Handbook of Turbomachinery Second Edition Revised - Ventech!

When Rohlik performed his analytical study in 1968, he limited the
rotor exit hub-to-tip radius ratio to values greater than 0.4. The turbine
investigated by Kofskey and Nusbaum [25] had a hub-to-tip radius ratio at
the exit of 0.53. However, with the desire for smaller and less expensive
turbine wheels, hub-to-tip radius ratios now are seen as low as 0.25 and less.
Along with inertia and stress considerations, this limits rotor blade count
from 10 to 14 (Rodgers [27]).
Typical materials for radial-inflow turbine wheels are cast superalloys
for high-temperature applications and cast or forged steel for lower
temperatures. Ceramics have been used in production turbochargers and
are in a research stage for small gas turbines. Radial-inflow turbine wheels
have three critical stress locations: inlet blade root, exducer blade root, and
hub centerline. Rodgers [27] notes that the tip speed of current superalloy
radial-inflow turbine wheels is limited to approximately 2,200 ft/sec. The
exact value is dependent on both operating temperature and desired life. For
moderate inlet temperatures and pressure ratios ðT 0 in
< 500 F and
p 0 in =pdis < 4Þ, stress considerations, while they must be addressed in the
mechanical design, usually do not constrain the aerodynamic design of the
turbine. This includes free-run operation.
As previously mentioned, radial-inflow turbine blades are usually
radial at the inlet to eliminate bending loads. At the exit, the rotor blade
angle is limited to about 608 from axial for manufacturing reasons. With
casting being the preferred method of construction, rotor trailing-edge
thickness should be greater than 0.020 in. Limitations on the radial-inflow
stator are similar to those for an axial-flow stator: exit blade angle should be
less than 758 (for a radial-inflow stator, this is measured from the radial
direction) and trailing-edge thickness should be 0.015 in. or greater.
Significantly thicker trailing edges are needed if the stator vanes are cooled.
Trailing-edge blockage for both stators and rotors should be kept below
10% for best performance. With low hub-to-tip radius ratios at rotor exit,
this guideline is frequently violated at the hub, where the blade spacing is
smallest and the trailing-edge thickness is large for mechanical reasons.
Overall package diameter is determined by rotor tip diameter, radius
ratio across the stator, and the size of the scroll. In addition, there is
normally a vaneless space between the stator and rotor, similar to the axial
gap between the stator and rotor in an axial-flow turbine. The vaneless space
radius ratio is usually held to 1.05 or less. Stator vane radius ratio is
controlled by stator vane count and stator turning. In most radial-inflow
turbines, a scroll provides a significant amount of tangential component at
stator inlet, resulting in relatively low amounts of flow deflection in the
stator vane row. This results in reduced solidity requirements, so that fewer
and shorter stator vanes can be used. Rodgers [24] states that a common
Copyright © 2003 Marcel Dekker, Inc.