Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 112

The aerofoil sections of the outboard half of the blade are responsible for extracting the major part of the wind energy. These sections should therefore be efficient with a high lift/drag ratio, hence reasonably thin, consistent with adequate structural strength. Thickness ratios around 18% are usual with relatively high C_Lmax so that the operating C_L where the best C_L/C_D ratio occurs is significantly below C_Lmax. This allows efficient operation while keeping sufficiently clear of the stall to avoid its adverse effects when wind gusts momentarily push up the angle of attack too quickly for pitch regulation to respond sufficiently.

Figure 3.73 The Delft University series of aerofoil profiles.

Table 3.7 The principal characteristics of the Delft University series.

Aerofoil	Max t/c %	x/c at max t/c	y/c at TE	Re × 10−6	α ο	cl max	Design α	Design cl	Max cl/cd
DU 96‐W‐180	18	0.3	0.0018	3.00	−2.7	1.26	6.59	1.07	145
DU 00‐W‐212	21.2	0.3	0.0023	3.00	−2.7	1.29	6.5	1.06	132
DU 91‐W2–250	25	0.3	0.0054	3.00	−3.2	1.37	6.68	1.24	137
DU 97‐W‐300	30	0.3	0.0048	3.00	−2.2	1.56	9.3	1.39	98
DU 00‐W‐350	35	0.3	0.01	3.00	−2.0	1.39	7.0	1.13	81
DU 00‐W‐401	40.1	0.3	0.01	3.00	−3.0	1.04	5.0	0.82	54

Figure 3.74 Flat‐back aerofoil derived from DU‐97‐W‐300.

The inboard sections of a wind turbine blade are much more strongly dictated by structural bending strength requirements. Hence increasingly thick sections are used as the radius reduces to the root. Blade sections at the root end may be up to 40% thick. Inboard of the root end of the aerodynamic sections of the blade, the blade often merges continuously into a circular or other bluff section joining the blade to the hub. To accommodate these very thick sections and at the same time retain a high C_Lmax for power purposes, blades are often fitted with vortex generators (VGs) (see later) near the location of early separation, and so‐called ‘flat‐back’ sections with blunt trailing edges have been designed. An example is the aerofoil shown in Figure 3.74, which is derived by thickening the rear half of the more conventional DU‐97‐W‐300. A computational analysis of the aerodynamics and aeroacoustics of this aerofoil has been given by Lynch and Smith (2009).