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Introduction 181
Compressor inlet injection 183
Combustion chamber
injection 184
INTRODUCTION
1. The maximum power output of a gas turbine
engine depends to a large extent upon the density or
weight of the airflow passing through the engine.
There is, therefore, a reduction in thrust or shaft
horsepower as the atmospheric pressure decreases
with altitude, and/or the ambient air temperature
increases. Under these conditions, the power output
can be restored or, in some instances, boosted for
take-off by cooling the airflow with water or
water/methanol mixture (coolant). When methanol is
added to the water it gives anti-freezing properties
and also provides an additional source of fuel. A
typical turbo-jet engine thrust restoration curve is
shown in fig. 17-1 and a turbo-propeller engine
power restoration and boost curve is shown in fig.
17-2.
2. There are two basic methods of injecting the
coolant into the airflow. Some engines have the
coolant sprayed directly into the compressor inlet,
but the injection of coolant into the combustion
chamber inlet is usually more suitable for axial flow
compressor engines. This is because a more even
distribution can be obtained and a greater quantity of
coolant can be satisfactorily injected.
3. When water/methanol mixture is sprayed into the
compressor inlet, the temperature of the compressor
181
Water injection
182
Fig. 17-1 Turbo-jet thrust restoration.
Fig. 17-2 Turbo-propeller power boost.
inlet air is reduced and consequently the air density
and thrust are increased. If water only was injected,
it would reduce the turbine inlet temperature, but with
the addition of methanol the turbine inlet temperature
is restored by the burning of methanol in the
combustion chamber. Thus the power is restored
without having to adjust the fuel flow.
4. The injection of coolant into the combustion
chamber inlet increases the mass flow through the
turbine, relative to that through the compressor. The
pressure and temperature drop across the turbine is
thus reduced, and this results in an increased jet pipe
pressure, which in turn gives additional thrust. The
consequent reduction in turbine inlet temperature,
due to water injection, enables the fuel system to
schedule an increase of fuel flow to a value that gives
an increase in the maximum rotational speed of the
engine, thus providing further additional thrust,
Where methanol is used with the water, the turbine
inlet temperature is restored, or partially restored, by
the burning of the methanol in the combustion
chamber.
COMPRESSOR INLET INJECTION
5. The compressor inlet injection system shown in
fig. 17-3 is a typical system for a turbo-propeller
engine. When the injection system is switched on,
water/methanol mixture is pumped from an aircraftmounted
tank to a control unit. The control unit
meters the flow of mixture to the compressor inlet
through a metering valve that is operated by a servo
piston. The servo system uses engine oil as an
operating medium, and a servo valve regulates the
Water injection
183
Fig. 17-3 A typical compressor inlet injection system.
supply of oil. The degree of servo valve opening is
set by a control system that is sensitive to propeller
shaft torque oil pressure and to atmospheric air
pressure acting on a capsule assembly.
6. The control unit high pressure oil cock control
lever is interconnected to the throttle control system
in such a manner that, until the throttle is moved
towards the take-off position, the oil cock remains
closed, and thus the metering valve remains closed,
preventing any mixture flowing to the compressor
inlet Movement of the throttle control to the take-off
position opens the oil cock, and the oil pressure
passes through the servo valve to open the metering
valve by means of the servo piston.
COMBUSTION CHAMBER INJECTION
7. The combustion chamber injection system
shown in fig. 17-4 is a typical system for a turbo-jet
engine. The coolant flows from an aircraft-mounted
tank to an air-driven turbine pump that delivers it to a
water flow sensing unit. The water passes from the
Water injection
184
Fig. 17-4 A typical combustion chamber injection system.
sensing unit to each fuel spray nozzle and is sprayed
from two jets onto the flame tube swirl vanes, thus
cooling the air passing into the combustion zone. The
water pressure between the sensing unit and the
discharge jets is sensed by the fuel control system,
which automatically resets the engine speed
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