Research On Cavitation And Its Induced Vibration And Noise In Centrifugal Pumps

With the development of science and technology, the application field of pump is expanding rapidly and playing a vital role. However, cavitation, as a century problem, has been restricting the development of the pump industry, not only in limiting the scope of the pump’s high efficiency and the realization of miniaturization, but also in affecting the operation stability and reliability of pumps. In order to obtain the cavitation characteristics, its induced vibration noise mechanism and spread rule, and to provide the foundation for setting up low vibration and low noise centrifugal pump hydraulic design method, the cavitation characteristics and its induced vibration noise are researched by theoretical analysis, experiments and numerical simulation. The main research contents and creative achievements include:1. The numerical simulation of cavitation characteristics in centrifugal pumps is divided into three stages——single-phase steady calculation, cavitation steady calculation and cavitation unsteady calculation. At first, the uncertain factors (convergence precision, boundary condition and turbulence model etc.) are compared in single-phase steady calculation. The calculation results show that:(1) The calculation time of convergence precision 10-4 is very short and the calculation accuracy is satisfactory. (2) The impact of the two commonly used inlet and outlet boundary conditions, total pressure inlet mass flow outlet and velocity inlet pressure outlet, on calculation results is quite similar. (3) The SST turbulence model is better for low specific speed centrifugal pumps and the k-εturbulence model for medium and high specific speed centrifugal pumps. Then single-steady calculation results are initialed for cavitation calculations. The Net Positive Suction Head required are obtained, the distributions of vapors and blade load in impeller are analyzed. Simultaneously the pressure pulsations in volute are calculated and the pressure pulsation at outlet of pump is measured. The research results show that:(1) The calculation results correspond to the experiment data quite well, the maximum absolute deviation of Net Positive Suction Head required is only 0.36m. (2) The vapors generated at suction side of blade near the leading edge at first and then expanded to the outlet of impeller, the vapor fraction reduces with the increase of the dimensionless distance S. (3) The blade load is affected greatly by cavitation, but the influence reduced with the rise of specific speed of pump. (4) With the deterioration of cavitation, the peak value of shaft frequency varied obviously, and the average pressures of the monitor points keep constant at first and then decline. Meanwhile the pressure pulsation intensifies and the pulsation amplitude reach the maximum near the volute tongue.2. The pump cavitation induced vibration and noise test rig is established, the synchronous acquisition of pump performance parameters and cavitation induced vibration and noise signal is realized. The cavitation performance, vibration and noise characteristics of a centrifugal pump under five different conditions are measured. Experiment results show that:as the cavitation intensifies, vibration acceleration and noise level stabilize at first and then increase apparently, which could be used to decide the inception net positive suction head, the vibration strength of pump volute is higher than that of other measure points and augments with the increase of flow. The cavitation noise signals focus on the range of 0～1000Hz, with the cavitation intensifies, there is a apparent change of signal between 500Hz and 1000Hz.3. The prediction models of caviation performance are established for the first time by using optimization algorithm BP neural network and RBF neural network based on MATLAB7.0. The relation between key geometry parameters of impeller and Net Positive Suction Head required is established. The models are verified by six sets of non-sample data and the results show that:both models have good adaptability, the average relative declination of BP and RBF are 5.69% and 6.32% respectively.4. The influence of blade number, incidence angle and wrap angle on cavitation performance of different specific centrifugal pumps are researched by CFD method, and the simulation results are verified by experiments. The results indicate that:CFD method has the prediction accuracy to some extent; the complicated influence rule of geometric parameters on cavitation performance of centrifugal pumps under design condition can be obtained accurately. There is an optimum value of three geometric parameters for pump have the best cavitation performance, the effects of incidence angle and wrap angle on cavitation performance is less than that of blade number.5. The cavitation induced vibration and noise characteristics of model pump with different blade number, incidence angle and wrap angle under design condition are measured. Experiment results show that:(1) With the decrease of NPSHa, the vibration strength of three parameter schemes all keep constant at first and then escalation, with the maximum of vibration strength at the measured point of the eighth section of volute. (2) The influence of blade number, incidence angle and wrap angle on vibration strength is obvious, but the change rule is complicated. (3) There is a great influence of three parameters on noise signal in non-cavitation condition, but change rules are different, and the rule of wrap angle is the most complex. With the deterioration of cavitation, the vary of three parameter have the energy peak of noise signal increase obviously between 500Hz-1000Hz and1750Hz-2250Hz.6. With the application of high-speed photography image acquisition system, a centrifugal pump with fixed guide vane are selected as research model, the relationship between vapor distribution and cavitation induced vibration and noise is acquired by the method of CFD and experiment test. The research results show that:(1) The vapor distribution at impeller inlet obtained by unsteady calculation corresponds well to the measured results. (2) The vibration strength and noise level of all the monitor points decrease at low flow condition and increase obviously at the large flow condition. (3) The frequency spectrum of vibration and noise signals at different NPSHa conditions are nearly similar. Under non-cavitation condition, spectrum mainly concentrated within 1000 Hz, and with the discretion frequency as the main frequency. With the decline of NPSHa, the energy peak of vibration and noise signals in broadband increase, the influence of cavitation on vibration and noise characteristics at large flow condition is bigger.