MAE 4261: AIRBREATHING ENGINES

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MAE 4261 AIR-BREATHING ENGINES

• Air-Breathing Engine Performance Parameters
• and Future Trends
• September 1, 2009
• Mechanical and Aerospace Engineering Department
• Florida Institute of Technology
• D. R. Kirk

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LECTURE OUTLINE

• Review
• General expression that relates the thrust of a
  propulsion system to the net changes in momentum,
  pressure forces, etc.
• Efficiencies
• Goal Look at how efficiently the propulsion
  system converts one form of energy to another on
  its way to producing thrust
• Overall Efficiency, hoverall
• Thermal (Cycle) Efficiency, hthermal
• Propulsive Efficiency, hpropulsive
• Specific Impulse, Isp s
• (Thrust) Specific Fuel Consumption, (T)SFC
  lbm/hr lbf or kg/s N
• Implications of Propulsive Efficiency for Engine
  Design
• Trends in Thermal and Propulsive Efficiency

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FLUID MECHANICS DERIVATION OF THRUST EQUATION
Chemical Energy
Thermal Energy
Kinetic Energy

• Flow through engine is conventionally called
  THRUST
• Composed of net change in momentum of inlet and
  exit air
• Fluid that passes around engine is conventionally
  called DRAG

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THERMODYANMICS BRAYTON CYCLE MODEL

• 1-2 Inlet, Compressor and/or Fan Adiabatic
  compression with spinning blade rows
• 2-3 Combustor Constant pressure heat addition
• 3-4 Turbine and Nozzle Adiabatic expansion
• Take work out of flow to drive compressor
• Remaining work to accelerate fluid for jet
  propulsion
• Thermal efficiency of Brayton Cycle, hth1-T1/T2
• Function of temperature or pressure ratio across
  inlet and compressor

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P-V DIAGRAM REPRESENTATION

• Thermal efficiency of Brayton Cycle, hth1-T1/T3
• Function of temperature or pressure ratio across
  inlet and compressor

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EXAMPLE OF LAND-BASED POWER TURBINE GENERAL
ELECTRIC LM5000

• Modern land-based gas turbine used for electrical
  power production and mechanical drives
• Length of 246 inches (6.2 m) and a weight of
  about 27,700 pounds (12,500 kg)
• Maximum shaft power of 55.2 MW (74,000 hp) at
  3,600 rpm with steam injection
• This model shows a direct drive configuration
  where the LP turbine drives both the LP
  compressor and the output shaft. Other models can
  be made with a power turbine.

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EFFICIENCY SUMMARY

• Overall Efficiency
• What you get / What you pay for
• Propulsive Power / Fuel Power
• Propulsive Power TUo
• Fuel Power (fuel mass flow rate) x (fuel energy
  per unit mass)
• Thermal Efficiency
• Rate of production of propulsive kinetic energy /
  fuel power
• This is cycle efficiency
• Propulsive Efficiency
• Propulsive Power / Rate of production of
  propulsive kinetic energy, or
• Power to airplane / Power in Jet

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PROPULSIVE EFFICIENCY AND SPECIFIC THRUST AS A
FUNCTION OF EXHAUST VELOCITY
Conflict
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COMMERCIAL AND MILITARY ENGINES(APPROX. SAME
THRUST, APPROX. CORRECT RELATIVE SIZES)
GE CFM56 for Boeing 737 T30,000 lbf, a 5

• Demand higher efficiency
• Fly at lower speed (subsonic, M8 0.85)
• Engine has large inlet area
• Engine has lower specific thrust
• Ue/Uo ? 1 and hprop ?

• Demand high T/W
• Fly at high speed
• Engine has small inlet area (low drag, low radar
  cross-section)
• Engine has high specific thrust
• Ue/Uo ? and hprop ?

PW 119 for F- 22, T35,000 lbf, a 0.3
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EXAMPLE SPECIFIC IMPULSE
SSME
PW4000 Turbofan

• Airbus A310-300, A300-600, Boeing 747-400,
  767-200/300, MD-11
• T 250,000 N
• TSFC 17 g/kN s 1.7x10-5 kg/Ns
• Fuel mass flow 4.25 kg/s
• Isp 6,000 seconds

• Space Shuttle Main Engine
• T 2,100,000 N (vacuum)
• LH2 flow rate 70 kg/s
• LOX flow rate 425 kg/s
• Isp 430 seconds

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PROPULSIVE EFFICIENCY FOR DIFFERENT ENGINE TYPES
Rolls Royce
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OVERALL PROPULSION SYSTEM EFFICIENCY

• Trends in thermal efficiency are driven by
  increasing compression ratios and corresponding
  increases in turbine inlet temperature
• Trends in propulsive efficiency are due to
  generally higher bypass ratio

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FUEL CONSUMPTION TREND

• U.S. airlines, hammered by soaring oil prices,
  will spend a staggering 5 billion more on fuel
  this year or even a greater sum, draining already
  thin cash reserves
• Airlines are among industries hardest hit by high
  oil prices.
• Example Airline stocks fell at the open of
  trading Tuesday as a spike in crude-oil futures
  weighed on the sector

JT8D
Fuel Burn
JT9D
PW4084
Future Turbofan
PW4052
NOTE No Numbers
1950
1960
1970
1980
1990
2000
2010
2020
Year
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CRUISE FUEL CONSUMPTION vs. BYPASS RATIO
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SUBSONIC ENGINE SFC TRENDS(35,000 ft. 0.8 Mach
Number, Standard Day Wisler)
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AEROENGINE CORE POWER EVOLUTION DEPENDENCE ON
TURBINE ENTRY TEMPERATURE Meece/Koff
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PRESSURE RATIO TRENDS (Janes 1999)