MAE 4261: AIR-BREATHING ENGINES

Air-Breathing Engine Performance Parameters

and Future Trends

Mechanical and Aerospace Engineering Department

Florida Institute of Technology

D. R. Kirk

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, overall

• Thermal (Cycle) Efficiency, thermal

• Propulsive Efficiency, propulsive

– 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

FLUID MECHANICS: DERIVATION OF THRUST EQUATION

• 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, th=1-T1/T2

– Function of temperature or pressure ratio across inlet and compressor

P-V DIAGRAM REPRESENTATION

• Thermal efficiency of Brayton Cycle, th=1-T1/T3

– Function of temperature or pressure ratio across inlet and compressor

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.

BYPASS RATIO: TURBOFAN ENGINES

Bypass Air

Core Air

Bypass Ratio, B, :Ratio of bypass air flow rate to core flow rateExample: Bypass ratio of 6:1 means that air volume flowing through fan and bypassing core engine is six times air volume flowing through core

TRENDS TO HIGHER BYPASS RATIO

1958: Boeing 707, United States' first commercial jet airliner 1995: Boeing 777, FAA Certified

PW4000-112: T=100,000 lbf , ~ 6Similar to PWJT4A: T=17,000 lbf, ~ 1

GE J85

• 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

TURBOJET / MODERATE BYPASS TURBOFAN

P&W F100 and 229• 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)

UNDUCTED FAN, ~ 30ANTONOW AN 70 PROPELLER DETAIL

“HYBRID” DUCTED FAN + TURBOJET

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

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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 prop ↓ P&W 119 for F- 22, T~35,000 lbf, ~ 0.3

• 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 prop ↑

GE CFM56 for Boeing 737 T~30,000 lbf, ~ 5

EXAMPLE: SPECIFIC IMPULSE

• 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

PW4000 Turbofan SSME

PROPULSIVE EFFICIENCY FOR DIFFERENT ENGINE TYPES [Rolls Royce]

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

FUEL CONSUMPTION TREND

1950 1960 1970 1980 1990 2000 2010 2020

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FutureTurbofan

• 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”

NOTE: No Numbers

CRUISE FUEL CONSUMPTION vs. BYPASS RATIO

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

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

PRESSURE RATIO TRENDS (Jane’s 1999)

AIR-BREATHING PROPULSION SYSTEMS

RAMJETSTURBOJETSTURBOFANS

Daniel R. Kirk

Assistant Professor

Mechanical and Aerospace Engineering Department

Florida Institute of Technology

RAMJETS

• Thrust performance depends solely on total temperature rise across burner

• Relies completely on “ram” compression of air (slowing down high speed flow)

• Ramjet develops no static thrust

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Energy (1st Law) balance across burnerCycle analysis employing general form of mass, momentum and energy

TURBOJET SUMMARY

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Cycle analysis employing general form of mass, momentum and energy

Turbine power = compressor power

How do we tie in fuel flow, fuel energy?Energy (1st Law) balance across burner

TURBOJET TRENDS: IN-CLASS EXAMPLE

Plot of Non-Dimensional Thrust and Specific Impulse for Maximum Thrust Condition

Heating Value of Fuel = 4.3x107 J/kg, Specific Heat Ratio = 1.4, T0=200K

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Max Non-Dim Thrust: Theta_t=6Max Non-Dim Thrust: Theta_t=9Max Thrust Isp: Theta_t=6Max Thrust Isp: Theta_t=9

TURBOJET TRENDS: IN-CLASS EXAMPLE(SEE INLET SLIDES FOR MORE DETAILS)

Plot of Thrust Normalized by Compressor Inlet Area and Ambient Pressurevs. Flight Mach Number for Compressor Inlet Mach Number, M2=0.5

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Theta_t=6

Theta_t=9

TURBOJET TRENDS: HOMEWORK #3, PART 1Tt4 = 1600 K, c = 25, T0 = 220 K

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Specific ThrustPropulsive EfficiencyThermal EfficiencyOverall Efficiency

TURBOJET TRENDS: HOMEWORK #3, PART 2a Tt4 = 1400 K, T0 = 220 K, M0 = 0.85 and 1.2

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Compressor Pressure Ratio

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Specific Thrust, M=0.85Specific Thrust, M=1.2Propulsive Efficiency, M=0.85Thermal Efficiency, M=0.85Overall Efficiency, M=0.85Propulsive Efficiency, M=1.2Thermal Efficiency, M=1.2Overall Efficiency, M=1.2

TURBOJET TRENDS: HOMEWORK #3, PART 2b Tt4 = 1400 K and 1800 K, T0 = 220 K, M0 = 0.85

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Specific Thrust, Tt4=1400KSpecific Thrust, Tt4=1800 KPropulsive Efficiency, Tt4=1400 KThermal Efficiency, Tt4=1400 KOverall Efficiency, Tt4=1400 KPropulsive Efficiency, Tt4=1800 KThermal Efficiency, Tt4=1800 KOverall Efficiency, Tt4=1800 K

TURBOFAN SUMMARY

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Two streams:Core and Fan Flow

Turbine power = compressor + fan powerExhaust streams have same velocity: U6=U8

Maximum power, c selectedto maximize f

TURBOFAN TRENDS: IN-CLASS EXAMPLE

Non-Dimensional Thrust vs. Flight Mach Numbert=6, To=200 K (PW4000 Series, ~ 5-6)

Higher of interest in range of Mo < 1 and lower of interest for supersonic transport

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0 0.5 1 1.5 2 2.5 3Flight Mach Number, M0

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Bypass Ratio = 1Bypass Ratio = 5Bypass Ratio = 10Bypass Ratio = 20

TURBOFAN TRENDS: IN-CLASS EXAMPLE

Non-Dimensional Thrust vs. Flight Mach Numbert=6, To=200 K (PW4000 Series, ~ 5-6)

Higher of interest in range of Mo < 1 and lower of interest for supersonic transport

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0 0.5 1 1.5 2 2.5 3Flight Mach Number, M0

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Bypass Ratio = 1Bypass Ratio = 5Bypass Ratio = 10Bypass Ratio = 20

Plot of Non-Dimensional Thrust and Specific Impulse for Maximum Thrust Condition

Heating Value of Fuel = 4.3x107 J/kg, Specific Heat Ratio = 1.4, T0=200K

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Max Non-Dim Thrust: Theta_t=6Max Non-Dim Thrust: Theta_t=9Max Thrust Isp: Theta_t=6Max Thrust Isp: Theta_t=9

Improvement over turbojet:4 – 2.4 → 66% at Mach 18 – 3.3 → 142% at Mach 0

TURBOFAN TRENDS: IN-CLASS EXAMPLE

Propulsive Efficiency vs. Flight Mach Numbert=6, To=200 K

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Bypass Ratio = 1Bypass Ratio = 5Bypass Ratio = 10Bypass Ratio = 20