Power Augmentation Systems (15.15) PDF

Power Augmentation Systems (15.15) Learning Objectives 15.15.1 Recall the operation and applications of power augmentation systems (Level 1). 15.15.2 Provide a simple description of water injection and water methanol injection (Level 1). 15.15.3 Provide a simple description of afterbu...

Power Augmentation Systems (15.15) Learning Objectives 15.15.1 Recall the operation and applications of power augmentation systems (Level 1). 15.15.2 Provide a simple description of water injection and water methanol injection (Level 1). 15.15.3 Provide a simple description of afterburner systems (Level 1). 2024-07-23 B1-15b Gas Turbine Engine Page 225 of 290 CASA Part 66 - Training Materials Only  Jet Engine Thrust Water Injection Systems The amount of thrust a jet engine can produce may be increased in two ways: by water injection and by the use of afterburners. Not many large aircraft today use water injection because the modern turbofan engine generally has enough thrust capability to offset the negative effect of high ambient temperatures and high runway altitudes on thrust. Many smaller commuter-size aircraft do need water injection to meet performance requirements. An advantage here is that when the water is all used, the aircraft is lighter. If a larger engine were used, the added engine and subsequent aircraft weight would still be present. One of the most important factors determining the output of a gas turbine engine is the weight of air ow passing through it. A reduction in atmospheric pressure due to altitude or temperature reduces gas turbine engine performance. Engine performance, thrust or shaft horsepower, can be restored or even boosted by using water or water/methanol injection. The graph shows the improvements gained in engine performance when using a water injection system. Max static thrust vs air temperature 2024-07-23 B1-15b Gas Turbine Engine Page 226 of 290 CASA Part 66 - Training Materials Only  Water Injection The uid can be added to the engine at either the compressor inlet or the combustion section. Water injected at the compressor primarily cools the air and increases its density, and therefore the mass air ow through the engine. Water added at the diffuser increases the mass air ow through the turbine compared to the mass air ow through the compressor. Fluid is injected into the compressor and/or diffuser as a ne spray. The injection of this uid into the gas path causes a heat transfer. As the uid evaporates, heat in the air is transferred to the uid droplets, cooling the air and increasing gas ow density. Greater mass ow, compression and extra fuel in the combustor increase the velocity and pressure of the gas ow in the jet pipe, which in turn gives additional thrust. The purpose of the water injection system is to boost lagging thrust caused by these poor ambient conditions. The water injection system is used only during take-off and climb. Adding water to the air ow through the engine improves thrust in two ways: By increasing the mass air ow By cooling the air ow through the engine, lowering the combustion temperature, which allows extra fuel to be burned in the engine without exceeding engine operating temperature limits. Demineralised water is used in these systems to prevent any mineral deposits forming on engine components from normal water. The picture below shows damage to the turbines caused by not using demineralised water for water injection. Turbine damage 2024-07-23 B1-15b Gas Turbine Engine Page 227 of 290 CASA Part 66 - Training Materials Only  All the water from the injection system must be used during take-off and climb. This is to prevent the water freezing when the aircraft reaches higher altitudes. As water expands when it freezes, any water left in the system would expand and damage the system. Water injection systems incorporate a purging system to dump any unused water overboard. Water/Methanol Injection When alcohol in the form of methanol is added to the water, the problem of freezing of the injection system is alleviated. The methanol reduces the freezing point of the water to a point where purging the system after each take-off is no longer required. This allows an aircraft to operate in cold environments without having to re ll the injection system every time it has to y. The system can be lled, and the aircraft can take off and land continually until the system is empty. As alcohol is also a fuel, additional thrust or performance can be gained from the engine without any increase in the fuel ow. Water/methanol injection does not, however, increase the performance of the engine as much as water injection, because water injection allows fuel to be added to the engine. Gas turbine engine fuel has a much higher calori c value than methanol, so the resulting engine performance is higher. However, because of the freezing problems with the water injection system, water/methanol injection systems offer bene ts other than higher increases in engine performance. Water/methanol injection may be used in turbojet, low-bypass turbofan and some turbo-propeller applications. Turbojet water/methanol injection system 2024-07-23 B1-15b Gas Turbine Engine Page 228 of 290 CASA Part 66 - Training Materials Only  Relevant Youtube link: Thrust Augmentation System (Video) Afterburner Afterburning was rst designed as a military application to increase thrust, speed and rate of climb. The only civil aircraft equipped with afterburner/reheat was the Concorde supersonic aircraft. Afterburning or reheat is a method of increasing take-off, climb and combat performance without carrying around a larger engine. It relies on injecting fuel between the turbine and propelling nozzle and igniting it. The ame temperature can reach 1700 °C, so the ame must be centred on the axis of the jet pipe. A proportion of the turbine discharge gas ows along the wall of the jet pipe to maintain the wall temperature at a safe level. The area of the jet pipe must be increased when afterburner is selected as the gas ow volume increases and, if not able to escape, will cause a back-pressure on the turbine. This is provided by a two-position or variable-geometry propelling nozzle. The nozzle is closed during normal operation and opened when afterburning is selected. The purpose of the afterburner is to increase thrust, and it is accomplished by burning (a lot of) fuel. Fuel is injected into the tailpipe of the engine and ignited. This produces a gross increase in the velocity and volume of exhaust gases. Since only about 25% of the air through the engine is used for combustion, there is enough oxygen left for a further combustion process in the tailpipe. The afterburner gas stream goes supersonic. The gas stream entering the jet pipe has a velocity of 750–1200 ft/s. This is too fast for a stable ame to be maintained, so the gas ow must be diffused before it enters the afterburner combustion zone (velocity reduced and pressure increased). To aid this process, a ame holder is tted. An atomised fuel spray is fed into the jet pipe through burners which distribute the ame evenly. When the afterburner is in operation, the ow of fuel is extremely large. The increase in thrust is accomplished by accelerating the air to a high velocity; the additional mass of exhaust gas is accelerated out of the propelling nozzle. A typical afterburner con guration includes fuel manifold, ignition source and ame holder. The ame holder and fuel manifold cause jet pipe losses during normal operation. The amount of fuel is metered to match the air ow, to burn relatively smokeless under most operating conditions. The amount of fuel sent to the afterburner is controlled by the position of the power lever. 2024-07-23 B1-15b Gas Turbine Engine Page 229 of 290 CASA Part 66 - Training Materials Only  Afterburner components 2024-07-23 B1-15b Gas Turbine Engine Page 230 of 290 CASA Part 66 - Training Materials Only  Ignition Ignition in an afterburner is initiated by: A catalytic igniter, which creates a ame by chemical reaction of the air-fuel mixture being sprayed onto a platinum-based element An igniter plug adjacent to the burner Diversion of a ame from the combustion chamber, called a Hot Shot. Afterburner ignition systems 2024-07-23 B1-15b Gas Turbine Engine Page 231 of 290 CASA Part 66 - Training Materials Only  Nozzles Exhaust nozzle geometry is very important as the dynamics and duct shape requirements change when afterburning. The exhaust nozzle and intake geometry must be controlled, and computers are used to position the various components as the needs change. The nozzle area depends on fuel ow at the afterburner. The pilot controls the afterburner fuel ow and the nozzle area via the thrust lever, which must enter the afterburner range. Since there are large fuel ows, the system has its own FCU and fuel pump, and all operate automatically. When afterburning is selected, the afterburner FCU controls the distribution of fuel to the burners, depending on how much afterburn is selected (lever range). Fuel is ignited and jet pipe pressure increases. This alters the pressure across the turbine, and the propelling nozzle area is automatically increased until the correct ratio has been restored. Variable exhaust nozzle operation 2024-07-23 B1-15b Gas Turbine Engine Page 232 of 290 CASA Part 66 - Training Materials Only