Page 258 - Fluid Mechanics and Thermodynamics of Turbomachinery

Radial Flow Gas Turbines 239

FIG. 8.3. Layout and velocity diagrams for a 90 deg inward flow radial turbine at the
nominal design point.

at rotor exit, c 3 , is axial. This configuration of the velocity triangles, popular with
designers for many years, is called the nominal design condition and will be consid-
ered in some detail in the following pages. Following this the so-called optimum
efficiency design will be explained.

Thermodynamics of the 90 deg IFR turbine

The complete adiabatic expansion process for a turbine comprising a nozzle blade
row, a radial rotor followed by a diffuser corresponding to the layout of Figure 8.3,
is represented by the Mollier diagram shown in Figure 8.4. In the turbine, frictional
processes cause the entropy to increase in all components and these irreversibilities
are implied in Figure 8.4.
Across the nozzle blades the stagnation enthalpy is assumed constant, h 01 D h 02
and, therefore, the static enthalpy drop is,
2
1
h 1 h 2 D .c 2 2 c / (8.1)
1
2
corresponding to the static pressure change from p 1 to the lower pressure p 2 . The
ideal enthalpy change (h 1 h 2s ) is between these same two pressures but at constant
entropy.
1 2
In Chapter 7 it was shown that the rothalpy, I D h 0rel U , is constant for an
2
adiabatic irreversible flow process, relative to a rotating component. For the rotor