Investigation On The Unteady Dynamic Characteristics And Its Mechanism Induced By Cavitation In A Centrifugal Pump

With the development of industry and the improvement of science and technology,the excellent performances which include high efficiency,high rotating speed,good reliability and low level of running noise in centrifugal pumps are becoming an increasingly important trends for the construction of national economy,and for the requirements of energy and environment as well.However,cavitation is the critical factor in a series of problems caused by the increasing of rotating speed,which affects the efficiency and running stability of centrifugal pumps and other hydraulic machineries.As a kind of complicated phenomenon,cavitation is regarded as the “cancer” of the hydraulic machinery owing to the development and operation life of hydraulic machinery is restricted by cavitation.In the current research,the theoretical analysis,numerical calculation and experimental methods are combined to investigate the development of cavitation in a centrifugal pump.The flow-state recognition techniques are used to extract and analyze the unsteady dynamic characteristics induced by the flow instabilities in the pump.Meanwhile,the pump internal flow and cavitation instabilities are effectively detected to obtain the typical signals of the unsteady flow and to monitor the whole development of cavitation.The relationship between the instabilities of cavitation and pump performance curves are established.The main research work and creative achievements of the dissertation include:(1)The numerical simulation is clarified to be feasible for the cavitation investigation via the comparison between the numerical and experimental results.The locations of the generation of vapor bubbles and the internal flow characteristics induced by cavitation are analyzed based on the whole flow field model of a centrifugal pump with the generating and development of cavitation.It is revealed that affected by the asymmetric structure of the volute,the distributions of the vapor bubbles are uneven in each blade channel.The length of the cavities which are close to the volute tongue is larger than that of the cavities which are far away from the tongue during the development of cavitation.The uneven radial force is induced by the asymmetric cavities distribution during the rotation of impeller,increasing the vibration of shaft and causing the instabilities of the operating system.Meanwhile,it is considered that the shedding of cloud cavitation bubbles is produced by the reverse re-entrant jet in the cavitating wake.Because it is affected by the viscous shear stress of mainstream,the shedding cavities rotate,which leads to the intensification of unsteady flow and turbulence in this region.(2)A closed hydraulic experimental system is established to investigate the instabilities of pump cavitation phenomenon.The experimental scheme and data acquisition methods are proposed to collect the data for the unsteady cavitation experiment and the flow induced unsteady dynamic characteristic test.The hydraulic performance and the cavitation performance of a centrifugal pump have been measured.At the same time,the unsteady dynamic signals induced by cavitation and unstable flow are obtained,which include the pump inlet and outlet pressure pulsations,noise,vibration and the stator current of electricmotor.(3)The good relationships between the internal unsteady flow and the external unsteady dynamic characteristics are established.The unsteady dynamic signals induced by unstable flow and the internal flow characters calculated by numerical simulation under a variety of flow rates conditions are combined to investigate the characteristic of flow instabilities and its typical information in the centrifugal pump.It is obvious that the corresponding external unsteady dynamic characteristics can be induced by the back flow,rotating stalls,hump and cavitation.Those flow instabilities in the pump can be apparently reflected by the pump inlet and outlet pressure pulsations,noise,vibration and stator current,which helps to obtain the safe and steady operating conditions of the system.(4)In order to investigate the cavitation characteristics under the same rotating speed and different flow rate conditions,the transformation laws of the pump head and relevant reasons are analyzed with the development of cavitation.It is found that little exclusion caused by backflow or cavitation to the flow rate can be found under the part load conditions,which has less impact on the pump performance.The pressure gain coefficients under the part load conditions are less than that under the overload conditions before the sudden drop in head happens.However,contrary circumstance occurs after the dramatic head drop.It is apparent that the critical net positive suction head presents a gradual increasing trend with the increase of flow rate,which is almost proportional to the increase of the flow rate when it is greater than the designed flow rate.(5)Corresponding relationships are established between the occurrence and development of cavitation and the unsteady dynamic characteristics.Therefore,the unsteady dynamic signals can be applied as the typical signals to detect the development of cavitation which includes the incipient cavitation and the unsteady cavitation generating point at the designed flow rate condition.However,the vibrating signals is not applicable for the incipient cavitation detection under the off-design conditions which have a large attack angle.It can be found that the intensity of vibration induced by cavitation is much stronger than that of rotor-stator interaction.Actually,a single accelerometer can be used to detect the vibration produced by cavitation,and the better position for the measurement of vibration should be as close as possible to the runner and the location where cavitation generated and collapsed.It is found that the spectrum characteristics of the pump outlet pressure pulsations and acoustic pressures can be considered as the signals for the super cavitation detection.(6)A method based on the application of unsteady dynamic characteristics induced by cavitation is proposed for the first time to detect the unsteady cavitation generating point under different flow rate conditions in the centrifugal pump.Corresponding relationships are established between the occurrence and development of cavitation and the unsteady dynamic characteristics,which provides a creative method for cavitation and flow instabilities detection.At the same time,the deductions of the pump head which are under the critical typical points are discussed and its reasons are analyzed.(7)The relationship between the centrifugal pump cavitation instability and the instability of pump performance curve is revealed.According to the whole developing process of cavitaiton,the continuous changes of pump performance curve are able to reflect the incipient cavitation and the occurrence and development of unsteady cavitation.The intensification and its frequency of the cavitation instabilities are clearly reflected by the external performance curve's intensification and fluctuating frequency,providing a significant reference for the mechanism investigation of the head affected by unsteady cavitation.