Page 285 - Fluid Mechanics and Thermodynamics of Turbomachinery

266 Fluid Mechanics, Thermodynamics of Turbomachinery
to favourable design conditions with high values of flow Reynolds number, efficient
diffusers and low leakage losses at the blade tips. It is seen that over a limited range
of specific speed the best radial-flow turbines match the best axial-flow turbine
efficiency, but from  s D 0.03 to 10, no other form of turbine handling compressible
fluids can exceed the peak performance capability of the axial turbine.
Over the fairly limited range of specific speed .0.3 5  s < 1.0/ that the IFR
turbine can produce a high efficiency, but it is difficult to find a decisive performance
advantage in favour of either the axial flow turbine or the radial-flow turbine. New
methods of fabrication enable the blades of small axial-flow turbines to be cast
integrally with the rotor so that both types of turbine can operate at about the same
blade tip speed. Wood (1963) compared the relative merits of axial and radial gas
turbines at some length. In general, although weight, bulk and diameter are greater
for radial than axial turbines, the differences are not so large and mechanical design
compatibility can reverse the difference in a complete gas turbine power plant. The
NASA nuclear Brayton cycle space power studies were all been made with 90 deg
IFR turbines rather than with axial flow turbines.
The design problems of a small axial-flow turbine were discussed by Dunham and
Panton (1973) who studied the cold performance measurements made on a single-
shaft turbine of 13 cm diameter, about the same size as the IFR turbines tested by
NASA. Tests had been performed with four different rotors to try and determine the
effects of aspect ratio, trailing edge thickness, Reynolds number and tip-clearance.
One turbine build achieved a total-to-total efficiency of 90 per cent, about equal to
that of the best IFR turbine. However, because of the much higher outlet velocity,
the total-to-static efficiency of the axial turbine gave a less satisfactory value (84 per
cent) than the IFR type which could be decisive in some applications. They also
confirmed that the axial turbine tip-clearance were comparatively large, losing two
per cent efficiency for every one per cent increase in clearance. The tests illustrated
one major design problem of a small axial turbine which was the extreme thinness
of the blade trailing edges needed to achieve the efficiencies stated.

Optimum design selection of 90 deg IFR turbines

Rohlik (1968) has examined analytically the performance of 90 deg inward flow
radial turbines in order to determine optimum design geometry for various appli-
cations as characterised by specific speed. His procedure, which extends an earlier
treatment of Balje (1981) and Wood (1963) was used to determine the design point
losses and corresponding efficiencies for various combinations of nozzle exit flow
angle ˛ 2 , rotor diameter ratio D 2 /D 3av and rotor blade entry height to exit diameter
ratio, b 2 /D 3av . The losses taken into account in the calculations are those associ-
ated with,
(i) nozzle blade row boundary layers,
(ii) rotor passage boundary layers,
(iii) rotor blade tip clearance,
(iv) disc windage (on the back surface of the rotor),
(v) kinetic energy loss at exit.