Investigation Of Cavitation Instabilities And It's Control In A Centrifugal Pump

The cavitation phenomena are commonly imperative in the process of operating hydraulic machinery.The damages of cavitation mainly inhabit three facets going as follows: the first one is that the machinery's performance is dramatically degraded by cavitating flow;the second is the material surfaces are broken by the collapsed bubble when delivering to the high-pressure region caused by interial development,which are associated with the problems of vibrating and noising;the last one is that the cavitation influences the characteristics of both transient flow and dynamic response.The flow instabilities occur with the changes of dynamic response including rotating cavitation and cavitation choke etc.These instabilities result in the massive fluctuations of flow rates and pressures,which are responsible for the unsteady rotor dynamics and the structure damages.Currently,the studies on the unsteady flow instabilities in hydraulic machinery still stay in the initiative stage related to an interesting and difficult concern.It is almost impossible to completely eliminate the cavitation damages in terms of engineering applications.Therefore,the research directions have adjusted to reduce and suppress the negative effects of cavitation instabilities,namely cavitation control.The present methods to control cavitation include re-arranging the pipe configurations,mounting the obstacle plate and circumferential grooves,air injecting and so on,which perform limited effects on some specific cavitating pattern instead of perfect control.The mechanisms of suppressing are still not totally clear.So,it is definitely meaningful and worthwhile to explore and discover practical theory and technique on cavitation instabilities in order to improve the cavitation performance,build energy-saving system,constitute environment-friendly modern hydraulic machinery.This research was based on the numerical and experimental means to investigate the cavitation instabilities in a centrifugal pump.It was proposed to suppress cavitation by obstacles attached on the pressure surface,which classified into an active category.The effects of obstacles on steady and unsteady cavitating flow were comprehensively studied.The optimum geometrical parameters of suppressing were obtained by investigations on the inverse problem,which have developed into a primary design system with the meaning of widely spreading and instructing.The primary coverage and creative achievements are focusing on:1.Sum up the current research status on cavitating flow instabilities at home and abroad.Including: the transient flow theory in hydraulic machinery related to rotating stall and choke flow instabilities;the cavitating flow instabilities in hydraulic machinery,such as the necessary conditions and dominant features of backflow vortex cavitation,rotating cavitation,cavitation choke and alternate blade cavitation.Meanwhile,sum up the frontier research progresses on cavitation control at home and abroad.Including: the control mechanism and available range of various manipulations,such as the modification of blade profile,re-arranging the blade configuration,obstacles attached on the hydrofoil surface to stop the re-entrant jets,enlarged hydrofoil roughness to advance the separation of boundary layer in order to delay the cavitation inception,air or water injection to improve the boundary layer velocity distributions for cavitation suppression,changing the spanwise circulation distributions to delay the inception of tip-vortex cavitation,polymer injection into the vortex core to suppress tip-vortex cavitation,accumulators and circumferential grooves mounted at the pipe to suppress the backflow cavitation and so on.2.Based on the closed centrifugal pump test platform,the high-precision tests was conducted to measure the hydraulic performance,cavitation performance and pressure fluctuations in a centrifugal pump in order to investigate the response relationships between cavitating pattern and pressure excitation.The parimary contens include: external performance curves,cavitation curves,pressure pulsations under different flow rates without cavitation,pressure pulsati ons under designed flow rates with cavitation.3.Observe and analyze the mechanisms of cloud cavitation shedding on Clark-Y hydrofoil objective by Large Eddy Simulation(LES)method.Meanwhile,observe the re-entrant jet and side-entrant jet to investigate the unsteady flow characteristics caused by cvitating flow.Derive continue and momentum equation based on one-element theory on cavitating flow instabilities.Verify and validate the conclusions of the flow rates fluctuations equal to the bubble growth rat e corresponding to the first derivative of cavity volume,the pressure fluctuations are linear to the acceleration of bubble,namely the second derivative of cavity volume.4.Numerically simulate the 3D unsteady flow in a low-specific speed centrifugal pump combined with the modified SST k-? turbulence model and Kubota cavitation model.The hydraulic experiments are conducted on the closed centrifugal pump test stand in Key Laboratory of Fluid Machinery and System affiliated to Lanzhou University of Technology.The experimental data validates the accuracy of numerical results.The conclusions show that the re-entrant jet is not the determinant reason for cavities shedding,and the magnitude of re-entrant jet velocity is extremely smaller than that of main flow.The variation of monitoring blade inlet angle indicates that the flow close to the suction surface is the separation flow dominated by adverse pressure gradients.That smaller reverse flow does not specifically contribute to the cavity shedding events.Interpret the reasons for asymmetric cavitation patterns in impeller passage caused by asymmetric volute part.5.Arrange obstacles on the pressure surface to suppress the cavitating flow in that above mentioned centrifugal pump.The effects of obstacles are detailed analyzed on cavity pattern,flow structure,turbulence behavior,cavity volume and the amplitude of pressure pulsation etc.The results show that the wake vortex induced by obstacles would increase the energy loss,which are reflected as the slight decrease of efficiency in case without cavitation.When cavitation occurs,the obstacles play a critical role to effectively optimize the flow structure,weaken the vortex intensity close to the suction side of blades and distinctly reduce the cavit y growth rates related to first derivative of cavity volume.When the bubbles develop close to the position where the obstacles arrange,the transient characteristics are suppressed to some extent.The primary mechanism to achieve the cavitation control is the optimize of flow structure and the change of transition characteristics.The induced high pressure waves drive massive bubbles collapsing.The non-dimensional geometrical parameters of performing excellent control achievements are obtained by the research on the inverse problem of obstacle method corresponding to the rectangular cross-section,the height of 1/2 outlet width,the radical position of 45% impeller radius.