier transform to analyze the signal in time-frequency domain,the results show that the time of high-frequency components of 130-170 k Hz in the time-frequency map corresponds to the time of cavitation bubble initiation in the image data of high-speed photography cavitation observation experiment,so the corresponding frequency band of cavitation is more determined.Based on this,the wavelet energy characteristics of 125-250 k Hz band including cavitation band are calculated to evaluate the cavitation intensity under various conditions.The results show that compared with the speed,the pressure difference has a more significant impact on the cavitation degree.At last,the relationship between AE event count,ring count,RMS change and cavitation is analyzed.The results show that when cavitation occurs,these characteristic parameters increase in varying degrees,which can be used as a basis for judging cavitation.(4)Finally,the results of high-speed photography and acoustic emission acquisition device are compared and analyzed.The results show that the conditions of cavitation,the location of cavitation and the intensity of cavitation evolution are consistent.High speed photography experiment can "see" a more intuitive cavitation evolution process,and can judge the type of cavitation,and "see" the impact of cavitation on the sealing oil film.In the acoustic emission experiment,after combining the "seen" results,the correlation between signal spectrum,characteristic parameter change characteristics and cavitation is established,that is,when 130-170 When the peak value of k Hz high frequency component appears,51.25% of the maximum average amplitude of the interval is used to determine the time point of cavitation phenomenon;meanwhile,32.37% increase of event count,23.18% increase of ring count and 21.89%increase of RMS are determined as the judgment basis of cavitation phenomenon.