Investgation On Unstable Flow And Vibration Characteristics Of A Centrifugal Pump

High speed,high-pressure and high-power is the development trend of vehicle-mounted fire-fighting centrifugal pump.The high-power centrifugal pump must be developed in a stable and reliable operation.Also,the performance stability of the pump will be directly affected by the pressure distribution and flow state in the centrifugal pump.The stability of vibration of the centrifugal pump will directly affected by the unstable flow resulted from the pressure pulsation and fluid exciting force in the pump.Therefore,it is of great importance to study the unstable flow characteristics,vibration characteristics and the function law of the unstable flow on the vibration of the centrifugal pump.At the same time,due to different fire-fighting media,the vehicle-mounted fire-fighting centrifugal pump must be equipped with different impeller structures.The most common impeller result is in the form of a closed impeller;while the semi-open and open impellers will be adopted when the fire-fighting media containing impurities are transformed.Therefore,it is also very important to explore the influence of different impeller structures on the stability and reliability of the fire-fighting centrifugal pump.In order to solve the above problems,the performance stability and the stability of vibration as well as the function law of the unstable flow on the vibration in the centrifugal pump for three typical impellers in the closed,semi-open and open models have been studied in this paper through the external characteristic test,dynamic pressure test and vibration test with the combination of numerical calculation and the theory of entropy production.Following conclusions could be obtained through the study:(1)Through the study of the stability test of the centrifugal pump,it’s found that the performance stability of the closed impeller was the best one,while the performance stability of the open impeller was the worst one when the three impellers were transporting clear water medium,especially in the high flow rate condition.Pressure pulsations in the tongue region and pump outlet have great influences on the stability of the pump operation.When the flow rate increases,the influence of the tongue area is decreased with gradually increasing influences in the pump outlet.It was discovered that the closed centrifugal pump had the best stability of vibration,while with the worst vibration in the vibration stability test of the centrifugal pump.The pressure pulsation at the blade passing frequency had a great influence on the vibration stability of the pump casing.And the pressure pulsation at the shaft frequency was a leading factor affecting the vibration stability of the pump rotors;It could be concluded that there was a positive correlation between the flow changes and the vibration of rotors,that is,when the vibration of the rotor system was strengthened,the influence of flow rate on the vibration stability of the pump shell would obtain a opposite result.(2)Based on the numerical calculation and entropy generation theory,the unstable flow structure,flow loss size and position in the model pump have been studied under different impellers and different flow conditions.It was found that the unstable flow structure and the flow loss of the clearance and at the intersection of the impeller flow channel and the balance hole.The flow loss inside the impeller flow channel of the semi-open impeller centrifugal pump was mainly concentrated inside the impeller flow channel near the clearance;The flow loss inside the flow channel inside the open impeller centrifugal pump was mainly concentrated in the middle and rear area of the blade flow channel.There was a larger flow loss at the clearance of wear-ring;moreover,the flow loss was increased,as the flow increased.When the flow rate was small,the flow loss of the outlet tongue region of the impeller flow channel was the largest;and,the flow loss was reduced with the increase of the flow rate.The flow loss in the pump outlet tongue area was increased sharply when the flow rate was large.(3)Fluid-Structual coupling calculation was carried out on the model to study the influence of different impeller structures and different flow conditions on the dynamic characteristics of the model pump.According to the study,it was discovered that the radial force and the von mises stress of the closed impeller were the largest;also,the impeller was vibrated at the smallest speed,when the equivalent stress was concentrated in the impeller ring;the equivalent stress of the semi-open impeller was concentrated at the leading and tailing edges of the blade.The largest radial force and the von mises stress stress of the closed impeller indicated the best vibration performance of the impeller.Therefore,it’s shown that the front and rear covers of the closed impellers had an inhibitory effect on the pump vibration.Under various flow conditions,the radial force of the volute was always larger than that of the impeller;the radial force of the volute was more sensitive to the changes of the flow rates with a more intense fluctuation degree.When the flow rate was small,the impeller and the volute of the model pump were affected the most by the radial force and the fluctuation;the radial force and fluctuation were reduced with the result of reaching the smallest design flow,as the flow increased;the fluctuation of the large-flow radial force was intensified.(4)The relationship among the pressure pulsation,the transient radial force and vibration characteristics of the closed impeller centrifugal pump inside the sub-flow areas(including the impeller ring,volute clearance)of the closed impeller centrifugal pump as well as the affecting law of the condition changes on the pump vibration and the flow loss have been studied simultaneously.Based on the study,it was found that the flow loss of the tongue region in the volute with the largest flow loss was presented in a pulsation feature with the blade passing frequency as the main frequency due to the intervention effect of the blade and the tongue action;When the flow was large,the largest flow in the volute outlet was lost at the outlet of the volute,which was also insensitive to the intervention action;The unstable flow pulsation within the pump was regarded the shaft frequency as the main frequency.The pressure pulsation at the gap of the impeller ring was positively related to the radial force of the impeller and the vibration of the impeller system.The changing trend of the radial force and the impeller vibration has been largely determined by the unstable flow in the clearance of the impeller ring to a certain extent.In small flow conditions,the radial force and the acceleration change of the low-frequency vibration have been dominated by the pressure pulsation in the tongue region of the volute.With the increase of the flow rate,the influence of the tongue area on the radial force and vibration acceleration was gradually reduced.At the same time,the influence of the unsteady flow on the radial force and the acceleration of the vibration at the low-pressure area were gradually increased.The vibration generated due to the unstable flow inside the pump model was mainly gathered in the low frequency;meanwhile,the reflective relation between the vibration energy and the flow could be obtained according to the change law of the vibration energy of the pump casing in the low frequency with the flow.