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.
Pratt & Whitney