Chapter 13 Gas Turbine Power Plants

in the cycle 1-2-3-4-1, the cycle is called the ideal Brayton cycle.
The thermal efficiency of the ideal Brayton cycle is a function of
pressure ratio pjpi, and its value is the highest possible efficiency
for any Brayton cycle at a given pressure ratio.
The thermal efficiency of any cycle is defined by (5.30). In the
Brayton cycle the net work is the algebraic sum of the turbine
work W t , which is positive, and the compressor work W c , which is
negative; thus, the thermal efficiency is written as
W +Wc
'
QA
(13.1)
where Q A is the energy added to the flowing gas in the combustor
as a result of the exothermic chemical reaction which occurs as
the fuel burns in air.
In the following sections the methods for computing W t , W c ,
and Q A will be shown for the ideal Brayton cycle, the standard
Brayton cycle, and for variations on the Brayton cycle which involve
the use of heat exchangers. Finally, the combined cycle,
Brayton plus Rankine, is considered.
13.2 Ideal Brayton Cycle
For the ideal cycle we can assume that the working fluid is cold
air, i.e., a gas having a molecular weight of 28.96 and a ratio of
specific heats y of 1 .4, and that the air behaves as a perfect gas.
The compression and expansion processes are isentropic for the
ideal cycle. According to (5.21) work for compression is given by
W c =h ol -h m (13.2)
where any change in potential energy is assumed negligible, and
the solid boundaries of the compressor are assumed to be adiabatic.
Assuming that the working substance is a perfect gas and