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Handbook of Turbomachinery Second Edition Revised - Ventech!

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different turbines easier, dimensional analysis leads to the formation <strong>of</strong><br />

several dimensionless parameters that can be used to describe turbines.<br />

Specific Speed and Specific Diameter<br />

The specific speed <strong>of</strong> a turbine is defined as<br />

pffiffiffiffiffiffi<br />

o Q2<br />

Ns ¼<br />

ðDhidealÞ 3=4<br />

ð40Þ<br />

where Q2 is the volumetric flow rate through the turbine at rotor exit. The<br />

specific speed is used to relate the performance <strong>of</strong> geometrically similar<br />

turbines <strong>of</strong> different size. In general, turbine efficiency for two turbines <strong>of</strong><br />

the same specific speed will be the same, except for differences in tip<br />

clearance and Reynolds number. Maintaining specific speed <strong>of</strong> a turbine is a<br />

common approach to scaling <strong>of</strong> a turbine to different flow rates.<br />

The specific diameter is defined as<br />

1=4<br />

dtipðDhidealÞ<br />

Ds ¼ ffiffiffiffiffiffi<br />

Q2<br />

p ð41Þ<br />

where dtip is the tip diameter <strong>of</strong> the turbine rotor, either radial in-flow or<br />

axial flow. Specific diameter and specific speed are used to correlate turbine<br />

performance. Balje [3] presents extensive analytical studies that result in<br />

maps <strong>of</strong> peak turbine efficiency versus specific speed and diameter for<br />

various types <strong>of</strong> turbines. These charts can be quite valuable during initial<br />

turbine sizing and performance estimation.<br />

Blade-Jet Speed Ratio<br />

Turbine performance can also be correlated against the blade-jet speed<br />

ratio, which is a measure <strong>of</strong> the blade speed relative to the ideal stator exit<br />

velocity. Primarily used in impulse turbines, where the entire static enthalpy<br />

drop is taken across the stator, the ideal stator exit velocity, C0, is calculated<br />

assuming the entire ideal enthalpy drop is converted into kinetic energy:<br />

C0 ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi p<br />

2Dhideal<br />

ð42Þ<br />

The blade-jet speed ratio is then calculated from<br />

U<br />

C0<br />

U<br />

¼ p ð43Þ<br />

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />

2Dhideal<br />

The value <strong>of</strong> blade speed at the mean turbine blade radius is typically used in<br />

Copyright © 2003 Marcel Dekker, Inc.

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