Flame initiation by nanosecond plasma discharges: Putting some new spark into ignition Paul D. Ronne

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Flame initiation by nanosecond plasma
dischargesPutting some new spark into
ignitionPaul D. RonneyUniversity of Southern
California, USANational Central
UniversityJhong-Li, Taiwan, October 3,
2005Research supported by U.S. AFOSR, ONR
DOETravel supported by the Combustion Institute

• Faculty collaborator Martin Gundersen (USC-EE)
• Research Associates Nathan Theiss, Jian-Bang
  Liu
• Graduate students Jason Levin, Fei Wang,
• Jun Zhao, Tsutomu Shimizu
• Undergraduate students Brad Tallon, Matthew
  Beck
• Jennifer Colgrove, Merritt Johnson, Gary Norris

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University of Southern California

• Established 125 years ago this week!
• jointly by a Catholic, a Protestant and a Jew -
  USC has always been a multi-ethnic,
  multi-cultural, coeducational university
• Today 32,000 students, 3000 faculty
• 2 main campuses University Park and Health
  Sciences
• USC Trojans football team ranked 1 in USA last 2
  years

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USC Viterbi School of Engineering

• Naming gift by Andrew Erma Viterbi
• Andrew Viterbi co-founder of Qualcomm,
  co-inventor of CDMA
• 1900 undergraduates, 3300 graduate students, 165
  faculty, 30 degree options
• 135 million external research funding
• Distance Education Network (DEN) 900 students in
  28 M.S. degree programs 171 MS degrees awarded
  in 2005
• More info http//viterbi.usc.edu

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Paul Ronney

• B.S. Mechanical Engineering, UC Berkeley
• M.S. Aeronautics, Caltech
• Ph.D. in Aeronautics Astronautics, MIT
• Postdocs NASA Glenn, Cleveland US Naval
  Research Lab, Washington DC
• Assistant Professor, Princeton University
• Associate/Full Professor, USC
• Research interests
• Microscale combustion and power generation
• (10/4, INER 10/5 NCKU)
• Microgravity combustion and fluid mechanics
  (10/4, NCU)
• Turbulent combustion (10/7, NTHU)
• Internal combustion engines
• Ignition, flammability, extinction limits of
  flames (10/3, NCU)
• Flame spread over solid fuel beds
• Biophysics and biofilms (10/6, NCKU)

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Paul Ronney
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Transient plasma ignition - motivation

• Multi-point ignition of flames has potential to
  increase burning rates in many types of
  combustion engines, e.g.
• Pulse Detonation Engines
• Reciprocating Internal Combustion Engines
• (Simplest approach) Leaner mixtures (lower NOx)
• (More difficult) Redesign intake port and
  combustion chamber for lower turbulence since the
  same burn rate is possible with lower turbulence
  (reduced heat loss to walls, higher efficiency)
• High altitude restart of gas turbines
• Lasers, multi-point sparks challenging
• Lasers energy efficiency, windows, fiber optics
• Multi-point sparks multiple intrusive electrodes
• How to obtain multi-point, energy efficient
  ignition?

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Transient plasma (pulsed corona) discharges

• Not to be confused with plasma torch
• Initial phase of spark discharge (highly conductive (arc) channel not yet formed
• Characteristics
• Multiple streamers of electrons
• High energy (10s of eV) electrons compared to
  sparks (1 eV)
• 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

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Corona vs. arc discharge
Corona phase (0 - 100 ns) Arc phase (
100 ns)
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Images of corona discharge flame

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

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Characteristics of corona discharges

• For short durations (1s to 100s of ns depending
  on pressure, geometry, gas, etc.) DC breakdown
  threshold of gas can be exceeded without
  breakdown if high voltage pulse can be created
  and stopped quickly enough

 
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Characteristics of corona discharges
Corona arc
Corona only

• If arc forms, current increases some but voltage
  drops more, thus higher consumption of capacitor
  energy with little increase in energy deposited
  in gas (still have corona, but followed by
  (relatively ineffective) arc)

 
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Corona discharges are energy-efficient

• Discharge efficiency ?d 10x higher for corona
  than conventional sparks

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Objectives

• Compare combustion duration and ignition energy
  requirements of spark-ignited and corona-ignited
  flames in constant-volume vessel
• Determine effect of corona electrode geometry and
  ignition energy on combustion duration
• Determine if reduced combustion duration observed
  for corona ignition in quiescent, constant-volume
  experiments also applies to turbulent flames
• Integrate pulsed corona discharge ignition system
  into premixed-charge IC engines
• Compare performance of corona-ignited and
  spark-ignited engines
• Efficiency
• Emissions

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Experimental apparatus (constant volume)

• Pulsed corona discharges generated using
  thyratron or pseudospark gas switch Blumlein
  transmission line
• 2.5 (63.5 mm) diameter chamber, 6 (152 mm) long
• Rod electrode (shown below) or single-needle
• Energy release (stoich. CH4-air, 1 atm) 1650 J
  energy release
• Discharge energy input for ignition is trivial
  fraction of heat release!

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Definitions

• Delay time 0 - 10 of peak pressure
• Rise time 10 - 90 of peak pressure

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Electrode configurations
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Pulsed corona discharges in IC engine-like
geometry

• Top view Side view

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Minimum ignition energy vs. mixture

• 1 pin corona discharge vs. spark - same
  geometry
• MIE significantly higher ( 100x) for corona -
  more distributed energy deposition in streamers?
• Minimum spark kernel diameter 0.2 mm for
  stoich. CH4-air

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Pressure effects on MIE

• MIE for pulsed corona does NOT follow Emin P-2
  as spark ignition does more like P-1 at low P,
  P0 at higher P
• Smaller chamber diameter enables ignition at
  higher P - higher voltage gradient

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Effect of geometry on delay time
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Effect of geometry on delay time

• Delay time of spark larger ( 1.5 - 2x) than
  1-pin corona ( same geometry)
• Consistent with computations by Dixon-Lewis,
  Sloane that suggest point radical sources improve
  ignition delay 2x compared to thermal sources
• More streamer locations (more pins, rod) yield
  lower delay time ( 3.5x lower for rod than
  spark)
• Suggests benefit of corona is both chemical (1.5
  - 2x) and geometrical ( 2x)

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Effect of geometry on rise time
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Effect of geometry on rise time

• Rise time of spark larger same as 1-pin corona
  ( same flame propagation geometry)
• More streamer locations (more pins, rod) yield
  lower rise time ( 3 - 4x lower for rod than
  spark), but multi-pin almost as good with less
  energy

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Peak pressures
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Peak pressures

• Peak pressures significantly higher for
  multi-point corona that one-pin corona or spark
• Improvement (for rod) nearly independent of
  mixture
• Probably due to change in flame propagation
  geometry, not heat losses
• 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 preliminary measurements of
  residual fuel

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Energy geometry effects on delay time

• What is optimal electrode configuration to
  minimize delay/rise time for a given energy?
• Delay time 2-ring, 4-ring plain rod similar
  (all are much better than spark)

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Energy geometry effects on rise time

• Rise time 2-ring or 4-ring best
• Note step behavior for multi-point ignition at
  low energies - not all sites ignite
• Delay time doesnt show step behavior

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Energy geometry effects (lean mixture)

• Delay time same conclusion as stoichiometric
  mixture

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Energy geometry effects (lean mixture)

• Rise time 4-ring stands out

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Rod diameter effects

• Plain rod optimal diameter exists ( 0.15),
  drod/dcyl 0.06
• Large d low field concentration, few streamers?
• Small d Too many streamers, too much energy
  deposition?

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Effect of number of pins on 1 ring
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Effect of number of pins on 1 ring

• MIE lower (!!) with more pins, optimal 4
• More pins Slightly beneficial effect on delay
  time, slightly adverse effect (!) on rise time
• More is not necessarily better!

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Thyratron vs. pseudospark generator

• Little effect of discharge generator type
  (pseudospark 1/2 discharge duration compared
  to thyratron)

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Turbulent test chamber
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Turbulence effects

• Simple turbulence generator (fan grid)
  integrated into coaxial combustion chamber, rod
  electrode
• Turbulence intensity 1 m/s, u/SL 3
  (stoichiometric)
• Benefit of corona ignition same in turbulent
  flames - shorter rise delay times, higher peak
  P
• Note quiescent corona faster than turbulent
  spark! (Faster burn with less heat loss)

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Turbulence effects

• Similar results for lean mixture but benefit of
  turbulence more dramatic - higher u/SL ( 8)

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Engine experiments

• 2000 Ford Ranger I-4 engine with dual-plug head
  to test corona spark at same time, same
  operating conditions
• National Instruments / Labview data acquisition
  control
• Horiba emissions bench, samples extracted from
  corona - equipped cylinder
• Pressure / volume measurements
• Optical Encoder mounted to crankshaft
• Spark plug mounted Kistler piezoelectric pressure
  transducer

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Electrode configuration

• Macor machinable ceramic used for insulator
• Coaxial shielded cable used to reduce EMI
• Simple single-point electrode tip, replaceable
• Point to plane geometry first step - by no
  means optimal

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On-engine corona ignition system

• Corona electrode and spark plug with pressure
  transducer in 1 cylinder
• Wired for quick change between spark and corona
  ignition under identical operating conditions
• 500 mJ/pulse (equivalent wall plug energy
  requirement of 50 mJ spark)
• Range of ignition timings for both spark corona
• 3 modes tested
• Corona only
• Single conventional plug
• Two conventional plugs (results very similar to
  single plug)

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On-engine corona ignition system
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On-engine results

• Corona ignition shows increase in peak pressure
  under all conditions tested

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On-engine results

• Corona ignition shows increase in IMEP under all
  conditions tested

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IMEP at various air / fuel ratios

• Indicated mean effective pressure (IMEP) higher
  for corona than spark, especially for lean
  mixtures (nearly 30)
• Coefficient of variance (COV) comparable

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IMEP at various loads

• Corona showed an average increase in IMEP of 16
  over a range of engine loads

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Burn rate

• Integrated heat release shows faster burning with
  corona leads to greater effective heat release

2900 RPM, ? 0.7
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Burn rates

• Corona ignition shows substantially faster burn
  rates at same conditions compared to 2-plug
  conventional ignition

2900 RPM, ? 0.7
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Emissions data - NOx

• Improved NOx performance vs. indicated efficiency
  tradeoff compared to spark ignition by using
  leaner mixtures with sufficiently rapid burning

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Emissions data - hydrocarbons

• Hydrocarbons emissions similar, corona vs. spark

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Emissions data - CO

• CO emissions similar, corona vs. spark

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Conclusions

• Flame ignition by transient plasma or pulsed
  corona discharges is a promising technology for
  ignition delay rise time reduction
• More energy-efficient than spark discharges
• Shorter ignition delay and rise times
• Rise time more significant issue
• Longer than delay time
• Unlike delay time, cant be compensated by spark
  advance
• Higher peak pressures
• Benefits apply to turbulent flames also
• Demonstrated in engines too
• Higher IMEP for same conditions with same or
  better BSNOx
• Shorter burn times and faster heat release
• Improvements due to
• Chemical effects (delay time) - radicals vs.
  thermal energy
• Geometrical effects - (delay rise time) - more
  distributed ignition sites

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Future work

• Improved electrode designs
• Solid-state discharge generators
• Multi-cylinder corona ignition
• Corona-ignited, low turbulence (thus low heat
  loss) engines???
• Transient plasma discharges for fuel electrospray
  dispersion?

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Thanks to

• National Central University
• Prof. Shenqyang Shy
• Combustion Institute (Bernard Lewis Lectureship)
• AFOSR, ONR, DOE (research support)