| Hydraulic stability of pump units and the flow-induced vibration are key points of the safe operation of pumping stations. Unsteady hydraulic pulsation in flow fields and nonuniform flow pattern in a forebay are important reasons for the increase of hydraulic vibration in pump units. Aimed at the question of the hydraulic stability, this thesis simulates numerically the pressure pulsation in a centrifugal pump and the flow pattern of a forebay. According to the numerical results, measures to mitigate the pressure pulsation of the double suction centrifugal pump and to improve the flow pattern of the forebay have been put forward. The main contents are as follows:1. Precise 3D modeling of the double suction centrifugal pump. Laser scanning technology is applied to obtain the three-dimensional data of a large double suction pump, and the geometrical model of the large double suction pump is reconstructed. Based on the method of automatic and semi-automatic data processing, precise three-dimensional solid model and water model are constructed, which can be used for the further analysis of flow characteristics inside the pump.2. Research on the methods to improve the accuracy of calculation for external characteristics of pumps. According to the precise model of pump and external characteristics provided by the manufacturer, Standard k-εmodel, RNG k-εmodel, Realizable k-εmodel, SST k-ωmodel and Reynolds stress model are used in the steady numerical simulation, and the comparison between the computational results with the experimental ones has been conducted. The RNG k-εmodel and the large eddy simulation using the Smagorinsky-lilly model are used in the unsteady simulation. Also the results of unsteady simulation are compared with the true value. It is revealed that the choice of steady or unsteady numerical is the most important factor which influences the accuracy of the numerical simulation of double suction pumps. The results of the unsteady simulation are more accurate than the steady simulation.3. Research on the unsteady pressure pulsation of the double suction centrifugal pump and the measures to mitigate it. The unsteady simulation with RNG k-εmodel is used to compare the external and internal characteristics of the double suction centrifugal pump with symmetrical and staggered impellers. The influence of pressure pulsation in double suction centrifugal pump with staggered impeller is main considered. The results show that the efficiency of double suction centrifugal pump with staggered impeller is somewhat lower than the one with symmetrical impeller, but the pressure pulsation is much lower. The periodicity of pressure pulsation at impeller outlet and volute tongue is obvious. The staggered impeller is an effective way to mitigate the pressure pulsation in the double suction centrifugal pump as well as to strengthen the operating stability of the pump unit.4. Research on the flow field of forebay and methods of improvement. The numerical simulation of flow field is established and compared with the results of model experiments to verify the reliability. The applicability of two turbulence models is discussed, reveals that the Realizable k-εmodel is more applicable than RNG k-εmodel in the simulation of flow field in forebay of large pumping stations. The forebay and part of the channel which is connected with the forebay are simulated. The flow uniformity and the vortex state are analyzed. The methods of vortex elimination and flow field improvement of the forebay are presented. |
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