Page 27 - The Jet Engine

Working cycle and airflow

AIRFLOW principle is conducive to improved propulsive
efficiency and specific fuel consumption.
15. The path of the air through a gas turbine engine
varies according to the design of the engine. A 18. An important design feature of the by-pass
straight-through flow system (fig. 2-5) is the basic engine is the by-pass ratio; that is, the ratio of cool air
design, as it provides for an engine with a relatively by-passed through the duct to the flow of air passed
small frontal area and is also suitable for use of the through the high pressure system. With low by-pass
by-pass principle. In contrast, the reverse flow ratios, i.e. in the order of 1:1, the two streams are
system gives an engine with greater frontal area, but usually mixed before being exhausted from the
with a reduced overall length. The operation, engine. The fan engine may be regarded as an
however, of all engines is similar. The variations due extension of the by-pass principle, and the
to the different designs are described in the requirement for high by-pass ratios of up to 5:1 is
subsequent paragraphs. largely met by using the front fan in a twin or triple-
16. The major difference of a turbo-propeller engine spool configuration (on which the fan is, in fact, the
is the conversion of gas energy into mechanical low pressure compressor) both with and without
power to drive the propeller. Only a small amount of mixing of the airflows. Very high by-pass ratios, in the
'jet thrust' is available from the exhaust system. The order of 15:1, are achieved using propfans. These
majority of the energy in the gas stream is absorbed are a variation on the turbo-propeller theme but with
by additional turbine stages, which drive the propeller advanced technology propellers capable of operating
through internal shafts (Part 5). with high efficiency at high aircraft speeds.

17. As can be seen in fig. 2-5, the by-pass principle 19. On some front fan engines, the by-pass
involves a division of the airflow. Conventionally, all airstream is ducted overboard either directly behind
the air taken in is given an initial low compression the fan through short ducts or at the rear of the
and a percentage is then ducted to by-pass, the engine through longer ducts; hence the term 'ducted
remainder being delivered to the combustion system fan'. Another, though seldom used, variation is that of
in the usual manner. As described in Part 21, this the aft fan.

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