Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

Radial Flow Gas Turbines 249
FIG. 8.3. Layout and velocity diagrams for a 90deg inward-flow radial turbine at the
nominal design point.
turbines (discussed in Chapter 9) a diffuser is invariably used and is called the
draught tube.
In Figure 8.3 the velocity triangles are drawn to suggest that the inlet relative velocity,
w2, is radially inward, i.e. zero incidence flow, and the absolute flow 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 considered in some detail in the following
pages. Following this the so-called optimum efficiency design will be explained.
Thermodynamics of the 90deg 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, h01 = h02 and,
therefore, the static enthalpy drop is
(8.1)
corresponding to the static pressure change from p 1 to the lower pressure p2. The ideal
enthalpy change (h1 - h2s) is between these same two pressures but at constant entropy.