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MAE 4261: AIR-BREATHING ENGINES

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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

- 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]

- Goal: Look at how efficiently the propulsion system converts one form of energy to another on its way to producing thrust
- Implications of Propulsive Efficiency for Engine Design
- Trends in Thermal and Propulsive Efficiency

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

- 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

- Thermal efficiency of Brayton Cycle, hth=1-T1/T3
- Function of temperature or pressure ratio across inlet and compressor

- 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 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

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

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

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)

ANTONOW AN 70 PROPELLER DETAIL

- 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

Conflict

- 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

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

- 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

- 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

AIR-BREATHING PROPULSION SYSTEMSRAMJETSTURBOJETSTURBOFANS

Daniel R. Kirk

Assistant Professor

Mechanical and Aerospace Engineering Department

Florida Institute of Technology

- 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

Cycle analysis employing general form of mass, momentum and energy

Energy (1st Law) balance across burner

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

Two streams:

Core and Fan Flow

Turbine power = compressor + fan power

Exhaust streams have same velocity: U6=U8

Maximum power, tc selected

to maximize tf

Improvement over turbojet:

4 – 2.4 → 66% at Mach 1

8 – 3.3 → 142% at Mach 0