Investigation On Cavitation/Cavitation Erosion Characteristics And Prediction Method Of Multiphase Flow In Valves And Associated Pipes

The high temperature,high pressure and complicated medium are easy to cause the cavitation erosion of throttling decompression device in the chemical process,resulting in the perforation and leakage accident of equipment.The pressure relief valve and its connected pipes are typical equipments with frequent failures,and the upgrading of materials is difficult to ensure the long-term stable operation of device for cavitation erosion damage.For early warning of cavitation damage in valve and its connected pipes and even eliminating the cavitation erosion in cavitation flow,there are great significant to stud the cavitation and cavitation erosion characteristics of valve type structure by combining experiments and numerical simulation method,and to explore the prediction method of cavitation and cavitation erosion.In this paper,the test section with valve model structure components and visual observation window is manufactured based on the actual pressure relief valve.Through the experimental research for cavitation and cavitation erosion characteristics with different conditions by our own cavitation tunnel device,the variations of cavitation and cavitation erosion phenomenon with cavitation number and time are obtained.Then the numerical simulation research for test section is carried out,and the coefficients of cavitation model are corrected by combining the results of cavitation experiment,a more accurate result of numerical reconstruction for cavitation phenomenon is obtained.Finally,in order to make the prediction of cavitation erosion,the condensation rate of vapor phase is considered as characterization parameter based on the improved cavitation model.And the reliability of the prediction method is verified through the analysis of practical failure case with cavitation erosion.The main work and conclusions are as follows:(1)The cavitation characteristic lines and characteristic length that the lines cross the cavitation area are obtained in visual cavitation experiment by the method of image post-processing,and the variation of cavitation phenomenon can be reflected by characteristic length.There are three typical cavitation phenomena are observed with the decrease of the cavitation number:(a)the length of attached cavitation periodical changed significantly,but there is no cavitation shedding;(b)the length of attached cavitation changed in a small range,and the periodicity is not obviously;(c)Accompanied by the phenomenon of cavitation shedding and collapse,the length of cloud cavitation changed significantly with periodicity.(2)The cavitation phenomenon is not symmetric.As the cavitation number decreases,the incipient cavitation will appear at the top valve seat,the bottom valve seat and the head of valve plug in turn at the cavitation number of 1.72,1.55 and 1.33.The cavitation shedding is only occurred at upper region.The cavitation length will increase with the decrease of cavitation number,and the growth rate is gradually increased.(3)In the cavitation erosion experiment,the length of erosion region on the surface of aluminum foil basically remains the same,and the area of erosion pits is gradually increased with the experimental duration.There is a sharply increase for erosion area between 15 min to 30 min,and finally tend to be stable at 60 min.In this research,the bottom valve seat has a larger erosion area that without cavitation shedding,and suffer more serious cavitation damage.(4)The two-dimensional numerical simulation of test section is conducted,and different model coefficients of Zwart-Gerber-Belamri cavitation model are comparative analyzed.The cavitation length and intensity of top valve seat are significantly reduced while decreasing the evaporation coefficient Fv or increasing the condensation coefficient Fc,the cavitation length and intensity of bottom valve seat are significantly increased while decreasing Fv or increasing Fc.Finally,for stable attached cavitation and transient cloud cavitation,the better results are obtained by improved cavitation model with coefficient group of Fv=200 and Fc=0.1.(5)Based on the improved cavitation model,the condensation rate of vapor phase is considered as characterization parameter to predict the region of cavitation erosion.The results have good accuracy comparing to the experiment.They have same results for the incept location of cavitation erosion,and an error of 9% for cavitation length.There are close relations between the location and range of vapor condensation region and the morphological characteristics of cavitation.So,the accurate numerical simulation of cavitation phenomenon is the precondition of cavitation erosion prediction.And the prediction methods for cavitation and cavitation erosion in this paper get good results in practical application.The innovation of this thesis is:(1)Based on the cavitation tunnel device and aluminum foil method,the visual cavitation experiment and cavitation erosion experiment are carried out.The results get several typical cavitation phenomena,the variations of cavitation type,cavitation length,and the location and area of cavitation erosion region with time scale are analyzed.(2)Combined with the cavitation characteristics experiment,improved cavitation model with coefficients of Fv=200 and Fc=0.1 is conducted.And then the condensation rate is used for prediction of cavitation erosion,good results are obtained both in experiment and practical application.