Handbook of Turbomachinery Second Edition Revised - Ventech!

and its attachments. A substantial fraction of the flow passes through the
TOBI (tangential on-board injector). This may be a set of nozzles or an
appropriately drilled set of holes designed to increase the tangential velocity
of the air. The air emerging from the TOBI has a low static temperature and
a tangential velocity that approximates that of the rotating disk. Hence, it
cools the turbine disk and the blade and purges the HP turbine cavities as
well. The second stream drawn from the impeller exit (marked HP in the
diagram) flows down the back face of the impeller. It may be used to cool
the bore of the turbine and for bearing cooling purposes where possible. In
the illustration, it is used partly to cool the first LPT disc. Still another
stream drawn from the impeller inlet flows axially through the impeller bore
to purge the second and the third LPT disk cavities.
b. Provide buffer air to bearing seals. This is the second important
purpose of secondary flow design. It is necessary to provide a sufficient
pressure difference between bearing seal faces so that oil leakage is
minimized. The air used for buffering should be at temperatures not higher
than 400 8F since the oil coming in contact with the air tends to coke and
form hard deposits if its temperature becomes excessive.
c. Maintain bearing thrust loads at low levels/thrust balance. The
thrust on the turbine disk due to aerodynamic and other loads acts on the
shaft bearing. The bearing should be designed to withstand the thrust so
that the shaft is held in place. The larger the bearing load, the bigger and
more expensive the bearing becomes. By using a thrust piston arrangement,
secondary air at the appropriate pressure is made to exert a force on the disk
to reduce the net thrust on the bearing. Thrust load calculation is a detailed
bookkeeping effort to account for all the aerodynamic forces, momentum
changes, and pressure forces acting on the surfaces of a control volume.
Since the calculations involve differences between large numbers of similar
magnitudes, there is a considerable amount of uncertainty in the estimated
net thrust. It may therefore be necessary to design the bearing to withstand a
larger-than-calculated load. In some cases, the calculations may lead to a
very small estimate for the thrust load. Then, secondary flow may be used to
ensure that the load acts in only one direction and does not reverse due to
changing operating conditions. It is a good practice to check the thrust on
the bearing at several operating conditions, say full power, 50% power, and
idle conditions, to ensure that there is no thrust reversal in the operating
range. Secondary flows have also been used to reduce the loads and thus the
stresses acting on nozzles and such other components in some experimental
engines. (For details regarding secondary flow design, see the chapter by
Bruce Johnson.)
d. Cooled and uncooled airfoil design. Airfoils may be cooled or
uncooled depending on the temperature of the gas and the material of the
Copyright © 2003 Marcel Dekker, Inc.