Handbook of Turbomachinery Second Edition Revised - Ventech!

Figure 1 Cross-section showing the main components of a gas turbine engine.
compressor with several axial and/or centrifugal stages. The engine shown in
Fig. 1has only centrifugal compressors. (Figure 8shows an engine with
axial and centrifugal stages for compression.) The pressure ratio in
commercial aircraft engine compressors is often in the range of 10–25,
though some experimental engines have pressure ratios in the range 17–35.
The turbine driving the compressor is usually an axial-flow device though in
small engines (auxiliary power units, or APUs), it is often a radial inward
flow device. Energy addition as heat and a slight pressure drop (3–5%) occur
in a combustor where a fine spray of fuel burns in the air from the
compressor. The maximum temperature of the gas is limited by material
considerations, being 2200–2500 8F in most engines. The mixture of burned
fuel and air at a high temperature enters the turbine. Work output is due to
the expansion of the gas while flowing over the rotating turbine blades.
Thermal energy rejection from the engine, as in all practical propulsion
engine cycles, occurs due to the gas that is exhausted from the turbine to the
atmosphere. There is no heat exchanger to reject thermal energy at constant
pressure from the system.
In aircraft engines, the air flowing through a propeller or a fan driven
by the turbine gives rise to the propulsive force on the aircraft. For example,
in propeller-driven engines, the change in momentum of the air flowing
through the propeller causes a reactive force, resulting in a forward thrust
on the engine. In engines with fans, the reactive force is due to the exhaust
jet at the exit of the turbine. Power generation units use the turbine output
to drive a gearbox or a load compressor.
Real Gas Engine Cycle
The real engine cycle differs from the theoretical Brayton cycle in several
respects. First, the processes of compression and expansion are not
isentropic. So the work input needed at the compressor is higher than in
Copyright © 2003 Marcel Dekker, Inc.