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From Atomic to Cosmic: A Panoramic View of Combustion

• What is combustion?- as a driver of technology- as a scientific discipline

• Examples of new challenges and frontiers

PrincetonPrinceton UniversityUniversityC. K. Law

Hong Kong Polytechnic University

April 13, 2007

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What is Combustion?

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The Dawn of Civilization

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Companion in Learning and Love

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Provider of Warmth and Joy

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Furnace for Industry and Pleasure

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Power for Mobility

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Agent of Destruction, and…

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…of Catastrophe

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Combustion as A Major Driver of Technology

• Energy & power

• Environment, climate & health

• Fuels

• Fire & explosion hazards

• Aerospace & defense

• New technologies

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Combustion as A Major Scientific Discipline

• Study of flows with highly exothermic, temperature-sensitive reactions

• Interdisciplinary:– Fluid mechanics– Chemical kinetics

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Reaction Kinetics at Atomic Scale• Characteristic time needed to resolve reaction

dynamics at the atomic scale:

Kinetic energy of colliding atoms

~ vibration energy of activated complex

• Use femto-second pump-probe laser pulses to study detailed reaction dynamics (Zewail, Nobel prize in chemistry, 1999)

300Kat 160 ~

motion atomicoftimesticCharacteri

1- fsTk

ho ≈=

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New Challenges & Frontiers in Combustion

• Energy sustainability & climate change

• Hydrogen economy

• Micro-engines & -burners

• Combustion synthesis of materials

• Bio-inspired interests

• Combustion in space exploration

• Cosmic combustion

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Energy Sustainability &

Climate Change

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Energy & Fuels

– Dwindling petroleum reserve (50 – 75 years)

– Geopolitical uncertainty in supply stresses global economy & harmony

– Increasing prosperity of developing, populous countries aggravates demand & competition

• Fossil fuels supply 85%of the US energy needs; petroleum: 40%

• Energy crisis = Fuel crisis

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Pollution & Climate• Pollution continues to be a

major concern; however, it is regional and mostly reversible on short time scales

• Global warming is reversible only on geological time scales, hence considerably more worrisome

Glacier National Park, Montana

1911

2000

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Effects of Global Warming• Sea level rise due to

- glacier melting- thermal expansion of ocean water

• Change in climate• Change in ecology; desertization• Spread of disease vectors

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Urgency of Global Warming• Global temperature will rise about 2-3 °C by

doubling the pre-industrial CO2 concentration in the atmosphere:

Pre-industrial 280 ppmPresent 370 ppmDoubling 560 ppm

• Doubling will occur within roughly 50-75 years if business as usual

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Climate-Energy Coupling:The Feedback Loop

Fossil fuel burning aggravates

global warming

Deteriorating climate requires

more energy expenditure to sustain

living environment

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Climate-Energy Coupling:The 50-75 Year Window

Period of petroleum depletion

20502000

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(280 ppmv)( ΔT = 0 °C)

(370 ppmv)( ΔT = 0.8 °C)

(560 ppmv)( ΔT = 3 °C)

Historicalemissions

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Some Roles of Combustion

• Combustion characteristics of fuels that are hydrogen-enriched, carbon-neutral in production, or coal-derived

• System studies of combustor, sensors, fuels, & processes as an integral unit (e.g. design engine based on fuel)

• Apply combustion knowledge to alternate energy/fuel productions: fuel cells, nuclear, & biological

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Hydrogen Economy

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Case for Developing Hydrogen Economy

• Potential benefits of hydrogen as energy carrier:- Zero GHG emission at point of use- Derivable from widely available primary

sources (fossil, renewable, nuclear)- Rapid progress in hydrogen and fuel cell

technologies

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Relevant Combustion Issues (1/3)

• Hydrogen-based powerplants- Fuel cells (long term)- Engines & burners (short to long term)

• Problem areas in internal combustion engines- Low power density ⇒ needs supercharging and

near-stoichiometric burning- High flame temperature ⇒ high NOx emissions- High initial pressure and reactivity ⇒ pre-ignition

and knock

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Relevant Combustion Issues (2/3)• High reactivity permits ultra-lean operations ⇒

Enhanced engine efficiency - Reduced NOx emission- Requires novel engine and combustion

process designs

• No soot formation and emission- Environmentally beneficial- Impedes radiation transfer in stationary

powerplants

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Relevant Combustion Issues (3/3)

• Explosion hazards- Highly explosive- Nonluminous flame- The Hindenburg syndrome

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Micro-Engines & -Burners

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Technological Interests• Micro-scale thrust for fine locomotion

and steering of satellites

• High energy density (100×) mobile power sources to replace conventional battery– Inexpensive– Nearly infinite shelf life– More constant voltage, no memory

effect, instant recharge– Environmentally superior to

disposable batteries– Applications: foot soldiers; portable

electronics (laptops, cell phones, …)

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Technological Challenges

• Large surface-to-volume ratio• Increased heat loss• Flame quenching• Laminar flows: hard to mix• Increased friction• Fabrication

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Possible Remedies

• Reduce heat loss at wall

• Preheat mixture

• Implement surface catalytic reaction

• Reduce moving parts

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Micro-Engines:Miniaturized version of conventional

engines (Berkeley)

12.5 mm

3.6 mm9.5 mm

12.5 mm

3.6 mm

12.5 mm

3.6 mm9.5 mm

Abo

ut 1

mm

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• “Swiss roll” heat recirculating burner - minimizes heat losses

• Toroidal 3-D geometry - further reduces losses - minimizes external temperature on all surfaces

Micro-Engines:Swiss Roll Design (USC)

One-dimensional counterflowcombustor / heat exchanger

Two-dimensional “Swiss-roll” burner

Products

Reactants

Combustionvolume

1600 1200 400 300 K500

1400 600 5007001600

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Combustion Synthesis of

Materials

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Gaseous Flame Synthesis of Nano-Particle Materials

• High production rate & purity• Nano-sized particles; increased catalyticity• Metal & metal oxide particles: Al, SiO2, TiO2, Al2O3,

YBa2Cu3O7-x– Lower sintering temperature– Higher theoretical density– Higher fracture toughness and ductility

• Carbonaceous particles: C, fullerenes, carbon nano-tubes Hi-Mag

Low-Mag

Hi-Mag

Low-Mag

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Solid Flame Synthesis: Process

Undesirablelaminated product

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• Extreme high temperature process (~3000 to 4000 ºC)

• No heating source• Rapid process• Self-purifying• Preform• Vast variety of products

Solid Flame Synthesis: Advantages

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Solid Flame Synthesis: Applications• Carbides: Abrassives, cutting tools, ceramic

reinforcements

• Borides: Abrasives, cutting tools, cathodes• Silicides: Heating elements, electrical connectors• Aluminides & Titanites: Aerospace materials, shape

memory alloys• Nitrides: Ceramic engine parts, ball bearings, nuclear

safety shields• Hydrides: Hydrogen storage, catalytic materials• Oxides: High-temperature superconductors, gas sensors

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Bio-Inspired Combustion

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Some Practical Interests• Obvious interests

- Bio-fuels from biomass- Health effects of inhaling

combustion-generated aerosols

• Bacteria in the service of mankind- Waste scavenging- Fuels production- Microbial fuel cells

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Origin of Life

• Previous theory: Formation of amino acids through lightning in atmosphere containing methane, ammonia, hydrogen, and water

• New hypothesis: Formation of organic compounds in geothermal vents on ocean floor

• Role of combustion: Biological reactions in turbulent, buoyant jets

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Relevance in Molecular & Cellular Biology

• Diffusive transport and reaction of ions and enzymes• Example: pattern formation in aggregating slime molds,

developing oocytes, cardiac muscles, spreading depression in chicken retina, and over flame surfaces!

c

slime mold flame surface

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Movie on Spiral Development over Surface of Expanding Flame

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Combustion in Space

Exploration

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Candle in Microgravity

• Microgravity produces a round, cooler candle flame with no soot

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Microgravity CombustionBuoyancy-affected phenomena suitable for microgravity investigation:

• Distortion from symmetry (e.g. spherical and cylindrical flames)

• Large flame dimensions• Slow burning flames (e.g. near-limit flames,

smoldering)• High pressure flames• Suspensions• Fire safety in space

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NASA’s 3M Mission: Mars Exploration & Colonization

• 3M: Man, Moon & Mars

• Fire hazard and detection

• CO2-breathing propulsion

• In-situ heat/power generation on Mars:

Thermite-class reactions

Fe2O3 + 2Al → Al2O3 + 2Fe + heat

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Cosmic Combustion:

Supernovae

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Phenomenon

• Exploding stars of few seconds duration; light emission ~ entire galaxy

• Fundamental phenomenon:- Accretion of white dwarf in binary star system to Chandrasekhar mass (~ 1.4 solar mass) - Violent explosion after ~102years of thermonuclear “cooking”

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Possible Mechanism• Theory has to fit light curve:

composition, temperature & velocity

• Combustion characteristics:- Buoyantly unstable (106g)

- Highly turbulent (Re~1014)

- Highly temperature sensitive (T12 at 1010 K) and complex nuclear reactions

- Transition from subsonic to supersonic combustion

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Summary• Combustion intimately affects almost every aspect of

our daily life• Combustion is at the center of studies on reacting flows

– physical, chemical, biological; traditional & new frontiers

• Combustion is:- Rich in phenomena- Technologically relevant - Intellectually stimulating

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Thanks for Coming!

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