Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

230 Fluid Mechanics, Thermodynamics of Turbomachinery
and, therefore,
(7.24a)
This formulation is useful if the flow angles can be specified. Alternatively, and more
usefully, as cq2 = sc¢q2 = s(U2 - cr2tan b¢2), then
(7.24b)
where Mu = U2/a01, is now defined as a blade Mach number.
It is of interest to calculate the variation of the pressure ratio of a radially vaned (b¢2
= 0) centrifugal air compressor to show the influence of blade speed and efficiency on
the performance. With g = 1.4 and s = 0.9 (i.e. using the Stanitz slip factor, s = 1 -
1.98/Z and assuming Z = 20, the results evaluated are shown in Figure 7.12. It is clear
that both the efficiency and the blade speed have a strong effect on the pressure ratio.
In the 1970s the limit on blade speed due to centrifugal stress was about 500m/s and
FIG. 7.12. Variation of pressure ratio with blade speed for a radial-bladed compressor
(b¢ 2 = 0) at various values of efficiency.