| As one of universal machines, fans have been widely used in lots of departments of the national economy. It is one of major devices for ensuring safe and economic operation for power plants as well as realizing energy saving and emission reduction, thus it's of great significance to study fan's economy and safety. In this paper, G4-73 centrifugal fan is chosen as the study object. Combined with numerical simulation and experimental research, the flow mechanism and influence law of structure parameters on fan performance is analyzed. Then, the impeller structure parameters are optimized by genetic algorithm and vortex-broken device is designed for improving fan performance. Moreover, the mechanism and dynamic characteristics of rotating stall are investigated. Main contents and results of the paper are as follows.(1) Study on dynamic characteristics of flow field for centrifugal fan and development of parametric numerical calculation platform. After analyzing the influence of mesh division strategy and turbulence models on computational accuracy, the flow field is solved by numerical simulation and entropy generation calculating. Numerical results are in good agreement with the experimental data, setting a basis for the realization of performance prediction and optimization of fan. Results show that turbulent dissipation is the primary cause of entropy generation while viscous dissipation is almost negligible. Entropy generation is largest inside the impeller. After confirming the calculation models, parametric numerical calculation platform for centrifugal fan is developed with VB programming language, including three modules, parametric modeling, FLUENT calculating and data analysis. The platforms can realize full functional automation and integration of geometric modeling, high-quality mesh generation, model setup, post-processing, data-base query and so on when structural parameters of the impeller change, to set a basis for in-depth study on the impacts of structural parameter variation on fan's performances.(2) Multi-objective optimization and experimental research of centrifugal fan performance. The influence laws of impeller structural parameters including the blade number, outlet installation angle of blade, and width of the impeller outlet on fan performance is studied with parametric numerical calculation platform. After obtained the training samples by orthogonal test, prediction model for centrifugal fan performance parameters is constructed based on BP neural network, and we obtain a new type of impeller structure parameters through multi-objective optimization to full pressure and efficiency of the fan by genetic algorithms. In order to verify the accuracy of optimization results, numerical and experimental studies are conducted to the fan with the new impeller. The results show that full pressure and efficiency of the fan improved after installation of the new impeller, the effective area expanded and the fan noise reduces as relative flowrate ranged from 65%to 100%.(3) The characteristics analysis and improvement for flow field of fan volute. The internal flow rules of volute are studied and S-like large-scale spiral whirls in volute are push forward. A vortex-broken device is designed for reducing flow loss and leak loss. Numerical simulation, performance and noise experiments are carried out respectively before and after the installation of the device. The simulation results show that large-scale swirl is broken and its intensity becomes weaker, entropy generation inside volute is reduced sharply, as well as the leak loss is decreased with the device. The experimental results illustrate that after the device was added, when relative flowrate more than 45%, full pressure increase significantly and efficiency area to broaden. Fan noise reduction, mid-frequency and low-frequency noise significantly decrease at the maximum efficiency point.(4) The mechanism investigation on dynamic characteristics of rotating stall for centrifugal fan. Phenomenon of rotating stall is numerical simulated with the throttle valve function compiled with user defined function. The speed of stall cell and stall frequency are obtained by the terms of setting monitoring points in flow field. Production mechanism of rotating stall was discussed. The full three dimensional unsteady transport processes including the radial, axial and circumferential directions is analyzed from the stall inception become to stall cell. Also, the change of stall characteristics and numbers of stall cell is studied. Mechanism of circumference transmission for stall cell, reasons that full pressure fluctuation and its frequency in low flowrate condition are explored. The results will provide a foundation for fault prevention, monitoring and diagnosis of rotating stall. |
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