Study On Internal Flow Characteristics And Optimal Design Of Large-Scale Pumps With Low-Head

Today an increasing number of people concern global energy issues which influence economic development. In the meantime, it is the base of improving national standard of living conditions. According to incomplete statistics, power consumption of pumps accounts approximately 17% of total generating capacity. Thereby increasing the efficiency of the pump is necessary to energy conservation which also has a very important significance on creating a resource-saving society today.Low-head pumping stations,which are widely used in the Yangtze River Basin, the North China Plain, Northeast Plain, the Pearl River Delta and the other economically developed areas, are built for the purpose to allocate water resources, improve water environment, control urban flood, irrigate and drainage. Low-head pumping stations play a significant role in preventing natural disasters, promoting economic development rapidly and sustainably. The number and scale of China low-head pump stations ranks the first in the world, in the meantime, China annual power consumption is also considerable. With energy increasingly decrease, improving the efficiency of low-head pump station and reducing operating costs and saving energy are of great importance. In addition, compared with other pumps, large low-head pumps have advantages as follows:1) The impeller submerged in the water which ensure that cavitation will not occur.2) It does not require irrigation water (exhausting air) at start time which is beneficial for running automatically.3)It only needs a small area. Although the centrifugal pumps have good hydraulic performance, but they own complex structures at the same time. Therefore, in addition to be used widely in various pump stations, the axial flow pumps and mixed-flow pumps are also extensively used in our social life.Because of the issue of match between blades and guide vanes affect the hydraulic performance of axial flow pump, this paper will do research on this issue to improve pump performance. But when it comes to the stable operation, efficiency indicators and cavitation performance and other aspects, the effective operating range of axial flow pump is relatively narrow, meanwhile, the axial pump can not be applied in case of changes with large head. With respects to the axial flow pump, mixed-flow pump has not only wide high efficiency district, and large range of flow change, head variation, but also has stable operation and other characteristics. So it is an ideal pump type.Theoretical analysis, model test and numerical calculation are adopted in this paper to research the match of runner and guide, design a high specific speed mixed-flow pump with double volute, and the unsteady pressure fluctuation characteristics, flow-induced noise and cavitation prediction of this model is also studied. The main research work and innovation of this dissertation are as follows:1、Raised and resolved the issue of matching between runner and guide of the axial flow pump. Creatively divided the guide vane in three parts including inlet, middle section and outlet, the middle and outlet sections are fixed to support guide bearing.2、Firstly proposed inlet sections of axial flow pump guides are adjustable so as to make the flow field of impeller blades match well with the guide inlet, which made hydraulic loss reduction in the non-design conditions; investigated hydraulic performance of the inlet of axial flow pump guide vane angle to pump device. Guide vane angle changes affect inlet flow pattern and hydraulic losses, thus affecting the energy performance of the pump; rotating clockwise appropriate angle on the basis of the original design of axial flow pump vane inlet angle may expand the scope of the low-head pump with efficient operation and significantly improve the efficiency when deviation from the design point.3、For the first time developed a high-performance mixed-flow pump model with double volutes. By conducting numerical simulation between the optimal pump and the former one, the results show that the flow state in the double-volute pump is better than in the single-volute pump; the radial thrust in the double-volute pump is also smaller than in the single-volute pump. The results indicate that the double-volute hydraulic model has a significant superiority in comparison with the single-double one. The hydraulic model of the double-volute mixed-flow pump has been made into a machined component and the performance testing has also been carried out on it. The pump efficiency and head deviation between numerical and calculated results were less than 7%, which illuminated that the numerical model and simulation methods predicted the hydraulic performance of double-volute mixed-flow pump well so that it could provide technical for the study on pressure fluctuation, flow-induced noise and cavitation characteristics in the mixed-flow pump.4、Obtained internal pressure pulsation characteristics of mixed-flow pump through research on pressure pulsation under different conditions. Studying unsteady flow field under different conditions and analyzing different monitoring points which located in this pump model. The results show that the pressure fluctuation amplitude increases from hub to shroud both at the inlet and outlet of the runner. So it is crucial to do some optimal design at the shroud location in order to weaken the pressure fluctuation. In the volute, the pressure fluctuation amplitude appears to be the largest near the tongue, the pressure fluctuation decreases with the increase of circular angles. The domain frequency of pressure fluctuation in the mixed-flow pump is always the blade passing frequency under different flow rate conditions. The pressure frequency characteristics at the outlet of the runner and the tongue are high and complex because of the rotor-stator interaction in these locations.5、On the basis of CFD/CA, the flow-induced noise caused by rotating blade dipole was studied. The tongue movement interference between impeller and volute is the main reason of noise. The results show that:rotor-stator interaction between the rotating blades and the stationary volute may be the major source of flow-induced noise in the mixed-flow pump. The dominant frequency of flow-induced noise is not equal to the dominant frequency of pressure fluctuation. The dominant frequency of pressure fluctuation has influence on flow-induced noise but it can not decide the dominant frequency of flow-induced noise directly. The dominant frequency of flow-induced noise is integral affected by the structure mode of the pump and the pressure fluctuation. It is easy to occur resonate especially when the dominant frequency of pressure fluctuation is closely equal to the dominant frequency of the structure mode. The flow rate has little effect on the exterior acoustic directional pattern of pump but it has significant effect on the intense of flow-induced noise. The flow-induced noise radiation level at small flow rate(0.8Q) condition is high, and at large flow rate(1.2Q) conditions is lower than small flow rate(0.8Q) condition, which is accordance with the conclusion that the smaller the flow rate is, the more intense the pressure pulsation is. It shows that the pressure fluctuation has direct influence on the flow-induced noise. The study on pressure fluctuation has important significance for the study on flow-induced noise.6、Obtained preliminary cavitation laws of mixed-flow pump impeller via studying the mechanism head decline of mixed-flow pump under cavitation conditions. In this paper, the basic parameters of the pump and cavitation margin measurement are briefly introduced, then using CFD techniques to predict cavitation characteristics of mixed-flow pump, which initially reveals the mechanism of cavitation conditions of mixed-flow pump with head drops; cavitation occurs in impeller inlet side bias rim at the beginning, the back of the blade near the rim appears cavitation with the lower margin of cavitation, the cavitation area extending from the rim to the hub, and gradually spread to the middle blade. When the cavitation is serious, it will lead to the impeller blockage of the normal flow of fluid, resulting in decrease of mixed-flow pump head and efficiency.