|Fig. 17-1 Turbo-jet thrust restoration|
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 isshown 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 eve 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 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.
|Fig. 17-2 Turbo-propeller power boost.|
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, bythe burning of the methanol in the combustion chamber.