Handbook of Turbomachinery Second Edition Revised - Ventech!

and the measurement of local heat-transfer coefficients may pose more
difficulties than for the liquid-crystal test.
1. Rotating tip shrouds. Figure 11 portrays schematically an
uncooled LPT blade with a rotating tip shroud. The tip shroud is necessary
due to the large radius and blade span in the low-pressure stages. The same
blade-tip clearance will cause a much larger fraction of leakage at a lowpressure
stage than at a high-pressure stage. The gain in power output and
turbine efficiency due to the tip shroud should be balanced against the cost
and complexity of design. Tip shrouds may have one or more teeth
depending on the permissible leakage and the required aerodynamic
efficiency. All the design parameters for a cooled blade are similar to those
of the cooled blade, except the rotating tip shroud and the number of teeth
on it.
2. Blade-to-disk attachment. There exist two types of attachments
between the blade and the disk. The first is the integral design where the
blade and the disk are made of the same material. These are cast as an
integral unit and cannot be detached. This design is usually used in
compressor and turbine stages in which the disk stresses are so low that the
blade material can stand the temperature and the stresses. Typically in
turbines, a high-strength forging that has a lower tolerance to temperature is
used for the disk, whereas a material casting that can stand higher
temperatures is used for the blades. In HP turbines where the blade needs
cooling, the procedure is to cast the blade as a separate piece that can be
inserted in the rim of the disk at the fir tree. Then the blade can be made of
special alloys of nickel with high strength and tolerance to temperature.
Moreover, the blade can be made with serpentine or other passages through
which cooling air can be passed to maintain the surface at low temperatures
to ensure long life.
3. Turbine disk design. Disk design should ensure that the stresses
due to thermal transients and those induced by rotation do not become
excessive at any location. The main areas of concern are the disk rim, the
web, and the disk bore. At the rim, the blade-disk attachment area (fir-tree)
is subjected to high stresses due to the centrifugal forces on the blades. The
disk rim is heated by conduction at the blade attachment, by convection
near the blade platform, and by the ingested gas in the cavities. It should
therefore be cooled by providing adequate amounts of air at both faces of
the disk. The flow should also be sufficient to keep the disk cavities purged
such that gas ingestion is minimized. The main aspect of the design is to
guard against LCF failure. It is necessary as well to guard against stressrupture
at specific points (e.g., fir-tree attachment), due to excessive creep,
thermal, and bearing stresses. At the disk bore where the stresses often
exceed yield limits, it is necessary to guard against the possibility of failure
Copyright © 2003 Marcel Dekker, Inc.