Research On Cavitation Flow In Low Specific Speed Centrifugal Pump

As one kind of centrifugal pumps,low specific speed centrifugal pump has the characteristics of low flow-rate and large head,which is widely used in modern society.With the development of modern industry and human society,the demand for centrifugal pumps are gradually trending for high-speed.However,some challenges to the reliability and stability of the pump system were brought by the high speed of centrifugal pumps,and cavitation is an important factor that influencing efficiency and running stability of centrifugal pumps.Cavitation is one of the most popular and difficult researching topics in hydraulic machineries.Based on theoretical analysis,numerical simulation and experimental research,the flow characteristics and pressure fluctuation in a low specific speed centrifugal pump under different cavitation stages were studied in this thesis.The cavitation development processes of the centrifugal pump is deeply understood,the formation mechanism of asymmetric cavitation of the centrifugal pump was revealed,and the influence of balancing-hole diameter on cavitation flow was analyzed too.The main research contents and creative achievements of this thesis include:(1)Based on the open test platform system,the high-precision hydraulic performance and cavitation performance of the low specific speed centrifugal pump were tested.The RNG k-εmodel was modified by FBM method,and used in the single-phase flow field of the centrifugal pump simulation.By the comparation with the experiment reaults,the reliability of FBM RNG k-ε model in the numerical simulation of the low specific speed centrifugal pump was verified.By analyzing the internal flow field of the impeller and volute,it is found that the inlet flow distributions of each passage of the impeller is quite different under different flow conditions.The applicability of ZGB model,Kunz model and Schnerr-Sauer model in cavitation flow simulation of the low specific speed centrifugal pump were studied.And ZGB cavitation model is the most suitable model in the centrifugal pump cavitation simulation.(2)The ZGB cavitation model was used to simulate the cavitation flow of the low specific speed centrifugal pump.It is found that cavitation firstly occurs near the impeller ring and balancing-holes,and the lower the flow rate is,the more obvious the phenomenon is.Under the condition of cavitation,the high-intensity vortices mainly concentrate in the rotating region,and the volume of vortex core increases with the increase of cavitation.At the inlet of the impeller,the asymmetric distribution of cavitation is resulted by the non-uniformity of the flow in each flow path.At the condition of low flow rate,the development of cavitation in the impeller ring region has a great influence on the inlet radial velocity of each channel.With the increase of the flow rate,the critical cavitation velocity on the blade near the wall first increases,then decreases,and then increases,and there is countercurrent on the suction side.A reverse jet is observed at the end of the cavity,but the shedding of the end cavity is not caused by the reverse jet.The distribution of radial force of the impeller is corresponding to the number of blades.With the deepening of cavitation,the radial force of the impeller increases gradually.In addition,cavitation will destroy the regularity of radial force direction.(3)Monitors were setted in impeller inlet,impeller passage and the volute to detect the pressure pulsation of the centrifugal pump.The pressure pulsation in the impeller of low specific speed centrifugal pump is not periodical.The main frequency of the pressure pulsation is the rotation frequency or the doubling of the rotation frequency.When cavitation occur,the main frequency becomes the doubling of the rotation frequency.The time-domain distribution period of the pressure fluctuation in the volute is consistent with the number of blades,and the fluctuation at the tongue is the most serious,where the main frequency is the blade frequency,and is not affected by cavitation.The amplitude of the main frequency of the monitoring points in the impeller increase with the deepening of cavitation,while that in the volute is opposite to that in the impeller,and the amplitude of the main frequency of the pressure fluctuation in the volute with cavitation is smaller than that of non cavitation.(4)The cavitation performance,vapor distribution and axial force distribution of the low specific speed centrifugal pump with different specific area k of balance holes were studied.The results show that the anti cavitation performance of the low specific speed centrifugal pump increase with the increasing of the specific area of the balance holes.With the increase of k value,the balance chamber pressure of the low specific speed centrifugal pump decreases gradually,and the jet velocity of balance holes decrease too.When cavitation occurs in the impeller,the axial force points to the positive z-axis and increases with the decrease of cavitation number.In the critical cavitation state,when the specific area k = 1,the axial force points to the positive half axis of z-axis and decreases with the increase of k value.When the specific area k ≥ 3,the axial force turns direction and increases with the increaseing of k value.