Environmental Engineering Reference
In-Depth Information
Dynamic stall produces higher loads on the blades and larger power output than the predictions
from the performance codes using the steady-state data for lift and drag. Dynamic stall may occur
during operation in high winds due to a wind gust for constant-pitch blades, or for variable-pitch
blades in the run position. During this increasing angle of attack a vortex forms near the leading
edge and moves to the trailing edge of the blade, resulting in higher lift, hence the name. Once
the vortex is shed off the trailing edge, deep stall occurs. The other condition for occurrence of
dynamic stall in high winds is during shutdown, as variable-pitch blades are moved to the feather
position. The Westinghouse wind turbines, rated at 600 kW, in Hawaii had this problem as power
spikes to 800 kW occurred during shutdown. Their solution was to change the blade pitch in the run
position to lower the rated power, so when the spike occurred during high wind shutdown, the loads
and power were not too high. Now lift and drag data for some airfoils are available as the attack
angle is changing, which show the dynamic stall, and these data can be used in the performance
prediction codes.
The dynamic stall vortex has been visualized and also noted by the analysis of time-varying
surface pressure data from field tests and wind tunnel experiments [20]. Blades with pressure taps
were used for the Unsteady Aerodynamics Experiment [21], which included a test of an extensively
instrumented wind turbine in the giant NASA-Ames wind tunnel, 24.4 by 36.6 m. Results from
computer models at high wind speeds under stall were significantly different, as power predictions
range from 30% to 275% of the measured values. So the aerodynamic performance prediction pro-
grams are used as a design tool, not the final answer.
Aerodynamic performance prediction programs [3] are now available for personal comput-
ers with menu-driven interactive editing and graphical display to facilitate its use as a design
TABLE 6.1
Sample Output from PROP93
Propprint3
Blade element data for delta beta
0.00,
X
= 6.11, yaw = 0.00
Element
1
2
3
4
5
6
7
8
9
10
Theta
180
180
180
180
180
180
180
180
180
180
Vel
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
A
0.296
0.140
0.188
0.204
0.230
0.213
0.195
0.206
0.231
0.308
AP
0.073
0.021
0.016
0.012
0.010
0.008
0.006
0.006
0.006
0.007
CL
0.813
1.005
1.160
1.206
1.334
1.311
1.168
1.037
0.918
0.772
CD
0.014
0.098
0.053
0.043
0.020
0.019
x0.016
0.014
0.013
0.011
PHI
49.92
42.48
27.54
20.14
15.45
13.03
11.35
9.74
8.34
6.72
ANG
7.92
19.18
15.74
14.84
13.35
12.93
11.35
9.74
8.34
6.72
TC
0.384
0.526
0.622
0.656
0.707
0.665
0.609
0.610
0.609
0.572
QC
0.040
0.059
0.073
0.075
0.083
0.079
0.074
0.073
0.069
0.056
PC
0.243
0.363
0.443
0.459
0.508
0.485
0.453
0.443
0.421
0.344
TD, lb/ft
2.64
6.03
11.90
17.57
24.37
28.01
30.31
35.04
329.6
41.60
QD, ft-lb/ft
4.38
10.92
22.21
32.26
45.86
53.47
59.02
66.73
71.85
65.54
PD kW
0.024
0.298
0.606
0.880
1.251
1.458
1.610
1.820
1.959
1.788
Rey, *10
6
0.920
0.862
0.922
0.931
0.910
0.868
0.890
1.004
1.132
1.132
Rotor
2 blades
Pitch
X
TC
QC
PC
V0
m/s
TD
lb
MD
ft-lb
QD
ft-lb
PD
kW
0.0
6.1
0.614
0.070
0.427
10.0
752
3,984
1,372
23.3
Note:
Output for one blade (Carter 25, 10 m diameter, pitch 0°), divided into ten stations, and then the total is summarized
at the bottom. Wind speed is 10 m/s and tip speed ratio,
X
6.11.
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