Handbook of Turbomachinery Second Edition Revised - Ventech!

the theoretical cycle and the work output of the turbine is lower than in the
theoretical cycle. (In most gas turbines used for propulsion, the adiabatic
compressor efficiency ranges from 0.83 to 0.88. In turbines the adiabatic
efficiency ranges from 0.85 to 0.92.) In addition, there are pressure losses
associated with flow through the combustor and several other parts of the
machine. These as well as other deviations from ideality reduce the net work
output and the thermal efficiency as compared with that of the theoretical
Brayton cycle. For engines with a pressure ratio in the range of 13–15, the
typical thermal efficiency for operation at 2000 8F is about 35%. A measure
of thermal efficiency is the specific fuel consumption, SFC, which is the rate
of fuel consumed (lbm/hr) per unit of output. For efficient operation, it is
necessary to have as low a fuel consumption and, hence, as low an SFC as
possible. Reduction in SFC may require an increased inlet temperature or
the use of a recuperator (a heat exchanger inserted between the compressor
and the combustor). The recuperator transfers part of the thermal energy of
the exhaust gases to the high-pressure air entering the combustor and
reduces the fuel consumption. The engine cycle that uses a recuperator is
called the regenerative Brayton cycle [2]. The thermal efficiency of the ideal
regenerative cycle fitted with a recuperator where there are no pressure
drops is given by the expression
Z R ¼ 1 ðPrÞ ðg 1Þ=g =Tr ð5Þ
Unlike the ideal Brayton cycle without regeneration, the thermal efficiency
of this cycle diminishes with increasing pressure ratio. However, it increases
with increasing temperature ratio as in the Carnot cycle.
Gas Turbine Engine Applications
The following are the areas of use of gas turbines:
1. Propulsion of aircraft as well as ground-based vehicles. There exist
four types of gas turbine engines: the ‘‘turboprop,’’ the ‘‘turbofan,’’ the
‘‘turbojet,’’ and the ‘‘turboshaft,’’ based on their use in propulsion. The first
three are designed for use where thrust is important. The turboprop uses a
propeller to move large masses of air and has a low specific thrust. It
operates at relatively low Mach numbers, usually on the order of 0.25.
(There are some engines that run at higher Mach numbers, on the order of
0.6.) Figure 2 exhibits a typical turboprop engine manufactured by
Honeywell Engines & Systems. Turboprops usually range in power between
600 and 6000 HP. Turboprops used in both commercial and military
applications are relatively small compared with turbofans, which employ
high-speed fans to move the air.
The turbofan requires large masses of air flow, though only a fraction,
Copyright © 2003 Marcel Dekker, Inc.