Handbook of Turbomachinery Second Edition Revised - Ventech!

The Thermodynamic Cycle
The thermodynamic cycle for the gas turbine engine is the Brayton cycle,
which consists of four theoretical processes. The first process is one of
isentropic compression where the pressure of the air drawn from the
atmosphere is raised to the operating level in a compressor. This constitutes
the work input part of the cycle. The second is the thermal energy input in a
combustor—an isobaric (constant-pressure) process to raise the temperature
of the air to the highest level permitted in the engine. The third and fourth
processes are, respectively, an isentropic expansion (work output) in the
turbine and an isobaric cooling process (energy rejection to the atmosphere),
to complete the cycle. The ideal thermal efficiency of the cycle is given by the
expression [1, 2]
ðg 1Þ=g
ZB ¼ 1 ½1=PrŠ
It is seen that the thermal efficiency of the Brayton cycle is the same as that
of the Carnot cycle with the same isentropic compression ratio. Nevertheless,
its thermal efficiency for operation between the same temperature
limits is lower than that of the Carnot cycle. The efficiency increases as the
pressure ratio increases. For this reason, efforts are made to operate the
engine at as high a pressure ratio as possible.
The work output of the theoretical Brayton cycle is a function of the
maximum temperature in the cycle, the temperature of energy rejection, and
the pressure ratio. There exists an optimum pressure ratio at which the work
output becomes a maximum. The pressure ratio for maximum work output
is given by the expression
ðPrÞopt ¼ðT3=T1Þ g=½2ðg 1ÞŠ ¼ðPrÞ 1=2
The corresponding maximum work output of the Brayton cycle is
Wmax ¼ CpT1 ðT3=T1Þ 1=2
h i2 1
ð3Þ
Engines may be designed to operate close to this condition. The thermal
efficiency of the Brayton cycle operating at the optimum pressure ratio is
ðZ BÞopt ¼ 1 ðT1=T3Þ 1=2
Components of the Gas Turbine Engine
Figure 1 shows the main components of the gas turbine engine—the
compressor, the combustor, and the turbine. Work input occurs in a
Copyright © 2003 Marcel Dekker, Inc.
ð1Þ
ð2Þ
ð4Þ