Environmental Engineering Reference
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Fig. 5.7 Predicted effect of
changing the number of
blades and pitch on the power
coefficient for the
measurements of Anderson
et al. [ 5 ]
0.5
0.4
0.3
N = 2
N = 3
N = 2 pitch = 5
°
0.2
6
7
8
9
10
11
12
13
14
Tip speed ratio,
λ
Fig. 5.8 Predicted effect of
changing the number of
blades and pitch on the thrust
coefficient for the
measurements of Anderson
et al. [ 5 ]
N = 2
N = 3
N = 2 pitch = 5
1.2
1
0.8
0.6
0.4
0.2
9
10
11
12
13
14
Tip speed ratio,
In turn, this suggests that a turbine using a more modern blade section would be
more efficient. Before discussing this issue, however, it is worthwhile to consider
the effects of changing the number of blades, and altering the blade pitch angle
without using a different aerofoil.
Figure 5.7 shows the two-bladed results from Fig. 5.2 compared to N = 3
along with the calculations for a two-bladed rotor with a pitch of 5; the whole
blade has been rotated such that h P in Fig. 3.2 is increased by 5 for all elements.
Thus a typical chord line has moved away from the plane of rotation. The cor-
responding thrust data are plotted in Fig. 5.8 . It is typical that increasing N does
not have a large effect on the thrust and power levels; the main change is to the
value of k at which they occur. This is not surprising given that the blade chord is
absorbed into the solidity, r, in the blade element equations in Chap. 3 . Thus it is
the product Nc, rather than c that is important. In other words, apart from the
effects of changing the Re, three blades with a certain chord will perform very
similarly to two blades with the chord increased by 50%. As shown by the very
large reductions in power and thrust, a pitch change of 5 has a marked impact on
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