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effective over the nozzle exit area. This is additional
thrust to that obtained due to the momentum change
of the gas stream (Part 20).
Exhaust system
61
Fig. 6-3 Gas flow through a convergentdivergent
nozzle.
Fig. 6-4 A low by-pass air mixer unit.
6. With the convergent type of nozzle a wastage of
energy occurs, since the gases leaving the exit do
not expand rapidly enough to immediately achieve
outside air pressure. Depending on the aircraft flight
plan, some high pressure ratio engines can with
advantage use a convergent-divergent nozzle to
recover some of the wasted energy This nozzle
utilizes the pressure energy to obtain a further
increase in gas velocity and, consequently, an
increase in thrust.
7. From the illustration (fig. 6-3), it will be seen that
the convergent section exit now becomes the throat,
with the exit proper now being at the end of the flared
divergent section. When the gas enters the
convergent section of the nozzle, the gas velocity
increases with a corresponding fall in static pressure.
The gas velocity at the throat corresponds to the
local sonic velocity. As the gas leaves the restriction
of the throat and flows into the divergent section, it
progressively increases in velocity towards the exit.
The reaction to this further increase in momentum is
a pressure force acting on the inner wall of the
nozzle. A component of this force acting parallel to
the longitudinal axis of the nozzle produces the
further increase in thrust.
Exhaust system
62
Fig. 6-5 High by-pass ratio engine exhaust systems.
8. The propelling nozzle size is extremely important
and must be designed to obtain the correct balance
of pressure, temperature and thrust. With a small
nozzle these values increase, but there is a
possibility of the engine surging (Part 3), whereas
with a large nozzle the values obtained are too low,
9. A fixed area propelling nozzle is only efficient
over a narrow range of engine operating conditions.
To increase this range, a variable area nozzle may
be used. This type of nozzle is usually automatically
controlled and is designed to maintain the correct
balance of pressure and temperature at all operating
conditions. In practice, this system is seldom used as
the performance gain is offset by the increase in
weight. However, with afterburning a variable area
nozzle is necessary and is described in Part 16.
10. The by-pass engine has two gas streams to
eject to atmosphere, the cool by-pass airflow and the
hot turbine discharge gases.
11. In a low by-pass ratio engine, the two flows are
combined by a mixer unit (fig. 6-4) which allows the
by-pass air to flow into the turbine exhaust gas flow
in a manner that ensures thorough mixing of the two
streams.
12. In high by-pass ratio engines, the two streams
are usually exhausted separately. The hot and cold
nozzles are co-axial and the area of each nozzle is
designed to obtain maximum efficiency. However, an
improvement can be made by combining the two gas
flows within a common, or integrated, nozzle
assembly. This partially mixes the gas flows prior to
ejection to atmosphere. An example of both types of
high by-pass exhaust system is shown in fig, 6-5.
CONSTRUCTION AND MATERIALS
13. The exhaust system must be capable of withstanding
the high gas temperatures and is therefore
manufactured from nickel or titanium. It is also
necessary to prevent any heat being transferred to
the surrounding aircraft structure. This is achieved by
passing ventilating air around the jet pipe, or by
lagging the section of the exhaust system with an
insulating blanket (fig. 6-6). Each blanket has an
inner layer of fibrous insulating material contained by
an outer skin of thin stainless steel, which is dimpled
to increase its strength. In addition, acoustically
absorbent materials are sometimes applied to the
exhaust system to reduce engine noise (Part 19).
14. When the gas temperature is very high (for
example, when afterburning is employed), the
complete jet pipe is usually of double-wall construction
(Part 16) with an annular space between the two
walls. The hot gases leaving the propelling nozzle
induce, by ejector action, a flow of air through the
annular space of the engine nacelle. This flow of air
cools the inner wall of the jet pipe and acts as an
insulating blanket by reducing the transfer of heat
from the inner to the outer wall.
15. The cone and streamline fairings in the exhaust
unit are subjected to the pressure of the exhaust
　
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