Handbook of Turbomachinery Second Edition Revised - Ventech!

during engine operation are compatible with the maximum stress limits
imposed by the specified operating life of the component. The cooling
system design and development process requires innovative design
experience, proven analytical methods, and sophisticated experimental
facilities, as well as advanced materials and manufacturing techniques. This
establishes the required confidence in the prediction of turbine component
temperatures affecting life and performance of the engine.
A major design challenge in achieving high turbine efficiency is to
minimize the flow rate of turbine cooling air with the best utilization of its
cooling potential for providing the required component temperatures. An
additional factor that should be considered, particularly in the industrial
environment, is the fuel/air quality, which is often responsible for erosion of
the coatings that protect the hot section components. Operation in such
environment also requires larger cooling passages to avoid their blockage.
Major gas turbine components typically requiring cooling include:
Stage 1 and Stage 2 nozzle vanes
Stage 1 blades
Support structure for the nozzles and tip shrouds (nozzle case,
diaphragms)
Turbine disc/rotor assemblies
Combustor liners
In addition to component temperature reduction, another important role
for the cooling system is to control ‘‘passively’’ or ‘‘actively’’ the relative
position between rotor and stator, maintaining turbine blade-tip clearances.
Commonly Used Cooling Techniques
The most common cooling techniques, for the reasons stated above, are
based on the application of the air bled from the compressor discharge or
intermediate stages. Figure 2illustrates ageneric turbine hot section with
main elements of turbine and combustor cooling system. Introduction of the
turbine cooling air downstream of the combustor results in degradation of
turbine performance because less work is extracted from the compressed
cooling air. At the same time the reduced amount of air available for the
combustor makes liner cooling and emission control more difficult. This
poses a major challenge for the designer of the cooling system: select a
system that requires a minimal amount of cooling air to achieve targeted
metal temperature of the turbine components and produces the smallest
negative impact on engine durability, performance, weight, emission, cost,
and fabrication complexity. Out of these most important engine characteristics,
weight reduction is a major design criterion for aero engines, and
Copyright © 2003 Marcel Dekker, Inc.