Intelligent wind power unit with tandem wind rotors - Wind Power Generation and Wind Turbine Design - page 357

the desirable diameter ratio getting the suffi cient output and playing the expected
rotational behaviour is in close to D RF = 0.84.

6.3 Optimum axial distance between front and rear wind rotors

Figure 35 shows the effect of the axial distance between the front and the rear wind
rotor, measured at the twist centre of the blade, on the performances, where D RF of
Tandem Wind Rotor GE is 0.84 and D RF of Tandem Wind Rotor EE is 0.71. The
rotational speed of the front wind rotor N Fopt becomes slower with the decrease of
L because of the fl ow interaction between both wind rotors, regardless of the blade
profi le. The rotational speed of the front wind rotor with blade G is slightly slower
than that of the front blade E, but the front blade G makes the rear wind rotor
speed faster. Resultantly, the relative rotational speed N Topt of Tandem Wind Rotor
GE is slightly faster than that of Tandem Wind Rotor EE. Such characteristics
are scarcely affected by the diameter ratio D RF . The maximum output coeffi cient
C P , max is higher at the smaller L because the rear wind rotor can get effectively the
wind energy as recognized in Fig. 36.
Besides, C P , max of Tandem Wind Rotor GE is markedly higher than that of Tandem
Wind Rotor EE because GE has not only the optimum but also the desirable
blade profi le.
These data suggest that the rear wind rotor should be set as close as possible to
the front wind rotor, taking the bending moment and the vibration of the blade into
account.

3000

N Topt

1500

N Fopt

0

V = 10 m/s N Ropt

-1500

0.4

Tandem Wind Rotor GE

b F =10 deg. b R =16 deg. D RF =0.84

0.3

0.2

0.1

Tandem Wind Rotor EE

b F = b R = 11 deg. D RF =0.71

0

0 0.1 0.2 0.3 0.4

L ( = l / d F )

Figure 35: Effect of the axial distance between front and rear wind rotors on the
performances.