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Handbook of Turbomachinery Second Edition Revised - Ventech!

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adial-inflow turbine will be higher than that for an axial-flow turbine,<br />

which can have Vy1=U1 > 1 with only a small impact on efficiency. This<br />

assumes zero exit swirl. For a fixed shaft speed, this means that the radialinflow<br />

turbine will be larger (and heavier) than an axial-flow turbine. Stage<br />

work can be increased by adding exit swirl; however, the radial-inflow<br />

turbine is again at a disadvantage. The lower wheel speed at exit for the<br />

radial-inflow turbine means that more Vy2 must be added for the same<br />

amount <strong>of</strong> work increase, resulting in higher exit absolute velocities<br />

compared to an axial-flow turbine. In addition, high values <strong>of</strong> exit swirl<br />

negatively impact obtainable diffuser recoveries.<br />

Packaging considerations may lead to the selection <strong>of</strong> a radial-inflow<br />

turbine. The outside diameter <strong>of</strong> a radial-inflow turbine is considerably<br />

larger than the rotor tip diameter, due to the stator and inlet scroll or torus.<br />

Compared to an axial-flow turbine, the radial-inflow package diameter may<br />

be twice as large or more. However, the axial length <strong>of</strong> the package is<br />

typically considerably less than for an axial turbine when the inlet and<br />

diffuser are included. Thus, if the envelope is axially limited but large in<br />

diameter, a radial-inflow turbine may be best suited for the application,<br />

considering performance requirements can be met.<br />

For auxiliary turbine applications where free run may be encountered,<br />

radial-inflow turbines have the advantage <strong>of</strong> lower free-run speed than an<br />

axial turbine <strong>of</strong> comparable design-point performance. Figure 11 shows the<br />

<strong>of</strong>f-design performance characteristics <strong>of</strong> both radial-inflow and axial-flow<br />

turbines. At higher shaft speeds, the reduction in mass flow for the radialinflow<br />

turbine leads to lower torque output and a lower free-run speed.<br />

Because <strong>of</strong> the change in radius in the rotor, the flow through the rotor must<br />

overcome a centrifugal pressure gradient caused by wheel rotation. As shaft<br />

speed increases, this pressure gradient becomes stronger. For a given overall<br />

pressure ratio, this increases the pressure ratio across the rotor and<br />

decreases the pressure ratio across the stator, leading to a reduced mass flow<br />

rate. A complete description <strong>of</strong> this phenomenon and its effect on relative<br />

temperature at free-run conditions is presented by Mathis [23]. However, the<br />

rotor disk weight savings from the lower free-run speed <strong>of</strong> a radial-inflow<br />

turbine is <strong>of</strong>fset by the heavier containment armor required due to the<br />

increased length <strong>of</strong> a radial-inflow turbine rotor compared to an axial<br />

turbine.<br />

Radial-Inflow Turbine Performance<br />

The literature on performance prediction and loss modeling for radialinflow<br />

turbines is substantially less than that for axial-flow turbines. Wilson<br />

[2] states that most radial-inflow turbine designs are small extrapolations or<br />

Copyright © 2003 Marcel Dekker, Inc.

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