Research On Hydraulic Characteristics Of Asymmetrical Blade Of The Low Specific Speed Centrifugal Pump

As consuming machines, pumps are widely used. The low specific speed centrifugal pumps with high head, small flow rate have been widely used in agricultural production,urban water supplying, energy transport and other industries. As an important part of the centrifugal pump, the shape, size and processing technology of the impeller have a decisive influence on pump performance. However, in the actual manufacturing process, due to defects in the casting, precision machining and assembly error, mass unbalance of the impeller inevitably exist, and then causes the hydraulic imbalance in the operating, inducing vibration, generating noise, accelerating the bearing wear, shortening the life, and even causing destructive accidents.To fully understand the influence of eccentric impeller on the internal flow field of centrifugal pumps, a low specific speed centrifugal pump with asymmetrical blades was proposed in this paper. Under the conditions of 0.8Q, 1.0Q and 1.2Q, the steady and unsteady numerical simulations were carried out for the entire flow field of pump with eccentric distances of 0mm, 1mm, 3mm and 5mm. The variation of energy characteristics, pressure fluctuation, radial force characteristics and cavitation characteristics of the entire pump was compared and analyzed. The energy characteristics, pressure fluctuation and cavitaion characteristics tests were carried out for the pumps with eccentric distance of 0mm and 3mm,and compared with the results of numerical simulations. The main contents and conclusions are as follow:1、The three-dimensional numerical simulation model and structure grids of low specific speed centrifugal pump with asymmetrical blade were built. The three-dimensional steady numerical simulations were carried out for the model pumps of four schemes based on the k-? turbulence model, and the numerical results were compared with those of the experiments. It was shown that the performance of pump becomes worse when the impeller is eccentric. And with the increase of the eccentric distance, the head and efficiency gradually decrease, but the influence is very limited. Numerical simulations could predict the hydraulic performance of the model pump accurately.2 、 On the basis of the steady simulations results, the three-dimensional unsteady numerical simulations were carried out for the model pumps of four schemes based on the k~? turbulence model, and compared with the results of numerical simulations. The resultsshow that:(1)The blade frequency and the amplitude of its harmonic frequencies are essentially consistent at the same monitoring point for the four schemes. The shaft frequency and the amplitude of its harmonic frequencies, except the blade frequency and its harmonic frequencies, become larger successively. Under the conditions of 0.8Q, 1.0Q and 1.2Q, the variations of pressure fluctuation characteristics are basically the same at four monitoring points. At the same monitoring point, the amplitudes of the pressure fluctuation decreases with the increase of flow rate(2)Near the rated conditions, the radial forces of the impeller and the volute decreases with the increase of flow rate, and the radial force in the impeller is far smaller than that in the volute. Compared to the scheme of 0mm eccentricity, the fluctuation amplitudes of the radial force in the impeller of the schemes of 1mm, 3mm and 5mm are sharp and irregular.The radial forces in the volute have obvious periodicity with small change in the size, but the fluctuation amplitudes are obviously different, becoming larger with the increase of the eccentric distance. When the impeller is eccentric, the characteristics of the radial forces in the impeller and volute become worse with the increase of the eccentric distance.3、Based on the standard k~? turbulence model and the Rayleigh-Plesset cavitation model, the three-dimensional steady numerical simulations were carried out for the cavitation performance prediction of the model pumps with four schemes, and were compared with the experimental results. It was shown that, under the same condition, critical NPSH corresponding eccentric distances of 0mm, 1mm, 3mm, 5mm increases in turn. In another words, with the increase of eccentric distances in the impeller, the cavitation performances of the pump become worse. The critical NPSH increases with the increase of the flow rate,namely the cavitation performance becomes worse with the increase of the flow rate.