Gas turbine cycles for aircraft propulsion
• In shaft power cycles, power is in form of generated power. In air craft cycles, whole power is in the form of thrust.
• Propulsion units include turbojets, turbofans and turboprops
• In turbojets and turbofans, the whole thrust is generated in propelling nozzles. In turboprops, most of the thrust is produced by a propeller with only a small contribution from exhaust nozzle.
Gas turbine cycles for aircraft propulsion
Gas turbine cycles for aircraft propulsion
• Turbojet
The turbine is designed to produce just enough power to drive the compressor. The gas leaving the turbine at high pressure and temperature is expanded to atmospheric pressure in a propelling nozzle to produce high velocity jet. The propelling nozzle refers to the component in which the working fluid is expanded to give a high velocity jet.
Gas turbine cycles for aircraft propulsion
• Turbojet
Gas turbine parts
Gas turbine parts
Compressor and turbine of a Gas turbine
Gas turbine cycles for aircraft propulsion
• Turbojet
Turbojet
Turbojet Operation
Temperature and pressure distributions
Thrust
Turbofan
• Turbofan
Part of the air delivered by an LP compressor or fan bypasses the core of the engine (HP compressor, combustion and turbines) to form an annular propulsive jet or cooler air surrounding the hot jet. This results in a jet of lower mean velocity resulting in better propulsive efficiency and reduced noise.
Turbofan
Flow in a turbofan
Turbofan Thrust
Turboprop
• Turboprop
For lower speed, a combination of propeller and exhaust jet provides the best propulsive efficiency. It has two stage compressor and ‘can-type’ combustion chamber. Turboprops are also designed with a free turbine driving the propeller or propeller plus LP compressor (called free-turbine turboprop).
Turboprop
Flow in a turboprop
Turboprop
Comparison
Performance Criteria• The net momentum thrust is due
to the rate of change of momentum
)( ajj PPA
• Thus, the total thrust is
)(.
aj CCmF
• Ca is the velocity of air at inlet relative to engine
• Cj Velocity of air at exit relative to engine.
• The net pressure thrust is
)()(.
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The propulsion efficiency
• Propulsive efficiency is a measure of the effectiveness with which the propulsive dust is being used for propelling the aircraft but it is not the efficiency of energy conversion.
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The propulsion efficiency
• Specific fuel combustion: fuel consumption per unit thrust, i.e. kg/h N = 0.12 net
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Thermodynamics of air craft engines
• Diffuser: Velocity decreases in diffuser while pressure increases
• Nozzle: Velocity increases in nozzle while pressure decreases
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Nozzle Efficiency
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Thermodynamics of air craft engines
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Thermodynamics of air craft engines
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Thermodynamics of air craft engines
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Thermodynamics of air craft engines
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Thermodynamics of air craft engines
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Thermodynamics of air craft engines
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Thermodynamics of air craft engines
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Thermodynamics of air craft engines
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Thermodynamics of air craft engines
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Thermodynamics of air craft engines
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Thermodynamics of air craft engines
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Thermodynamics of air craft engines
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