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MAE 5350: Gas Turbines

MAE 5350: Gas Turbines. Air-Breathing Engine Performance Parameters and Future Trends Mechanical and Aerospace Engineering Department Florida Institute of Technology D. R. Kirk. LECTURE OUTLINE. Review

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MAE 5350: Gas Turbines

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  1. MAE 5350: Gas Turbines Air-Breathing Engine Performance Parameters and Future Trends Mechanical and Aerospace Engineering Department Florida Institute of Technology D. R. Kirk

  2. 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

  3. FLUID MECHANICS: DERIVATION OF THRUST EQUATION Chemical Energy Kinetic Energy Thermal 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

  4. 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, hth=1-T1/T2 • Function of temperature or pressure ratio across inlet and compressor

  5. P-V DIAGRAM REPRESENTATION • Thermal efficiency of Brayton Cycle, hth=1-T1/T3 • Function of temperature or pressure ratio across inlet and compressor

  6. 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.

  7. CROSS-SECTIONAL EXAMPLE: GE 90-115B Compressor Nozzle Fan Turbine Combustor Inlet • Why does this engine look the way that it does? • How does this engine push an airplane forward, i.e. how does it generate thrust? • What are major components and design parameters? • How can we characterize performance and compare with other engines?

  8. EXAMPLE OF MILITARY ENGINE:TURBOJET OR LOW-BYPASS RATIO TURBOFAN Extreme Temperature Environment Compressor Combustor Turbine Afterburner

  9. MAJOR GAS TURBINE ENGINE COMPONENTS • Inlet: • Continuously draw air into engine through inlet • Slows, or diffuses, to compressor • Compressor / Fan: • Compresses air • Generally two, or three, compressors in series • Raises stagnation temperature and pressure (enthalpy) of flow • Work is done on the air • Combustor: • Combustion or burning processes • Adds fuel to compressed air and burns it • Converts chemical to thermal energy • Process takes place at relatively constant pressure

  10. MAJOR GAS TURBINE ENGINE COMPONENTS • Turbine: • Generally two or three turbines in series • Turbine powers, or drives, the compressor • Air is expanded through turbine (P & T ↓) • Work is done by the air on the blades • Use some of that work to drive compressor • Next: • Expand in a nozzle • Convert thermal to kinetic energy (turbojet) • Burning may occur in duct downstream of turbine (afterburner) • Expand through another turbine • Use this extracted work to drive a fan (turbofan) • Nozzle: • Flow is ejected back into the atmosphere, but with increased momentum • Raises velocity of exiting mass flow

  11. BYPASS RATIO: TURBOFAN ENGINES Bypass Air Core Air Bypass Ratio, B, a: Ratio of bypass air flow rate to core flow rate Example: Bypass ratio of 6:1 means that air volume flowing through fan and bypassing core engine is six times air volume flowing through core

  12. TRENDS TO HIGHER BYPASS RATIO 1995: Boeing 777, FAA Certified 1958: Boeing 707, United States' first commercial jet airliner Similar to PWJT4A: T=17,000 lbf, a ~ 1 PW4000-112: T=100,000 lbf , a ~ 6

  13. ENGINE STATION NUMBERING CONVENTION 2.0-2.5: Fan 3: Combustor 0: Far Upstream 1: Inlet 4: Turbine 2.5+: Compressor 5: Nozzle One of most important parameters is TT4: Turbine Inlet Temperature Performance of gas turbine engine ↑ with increasing TT4 ↑

  14. Combustor Inlet Compressor Turbine Nozzle MAE 4261 REPRESENTATION OF AN ENGINE Freestream 0 1 2 5 3 4

  15. TYPICAL PRESSURE DISTRIBUTION THROUGH ENGINE

  16. J85-GE-1 - 2,600 lbf (11.6 kN) thrust J85-GE-3 - 2,450 lbf (10.9 kN) thrust J85-GE-4 - 2,950 lbf (13.1 kN) thrust J85-GE-5 - 2,400 lbf (10.7 kN) thrust, 3,600 lbf (16 kN) afterburning thrust J85-GE-5A - 3,850 lbf (17.1 kN) afterburning thrust J85-GE-13 - 4,080 lbf (18.1 kN), 4,850 lbf (21.6 kN) thrust J85-GE-15 - 4,300 lbf (19 kN) thrust J85-GE-17A - 2,850 lbf (12.7 kN) thrust J85-GE-21 - 5,000 lbf (22 kN) thrust GE J85

  17. TURBOJET / MODERATE BYPASS TURBOFAN

  18. P&W 229 Overview Type: Afterburning turbofan Length: 191 in (4,851 mm) Diameter: 46.5 in (1,181 mm) Dry weight: 3,740 lb (1,696 kg) Components Compressor: Axial compressor with 3 fan and 10 compressor stages Bypass ratio: 0.36:1 Turbine: 2 low-pressure and 2 high-pressure stages Maximum Thrust: 17,800 lbf (79.1 kN) military thrust 29,160 lbf (129.6 kN) with afterburner Overall pressure ratio: 32:1 Specific fuel consumption: Military thrust: 0.76 lb/(lbf·h) (77.5 kg/(kN·h)) Full afterburner: 1.94 lb/(lbf·h) (197.8 kg/(kN·h)) Thrust-to-weight ratio: 7.8:1 (76.0 N/kg) P&W F100 and 229

  19. UNDUCTED FAN, a ~ 30 ANTONOW AN 70 PROPELLER DETAIL

  20. “HYBRID” DUCTED FAN + TURBOJET

  21. 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

  22. PROPULSIVE EFFICIENCY AND SPECIFIC THRUST AS A FUNCTION OF EXHAUST VELOCITY Conflict

  23. COMMERCIAL AND MILITARY ENGINES(APPROX. SAME THRUST, APPROX. CORRECT RELATIVE SIZES) • 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 ↓ GE CFM56 for Boeing 737 T~30,000 lbf, a ~ 5 • Demand higher efficiency • Fly at lower speed (subsonic, M∞ ~ 0.85) • Engine has large inlet area • Engine has lower specific thrust • Ue/Uo → 1 and hprop ↑ P&W 119 for F- 22, T~35,000 lbf, a ~ 0.3

  24. 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

  25. PROPULSIVE EFFICIENCY FOR DIFFERENT ENGINE TYPES [Rolls Royce]

  26. 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

  27. FUEL CONSUMPTION TREND • U.S. airlines, hammered by soaring oil prices, will spend a staggering $5 billion more on fuel in 2007 or even a greater sum, draining already thin cash reserves • Airlines are among the industries hardest hit by high oil prices • “Airline stocks fell at the open of trading Tuesday as a spike in crude-oil futures weighed on the sector” JT8D Fuel Burn PW4084 JT9D Future Turbofan PW4052 NOTE: No Numbers 1950 1960 1970 1980 1990 2000 2010 2020 Year

  28. CRUISE FUEL CONSUMPTION vs. BYPASS RATIO

  29. SUBSONIC ENGINE SFC TRENDS(35,000 ft. 0.8 Mach Number, Standard Day [Wisler])

  30. AEROENGINE CORE POWER EVOLUTION: DEPENDENCE ON TURBINE ENTRY TEMPERATURE [Meece/Koff]

  31. PRESSURE RATIO TRENDS (Jane’s 1999)

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