Handbook of Turbomachinery Second Edition Revised - Ventech!

General rules for minimizing cooling air discharge penalties affecting
turbine performance can be summarized as follows:
Discharge any cooling flow as far upstream along the gas path as
possible.
Use benefits of a shower head cooling for the leading edge of the first
stages of airfoils as a preferred cooling technique, even using larger
cooling flows, if sufficient cooling budget is allowed and when
fabrication cost is not prohibitive.
Attempt to design the cooling system that discharges cooling air at a
temperature that is close to (or slightly below) allowable local metal
surface temperature.
Minimize mixing losses by closely matching velocity vectors between
main-stream and discharged cooling flows.
Avoid film discharge on the suction side of an airfoil at close vicinity
of the boundary-layer transition.
Exercise maximum effort in avoiding cooling flow discharge from the
suction side of an airfoil in the vicinity upstream of the throat and
especially downstream of the throat.
Minimize pressure losses in the internal cooling passages to retain the
total pressure in the cooling flow.
Use preswirling mechanism for blade cooling supply system, lowering
the relative temperature of the coolant and reducing disc friction
losses.
Combination of Cooling with Thermal Barrier Coatings
The use of thermal barrier coatings (TBC) presents a great benefit in
reducing the heat load on the airfoil alloy particularly for internally cooled
airfoils. Airfoils dressed in thermal barrier coatings are fabricated in a
conventional manner with the exception of a thin layer of a hightemperature
insulating material. TBC material usually can withstand very
high temperatures and has a thermal conductivity in order of one tenth that
of conventional superalloys. Due to the additional thermal resistance,
airfoils coated in this manner can operate with much less cooling air at a
given gas temperature or, conversely, can withstand much higher gas
temperatures at a given level of cooling flow than uncoated airfoils. It is
quite clear that TBC applied on the external surface cannot do the job of
protecting the airfoil material from high temperature without applying
cooling on the inner surface.
Designing cooled airfoils with thermal barrier coatings presents certain
problems. Even when polished, the material has an inherent roughness,
Copyright © 2003 Marcel Dekker, Inc.