| Summary: | 碩士 === 國立屏東科技大學 === 機械工程系所 === 100 === Traditional horizontal axis wind turbine (HAWT) uses single rotor blade to capture wind energy; however, the actual maximum power coefficient is only about 0.45. Considering the Betz Limit is 0.59, which means a lot of useable wind energy is not taken by the rotor blades. If this wasted energy could be use, then the overall efficiency of wind turbine will be improved significantly. To achieve this goal with lowest cost, the simplest way is to couple a new set of rotor blades behind the original rotor blades, which arrangement is called as “co-axial dual-rotor” wind turbine. Apart from increasing the power output, dual-rotor wind turbine can also provide higher starting torque, so that the wind turbine can be started with a lower wind speed.
However, the additional rear-rotor blades will affect and reduce the performance of the front-rotor blades, because in subsonic flow, the change of downstream flow field will interfere with the upstream flow field. This phenomenon is well known in the aerodynamic of an aircraft’s tails can reduce the lift of the main wings. Therefore, in order to reduce the interference of the air flow between the two rotor blades, the Taguchi optimization method is used in this study to get the best configuration parameters of the two rotor blades, such that the possible highest overall power output can be achieved.
In the Taguchi method, factors which affects the power output of the dual-rotor blades are: (1) the front-rotor blades’ angle of attack, (2) rear-rotor blades’ angle of attack, (3) The rotation speed of the dual-rotor, (4) space between the two rotors. Level number used is set to be three. With the orthogonal array table of Taguchi method, a set of experiments with different rotors’ shapes are built. Commercial software: Gambit and Fluent is used to generate the mesh system and then solves the flow field of the rotor. Result of the analysis reveals: torque generated by a single-rotor is 411 N-m per blade. For dual-rotor, torque generated is 544 N-m per blade, which is 32% more than the single-rotor case. Therefore, the idea to use the Taguchi method to obtain the optimal parameters of the dual-rotor is feasible and successful. Even though the dual-rotor’s power coefficient is 0.57, a little less than the Betz limit (0.59), but it has captured 96.6% of the theoretical usable wind energy, which is far exceeding the performance of a single rotor blade wind turbine.
In this study, impact of the mesh size and different turbulent models on the numerical solution accuracy is also investigated. Mesh density of the Y+ value used is between 30 to 130 in this study. Turbulent models are: the relatively simple Spalart-Allmaras’s one-equation model and the more complicated two-equation model, i.e. the k-εmodel. Analysis result shows the difference between the two turbulent models is only 2% in the power output. Therefore, all the analysis done is by taking the simpler one-equation Spalart-Allmaras model.
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