Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

132 Fluid Mechanics, Thermodynamics of Turbomachinery
of hub–tip ratio and solidity which have been found to allow self-starting of the Wells
turbine are indicated in Figure 4.28.
Pitch-controlled blades
Introduction
p * /p *
0
h /h 0
1.5
0.5
Over the last decade some appreciable improvements have been made in the performance
of the Wells turbine as a result of incorporating pitch-controlled blades into
the design. The efficiency of the original Wells turbine had a peak of about 80% but
the power output was rather low and the starting performance was poor. One reason
for the low power output was the low tangential force Y and low flow coefficient f as
a consequence of the fixed-blade geometry.
A turbine with self-pitch controlled blades
2
1
0
0 0.4 0.8
FIG. 4.27. Variation of peak efficiency and non-dimensional pressure drop (in
comparison to the values for an isolated aerofoil) vs solidity: ---- pressure_____
efficiency (adapted from Raghunathan et al. 1995).
Performance enhancement of the Wells turbine reported by Kim et al. (2002) was
achieved by incorporating swivellable vanes instead of fixed vanes in an experimental
test rig. The method they devised used symmetrical blades that pivot about the nose,
whose pitch angle changes by a small amount as a result of the varying aerodynamic
forces developed by the oscillating flow. This change to the turbine configuration
s