Corona discharge ignition of premixed flames

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Corona discharge ignition of premixed flames

• Jian-Bang Liu, Paul Ronney, Martin Gundersen
• University of Southern California
• Los Angeles, CA 90089-1453 USA

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Flame ignition by pulsed corona discharges

• Characteristics
• Initial phase of spark discharge (lt 100 ns) -
  highly conductive (arc) channel not yet formed
• Multiple streamers of electrons
• High energy (10s of eV) electrons - couple
  efficiently with cross-section for ionization,
  electron attachment, dissociation
• More efficient use of energy deposited into gas
• Enabling technology USC-built discharge
  generators having high wall-plug efficiency
  (gt50) - far greater than arc or laser sources

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Pulse detonation engine concept

• Advantages over conventional propulsion systems
• Nearly constant-volume cycle vs. constant
  pressure - higher ideal thermodynamic efficiency
• No mechanical compressor needed
• Can operate from zero to hypersonic Mach numbers

Courtesy Fred Schauer
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Pulse detonation engines - initiation

• Need rapid ignition and transition to detonation
  (? high thermal efficiency) and repetition rate
  (? thrust)
• Conventional spark ignition sources may initiate
  detonations, but need obstacles - heat
  stagnation pressure losses
• Multiple high-energy discharges may be too
  energy-intensive
• Need energy-efficient, minimally intrusive means
  to initiate detonations

Courtesy Fred Schauer
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Transient plasma (corona) discharge

• Not to be confused with plasma torch
• Initial phase of spark discharge (lt 100 ns) -
  highly conductive (arc) channel not yet formed
• High field strength
• Multiple streamers of electrons

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Corona vs. arc discharge
Corona phase (0 - 100 ns) Arc phase (gt
500 ns)
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Transient plasma (corona) discharge

• Not to be confused with plasma torch
• Initial phase of spark discharge (lt 100 ns) -
  highly conductive (arc) channel not yet formed
• High field strength
• Multiple streamers of electrons
• High energy (10s of eV) electrons - couple
  efficiently with cross-section for ionization,
  electron attachment, dissociation

8
Corona vs. arc discharges for ignition
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Transient plasma (corona) discharge

• Not to be confused with plasma torch
• Initial phase of spark discharge (lt 100 ns) -
  highly conductive (arc) channel not yet formed
• High field strength
• Multiple streamers of electrons
• High energy (10s of eV) electrons - couple
  efficiently with cross-section for ionization,
  electron attachment, dissociation
• Electrons not at thermal equilibrium with
  ions/neutrals
• Ions are good chain branching agents

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Ions are energy-efficient chain-branching agents

• Rates
• Reaction Pre-exponential
  Activation energy
• H O2 ? OH O 3.1 x 10-10 s/cm3mol 16.81
  kcal/mol
• H O2- ? OH- O 1.2 x 10-9 0
• Rate ratio at 1000K 1/18,000
• Energy cost of O2- higher than H, but not
  18,000x higher!
• Reaction Energy
• CH4 ? CH3 H 4.6 eV
• vs.
• O2 e- ? O2 e- e- 12.1 eV
• N2 O2 e- ? N2 O2-

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Transient plasma (corona) discharge

• Not to be confused with plasma torch
• Initial phase of spark discharge (lt 100 ns) -
  highly conductive (arc) channel not yet formed
• High field strength
• Multiple streamers of electrons
• High energy (10s of eV) electrons - couple
  efficiently with cross-section for ionization,
  electron attachment, dissociation
• Ions are good chain branching agents
• Electrons not at thermal equilibrium with
  ions/neutrals
• Ions stationary - no hydrodynamics
• Low anode cathode drops, little radiation
  shock formation - more efficient use of energy
  deposited into gas
• USC-built discharge generators have high
  wall-plug efficiency (gt50) - far greater than
  arc or laser sources

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Comparison with conventional arc

• Single unnecessarily large, high current
  conductive path
• Low field strength (like short circuit)
• Large anode cathode voltage drops - large
  losses
• Low energy electrons (1s of eV)
• Flow effects due to ion motion - gasdynamic
  losses
• Less efficient coupling of energy into gas

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Experimental apparatus for corona ignition
(constant volume)
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Experimental apparatus for corona ignition
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USC corona discharge generator

• "Inductive adder" circuit
• Pulse shaping to minimize duration, maximize peak
  power
• Parallel placement of multiple MOSFETs (thyratron
  replacement) all referenced to ground potential
• gt 40kV, lt 100 ns pulse

 
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Images of corona discharge flame

• Axial (left) and radial (right) views of
  discharge
• Axial view of discharge flame
• (6.5 CH4-air, 33 ms between images)

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Characteristics of corona discharge
Corona only
Corona arc
 

• Arc leads to much higher energy consumption with
  little increase in energy deposited in gas
• Corona has very low noise light emission
  compared to arc with same energy deposition

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Characteristics of corona discharges

• Optimal energy above which ignition properties
  are nearly constant

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Ignition delay rise time (methane-air)

• Both ignition delay time (0 - 10 of peak P)
  rise time (10 - 90 of peak P) 3x smaller with
  corona ignition
• Rise time more significant issue
• Longer than delay time
• Unlike delay time, cant be compensated by spark
  advance
• Brush electrode provides localized field
  strength enhancement with minimal increase in
  surface area (? drag, heat loss)

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Peak pressures

• Peak pressure higher with corona discharge
• Radial propagation (corona) vs. axial propagation
  (arc)
• Corona more combustion occurs at higher
  pressure (smaller quenching distance)
• Corona lower fraction of unburned fuel
• Consistent with measurements of residual pressure
  (need GC verification)

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Modified electrode

• Brush electrode provides localized field
  strength enhancement with minimal increase in
  surface area (? drag, heat loss)
• 5x faster rise time than arc

Stoichiometric CH4-air, 1 atm Stoichiometric CH4-air, 1 atm Stoichiometric CH4-air, 1 atm
Ignition source Delay time (ms) Rise time (ms)
Arc at end plate 19 80
Arc at tip 17 40
Arc at center 19 41
Corona (plain electrode) 7.4 14
Corona (modified electrode) 8.3 8.7
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Pressure effects

• Results similar at reduced pressure -
• useful for high-altitude ignition

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Pressure effects

• Results similar at higher pressure

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Pressure fuel effects - propane-air

• Results similar with other fuels (e.g. propane)

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Fuel effects

• n-butane and iso-butane exhibit similar trends
  but greater difference between corona and arc for
  n-butane (more weaker secondary C-H bonds?)

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PDE testing at U.S. Naval Postgraduate School

• 1 day facility time
• Ethylene-air, 1 atm, 2 inch diameter tube, no
  obstacles
• Initial results promising - 3x shorter time to
  reach peak pressure than with arc ignition, much
  higher peak pressure (17 psig vs. 1 psig)

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Prior work Diesel Emission NO Plasma
Interactions

• Energy efficient 10 eV/molecule or less
  possible
• Transient plasma provides dramatically improved
  energy efficiency - by 100x compared to prior
  approaches employing quasi-steady discharges
• 10 eV/molecule corresponds to 0.2 of fuel
  energy input per 100 ppm NO destroyed
• Applicable to propulsion systems, unlike
  catalytic post-combustion treatments

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NO removal by corona discharge

• Diesel engine exhaust
• Needle/plane corona discharge (20 kV, 30 nsec
  pulse)
• Lower left before pulse
• Lower right 10 ms after pulse
• Upper difference, showing single-pulse
  destruction of NO ( 40)

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Conclusions

• Corona ignition is promising for ignition delay
  reduction
• More energy efficient than arc discharges
• More rapid ignition transition to detonation
• Higher peak pressures
• Reasons for improvements not yet fully understood
• Geometrical - more distributed ignition sites?
• Chemical effects - more efficient use of electron
  energy? (Radical ignition courses similar minimum
  ignition energies to thermal sources, but shorter
  ignition delays)
• Enabling technology corona generators - require
  sophisticated approach to electronics

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Potential applications

• PDE-related
• Integration into PDE test facility
• NPS (Brophy)
• WPAFB (Schauer)
• Coaxial geometry easily integrated into PDEs
• Multiple parallel electrodes to create
  imploding flame
• Electrostatic sprays charged with corona
  discharges
• Pipe dream integration of electrostatic fuel
  dispersion, ignition NOx remediation
• Others
• Flameholding
• Quasi-steady, constant pressure jet flames - USC
• Cavity-stabilized ramjet-like combustor - WPAFB
  (Jackson)
• High altitude relight
• Cold weather ignition
• Endothermic fuels
• Lean-burn internal combustion engines

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Future work - science-related

• Transient plasmas are a new area for applications
  
• Quantitative understanding of physics needed for
  applications, but theory almost nonexistent
• Temporal, spatial behavior of electron energy
  distribution
• Need integration of plasma into CFD codes (add
  field subroutine, radical generator, spatial
  distribution of energetic electrons relative to
  streamer head)
• Modeling of chemical reactions between ions /
  electrons / neutrals (no GRI Mech for ionized
  species!)