Handbook of Turbomachinery Second Edition Revised - Ventech!

1. Impinging air from the stator in the direction normal to rim
section of the turbine rotor disc
2. Providing radial outflow of the cooling air in the disc cavity
toward blade root air inlet passages
3. Preswirling cooling air prior to its discharge into a disc cavity
4. Supplying air close to the center of the disc into a channel formed
between turbine disc and corotating cover plate attached to the
disc
The first two methods are rarely found in the first stage of modern gas
turbines due to significant heat pick-up by the air from the disc and due to
work required for tangentially accelerating the air until the disc tangential
velocity is matched.
Application of preswirl nozzles accelerates the air in tangential
direction, reducing the relative temperature of the air and minimizing
frictional losses in the system (Fig. 23). Usually the relative temperature of
the swirled air should be set at about 100 8F below the allowable disc rim
temperature. The preswirl nozzles are often positioned at approximately the
same radius as the entries into the blade-cooling passages on the rotor (Fig.
24, left configuration). Asealed subplenum supplying the preswirled air is
recommended to prevent hot gas ingress and reduce mixing between the
blade-cooling and disc-cooling flows.
The fourth method (Fig. 24, right configuration), using various types
of corotating cover plates, is usually applied when additional air pumping is
required for the blades, particularly when a high-pressure margin has to be
provided, for example, for the blade shower head cooling. This method
makes cooling air delivery system more expensive but less sensitive to rim
seal leakages than open disc systems.
The cooling air from intermediate compressor stages is often provided
to the turbine components downstream of stage 1. This helps to improve the
overall engine performance by saving compression work and also provides
lower-temperature cooling air bled from an intermediate stage. The first two
methods, as described above, are typical options for delivering air to cool
stage 2 (and sometimes stage 3) blades and discs.
A small portion of the internal air flow has to be assigned for buffering
of the sealed bearing housing cavities. The air pressure in the cavities should
be carefully balanced as a part of the whole internal flow network to avoid
ingress of the hot gas, oil leakages into the turbine, and proper ventilation of
the oil vapor. Performance and long-term durability of the rotor-to-stator
seals are often crucial for reliable supply of the cooling air and for overall
engine performance. Figure 25 provides a good summary of the flow
characteristics for traditional labyrinth seals [99]. More recently, a new
Copyright © 2003 Marcel Dekker, Inc.