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Water Injection

The maximum power a turbine engine can output depends largely upon the density or weight of the flow of the gases through the engine. Therefore, when the atmospheric pressure decreases or ambient air temperature increases, there is a loss in thrust. The power output can be boosted or restored by cooling the airflow with water or coolant.

There are two main methods for injecting the coolant into the airflow. Where some engines spray directly into the compressor inlet, the most suitable method is injection into the combustion chamber inlet. With this, a more even distribution can be obtained and more coolant can be injected.
When the water/coolant is sprayed into the compressor inlet, the temperature of the air is reduced, increasing the density of the compressor inlet air, and consequently, the thrust is increased.

The system shown to the left is a typical compressor inlet injection system. When switched on, the coolant is pumped to a control unit which measures the flow of the mixture to the compressor inlet. A servo valve regulates the supply of oil using engine oil as an operating medium. The degree of the servo valve opening is sensitive to propeller shaft torque oil pressure and to atmospheric air pressure.

The injection of coolant into the combustion chamber inlet increases the mass flow rate through the turbine. The pressure and temperature drop across the turbine results in an increase in pressure in the jet pipe, giving additional thrust. Also, the consequent reduction in turbine inlet temperature allows the fuel system to increase the fuel flow to a valve giving an increase in rotational speed of the engine. This provides even more additional thrust.

The system shown is a typical combustion chamber injection system. Coolant flows to an air-driven turbine pump sending it to a water flow sensing unit. From there it passes to each fuel spray nozzle and is sprayed onto the flame tube swirl vanes. This cools the air passing into the combustion zone. The fuel control system senses the water pressure, which will automatically reset the engine speed governer to give a higher maximum engine speed. The water flow sensing unit will only open when the difference between the compressor delivery air pressure and the water pressure is correct.

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