Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

(a)
(2)
(1)
w 2
b2 ¢
w 2 ¢
b2, opt
Radial Flow Gas Turbines 271
Figure 8.11 shows the variation of the incidence loss function (1 - cos n i) for a range
of the incidence angle i using the appropriate values of n.
EXAMPLE 8.6. (a) For the IFR turbine described in Example 8.3, and using the data
and results in Examples 8.4 and 8.5, deduce a value for the rotor enthalpy loss coefficient,
zR, at the optimum efficiency flow condition.
(b) The rotor speed of rotation is now reduced so that the relative flow enters the
rotor radially (i.e. at the nominal flow condition). Assuming that the enthalpy loss
coefficients, zN and zR, remain the same, determine the total-to-static efficiency of
the turbine for this off-design condition.
Solution. (a) From eqn. (8.10), solving for zR,
h
1
2s
01
P01
P 1
02¢rel
2¢
3ss 3s¢ 3s 3
2
P02¢ rel
02rel
P 2
P02rel
FIG. 8.10. (a) Simple flow model of the relative velocity vector (1) immediately before
entry to the rotor, (2) immediately after entry to the rotor. (b) Mollier diagram
indicating the corresponding entropy increase, (s 3s - s 3s¢), and enthalpy “loss”, (h 2 -
h 2¢), as a constant pressure process resulting from non-optimum flow incidence.
P 3
s
(b)