Study On The Unsteady Flow And Dynamic Characteristics In A Centrifugal Pump

Centrifugal pump,as a kind of general equipment which converts the motor energy to liquid energy,has widely been used in various fields.With the development of modern industry,more and more high requirements are put forward for centrifugal pump,which not needs only wide hydraulic performance area and good anti-cavitation ability,but also excellent dynamic characteristics to ensure the operational stability of centrifugal pump.The dynamic characteristics of centrifugal pump are very complex,and the previous research is not systematic and in-depth,which has become a bottleneck of the development of centrifugal pump.It is generally accepted that the vibration and noise of centrifugal pump are mainly caused by electromagnetic excitation,system excitation due to mechanical structures,and fluid excitation generated by unsteady flow in centrifugal pump.The former two can be suppressed by continuously improving the process level,but the vibration induced by unsteady flow is the most difficult factor to effectively control due to the lack of distinct mechanism research.The research on the unsteady flow and its dynamic characteristics and the internal unsteady flow mechanism are crucial for centrifugal pump application.On this basis,the appropriate flow control methods are adopted to improve the hydraulic performance and anti cavitation performance of centrifugal pump,and achieve the purpose of vibration noise reduction.The theoretical foundation and technical support for the design of high-performance centrifugal pump has been provided in this thesis.The main works and results are as follows:?.Construction of the test rig and establishment of the experimental schemeIn order to fully obtain the hydraulic performance,cavitation performance,pressure pulsation and vibration characteristics of centrifugal pump,a closed experimental device and its signal synchronous acquisition system have been constructed,which could be used to test the hydraulic performance,cavitation performance,pressure pulsation,and vibration of centrifugal pump.The experimental device is used to measure the hydraulic performance and cavitation performance of the centrifugal pump under different flow rate conditions,meanwhile the pressure pulsation and vibration signals are simultaneously collected,which provides the foundation for the experimental research and the validity of the numerical method.?.Development of the hybrid RANS / LES method with a rotation curvature correctionIn order to obtain the abundant flow structure in a centrifugal pump,the hybrid RANS / LES method based on the von Kármán scale has been introduced.At the meantime,considering that the rotation and streamline bending of centrifugal pump system have important effects on the turbulence of centrifugal pump,referring to the rotation curvature correction method of Menter et al.,the SST k-? model has been modified.The hybrid RANS / LES-RC method(namely the hybrid RANS / LES with rotation curvature correction)based on the von Kármán scale and rotation curvature correction has been developed.The validity of the hybrid RANS/LES method is verified by comparing with the DNS and experimental results of a rotating channel flow and a U-tube bending flow.The hybrid RANS/LES-RC method is used to simulate the whole flow field of the centrifugal pump and is compared with the experiment results of the hydraulic performance and pressure fluctuation of the centrifugal pump to verify the applicability in centrifugal pump.?.Research on the complex flow and dynamic characteristics in a centrifugal pumpBased on the experimental and numerical simulation results,the complex flow and dynamic characteristics of a marine vertical single-stage centrifugal pump were studied by using the proper orthogonal decomposition method and the signal analysis method.The results show that when the fluid flows into the impeller,the flow separation occurs in the impeller due to the inlet attack angle,system rotation and blade bending leading to an unstable flow state inside the impeller and an obvious pressure fluctuation on the blade surface,which affect the hydraulic performance of the centrifugal pump.At the same time,the poor inflow in the impeller even leads to significant low pressure in the impeller,which affects the anti cavitation ability of the centrifugal pump.When the fluid flows out of the impeller and into the volute region,it is no longer constrained by the blade,and the unsteady flow in the volute is enhanced.There are abundant flow structures of different scales in the centrifugal pump,and the low-order modes correspond to the low-frequency large-scale flow with higher energy content,which represents the main flow characteristics in the centrifugal pump,and directly affect and determine the mixing and transport characteristics of turbulence in the centrifugal pump,as well as the vibration caused by the turbulent flow.The amplitude of pressure fluctuation in centrifugal pump is the lowest in design condition,and it is significantly enhanced in off design condition.The frequency of pressure fluctuation is mainly the rotating frequency and its multiple-frequency in impeller,while is mainly the blade passing frequency and its multiple frequency in volute.The vibration intensity of centrifugal pump is low in the small flow range,and it is significantly enhanced in the design condition and in the large flow condition.The vibration is mainly on the rotating frequency and blade passing frequency,and contains some broadband components.The fluid excitation generated in the impeller of centrifugal pump acts on the rotating shaft system of centrifugal pump,which has a direct impact on the vibration of centrifugal pump and its system.The pressure fluctuation and radial force caused by uneven pressure in impeller are the main reasons for the strong vibration of centrifugal pump.Therefore,the key to improve the vibration characteristics of centrifugal pump is to ameliorate the unsteady complex turbulence,energy content and fluid excitation characteristics in impeller.?.Research on the improvement mechanism of complex flow and the dynamic characteristics in a centrifugal pump based on the flow control methodCombined the internal flow characteristics and dynamic characteristics of centrifugal pump and based on the flow control idea,the original impeller of centrifugal pump is optimized and modified by using three-dimensional gap drainage blade structure,and the effectiveness of flow control method has been verified.Furthermore,the mechanism of improving the unsteady complex flow and the dynamic characteristics of centrifugal pump by the gap drainage blade structure has also been explored.The experimental results show that compared with the traditional centrifugal pump,the hydraulic performance and anti-cavitation performance of the centrifugal pump with gap drainage are significantly improved in the whole working conditions,and the pressure fluctuation and vibration characteristics are also significantly improved in the range of the design to large flow rate condition,which proves the effectiveness of the flow control method.Owing to the gap drainage blade which introduces some high-speed fluid from the pressure side to the suction side of the blade,the inflow state of the fluid at the inlet of the impeller is homogenized and puts the flow separation point backward,so that the flow in the gap drainage impeller could better fit the streamline blades.In addition,the pressure distribution on the blade surface is optimized,which enhances the work capacity of the blade.The structure of gap drainage blade could reduce the fluid exciting force and improve the fluid-induced vibration by optimizing the internal flow state of centrifugal pump.The gap drainage blade also redistributes the energy content of each stage flow in the centrifugal pump impeller,so that the fluid energy in the impeller is more concentrated in the low-order and low-frequency large-scale flow structure,while the energy content of the high-order small-scale flow is significantly reduced,which is beneficial to the improvement of the macro dynamic characteristics of the centrifugal pump.All in all,the complex flow and its dynamic characteristics in a centrifugal pump has been studied,and the mechanism of ameliorating unsteady flow and improving dynamic characteristics of centrifugal pump with a gap drainage blade structure based on the flow control idea has been explained.The remarkable effect of this flow control method indicates that we can develop a reasonable flow control method from the internal flow field characteristics and energy distribution of centrifugal pump,which is more conducive to the improvement of hydraulic performance and dynamic characteristics of centrifugal pump,and provides theoretical basis and new ideas for the design of high performance centrifugal pump or other fluid machinery.