Handbook of Turbomachinery Second Edition Revised - Ventech!

due to cyclic operation. In addition, disks have to be designed to operate at
speeds about 20% above the engine operating speed for maximum power.
Referred to as overspeed capability, this design ensures that in the event the
engine needs extra power during an emergency, there is adequate margin
with little risk of failure. It also provides safety against inadvertent
overspeeds.
4. Turbine stage design. The design of turbine stages requires
specification of the shapes of the airfoils, the rotational speeds of the
rotors, the velocity triangles, aerodynamic efficiencies, and the work output.
Figure 12 shows schematics of typical turbine HPT and LPT blades. Since
these two blades run at different speeds, have different velocities of gas flow,
and have different outputs, they are quite different in shape. The primary
element of aerodynamic design is the maximization of blade efficiency. For a
given stage work output, the stage efficiency increases with tip speed to
reach a maximum level at a certain speed. Efforts are made to design the
blade to operate close to this optimal speed. In the HP turbine stage,
because of the high density and high velocity of the gas, the flow area
required and the disk diameter are smaller than those of the LPT. Further,
the airfoils are usually of small span compared with those of the LPT stages
and turn the gas through large angles ranging to 1308. These requirements
(high tip speed for maximum efficiency and small diameter) imply that they
run at much higher rotational speeds than LPT blades. In gas turbines
designed by Honeywell Engines & Systems, there is no attached rotating tip
shroud at the HPT blade tips, since leakage across the blade is not as serious
as in the LPT. (Large engines designed by Rolls-Royce Plc., for example,
often have rotating tip shrouds on HPT blades as well.) As opposed to this,
the LPT stages have large diameters since they are driven by low-pressure,
low-density gas with large specific volumes. They tend to run at lower tip
Figure 12 Schematic diagrams of typical HP turbine and LP turbine blades.
Copyright © 2003 Marcel Dekker, Inc.