一垂直風機之三維模擬

Abstract

本研究係針對一結合升力型與阻力型風機特性的新型式垂直風機進行氣動力、性能與流場結構的研究。此種風機之葉輪具有三個S型葉片與兩個翼形葉片，並可將數個葉輪堆疊進而得到較佳的性能。為瞭解其氣動力特性，利用計算流體力學進行數值模擬，數值方法採用有限體積法，並利用移動網格處理風機葉片的旋轉，搭配RNG 紊流模式進行三維與二維暫態模擬。除數值模擬外，另進行實驗予以驗證模擬之準確性。 三維模擬結果顯示此風機之扭矩係數與風機端速比呈線性遞減關係，而功率係數則與端速比呈二次式關係。與單層葉輪的實驗結果比較可發現，二維暫態的計算高估了扭矩的輸出，而三維暫態模擬則與實驗結果相當吻合。從流場中可發現，自風機後緣流出的氣流，在二維計算中形成一週期性變化的渦流，此渦流逐漸往下游逸散，三維計算中則呈現一穩定的流場，這表示在三維模擬中，葉輪上下的流場有穩定風機尾流之效果，此三維效應是二維模擬無法反應的。此外，模擬結果亦顯示，將葉輪堆疊而成的多層風機較同高度之單層風機僅具稍高的輸出功率，但扭矩隨時間的變化則隨風機層數的增加而減少，對於穩定風機旋轉有相當大的助益。

A vertical-axis wind turbine which combines the merits of the lift type and drag type wind turbines is studied in this project. It consist of three S-type and two airfoil-type turbine blades. Several stages of these turbine blades can be stacked together to achieve better performance. The aerodynamic characteristics and performance are investigated by using computational fluid dynamic. The numerical scheme is based on the finite volume method. The moving mesh technique is adopted to tackle the rotation of the turbine wheel.With use of the RNG model to characterize turbulence, unsteady calculations are undertaken. Experiments are also conducted to assess the accuracy of the numerical simulation. Two-dimensional and three-dimensional, unsteady simulations are conducted. It is shown that the resulting moment coefficient becomes a linearly decreasing function of the tip speed ratio and the power coefficient is a quadratic function of the tip speed ratio. Comparison with experimental measurements for a one-stage wind turbine indicates that good agreement is obtained by the 3-D unsteady simulation. However, the torques and powers are over-predicted by the 2-D unsteady calculations. It can be detected from the results that after the flow passing the wind turbine, a periodic shedding vortex is formed in 2-D calculations while a relativity steady flow is observed in 3-D calculations. This implies that the flows beyond the upper and lower sides of the wind turbine have an effect of stabilizing the flow in the near wake of the turbine. This 3-D effect cannot be accounted for in 2-D calculations. Comparing with the single-stage wind turbine, only slightly higher power performance can be obtained by stacking several stages of turbines together. However, the variation of the torque with respect to time becomes smoother as the number of stage increase. This character helps stabilize operation of the wind turbine.