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33. A non-metal based turbine blade can be manufactured
from reinforced ceramics. Their initial
production application is likely to be for small high
speed turbines which have very high turbine entry
temperatures. An example of a ceramic blade is
shown in fig. 5-14.
BALANCING
34. The balancing of a turbine is an extremely
important operation in its assembly. In view of the
high rotational speeds and the mass of materials,
any unbalance could seriously affect the rotating
assembly bearings and engine operation. Balancing
is effected on a special balancing machine, the
principles of which are briefly described in Part 25.
57
Rolls-Royce RB50 Trent
Late in 1943 the decision was taken at Rolls-
Royce to build a turbo-prop for aircraft speeds
of around 400 mph. The resulting engine,
known as the RB50 Trent, was basically a
Derwent II with a flexible quillshaft to
reduction gear and propeller. On 20
September 1945 a Gloster Meteor, fitted with
two Trents, became the world’s first turboprop
powered aircraft to fly.
Rolls-Royce RB211-535E4
6: Exhaust system
Contents Page
Introduction 59
Exhaust gas flow 61
Construction and materials 63
INTRODUCTION
1. Aero gas turbine engines have an exhaust
system which passes the turbine discharge gases to
atmosphere at a velocity, and in the required
direction, to provide the resultant thrust. The velocity
and pressure of the exhaust gases create the thrust
in the turbo-jet engine (para. 5) but in the turbopropeller
engine only a small amount of thrust is
contributed by the exhaust gases, because most of
the energy has been absorbed by the turbine for
driving the propeller. The design of the exhaust
system therefore, exerts a considerable influence on
the performance of the engine. The areas of the jet
pipe and propelling or outlet nozzle affect the turbine
entry temperature, the mass airflow and the velocity
and pressure of the exhaust jet.
2. The temperature of the gas entering the exhaust
system is between 550 and 850 deg. C. according to
the type of engine and with the use of afterburning
(Part 16) can be 1,500 deg. C. or higher. Therefore,
it is necessary to use materials and a form of construction
that will resist distortion and cracking, and
prevent heat conduction to the aircraft structure.
3. A basic exhaust system is shown in fig. 6-1. The
use of a thrust reverser (Part 15), noise suppressor
(Part 19) and a two position propelling nozzle entails
a more complicated system as shown in fig. 6-2. The
low by-pass engine may also include a mixer unit
(fig. 6-4) to encourage a thorough mixing of the hot
and cold gas streams.
59
Exhaust system
60
Fig. 6-1 A basic exhaust system.
Fig. 6-2 Exhaust system with thrust reverser, noise suppressor and two position propelling nozzle.
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 turbojet
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
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
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