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Numerical Simulation of Trash-barriering Flow of Trash Rack and Prediction of Trash-barriering Resis

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Author
Tutor: ChouBaoYun
School: Yangzhou University
Course: Fluid Machinery and Engineering
Keywords: Trash rack,Trash-barriering,Head loss,Velocity distribution,Numerical simulation
CLC: TV675
Type: Master's thesis
Year:  2013
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Abstract:
Trash interception by trash rack of pump station insures the safety operation of pump units, but trash-barriering increases the shaft power of pump units (for axial flow pump and high specific speed mixed-flow pump). When the trash rack is blocked seriously, the flow state of pump inlet will be affected and the energy characteristic and the cavitation performance of pump will deteriorate. At present, there are few researches at home and abroad about the head loss of trash rack and the flow fields before and after the trash rack when the trash blocks the trash rack. The research of numerical simulation of trash-barriering head loss and flow field of pumping station was developed under the financial of Project Supported by National Natural Science Foundation of China(No:51079125) and Author special Foundation of National Excellent Doctoral Dissertation of China (No:2007B41).According to the aggregation shape of waterweeds before trash rack and its water permeability, the aggregated waterweeds was simplified reasonably, and the corresponding trash-barriering calculation model was set up according to the actual size of trash-barriering test device and a prototype pumping station trash-barriering device. Different trash-barriering schemes were made, ANSYS CFX software and VOF model were adopted in the numerical simulation of trash-barriering head loss and the flow fields before and after the trash rack. And the influence of different velocity inlet on device operating efficiency of a prototype pumping station was calculated. Besides, it initially revealed the aggregation characteristic of waterweed lump and the ultimate resistance of trash tack. The results show that:(1) When the waterweeds dive to block trash rack, there is a critical velocity which is related with these factors, such as flow deflection angle a, the volume of waterweed lump Vct, the area of water affection A, the density of waterweed pc, the thickness of waterweed lump ¦Ä, the void percent of waterweed lump K, the coefficient of pressure drag CD and so on. When the flow velocity in front of trash rack doesn¡¯t reach the critical velocity, it is hard for the waterweed to dive to block the trash rack, and it only can roll out along the water surface. When it reaches the critical velocity, the waterweeds can dive to block the flow section of trash rack, and the trash-barriering resistance of trash rack increases. When the trash-barriering resistance reaches its ultimate resistance, the waterweed will not be able to dive to bock the trash rack. Finally, the waterweeds will gather in a triangular form in the longitudinal profile in front of trash rack. (2) According to the numerical calculation results, the trash-barriering head loss of model trash rack is related with the factors, such as the velocity in front of trash rack, the blockage ratio of the trash rack, and the angle of inclination of trash rack. When the water depth in front of trash rack was0.33m, the average velocity in front of trash rack was0.244m/s and the blockage ratio of the trash rack was0.6, the trash-barriering head loss of trash rack calculated by numerical calculation and that tested by experiment were almost equal, and their flow fields were similar. This calculation method can simulate water flow before and after the trash rack accurately.(3) The variation laws of trash-barriering head loss with related factors of a prototype pump station are similar with that of head loss of trash-barriering model. The trash-barriering head loss increases when the blockage ratio of trash rack and the velocity in front of trash rack increase, and it decreases when the angle of inclination of trash rack increases. The trash aggregated in front of the trash rack makes obvious influence on the flow fields before and after the trash rack. With the blocked ratio of trash rack surface increased, the flow velocity in front of trash rack and its deflection angle increases, and the velocity behind the trash rack increases in the bottom area and decreases in the upper area. Besides, backflow even appear in the upper area. When the blockage ratio of trash rack is0.4, and the average velocity is1.0m/s, the trash-barriering head loss is0.25m. When the blockage ratio is0.7and the average velocity was0.8m/s, the head loss increased sharply, up to0.89m. These are in accordance with the practical situation of pumping station.(4) When the inlet water level and inlet flow of pumping station are certain, the different velocity distributions of inlet main affects the internal flow state of the inlet passage, and they have little influence on the pump device efficiency, and the trash-barriering head loss is the main reason that decreases the pump device efficiency.
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