Gas turbine cycles for aircraft propulsion

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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.

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Gas turbine cycles for aircraft propulsion
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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.

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Gas turbine cycles for aircraft propulsion

• Turbojet

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Gas turbine parts
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Gas turbine parts
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Compressor and turbine of a Gas turbine
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Gas turbine cycles for aircraft propulsion

• Turbojet

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Turbojet
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Turbojet Operation
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Temperature and pressure distributions
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Thrust
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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.

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Turbofan
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Flow in a turbofan
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Turbofan Thrust
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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).

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Turboprop
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Flow in a turboprop
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Turboprop
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Comparison
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Performance Criteria

• The net momentum thrust is due to the rate of
  change of momentum

• 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

• Thus, the total 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

• Energy conversion efficiency

• Overall efficiency

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The propulsion efficiency

• Specific fuel combustion fuel consumption per
  unit thrust, i.e. kg/h N 0.12

• Specific thrust, Fs

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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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Thermodynamics of air craft engines

• Isentropic efficiency of a diffuser

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Thermodynamics of air craft engines
The rest of the components ( compressor, turbine
combustion chamber) are treated before. The ram
efficiency is
Propelling nozzle Propelling nozzle is the
component in which the working fluid is expanded
to give a high velocity jet.
Nozzle Efficiency
for adiabatic flow
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Thermodynamics of air craft engines
for unchoked nozzle (Mjlt1) P5Pa For choked
nozzle ( Max. rate is reached) M1, P5Pc
To check if it is choked or not
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Thermodynamics of air craft engines
for choked condition M1
But isentropic efficiency is
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Thermodynamics of air craft engines
Pc is calculated as
substituting for
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Thermodynamics of air craft engines
To calculate A5 of nozzle
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Thermodynamics of air craft engines
Example Simple turbojet cycle
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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
Example2 Turbofan Analysis Overall pressure
ratio given
sea level Pa 1 bar Ta288 K
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Thermodynamics of air craft engines
State 1 is sea level since Ca0.0 Required sfc,
Fs
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Thermodynamics of air craft engines
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Thermodynamics of air craft engines
check for choking of both nozzles ( hot and cold)
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Thermodynamics of air craft engines
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Thermodynamics of air craft engines
C7 476 m/s
for cold nozzle ( do same)
note Nozzles are independent of each other
regarding choking.
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Thermodynamics of air craft engines
Notes a8344.2 M8lt1
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Thermodynamics of air craft engines