Page 71 - The Jet Engine

Exhaust system

EXHAUST GAS FLOW

4. Gas from the engine turbine enters the exhaust
system at velocities from 750 to 1,200 feet per
second, but, because velocities of this order produce
high friction losses, the speed of flow is decreased by
diffusion. This is accomplished by having an
increasing passage area between the exhaust cone
and the outer wall as shown in fig. 6-1. The cone also
prevents the exhaust gases from flowing across the
rear face of the turbine disc. It is usual to hold the
velocity at the exhaust unit outlet to a Mach number
of about 0.5, i.e. approximately 950 feet per second.
Additional losses occur due to the residual whirl
velocity in the gas stream from the turbine. To reduce
these losses, the turbine rear struts in the exhaust
unit are designed to straighten out the flow before the
gases pass into the jet pipe.
5. The exhaust gases pass to atmosphere through
the propelling nozzle, which is a convergent duct,
thus increasing the gas velocity (Part 2). In a turbo-
jet engine, the exit velocity of the exhaust gases is
subsonic at low thrust conditions only. During most
operating conditions, the exit velocity reaches the
speed of sound in relation to the exhaust gas

Fig. 6-4 A low by-pass air mixer unit.

temperature and the propelling nozzle is then said to
be 'choked'; that is, no further increase in velocity
can be obtained unless the temperature is increased.
As the upstream total pressure is increased above
the value at which the propelling nozzle becomes
'choked', the static pressure of the gases at exit
increases above atmospheric pressure. This
pressure difference across the propelling nozzle
gives what is known as 'pressure thrust' and is
effective over the nozzle exit area. This is additional
Fig. 6-3 Gas flow through a convergent- thrust to that obtained due to the momentum change
divergent nozzle. of the gas stream (Part 20).

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