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changes in velocity.
10. The efficiency with which these changes are
made will determine to what extent the desired
relations between the pressure, volume and
temperature are attained. For the more efficient the
compressor, the higher the pressure generated for a
given work input; that is, for a given temperature rise
of the air. Conversely, the more efficiently the turbine
uses the expanding gas, the greater the output of
work for a given pressure drop in the gas.
11. When the air is compressed or expanded at 100
per cent efficiency, the process is said to be
adiabatic. Since such a change means there is no
energy losses in the process, either by friction,
conduction or turbulence, it is obviously impossible
to achieve in practice; 90 per cent is a good adiabatic
efficiency for the compressor and turbine.
CHANGES IN VELOCITY AND PRESSURE
12. During the passage of the air through the
engine, aerodynamic and energy requirements
demand changes in its velocity and pressure. For
instance: during compression, a rise in the pressure
of the air is required and not an increase in its
velocity. After the air has been heated and its internal
energy increased by combustion, an increase in the
velocity of the gases is necessary to force the turbine
to rotate. At the propelling nozzle a high exit velocity
is required, for it is the change in the momentum of
the air that provides the thrust on the aircraft. Local
decelerations of airflow are also required, as for
instance, in the combustion chambers to provide a
low velocity zone for the flame to burn.
13. These various changes are effected by means
of the size and shape of the ducts through which the
air passes on its way through the engine. Where a
conversion from velocity (kinetic) energy to pressure
is required, the passages are divergent in shape.
Conversely, where it is required to convert the energy
stored in the combustion gases to velocity energy, a
convergent passage or nozzle (fig. 2-3) is used.
These shapes apply to the gas turbine engine where
the airflow velocity is subsonic or sonic, i.e. at the
local speed of sound. Where supersonic speeds are
encountered, such as in the propelling nozzle of the
rocket, athodyd and some jet engines (Part 6), a
convergent-divergent nozzle or venturi (fig. 2-4) is
used to obtain the maximum conversion of the
energy in the combustion gases to kinetic energy.
14. The design of the passages and nozzles is of
great importance, for upon their good design will
depend the efficiency with which the energy changes
are effected. Any interference with the smooth airflow
creates a loss in efficiency and could result in
component failure due to vibration caused by eddies
or turbulence of the airflow.
Working cycle and airflow
14
Fig. 2-4 Supersonic airflow through a
convergent-divergent nozzle or
venturi.
Working cycle and airflow
15
Fig. 2-5-1 Airflow systems.
Working cycle and airflow
16
Fig, 2-5-2 Airflow systems.
AIRFLOW
15. The path of the air through a gas turbine engine
varies according to the design of the engine. A
straight-through flow system (fig. 2-5) is the basic
design, as it provides for an engine with a relatively
small frontal area and is also suitable for use of the
by-pass principle. In contrast, the reverse flow
system gives an engine with greater frontal area, but
with a reduced overall length. The operation,
however, of all engines is similar. The variations due
to the different designs are described in the
subsequent paragraphs.
16. The major difference of a turbo-propeller engine
is the conversion of gas energy into mechanical
power to drive the propeller. Only a small amount of
’jet thrust’ is available from the exhaust system. The
majority of the energy in the gas stream is absorbed
by additional turbine stages, which drive the propeller
through internal shafts (Part 5).
17. As can be seen in fig. 2-5, the by-pass principle
involves a division of the airflow. Conventionally, all
the air taken in is given an initial low compression
and a percentage is then ducted to by-pass, the
remainder being delivered to the combustion system
in the usual manner. As described in Part 21, this
principle is conducive to improved propulsive
efficiency and specific fuel consumption.
18. An important design feature of the by-pass
engine is the by-pass ratio; that is, the ratio of cool air
by-passed through the duct to the flow of air passed
through the high pressure system. With low by-pass
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