MAE 4261: AIR-BREATHING ENGINES
Advanced Concepts
Mechanical and Aerospace Engineering Department
Florida Institute of Technology
D. R. Kirk
1
CONTENTS AND OVERVIEW
• Review
– What did we do in this course?
• Future / Advanced Topics
– What would be covered in a graduate course in air-breathing propulsion?
• SCRAMJETS
• Rocket-Based Combined Cycle Engines (RBCC)
• Pulse Detonation Engines
• Micro Propulsion
2
REVIEW OF MAE 4261
•
Review of governing equations of motion and energy
– General expressions for thrust and performance (continuity + SF = d/dt(mv))
•
General layout of an air-breathing engine
– Inlet, compressor, burner, turbine, nozzle
•
Air-Breathing engine performance parameters
– Thrust, specific thrust, Isp, T/W, thermal, propulsive, overall efficiency
– Relate to overall airplane performance
•
Cycle analysis
– Detailed examination of ideal and non-ideal Brayton cycle
– Expression for thrust, Isp, etc. for ramjets, turbojets, turbofans
•
Non-Rotating components
– Inlets, nozzles, combustors, after-burners
•
Energy exchange with turbomachinery
– Euler turbine equation, velocity triangles
– Implications for performance prediction and design
•
Design of axial compressors and turbines
– Radial variation (twist), axial area change, degree of reaction
•
Component matching
– Integrate compressor + burner + turbine on single shaft (gas generator)
– Usefulness of compressor and turbine maps
3
FUTURE / ADVANCED TOPICS
•
Compressors
– Axisymmetric Design
– Aspect Ratio, Radial changes in AR
– Blade row performance, 3-D effects, blade row interactions, performance limitations,
tip clearance and vortical flow behavior
– Instabilities: Rotating Stall and Surge
•
Combustors
– More detailed design on various geometries for performance and emissions, liner and
cooling flows, advanced concepts including pre-mixed combustion for NOx reduction
– Instabilities, flame stability, turbulent flame speed, fuel control
•
Turbines
– Detailed disk design
– Rotating seals
– Detailed analysis of blade heat load prediction and turbine cooling
– Flutter, vibration, fatigue, rotational dynamics and eccentricity
•
Engine Matching
– 2 and 3 spool engines, include effects of variable geometry
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COMPRESSOR PERFORMANCE LIMITATIONS
•
Compressor performance is limited
by two important (and often highly
detrimental) phenomena
1. Compressor stall
– Similar to stall on airfoils
– Can also be dynamic
phenomena, called rotating
stall
2. Compressor surge
– Static and dynamic phenomena
that can lead to violent flow
oscillations inside of device
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COMPRESSOR STALL
•
•
•
Usually not all blades are stalled
Rotating stall refers to progression around blade
annulus of stall pattern in which one or several
adjacent blade passages are instantaneously stalled
Zones of stall rotate at about ½ engine speed
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ENGINE SURGE: H&P SECTION 7.5
•
•
•
If downstream blades are stalled may act as a throttle
– Variable mass flow ingestion capability because of variable downstream area
Leads to stable vs. unstable dynamic response of turbomachine
Compressor characteristics (compressor map) vs. throttle characteristics (operating line)
– IF slope of throttle line > slope of compressor line: Stable
– IF slope of throttle line < slope of compressor line: Unstable (mass flow may go to zero)
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ENGINE SURGE
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SIMPLIFIED DYNAMIC MODEL
• Simplified representation called Moore-Greitzer model
• Reference, “The Stability of Pumping Systems – The 1980 Freeman Scholar
Lecture, Journal of Fluids Engineering, Vol. 103.
• Brilliance is in elegant simplicity of model (No CFD)
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ROTATING STALL AND SURGE
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HIGH CYCLE FATIGUE (HCF) IN GAS TURBINE ENGINES
• HCF results from vibratory stress cycles at frequencies
which can reach thousands of cycles per second and can be
induced from various aeromechanical sources (stall)
• Widespread phenomenon in aircraft engines that leads to
premature failure of major engine components (fans,
compressors, turbines) and in some instances has resulted in
loss of total engine and aircraft.
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WHAT IS GOING ON NOW?
•
Integrated High Performance Turbine Engine Technology
(IHPTET) program
– Advanced material developments (ceramics)
– Innovative structural designs
– Improved aerothermodynamics
– Drivers: Propulsion performance, reliability, and costsaving improvements
•
Examples:
– Turbomachinery
• Increase operating temperature and turbine life
• Increase bearing lifetime and reduce their wear by
incorporating hydrostatic bearings (magnetic)
• Reduction in number of compressor stages
– Improved injection, mixing and combustion modeling
– High Cycle Fatigue
– Burner-Turbine-Burner-Turbine Concepts
•
http://www.pr.afrl.af.mil/divisions/prt/ihptet/ihptet.html
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AEROENGINE CORE POWER EVOLUTION:
DEPENDENCE ON TURBINE ENTRY TEMPERATURE [Meece/Koff]
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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
Fuel Burn
JT8D
PW4084
JT9D
Future
Turbofan
PW4052
NOTE: No Numbers
1950
1960
1970
1980
1990
2000
2010
2020
Year
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BURNER-TURBINE-BURNER (ITB) CONCEPTS
Conventional
Intra Turbine Burner
•
•
Improve gas turbine engine performance using an interstage turbine burner (ITB)
– With a higher specific thrust engine will be smaller and lighter
– Increasing payload
– Reduce CO2 emissions
– Reduce NOx emissions by reducing peak flame temperature
Initially locate ITB in transition duct between high pressure turbine (HTP) and low
pressure turbine (LPT)
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SIEMENS WESTINGHOUSE ITB CONCEPT
Tt4
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UNDERSTANDING BENEFIT FROM CYCLE ANALYSIS
From “Turbojet and Turbofan Engine Performance Increases Through Turbine Burners, by
Liu and Sirignano, JPP Vol. 17, No. 3, May-June 2001
Conventional
Intra Turbine Burner
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UNDERSTANDING BENEFIT FROM CYCLE ANALYSIS
From “Turbojet and Turbofan Engine Performance Increases Through Turbine Burners, by
Liu and Sirignano, JPP Vol. 17, No. 3, May-June 2001
2 additional burners
5 additional burners
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UNDERSTANDING BENEFIT FROM CYCLE ANALYSIS
From “Turbojet and Turbofan Engine Performance Increases Through Turbine Burners, by
Liu and Sirignano, JPP Vol. 17, No. 3, May-June 2001
Continuous burning in turbine
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MAGNETIC BEARINGS
•
•
•
•
•
•
Magnetic bearings increase reliability
Reduce weight of engines by eliminating lubrication system
Increase DN2 limit on engine speed and allow active vibration cancellation systems to be used,
resulting in a more efficient, "more electric" engine
Actively control shaft vibration, damping, improve surge and stall characteristics
Integrated High Performance Turbine Engine Technology (IHPTET) program, identified a need for a
high-temperature (1200 °F) magnetic bearing
http://www.grc.nasa.gov/WWW/RT1997/5000/5930kascak.htm
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EXOSKELETAL ENGINE CONCEPT
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EXOSKELETAL ENGINE CONCEPT COMMENTS
NASA/TM – 2004-212621
•
•
•
•
An innovative approach to gas turbine design involves mounting compressor and turbine
blades to an outer rotating shell → exoskeletal engine
Compression (preferable to tension for high-temperature ceramic materials, generally)
becomes dominant blade force
Exoskeletal engine feasibility lies in structural and mechanical design (as opposed to cycle
or aerothermodynamic design)
Some Study Details:
– Focused on development and assessment of a structural-mechanical exoskeletal
concept using Rolls-Royce AE3007 regional airliner all-axial turbofan as a baseline
– Effort was further limited to definition of an exoskeletal high-pressure spool concept,
where major structural and thermal challenges are represented
– Mass of high-pressure spool was calculated and compared with mass of AE3007
engine components
• Exoskeletal engine rotating components can be significantly lighter than rotating
components of a conventional engine
• However, bearing technology development is required, since mass of existing
bearing systems would exceed rotating machinery mass savings
• Recommended that once bearing technology is sufficiently advanced, a "clean
sheet" preliminary design of an exoskeletal system be accomplished to better
quantify potential for exoskeletal concept to deliver benefits in mass, structural
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efficiency, and cycle design flexibility.
NASA's X-43A Scramjet Breaks Speed Record
• “NASA's X-43A research vehicle screamed into the record books again
Tuesday, demonstrating an air-breathing engine can fly at nearly 10 times
the speed of sound. Preliminary data from the scramjet-powered research
vehicle show its revolutionary engine worked successfully at nearly Mach
9.8, or 7,000 mph, as it flew at about 110,000 feet.”
• “NASA's X-43A scramjet program successfully smashed its own world
speed record for aircraft by flying at nearly 10 times the speed of sound.
The flight proves its radical, air-breathing engine can function at speeds
of nearly 12,000 kilometers per hour.”
• “Aviation history was made today as NASA successfully flew its
experimental X-43A research vehicle, a forerunner of craft that could well
offer alternate access to space in the future.”
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X-43 HIGHLIGHTS
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HYPER-X FLIGHT TRAJECTORY
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COMPARISON OF PROPULSION SYSTEMS
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RAMJETS VS. SCRAMJETS
•
Ramjets have a couple of important limits
1. Do not work until engine is moving at high speeds (also true for scramjets)
• Need a way to get plane moving at Mach speeds
• One way is combine ramjet and turbine engine: run turbine portion of engine
until move through air fast enough for ramjet to work efficiently
• At speed shut turbines down and let ramjet work.
2. At high speeds all temperature rise is due to deceleration
• Dissociation is dominant
• Combustion can not add any more heat to flow
3. At speeds of ~ Mach 6, air flowing into inlet is moving so quickly that it creates a
supersonic shock wave as it is compressed inside engine
• Shock wave may stop ignition of air-and-fuel mixture, shutting engine down
• Pressure and heat from shock wave tear engine to bits
•
Solution is a supersonic combustion engine or scramjet
– Variation of a ramjet where combustion of fuel air mixture occurs at supersonic
speeds
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IMPACT OF HIGH SPEED FLIGHT AND DISSOCIATION
• At high Mach numbers
affect of deceleration of
flow is to produce higher
temperature than
combustion!
• Energy is ‘trapped’ in
dissociated products (H,
H2, OH, O, O2, NO,
NO2, etc.)
• Combustion is actually
cooling the flow
• How do we get around
this?
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RAMJETS VS. SCRAMJETS
Ramjet:
Physical Throat
Sub-Sonic Combustion
Scramjet:
NO Physical Throat
(Thermal Choking)
Super-Sonic Combustion
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SCRAMJET CHALLENGES
• Engine inlet has less compression than ramjet, allowing air to speed through
engine at supersonic speeds
– Somewhat reduces shockwave problem
• When fuel is injected into onrushing air, small shock waves are created, so
combustion chamber must be able to withstand pressure
• At supersonic speeds, fuel injection and combustion must be accomplished in
milliseconds
• Making sure that fuel burns while still inside engine and not after it is ejected
from it is one of main challenges for scramjet builders
• Launching a scramjet-powered plane into orbit has huge advantages over
standard rocket-powered spacecraft
– Rockets must carry all their oxidizer with them
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EXAMPLE OF SCRAMJET EXPERIMENTAL MODEL
Combustor
Inlet
Nozzle
Cowl
Isolator
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RAMJETS VS. SCRAMJETS
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DUAL-MODE OPERATION
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LAUNCH ASSIST
Main Idea: Why waste fuel to get supersonic → launch at high speed from earth?
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ROCKET-BASED COMBINED CYCLE (RBC2, RBC3)
•
Combination of ROCKET, RAMJET and SCRAMJET
– Most efficient aspects of each form of propulsion
•
Rocketdyne, Boeing Co., Pratt & Whitney, Aerojet and
NASA
•
Engines completely reusable, take off and land at
airport runways, and ready to fly again within days
•
By 2006 ground test version of an air-breathing rocket
engine for a next-generation hypersonic flight vehicle
•
Flight-test self-powered vehicle to more than M=6,
demonstrating all modes of engine operation by end of
decade
"Air-breathing propulsion is one of the most promising concepts we've
seen for reaching NASA's future-generation spaceflight goals"
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RBCC: OPERATIONAL IDEAS
• KEY POINT
– Use ambient air as an oxidizer
– Compared with conventionally powered rocket vehicles significantly reduce
weight by eliminating a significant amount of its required on-board oxidizer
• PHASE 1: DUCTED ROCKET
– Innovative air-breathing rocket engine for operational vehicle would get
initial power boost from rockets in a duct that captures air
– Improves performance about 15 percent above conventional rockets
• PHASE 2: RAMJET / SCRAMJET MODES
– Once vehicle has accelerated to more than M~2, rockets are turned off and
engine relies solely on oxygen in atmosphere to burn hydrogen fuel
• PHASE 3: TRADITIONAL ROCKET
– When vehicle has accelerated to ~ 10 times the speed of sound, engine
converts to a conventional rocket-powered system to propel craft into orbit
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PDE: PULSE DETONATION ENGINE
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HOW DOES IT WORK: IN WORDS
• Tube, closed at one end and filled with a mixture of fuel and air
• A spark ignites fuel at closed end, and a combustion reaction propagates down
the tube
• In deflagration, even in "fast flame" situations ordinarily called explosions—that
reaction moves at tens of meters per second at most
• In detonation, a supersonic shock wave slams down the tube at thousands of
meters per second, close to Mach 5, compressing and igniting fuel and air almost
instantaneously in a narrow, high-pressure, heat-release zone
• The reaction to this detonation is used to propel the vehicle
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PDE: HOW DOES IT WORK?
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PDE: PULSE DETONATION ENGINES
• Main Idea: Detonates air/fuel mixture rather than just allowing it to
simply combust
• Constant volume combustion is more efficient than constant pressure
combustion, as demonstrated by a cycle analysis
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PULSE JET: SUBSONIC V1
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GAS TURBINE / PDE HYBRID
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WHAT DID THIS?
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PDE: AT HOME
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Isp
PDE POTENTIAL
Mach Number
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PDE SUMMARY
• Claimed Advantages
– No moving parts
– High thermodynamic efficiency
– Operating in a potential large Mach number range (from 0 to 4-5)
– Simplicity and flexibility of geometrical configuration
– Easy integration to vehicle
– Low cost
• Key Issues / Challenges
– Detonation initiation (PDE/PDRE)
– Air inlet design (PDE)
– Fuel/air injection and mixing (PDE/PDRE)
– Coupling with external flow (PDE)
– Design optimization (PDE/PDRE)
• PDE looks attractive for special missions but is not yet totally mastered
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MICRO PROPULSION
• Why Go So Small?
– T/W advantages
– Scalability
– Fine pointing
– Fine orbital corrections
– Constellation applications
• Traditional Manufacturing vs. MEMS (Micro-Electro-Mechanical Systems)
• Military Applications
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MICRO TURBOMACHINERY
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MICRO-ROCKET PROJECT
•
•
•
•
Design complete rocket system, tanks, valves, turbomachinery, thrust chamber
Mass production, silicon-carbide material
Launch and orbital applications, T~ 5-15 N, Isp ~ 300 sec
Higher T/W than Space Shuttle
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