Intelligent wind power unit with tandem wind rotors - Wind Power Generation and Wind Turbine Design - page 355

4000

N Topt

2000

N Fopt

0

N Ropt

-2000

0.3

Tandem Wind Rotor EE

b F = 10 deg. b R = 16 deg.

0.2

0.1

0

Tandem Wind Rotor GE

b F = b R = 11 deg.

0.4

0.3

0.2

0.1

L = 0.08
V = 10 m/s

0

0.4 0.6 0.8 1 1.2

D RF

Figure 33: Effect of the diameter ratio on the performance.

the maximum output coeffi cient at each D RF is denoted by C P ,max , where P is the
output, A is the rotational area of the front wind rotor, r is the ambient air density,
V is the wind velocity, N is the rotational speed [subscript F, R, T: the front, the
rear and the relative rotational speeds, N T = N F

N R (the rotational direction of N F
is positive), subscript 'opt' means the rotational speed giving C P,max and L is the
dimensionless axial distance between both wind rotors [= l / d F ]. Besides, the blade
setting angles b F and b R of these wind rotors were optimized [14, 15]. With the
increase of the rear rotor diameter, that is D RF becomes larger, the rotational speed
of the front wind rotor N Fopt becomes slower, with almost the same speed, regard-
less of the blade profi les. The front blade profi les, however, affects markedly the
rotational speed of the rear wind rotor N Ropt , with the decrease of D RF . The front
blade G with the reasonable twist makes the rear wind rotor speed N Ropt compara-
tively faster at the smaller D RF (see Tandem Wind Rotor GE), in comparison with
the front blade E without the twist. That may be caused from the difference of
the fl ow condition at the rear wind rotor inlet, namely, the front wind rotor outlet.
That is, the two-dimensional front blade E has unacceptable fl ow separation whose
scale is large at the smaller radius due to the poor and negative angle of attack, and

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