A "wet" takeoff of a KC-135 with Pratt & Whitney J57 engines.
From PWA OI 200, "The Aircraft Gas Turbine Engine and its Operation"
published by Pratt & Whitney:
The sensitivity of gas turbine engines to compressor inlet temperature
results in appreciable loss of the thrust (or power, in the case of a
turboprop) which is available for takeoff on a hot day. It is frequently
necessary, therefore, to provide some means of thrust augmentation for
nonafterburning engines during takeoff on warm or hot days. This is also
the case on at least one afterburning engine. Ten to thirty percent
additional thrust (power) can be gained by injecting water into the
engine, either at the compressor air inlet or at some other point, such as
the diffuser case. In a piston engine, during power augmentation by means
of water injection, the water acts primarily as a detonation suppressor
and a cylinder charge coolant. Induction air cooling is secondary.
Higher takeoff horsepower results chiefly because when water is added, the
engine can operate at the fuel/air ratio that will produce "best power."
Sometimes a higher manifold pressure may be obtained than would otherwise
be possible without experiencing detonation. Jet engines, however, have
no detonation difficulties. When water is added, thrust or power
augmentation is obtained principally by cooling the air entering the
engine, by means of vaporization of the water introduced into the
airstream. Cooling the air has the effect of reducing the compressor
inlet temperature. The reduction in temperature increases the air density
and the mass airflow. More and cooler air to the burners permit more fuel
to be burned before limiting turbine inlet temperatures are reached,
which, in turn, means more thrust.
Although methyl or ethyl alcohol (or a mixture of one or both of these and
water) has been used in the past for injection to augment jet engine
thrust, water has a higher heat of evaporation, and is therefore the only
liquid generally used for thrust augmentation today. The effect upon
engine thrust depends upon the rate of water flow into the engine.
Water injection is usually accomplished by spraying water into the engine
at the compressor air inlet or directly into the diffuser section, in the
case of axial flow compressor engines. Occasionally, a combination of
both methods is used. When the injection system permits a pilot to select
manually either a combination of both compressor and diffuser injected
water or diffuser water alone, the latter may be used at lower ambient
temperatures than is permissible when water is introduced into the
compressor, because the problem of possible ice formation at the front end
of the engine is eliminated. In addition to this, diffuser case
injection, although less efficient, also has the advantage of requiring
less change in the basic engine design when a water injection feature is
added, and avoids the problem of deposits forming on the compressor
blades.
James Beaver
Pratt & Whitney
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