Numerical Simulation Study For Pressure Fluctuation Of Axial-flow Pump Under Non-uniform Inflows

Hydraulics is the lifeline of national economy, hydraulic engineering is an indispensable precondition of local economic construction and social development, in which pumping station engineering is the main part. Axial-flow pump is widely used in drainage pumping stations in middle-lower Yangtze and other areas and large-scale water transfer projects, because of its characteristics such as larger flow and low head. But the non-uniform velocity before impeller in operation can not only produce paroxysmal vortex taking a dramatic decline of pump performance, even cause units serious vibration and noises, seriously affecting the safe and stable operation of pumping station. It has very important theoretical significance and engineering practical value to deeply research and predict the pressure fluctuation of axial-flow pump under different inflows. In this paper, axial-flow pump was studied by steady and unsteady numerical simulation in the following areas:Combining with the characteristics of the internal flow in axial-flow pump, the RNG k-ε turbulent model was used to simulate the axial-flow pump at seven different flow rates by steady and unsteady numerical simulation in this paper. Compared with the results of model test in external characteristic, the results of numerical simulation were proved feasible and credible. And this paper further explained the unsteady numerical simulation is more accurate than the steady numerical simulation in predicting the external characteristic of axial-flow pump. The internal flow characteristics of axial-flow pump were revealed by analyzing the results of numerical simulation witch were proved.Considering the pressure fluctuation of axial-flow pump caused by the interaction between impeller and guide vane, in this paper pressure fluctuation was monitored to get the time domain characteristics of different monitoring points under three different flow conditions at uniform inflow, and then the frequency domain characteristics were got by FFT transform. Through the analysis found that:the pressure fluctuation frequency in the axial-flow pump is mainly controlled by the impeller rotation frequency; the larger pressure fluctuation happens at the impeller inlet; the pressure fluctuation at the impeller inlet increases from hub to shroud gradually; and at the impeller outlet it decreases first and then increases, but at the guide vane outlet it increases first and then decreases. The pressure fluctuations under the conditions of small flow and large flow are greater than that of the optimal condition. Because of the impeller rotation, the pressure fluctuation affects to0.7D before the center of the impeller.This paper constructed two different velocity distributions of non-uniform inflow, and introduced uniformity. By comparing external characteristics with numerical simulation data of uniform inflow, it is concluded that the head, shaft power and efficiency prediction of non-uniform inflow I are higher than that of non-uniform inflow II, and the head and shaft power of non-uniform inflow I are higher than that of uniform inflow. Through comparing, it is known that the trend of pressure distribution at blade surface under non-uniform inflow I is same as uniform inflow, presenting the symmetrical distribution of four blades; but the trend of pressure distribution at blade surface under non-uniform inflow II is asymmetric distribution, and the pressure of small velocity side is less than that of the large velocity side. Differential pressures of pressure surface and suction surface under non-uniform inflow I are less than that under non-uniform inflow II.By analyzing the trends of the pressure fluctuation under the two different inflow conditions, it is obtained that the trends of the pressure fluctuations under non-uniform inflow I are the same as that under non-uniform inflow II at small flow. The pressure fluctuation of non-uniform inflow I is based on the dynamic and static interference of guide vanes and impellers, and the pressure fluctuation of non-uniform inflow II is based on the non-uniform inflow. By further analyzing the trends of the pressure fluctuation with the uniformity, it is concluded that as the uniformity increasing, the whole pressure fluctuations under the non-uniform inflow II at every section are growing larger, and the pressure fluctuations under non-uniform inflow I at every section are basically unchanged.