3-D Unsteady Numerical Simulation And Fluid-induced Vibration For Centrifugal Pumps

The centrifugal pump has been used widely in chemical processing and in industrial and agriculutural fields. Moreover it is also the very important equipment in the spaceflight and war engineering. Flow-induced vibration in the centrifugal pump is caused by cavitation, rotating stall and rotor-stator interaction. It induces mechanical failure and noise. The 3D numerical simulation of unsteady turbulent flow in centrifugal pump and the measurement of the unsteady flow is used for investigating the mechanism of vibration. The study is not only to maintain equipment and reduce environment noise but also the non-touch measurement of the interior flow in the pump. The main work and achievements include:1. The hydro-forces are caused by flow fluctuation, rotating stall, cavitation surge, rotating cavitation, suction vortex and discharge vortex, rotor-stator interaction and impeller make-error. They are divided into itself-excited vibration and forced vibration.2. The principle of the enlarged flow design method for low specific-speed centrifugal pump is briefly introduced. The constraint equations of two enlargement coefficients in the design of the enlarged flow of low specific-speed centrifugal pump are established and solved. The numerical calculation can obtain the series charts of the specific speed enlargement coefficient and the enlarged flow coefficient at the best efficiency of the running points. The relationship between the two coefficients and the calculation formula for the impeller outlet width are also drawn out. The results calculated with the enlarged flow design method agree with the excellent hydraulic models manufactured, and the calculation results also agree with the results of the impeller outlet width of empirical formula. A case study on the selection of key geometrical parameters is analyzed and tested using the enlarged flow design method for low specific-speed centrifugal pump. The experimental loop equipment and data processing apparatus are introduced. The analysis of the data at several conditions firstly proves that the signal of the collecting vibration by accumulation transducers contain the signal of rotor-stator interaction.3. By FLUENT, the three dimensional steady and unsteady turbulent flow is simulated within the centrifugal pump using different flow models. The distributions of relative velocity, static pressure and turbulence kinetic energy within the pump are obtained, which preliminary revealed the flow mechanism in centrifugal pump. With the increasing of flow rate, the relative velocity increased, turbulence kinetic energy is more uniformly and the total pressure difference between the inlet and outlet decreased. In greater flow conditions, wake is at outlet of the impeller.The unsteady flow characteristics are discussed considering the interaction between the impeller and volute. It can be mainly concluded that:(1) On design condition, the static pressure distribution is different at different time. Pressure fluctuation shows periodic variation in different radius of the impeller. The variable manner of fluctuation is close to sinusoid. The frequency at the point of maximum amplitude is close to the blade passing frequency. The peak values of the pressure fluctuation have five periods at different point of the interface. The peak is almost near the tongue. Pressure fluctuations of the different axis section have good coincidence. The peak values of pressure fluctuation have not high frequency in the volute.(2) On off-design operating conditions, jet-wake occurs at the impeller outlet. In greater flow conditions, the jet-wake area is larger. Blade tails left obvious wake area on the static pressure contour. Firstly pressure fluctuations have been studied in the different section of the volute. With the increasing of flow rate, the scopes of pressure fluctuation become smaller in the casing flow passage. In small flow condition and greater flow condition, high frequency fluctuation is caused by unsteady flow.4. According to the definition of NPSH_r, the method of predicting the performance curve of the NPSH_r is established. The pressure pulsation of the pump outlet is testedfor its cavitation feature. The results show that they have good coincidence in the greater flow conditions.The length of cavitation area at the leading-edge is periodic variation. The frequency of the quasi-synchronous vibration symmetrically appears on the two sides of the blade passing frequency, by which the cavitation incipiency can be detected. During the developing process of the cavitation, as the severe complexity of the unsteady flow, it is very difficult to detect the development of cavitation by classical analysis methods. Fractal method of Higuchi is successfully used to detect the incipiency, fully development of cavitation and the development between them.