Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

hub–tip ratio, u
1.0
0.8
0.6
0.4
0.2
Axial-flow Turbines: Two-dimensional Theory 133
Crawl region
Full self-start
region
0
0 0.4 0.8
s
FIG. 4.28. Self-starting capability of the Wells turbine (adapted from Raghunathan
et al. 1995).
enables a higher torque and efficiency to be obtained from the reciprocating airflow.
According to the authors the turbine is geometrically simpler and would be less expensive
to manufacture than some earlier methods using “active” pitch-controlled blades,
e.g. Sarmento et al. (1987), Salter (1993).
The working principle with self-pitch-controlled blades is illustrated in Figure 4.29.
This shows one of the turbine blades fixed to the hub by a pivot located near the leading
edge, allowing the blade to move between two prescribed limits, ±g. An aerofoil set at
a certain angle of incidence experiences a pitching moment about the pivot, which
causes the blade to flip. In this new position the blade develops a higher tangential force
and torque at a lower rotational speed than was obtained with the original fixed-blade
design of the Wells turbine.
Kim et al., using a piston-driven wind tunnel, measured the performance characteristics
of the turbine under steady flow conditions. To determine its running and starting
characteristics, a quasi-steady computer simulation of the oscillating through-flow
was used together with the steady-state characteristics. Details of the turbine rotor are
Blade profile NACA 0021 Hub–tip ratio 0.7
Blade chord, l 75mm Tip diameter 298mm
No. of blades, Z 8 Hub diameter 208mm
Solidity 0.75 Blade length, H 45mm