Corona discharge ignition Corona discharge ignition of premixed flamesof premixed flames

Jian-Bang Liu, Paul Ronney, Martin GundersenJian-Bang Liu, Paul Ronney, Martin GundersenUniversity of Southern CaliforniaUniversity of Southern California

Los Angeles, CA 90089-1453 USALos Angeles, CA 90089-1453 USA

Flame ignition by pulsed corona dischargesFlame ignition by pulsed corona discharges

• CharacteristicsCharacteristics• Initial phase of spark discharge (< 100 ns) - highly conductive (arc) Initial phase of spark discharge (< 100 ns) - highly conductive (arc)

channel not yet formedchannel not yet formed• Multiple streamers of electronsMultiple streamers of electrons• High energy (10s of eV) electrons - couple efficiently with cross-High energy (10s of eV) electrons - couple efficiently with cross-

section for ionization, electron attachment, dissociationsection for ionization, electron attachment, dissociation• More efficient use of energy deposited into gasMore efficient use of energy deposited into gas

• Enabling technology: USC-built discharge generators having Enabling technology: USC-built discharge generators having high wall-plug efficiency (>50%) - far greater than arc or laser high wall-plug efficiency (>50%) - far greater than arc or laser sourcessources

Pulse detonation engine conceptPulse detonation engine concept

Fill TubeDetonateMixture

Exhaust

Refill Tube, Repeat

Fuel

Air(or otheroxidizer)

• Advantages over conventional propulsion systemsAdvantages over conventional propulsion systems• Nearly constant-volume cycle vs. constant pressure - higher Nearly constant-volume cycle vs. constant pressure - higher

ideal thermodynamic efficiencyideal thermodynamic efficiency• No mechanical compressor neededNo mechanical compressor needed• Can operate from zero to hypersonic Mach numbersCan operate from zero to hypersonic Mach numbers

Courtesy Fred Schauer

Pulse detonation engines - initiationPulse detonation engines - initiation• Need rapid ignition and transition to detonation (Need rapid ignition and transition to detonation ( high high

thermal efficiency) and repetition rate (thermal efficiency) and repetition rate ( thrust) thrust)• Conventional spark ignition sources may initiate Conventional spark ignition sources may initiate

detonations, but need obstacles - heat & stagnation detonations, but need obstacles - heat & stagnation pressure lossespressure losses

• Multiple high-energy discharges may be too energy-Multiple high-energy discharges may be too energy-intensiveintensive

• Need energy-efficient, minimally intrusive means to initiate Need energy-efficient, minimally intrusive means to initiate detonationsdetonations

Courtesy Fred Schauer

Transient plasma (corona) dischargeTransient plasma (corona) discharge

• Not to be confused with “plasma torch”Not to be confused with “plasma torch”• Initial phase of spark discharge (< 100 ns) - highly Initial phase of spark discharge (< 100 ns) - highly

conductive (arc) channel not yet formedconductive (arc) channel not yet formed• High field strengthHigh field strength• Multiple streamers of electronsMultiple streamers of electrons

Corona vs. arc dischargeCorona vs. arc discharge

Arc channel

High voltage pulse

Corona StreamersPlasma Zone

Corona dies out in pulsed mode

Coaxial ground electrode - no dielectric barrier needed

High voltage pulse

Corona phase (0 - 100 ns)Corona phase (0 - 100 ns)

Arc phase (> 500 ns)Arc phase (> 500 ns)

Transient plasma (corona) dischargeTransient plasma (corona) discharge

• Not to be confused with “plasma torch”Not to be confused with “plasma torch”• Initial phase of spark discharge (< 100 ns) - highly Initial phase of spark discharge (< 100 ns) - highly

conductive (arc) channel not yet formedconductive (arc) channel not yet formed• High field strengthHigh field strength• Multiple streamers of electronsMultiple streamers of electrons• High energy (10s of eV) electrons - couple efficiently with High energy (10s of eV) electrons - couple efficiently with

cross-section for ionization, electron attachment, cross-section for ionization, electron attachment, dissociationdissociation

Corona vs. arc discharges for ignitionCorona vs. arc discharges for ignition

Transient plasma (corona) dischargeTransient plasma (corona) discharge

• Not to be confused with “plasma torch”Not to be confused with “plasma torch”• Initial phase of spark discharge (< 100 ns) - highly Initial phase of spark discharge (< 100 ns) - highly

conductive (arc) channel not yet formedconductive (arc) channel not yet formed• High field strengthHigh field strength• Multiple streamers of electronsMultiple streamers of electrons• High energy (10s of eV) electrons - couple efficiently with High energy (10s of eV) electrons - couple efficiently with

cross-section for ionization, electron attachment, cross-section for ionization, electron attachment, dissociationdissociation

• Electrons not at thermal equilibrium with ions/neutralsElectrons not at thermal equilibrium with ions/neutrals• Ions are good chain branching agentsIons are good chain branching agents

Ions are energy-efficient chain-branching agentsIons are energy-efficient chain-branching agents

• RatesRates

ReactionReaction Pre-exponential Pre-exponential Activation Activation energyenergy

H + OH + O22 OH + O OH + O 3.1 x 103.1 x 10-10-10 s/cm s/cm33mol mol 16.81 kcal/mol16.81 kcal/mol

H + OH + O22-- OHOH-- + O + O 1.2 x 101.2 x 10-9-9 00

Rate ratio at 1000K: 1/18,000Rate ratio at 1000K: 1/18,000

• Energy cost of OEnergy cost of O22- - higher than H, but not 18,000x higher! higher than H, but not 18,000x higher!

ReactionReaction EnergyEnergy

CHCH44 CH CH33 + H + H 4.6 eV 4.6 eV

vs.vs.

OO2 2 + e+ e-- OO22

++ + e+ e- - + e+ e- - 12.1 eV 12.1 eV

NN22 + + OO2 2 + e+ e-- NN22 + O + O22

--

Transient plasma (corona) dischargeTransient plasma (corona) discharge

• Not to be confused with “plasma torch”Not to be confused with “plasma torch”• Initial phase of spark discharge (< 100 ns) - highly Initial phase of spark discharge (< 100 ns) - highly

conductive (arc) channel not yet formedconductive (arc) channel not yet formed• High field strengthHigh field strength• Multiple streamers of electronsMultiple streamers of electrons• High energy (10s of eV) electrons - couple efficiently with High energy (10s of eV) electrons - couple efficiently with

cross-section for ionization, electron attachment, cross-section for ionization, electron attachment, dissociationdissociation

• Ions are good chain branching agentsIons are good chain branching agents• Electrons not at thermal equilibrium with ions/neutralsElectrons not at thermal equilibrium with ions/neutrals• Ions stationary - no hydrodynamicsIons stationary - no hydrodynamics• Low anode & cathode drops, little radiation & shock Low anode & cathode drops, little radiation & shock

formation - more efficient use of energy deposited into gasformation - more efficient use of energy deposited into gas• USC-built discharge generators have high wall-plug USC-built discharge generators have high wall-plug

efficiency (>50%) - far greater than arc or laser sourcesefficiency (>50%) - far greater than arc or laser sources

Comparison with conventional arcComparison with conventional arc

• Single unnecessarily large, high current conductive Single unnecessarily large, high current conductive pathpath

• Low field strength (like short circuit)Low field strength (like short circuit)• Large anode & cathode voltage drops - large lossesLarge anode & cathode voltage drops - large losses• Low energy electrons (1s of eV)Low energy electrons (1s of eV)• Flow effects due to ion motion - gasdynamic lossesFlow effects due to ion motion - gasdynamic losses• Less efficient coupling of energy into gasLess efficient coupling of energy into gas

Experimental apparatus for corona ignition Experimental apparatus for corona ignition (constant volume)(constant volume)

Pulse generator

Oscilloscope

TriggerDC power

supply

High voltage

DC power

supply

To thyratron

InputOutput spark plug

circuit

Current signal

Air

Fuel

Vacuum pump

Gas outlet

+

-

Probe

Pressure

Transducer

Spark plug

Pressure

gauge

Transformer

Experimental apparatus for corona ignitionExperimental apparatus for corona ignition

 

USC corona discharge generatorUSC corona discharge generator

• "Inductive adder" circuit"Inductive adder" circuit• Pulse shaping to minimize Pulse shaping to minimize

duration, maximize peak powerduration, maximize peak power• Parallel placement of multiple Parallel placement of multiple

MOSFETs (thyratron replacement) MOSFETs (thyratron replacement) all referenced to ground potentialall referenced to ground potential

• > 40kV, < 100 ns pulse> 40kV, < 100 ns pulse

Images of corona discharge & flameImages of corona discharge & flame

Axial (left) and radial (right) views of dischargeAxial (left) and radial (right) views of discharge

Axial view of discharge & flame Axial view of discharge & flame (6.5% CH(6.5% CH44-air, 33 ms between images)-air, 33 ms between images)

 

Characteristics of corona dischargeCharacteristics of corona discharge

• Arc leads to much higher energy consumption with little Arc leads to much higher energy consumption with little increase in energy deposited in gasincrease in energy deposited in gas

• Corona has very low noise & light emission compared to arc Corona has very low noise & light emission compared to arc with same energy depositionwith same energy deposition

-100

0

100

200

300

400

-20

-10

0

10

20

30

40

50

Current (amps)

Voltage (kV) or power (MW)

Current

Voltage

Power

-100

0

100

200

300

400

-20

-10

0

10

20

30

40

50

-50 0 50 100 150 200 250 300

Current (amps)Voltage (kV) or power (MW)

Time (ns)

Current

Voltage

Power

Start

of arc

Corona only

Corona + arc

Characteristics of corona dischargesCharacteristics of corona discharges

““Optimal” energy above which ignition Optimal” energy above which ignition properties are nearly constantproperties are nearly constant

0

50

100

150

100 1000

Rise time (ms)

Corona energy (mJ)

φ = 1

φ = 0.93

φ = 0.83

φ = 0.73

φ = 0.65

Ignition delay & rise time (methane-air)Ignition delay & rise time (methane-air)

• Both ignition delay time (0 - 10% of peak P) & rise time (10% - 90% of Both ignition delay time (0 - 10% of peak P) & rise time (10% - 90% of peak P) ≈ 3x smaller with corona ignitionpeak P) ≈ 3x smaller with corona ignition

• Rise time more significant issueRise time more significant issue• Longer than delay timeLonger than delay time• Unlike delay time, can’t be compensated by “spark advance”Unlike delay time, can’t be compensated by “spark advance”

• ““Brush” electrode provides localized field strength enhancement with Brush” electrode provides localized field strength enhancement with minimal increase in surface area (minimal increase in surface area ( drag, heat loss) drag, heat loss)

10

100

0.6 0.7 0.8 0.9 1 1.1

arc at center

arc at end plate

corona

corona+brush

Delay time (ms)

Equivalence ratio

10

100

0.6 0.7 0.8 0.9 1 1.1

arc at center

arc at end plate

corona

corona+brush

Rise time (ms)

Equivalence ratio

Peak pressuresPeak pressures

• Peak pressure higher with corona dischargePeak pressure higher with corona discharge• Radial propagation (corona) vs. axial propagation (arc)Radial propagation (corona) vs. axial propagation (arc)• Corona: more combustion occurs at higher pressure (smaller Corona: more combustion occurs at higher pressure (smaller

quenching distance)quenching distance)• Corona: lower fraction of unburned fuelCorona: lower fraction of unburned fuel• Consistent with measurements of residual pressure (need GC Consistent with measurements of residual pressure (need GC

verification)verification)

3.5

4

4.5

5

5.5

6

6.5

7

0.6 0.7 0.8 0.9 1 1.1

Equivalence ratio

arc at center

arc at tip

arc at end plate

corona

Modified electrodeModified electrode

• ““Brush” electrode provides localized field strength enhancement Brush” electrode provides localized field strength enhancement with minimal increase in surface area (with minimal increase in surface area ( drag, heat loss) drag, heat loss)

• ≈ ≈ 5x faster rise time than arc5x faster rise time than arc

Stoichiometric CHStoichiometric CH44-air, 1 atm-air, 1 atm

Ignition Ignition sourcesource

Delay Delay time (ms)time (ms)

Rise timeRise time(ms)(ms)

Arc at end Arc at end plateplate

1919 8080

Arc at tipArc at tip 1717 4040

Arc at centerArc at center 1919 4141

Corona Corona (plain (plain

electrode)electrode)

7.47.4 1414

Corona Corona (modified (modified electrode)electrode)

8.38.3 8.78.7

Pressure effectsPressure effects

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

5

10

15

20

2 4 6 8 10 12 14 16

Delay time (ms)Rise time (ms)

Initial pressure (psi)

Equivalence ratio = 1.0Corona ignition

Pressure effectsPressure effects

Results similar at higher pressureResults similar at higher pressure

10

100

0.6 0.7 0.8 0.9 1

Rise time (arc, 1 atm)Rise time (arc, 1.5 atm)Rise time (corona, 1 atm)Rise time (corona, 1.5 atm)

Equivalence ratio

CH4-air

Pressure & fuel effects - propane-airPressure & fuel effects - propane-air

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

10

100

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Arc (1 atm)

Arc (2 atm)

Corona (1 atm)

Corona (1.5 atm)

Corona (2 atm)

Rise time (ms)

Equivalence ratio

Fuel effectsFuel effects

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

10

100

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

Rise Time (iso-butane, arc)Rise Time (n-butane, arc)Rise Time (iso-butane, corona)Rise Time (n-butane, corona)

Equivalence ratio

C4H

10-air, 1 atm

-10

-5

0

5

10

15

20

0.005 0.01 0.015 0.02 0.025

5 in10.5 in16 in21.5 in32.5 in

Time (seconds)

PDE testing at U.S. Naval Postgraduate SchoolPDE testing at U.S. Naval Postgraduate School• 1 day facility time1 day facility time• Ethylene-air, 1 atm, 2 inch diameter tube, no obstaclesEthylene-air, 1 atm, 2 inch diameter tube, no obstacles• Initial results promising - ≈ 3x shorter time to reach peak Initial results promising - ≈ 3x shorter time to reach peak

pressure than with arc ignition, much higher peak pressure (17 pressure than with arc ignition, much higher peak pressure (17 psig vs. ≈ 1 psig)psig vs. ≈ 1 psig)

Prior work: Diesel Emission NO – Plasma Prior work: Diesel Emission NO – Plasma InteractionsInteractions

• Energy efficient: ≈ 10 eV/molecule or less possible Energy efficient: ≈ 10 eV/molecule or less possible • Transient plasma provides dramatically improved energy Transient plasma provides dramatically improved energy

efficiency - by 100x compared to prior approaches efficiency - by 100x compared to prior approaches employing quasi-steady discharges employing quasi-steady discharges

• 10 eV/molecule corresponds to 0.2 % of fuel energy input 10 eV/molecule corresponds to 0.2 % of fuel energy input per 100 ppm NO destroyed per 100 ppm NO destroyed

• Applicable to propulsion systems, unlike catalytic post-Applicable to propulsion systems, unlike catalytic post-combustion treatmentscombustion treatments

NO removal by corona dischargeNO removal by corona discharge

• Diesel engine Diesel engine exhaustexhaust

• Needle/plane Needle/plane corona discharge corona discharge (20 kV, 30 nsec (20 kV, 30 nsec pulse)pulse)

• Lower left: before Lower left: before pulsepulse

• Lower right: 10 ms Lower right: 10 ms after pulseafter pulse

• Upper: difference, Upper: difference, showing single-showing single-pulse destruction pulse destruction of NO (≈ 40%)of NO (≈ 40%)

m m m m

0 2 4 6 8 1 0 1 2 1 4 1 6 1 8

0

2

4

6

8

1 0

1 2

0 2 4 6 8 1 0 1 2 1 4 1 6 1 8

0

2

4

6

8

1 0

1 2

0

2 0

4 0

6 0

8 0

1 0 0

0 1 0 2 0 3 0 4 0 5 0

m m

0 2 4 6 8 1 0 1 2 1 4 1 6 1 8

0

2

4

6

8

1 0

1 2

G a s F l o w

2 2 6 n m l a s e r

s h e e t

.

ConclusionsConclusions

• Corona ignition is promising for ignition delay Corona ignition is promising for ignition delay reductionreduction

• More energy efficient than arc dischargesMore energy efficient than arc discharges• More rapid ignition & transition to detonationMore rapid ignition & transition to detonation• Higher peak pressuresHigher peak pressures

• Reasons for improvements not yet fully understoodReasons for improvements not yet fully understood• Geometrical - more distributed ignition sites?Geometrical - more distributed ignition sites?• Chemical effects - more efficient use of electron energy? Chemical effects - more efficient use of electron energy?

(Radical ignition courses similar minimum ignition (Radical ignition courses similar minimum ignition energies to thermal sources, but shorter ignition delays)energies to thermal sources, but shorter ignition delays)

• Enabling technology: corona generators - require Enabling technology: corona generators - require sophisticated approach to electronicssophisticated approach to electronics

Potential applicationsPotential applications• PDE-relatedPDE-related

• Integration into PDE test facilityIntegration into PDE test facility» NPS (Brophy)NPS (Brophy)» WPAFB (Schauer)WPAFB (Schauer)» Coaxial geometry easily integrated into PDEsCoaxial geometry easily integrated into PDEs

• Multiple parallel electrodes to create “imploding” flameMultiple parallel electrodes to create “imploding” flame• Electrostatic sprays charged with corona dischargesElectrostatic sprays charged with corona discharges• Pipe dream: integration of electrostatic fuel dispersion, ignition & Pipe dream: integration of electrostatic fuel dispersion, ignition &

NONOxx remediation remediation

• OthersOthers• FlameholdingFlameholding

» Quasi-steady, constant pressure jet flames - USCQuasi-steady, constant pressure jet flames - USC» Cavity-stabilized ramjet-like combustor - WPAFB (Jackson)Cavity-stabilized ramjet-like combustor - WPAFB (Jackson)

• High altitude relightHigh altitude relight• Cold weather ignitionCold weather ignition• Endothermic fuelsEndothermic fuels• Lean-burn internal combustion enginesLean-burn internal combustion engines

Future work - science-relatedFuture work - science-related• Transient plasmas are a new area for applications Transient plasmas are a new area for applications • Quantitative understanding of physics needed for applications, Quantitative understanding of physics needed for applications,

but theory almost nonexistentbut theory almost nonexistent• Temporal, spatial behavior of electron energy distributionTemporal, spatial behavior of electron energy distribution• Need integration of plasma into CFD codes (add field subroutine, Need integration of plasma into CFD codes (add field subroutine,

radical generator, spatial distribution of energetic electrons radical generator, spatial distribution of energetic electrons relative to streamer head)relative to streamer head)

• Modeling of chemical reactions between ions / electrons / neutrals Modeling of chemical reactions between ions / electrons / neutrals (no “GRI Mech” for ionized species!)(no “GRI Mech” for ionized species!)