Some gas turbine engines require the use of a water injection, power augmentation system to increase flat rated thrust or to regain thrust when operating under high ambient temperatures or high altitude runway conditions. Water, in a fine spray, is introduced into the compressor inlet, the combustion inlet, or both, in an attempt to increase thrust or to regain lagging thrust created by these poor ambient conditions.
Water injection in a gas turbine engine is a means of augmenting engine thrust in two ways. First, addition of water to air in the compressor increases compression and mass flow. Second, water cools the combustion gases, which allows additional fuel to be used without exceeding maximum temperature limits during takeoff. Increases in these three engine parameters result in a thrust increase in the range of 10 to 15 percent. This means that when the engine is operating at 100 percent thrust without water injection and water is injected, the thrust level will be raised to 115 percent. [Figure 1C]
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| Figure 1. Effect of water injection |
- Observe that flat-rated dry thrust can be extended to a higher ambient temperature by water injection. [Figure 1A]
- Observe that when an aircraft is taking off from a runway higher than sea level some of its lost thrust can be recovered by water injection. [Figure 1B]
- Observe that the flat-rated dry thrust can be increased by water injection. [Figure 1C]
Principles of Operation
The principle of latent heat of vaporization applies to the water injection process in the gas turbine engine; that is, injection of fluid into the gas path causes a heat transfer. When the fluid evaporates, heat in the air will be transferred into the fluid droplets, cooling the air and increasing gas flow density. When water is vaporized in the engine, it absorbs heat from the air at the rate of approximately 1,000 BTUs per pound of water.Water injection in a gas turbine engine is a means of augmenting engine thrust in two ways. First, addition of water to air in the compressor increases compression and mass flow. Second, water cools the combustion gases, which allows additional fuel to be used without exceeding maximum temperature limits during takeoff. Increases in these three engine parameters result in a thrust increase in the range of 10 to 15 percent. This means that when the engine is operating at 100 percent thrust without water injection and water is injected, the thrust level will be raised to 115 percent. [Figure 1C]
Inlet water injection is designed for use at ambient temperature above 40°F. Below this temperature, icing is likely to occur in the water injection system and in the engine inlet. There is no temperature restriction concerning combustor inlet water injection.
The principle of latent heat of evaporation lowers the compressor inlet temperatures during water injection. Water injection is only used at takeoff power settings because the combination of cooling effect of high velocity airflow and absorption of heat by water molecules sets up conditions for icing well above 32°F ambient temperature.
When the temperature approaches 40°F and water injection is required, heated water is serviced into the aircraft tanks. The tanks are also configured with heating elements to keep the water at the required temperature until use.
Typical fluid properties are as follows:
The following table shows the heat absorption or vaporization effect of the most common injection fluids.
The principle of latent heat of evaporation lowers the compressor inlet temperatures during water injection. Water injection is only used at takeoff power settings because the combination of cooling effect of high velocity airflow and absorption of heat by water molecules sets up conditions for icing well above 32°F ambient temperature.
When the temperature approaches 40°F and water injection is required, heated water is serviced into the aircraft tanks. The tanks are also configured with heating elements to keep the water at the required temperature until use.
Water Injection Fluids
Pure demineralized or distilled water is the most common water injection fluid. Ordinary tap water is not used because its high mineral solid content can cause severe turbine distress when the minerals impinge on the turbine blades. Pure water is also widely used because it produces a greater cooling effect than a mixture of water and methyl or ethyl alcohol. Airliners can take advantage of this and not worry about altitude freeze up by using the complete supply of water at takeoff. Aircraft such as helicopters and turboprops, which make frequent takeoffs and landings, are forced to use a water-alcohol mixture to protect against freeze up.Typical fluid properties are as follows:
- Demineralized water or distilled water must have less than 10 parts per million (ppm) of solids.
- Methyl/ethyl mixtures will generally be a blend of 35 to 50 percent alcohol in either demineralized or distilled water.
The following table shows the heat absorption or vaporization effect of the most common injection fluids.
Even though water does not contain the heating value of alcohol, because of its heat absorption capability more thrust can be obtained by injecting a given volume of water into the engine than an equal mixture of water and alcohol. Although alcohol can be used as fuel after it is used as a coolant, the thrust augmentation factor per unit volume in a water/alcohol mixture is less than that of pure water.
In terms of fuel flow, this engine has a takeoff fuel flow of 9,000 pounds per hour (22 gallons per minute). Therefore, with a 100 gallons per minute water flow, a 4.5:1 water to fuel ratio exists.
A typical water injection system is shown in Figure 2. Notice that it contains two independent injection nozzles, one to spray water into the compressor inlet and the other to spray into the diffuser/combustor area. Compressor injection increases mass air flow and also cools the combustion air-fuel mixture, allowing increased fuel flow. The addition of fuel increases acceleration of gases exiting the tailpipe. Both of these factors increase the thrust output of the engine.
Water Injection Systems
Not many large aircraft today use water injection because the modern turbofan engine generally has enough thrust capability to offset the negative effect that high ambient temperatures and high runway altitudes have on thrust. However, military transports converted to commercial freighter operations and other smaller aircraft like turboshaft helicopters, may need water injection to meet performance requirements. An advantage is that, when the water is all used, the aircraft is lighter. If a larger engine were to be used, the added engine and subsequent aircraft weight would still be present.Water Injection System (Large Engine)
Since available thrust quite often determines allowable aircraft takeoff weight, water injection is used almost exclusively at takeoff power settings. For instance, Boeing 707 and DC-8 airplanes carried approximately 300 gallons of water injection fluid per engine, using up the entire supply in a three-minute takeoff and climb. This would equal an air-water ratio of approximately 12 to 1, based on a mass airflow of 160 pounds per second and a water injection rate of 100 gallons per minute (13.6 lb/sec).In terms of fuel flow, this engine has a takeoff fuel flow of 9,000 pounds per hour (22 gallons per minute). Therefore, with a 100 gallons per minute water flow, a 4.5:1 water to fuel ratio exists.
A typical water injection system is shown in Figure 2. Notice that it contains two independent injection nozzles, one to spray water into the compressor inlet and the other to spray into the diffuser/combustor area. Compressor injection increases mass air flow and also cools the combustion air-fuel mixture, allowing increased fuel flow. The addition of fuel increases acceleration of gases exiting the tailpipe. Both of these factors increase the thrust output of the engine.
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| Figure 2. Typical water injection system |
In the system shown, full thrust augmentation, when required, will necessitate the use of both compressor and diffuser injection. In other installations, it is common to see injection at only one location, either the compressor or diffuser. Although diffuser injection alone will be less effective for a given water flow rate.
When the ambient temperature is low, only the diffuser injection system can be used. Below 40°F, at takeoff revolutions per minute, there is a danger of ice formation. At low ambient temperatures, thrust is usually high enough without water injection for almost any aircraft gross takeoff weight.
This water-injection system is controlled by a cockpit switch that arms the circuit and makes flow into both manifolds possible. When closed, the cockpit switch allows electrical current to flow to the fuel control microswitch. As the power lever reaches takeoff power, the microswitch is depressed and the water pump valve will be powered to open. This allows compressor bleed air to flow through the air-driven water pump, which supplies water under a pressure of 200 to 300 pounds p.s.i.g. to the dual manifold.
If compressor flow is not needed, a cockpit switch deactivates the flow valve. The pressure sensing tube to the fuel control alerts the control to increase fuel flow when water is flowing. This system is not generally needed if water/alcohol is used because combustion of the alcohol keeps turbine inlet temperature at its required value.
A tank float level circuit cuts off power to the pump when the tank is empty and prevents the system from operating if the circuit is activated when the water supply is low or unserviced. When the water injection system is not in use, the check valve at the diffuser prevents high temperature air from backing up into the water system.
Drains are present to drain the lines when the system is not in use, preventing freeze-up. The surge chambers alleviate water pressure peaks by providing an air cushioning effect to the system. In some installations, a bleed air system allows the pilot to purge the system of water after terminating water injection. In this system, it will occur automatically when the water supply is depleted, and the water pump control valve redirects bleed air through the purge valve.
When the ambient temperature is low, only the diffuser injection system can be used. Below 40°F, at takeoff revolutions per minute, there is a danger of ice formation. At low ambient temperatures, thrust is usually high enough without water injection for almost any aircraft gross takeoff weight.
This water-injection system is controlled by a cockpit switch that arms the circuit and makes flow into both manifolds possible. When closed, the cockpit switch allows electrical current to flow to the fuel control microswitch. As the power lever reaches takeoff power, the microswitch is depressed and the water pump valve will be powered to open. This allows compressor bleed air to flow through the air-driven water pump, which supplies water under a pressure of 200 to 300 pounds p.s.i.g. to the dual manifold.
If compressor flow is not needed, a cockpit switch deactivates the flow valve. The pressure sensing tube to the fuel control alerts the control to increase fuel flow when water is flowing. This system is not generally needed if water/alcohol is used because combustion of the alcohol keeps turbine inlet temperature at its required value.
A tank float level circuit cuts off power to the pump when the tank is empty and prevents the system from operating if the circuit is activated when the water supply is low or unserviced. When the water injection system is not in use, the check valve at the diffuser prevents high temperature air from backing up into the water system.
Drains are present to drain the lines when the system is not in use, preventing freeze-up. The surge chambers alleviate water pressure peaks by providing an air cushioning effect to the system. In some installations, a bleed air system allows the pilot to purge the system of water after terminating water injection. In this system, it will occur automatically when the water supply is depleted, and the water pump control valve redirects bleed air through the purge valve.
Water Injection System (Small Engine)
The system shown in Figure 3 uses only compressor inlet injection; compressor discharge air pressure is the motive force for pumping the fluid to the engine. The water line restrictor creates a predictable pressure drop and establishes the correct water schedule at the takeoff power setting. System water flow is between 1.2 and 1.3 gallons per minute at a discharge pressure of approximately 40 p.s.i.g. The duration time for this system is three minutes. |
| Figure 3. Small engine water-alcohol injection system (Allison 250 Turboshaft) |
In the example schematic, the following sequence occurs when selecting water injection:
1. Push in the Warning Light circuit breaker. The Water/Alcohol Low Level light will not illuminate because the circuit is open at the System Switch.
2. Push in the System circuit breaker. The Circuit is open at the System Switch.
3. Turn on the System Switch.
4. Turn on the Injection Switch.
5. Turn off the Injection Switch.
For normal termination of water injection with some water remaining in the tank, turning off the Injection switch will cause the solenoid valve to close and the pressure switch to open the Water/Alcohol Injection light circuit.
At standard conditions, an engine of this type will produce 310 shaft horsepower dry and 335 shaft horsepower wet. [Figure 4] This engine would also be capable of attaining its dry-rated power of 310 shaft horsepower, in water injection, up to 95° F.
1. Push in the Warning Light circuit breaker. The Water/Alcohol Low Level light will not illuminate because the circuit is open at the System Switch.
2. Push in the System circuit breaker. The Circuit is open at the System Switch.
3. Turn on the System Switch.
- The Low Fluid Warning Relay coil circuit is completed to Low Level Float switch.
- The Low Fluid Warning Relay coil is energized and contactors move down if the water/alcohol tank is serviced to cause the float switch to be closed.
- The circuit is now open to the Water/Alcohol Low Level Light. This light will only illuminate if fluid level is low in the tank, keeping the warning relay contractor closed.
- The Injection Switch is powered but the circuit is open.
- The Water/Alcohol Injection Light is powered but the circuit is open at the Pressure Switch.
4. Turn on the Injection Switch.
- The Solenoid Valve opens and water flows to the engine.
- The Pressure switch expands to complete the Water/Alcohol Injection light circuit through the differential pressure switch. Equal pressure on both sides of the diaphragm results in contactor closing.
- The Water/Alcohol Injection light illuminates to show normal water flow condition exists. The light will not illuminate if the water tank is not pressurized correctly due to system air leak, loose filler cap, etc., because weak air pressure on the contactor side of the diaphragm will allow contactor to open. This is a safety feature because low air pressure could result in a low water/alcohol flow and could cause low engine power to result.
- A low water level in the fluid tank will cause the Water/Alcohol Low Level light to illuminate. When the tank has 30 seconds of water remaining, the float contactor will open, de-energizing the low fluid warning relay.
- The Water/Alcohol Injection light will go out when the tank is empty and the contactor side of the differential pressure switch diaphragm experiences a pressure drop in excess of 4.5 to 7.0 pressure per square inch-differential.
- Loss of water pressure will also cause the pressure switch to open the circuit to the Water/Alcohol Injection light.
5. Turn off the Injection Switch.
For normal termination of water injection with some water remaining in the tank, turning off the Injection switch will cause the solenoid valve to close and the pressure switch to open the Water/Alcohol Injection light circuit.
At standard conditions, an engine of this type will produce 310 shaft horsepower dry and 335 shaft horsepower wet. [Figure 4] This engine would also be capable of attaining its dry-rated power of 310 shaft horsepower, in water injection, up to 95° F.
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| Figure 4. Effect of water-alcohol injection on shaft horsepower |
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