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66 Gas Turbine Handbook: Principles and Practices
Properties of a good burner are high combustion efficiency, stable
combustion, low NO x
formation, freedom from blowout, uniform
or controlled discharge temperature, low pressure loss, easy starting,
long life, and (for liquid fuel operation) minimum carbon accumulation.
Combustors must be able to withstand various conditions;
namely, a wide range of airflow, fuel flow, and discharge temperature,
rapid acceleration and deceleration, and variation in fuel properties.
Combustor efficiency 2 is defined as
η B = W ac p T o – T i
W f Q r (4-27)
The pattern or profile of the gas temperature leaving the combustor
has a direct impact on the operation and life of the turbine.
Therefore, the design of the combustor and the transition duct are
critical. As will be discussed later, the condition of the fuel nozzles,
burner, and transition duct can have a deleterious effect on the temperature
profile, and the turbine gas path components.
TURBINE
The turbine extracts kinetic energy from the expanding gases
that flow from the combustion chamber, converting this energy into
shaft horsepower to drive the compressor, the output turbine, and
select accessories.
The axial-flow turbine is made up of stationary nozzles (vanes
or diaphragms) and rotating blades (buckets) attached to a turbine
wheel (disc). Turbines are divided into three types: “ impulse,” “ reaction,”
and a combination of the two designs called “ impulse-reaction.”
The energy drop to each stage is a function of the nozzle area
and airfoil configuration. Turbine nozzle area is a critical part of the
design: too small and the nozzles will have a tendency to “choke”
under maximum flow conditions, too large and the turbine will not
operate at its best efficiency. It is important to note that approximately
3/4 - 2/3 of the turbine work drives the compressor leaving
approximately 1/4-1/3 for shaft horsepower (or thrust for the jet
engine).