Study On Transient Characteristics Of Centrifugal Pumps During Power-off Runaway Transient Process

In recent years,with the continuous operation of the field engineering applications,Transient processes of pumps such as rapid start-up,emergency power-off runaway,and rapid opening and closing of valves have occurred frequently,and the problem of stability has become increasingly prominet.In these processes,a scries of external characteristic parameters such as pump rotating speed and flow rate will produce streng instantaneous changes,which makes the internal flow field extremely unstable,the turbulence intensity of the fluid rises sharply,and will undergo various unstable turbulence phenomena such as the formation of boundary layer,flow separation,internal flow vortices,are quite different from the periodic unsteady flow generated in stable conditions,that is,a certain degree of transient effect will be generated,which is the key factor to cause the external instantancoushydraulic impact of the pump unit or the inscantanccus overload of the motor,it will also cause vibration and noise of the unit,which has a serious impact on the stable operation of the unit.Therefore,it is necessary to study the study the transient flow of the rapid transient process of the centrifugal pump.However,at present,the research on the transient process of centrifugal pumps mainly focuses on the start-up process of centrifugal pumps,and there is little research on the numerical simulation of centrifugal pump accidental powrer-off Therefore,this paper conducts an in-depth research on the runaway transition process,the ralvant work contents areas follows:(1)A transient numerical calculation mcthod is established based on ANSYS CFX software and user-dufined program,which can simulate the power-off runaway transient characteristics of a centrifugal pump.The variation rule of external characterstic parameters(rotational speed,flow rate)with time and the dynamic characteristics of the internal flow field can be obtained through numerical calculations.The comparison of results show that the simulation results agree well with experimental results,the proposed numerical method hasbeen therefore validated.The variation of the working parameters of the centrifugal pump were analyzed,including the external characteristic parameters,impeller inlet velocity moment,impeller inlet axial surface velocity,etc.In addition,the instantaneous characteristics of the internal flow field of the suction pipe and impeller are analyzed specifically,and the changes of the impeller blade surface pressure in this process are calculated and analyzed.(2)By setting pressure monitoring points in each flow components,the pressure changes of the suction pipe,impeller,guide vane,and volute during the power-off runaway transient process,and the reasons for the vibration of the centrifugal pump during the runaway process were revealed.In this paper,the dynamic mode decomposition method is used to analyze the characteristics of the flow field snapshot during the whole runaway process.The contribution and influence of flow structures with different frequencies on the flow field can be intuitively obtained by ordering the energy according to the modal norm,and then the modal and its eigenvalues which can represent the flow field information can be obtained.By comparing and analyzing the Fourier transform results of each component with the dynamic mode decomposition results,it can be found that the dynamic mode decomposition method has a higher frequency resolution under the same conditions,and each mode has a only frequency and magnification,which can analyze the stability and attenuation speed of any specific characteristic frequency in the flow field,which has a great advantage for the analysis of periodic flow.(3)In order to accurately get the energy loss of each component of the centrifugal pump during the power-off runaway transient process,based on the entropy production theory,the location and size of the flow field energy loss of each component in the process were qualitatively analyzed,and the mechanism of energy loss production and the relationship between the instantaneous change of internal and external characteristics were further explored.The results show that there is a clear correlation between the change in entropy production value and the external characteristics of the centrifugal pump during the entire accidental power-off runaway process,and the energy loss is closely related to a series of undesirable flows such as flow separation and vortex;The main losses of impeller,guide vane and water suction pipe are caused by turbulence dissipation,while the losses of volute area are mainly caused by the strong wall effect near the wall surface,the main loss of the two The mechanism is different.By analyzing the variation of the local entropy production rate distribution of each component flow field with time,it can be concluded that the flow field energy loss of the centrifugal pump is the largest under the braking condition.