Page 413 - Advanced thermodynamics for engineers

17.3 AIRCRAFT GAS TURBINES 403

It was shown that the ‘efficiency’ of an internal combustion engine could be defined by its brake
specific fuel consumption (bsfc). It is not appropriate to use this parameter for all gas turbines because
they do not necessarily produce shaft power. In the case of aircraft jet engines a more appropriate term
is the fuel consumption per unit thrust, e.g. kg/hN, known as the specific fuel consumption (sfc).
Then
_ m f
sfc ¼ (17.65)
_ m V j V a
and
V a 1
h ¼ (17.66)
o
sfc Q 0 p
It is also possible to introduce a parameter which gives an indication of the amount of power
obtained from a particular engine, somewhat similar to bmep for reciprocating engines. This is termed
the specific thrust, F s , which is the thrust per unit mass flow of air, i.e. F s ¼ F/m.
The performance of aircraft engines is normally defined at two operating conditions:
1. static performance at sea-level at maximum power (to give an indication of the power
available at takeoff);
2. cruising performance at the cruising speed and altitude.

17.3.2 INTAKE AND PROPELLING NOZZLES
Intake ducts will not be discussed in detail here, except to point out that they are essential in
decelerating the airflow from the aircraft forward velocity,V a , to one acceptable to the compressor
blades of the gas turbine (V 1 ). The design of nozzles for subsonic aircraft is much easier than for
supersonic ones, in which a series of shocks are required to achieve the necessary deceleration. The
processes occurring in the intake system are shown in Fig 17.18. The efficiency of the process can be
defined in a similar manner to other isentropic efficiencies, giving
T 01 T a
h ¼ (17.67)
i
T 0a T a
and this gives the pressure ratio across the intake as
k k
2 k 1
p 01 T 01 k 1 V a
¼ ¼ 1 þ h i (17.68)
p a T a 2c p T a
where p a , and T a are the ambient pressure and temperature respectively, h i is the isentropic efficiency of
the intake and V a is the velocity of the air entering the intake (the cruising speed).
Propelling nozzles are an important feature of both subsonic and supersonic engines. The aim of
these nozzles is to produce the best overall efficiency for the aircraft propulsion system. The propelling
nozzle is the part of the engine aft of the last turbine stage, and may contain all the components shown in
Fig. 17.19.
The diffuser slows down the stream of hot gas leaving the turbine, converting kinetic energy into a
pressure rise. On combat aircraft reheat may be fitted and this consists of burning fuel at constant