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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.

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Presentation on theme: "Corona discharge ignition of premixed flames Jian-Bang Liu, Paul Ronney, Martin Gundersen University of Southern California Los Angeles, CA 90089-1453."— Presentation transcript:

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2 Corona discharge ignition of premixed flames Jian-Bang Liu, Paul Ronney, Martin Gundersen University of Southern California Los Angeles, CA USA

3 Flame ignition by pulsed corona discharges CharacteristicsCharacteristics Initial phase of spark discharge (< 100 ns) - highly conductive (arc) channel not yet formedInitial phase of spark discharge (< 100 ns) - highly conductive (arc) channel not yet formed Multiple streamers of electronsMultiple streamers of electrons High energy (10s of eV) electrons - couple efficiently with cross- section for ionization, electron attachment, dissociationHigh energy (10s of eV) electrons - couple efficiently with cross- section 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 high wall-plug efficiency (>50%) - far greater than arc or laser sourcesEnabling technology: USC-built discharge generators having high wall-plug efficiency (>50%) - far greater than arc or laser sources

4 Pulse detonation engine concept Advantages over conventional propulsion systemsAdvantages over conventional propulsion systems Nearly constant-volume cycle vs. constant pressure - higher ideal thermodynamic efficiencyNearly constant-volume cycle vs. constant pressure - higher ideal 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

5 Pulse detonation engines - initiation Need rapid ignition and transition to detonation (  high thermal efficiency) and repetition rate (  thrust)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 lossesConventional spark ignition sources may initiate detonations, but need obstacles - heat & stagnation pressure losses Multiple high-energy discharges may be too energy- intensiveMultiple high-energy discharges may be too energy- intensive Need energy-efficient, minimally intrusive means to initiate detonationsNeed energy-efficient, minimally intrusive means to initiate detonations Courtesy Fred Schauer

6 Transient 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 conductive (arc) channel not yet formedInitial phase of spark discharge (< 100 ns) - highly conductive (arc) channel not yet formed High field strengthHigh field strength Multiple streamers of electronsMultiple streamers of electrons

7 Corona vs. arc discharge Corona phase ( ns) Arc phase (> 500 ns)

8 Transient 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 conductive (arc) channel not yet formedInitial phase of spark discharge (< 100 ns) - highly conductive (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 cross-section for ionization, electron attachment, dissociationHigh energy (10s of eV) electrons - couple efficiently with cross-section for ionization, electron attachment, dissociation

9 Corona vs. arc discharges for ignition

10 Transient 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 conductive (arc) channel not yet formedInitial phase of spark discharge (< 100 ns) - highly conductive (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 cross-section for ionization, electron attachment, dissociationHigh energy (10s of eV) electrons - couple efficiently with cross-section for ionization, electron attachment, dissociation 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

11 Ions are energy-efficient chain-branching agents RatesRates Reaction Pre-exponential Activation energy H + O 2  OH + O3.1 x s/cm 3 mol kcal/mol H + O 2 -  OH - + O1.2 x Rate ratio at 1000K: 1/18,000 Energy cost of O 2 - higher than H, but not 18,000x higher!Energy cost of O 2 - higher than H, but not 18,000x higher! Reaction Energy CH 4  CH 3 + H4.6 eV vs. O 2 + e -  O e - + e eV N 2 + O 2 + e -  N 2 + O 2 -

12 Transient 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 conductive (arc) channel not yet formedInitial phase of spark discharge (< 100 ns) - highly conductive (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 cross-section for ionization, electron attachment, dissociationHigh energy (10s of eV) electrons - couple efficiently with cross-section for ionization, electron attachment, dissociation 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 formation - more efficient use of energy deposited into gasLow anode & cathode drops, little radiation & shock formation - more efficient use of energy deposited into gas USC-built discharge generators have high wall-plug efficiency (>50%) - far greater than arc or laser sourcesUSC-built discharge generators have high wall-plug efficiency (>50%) - far greater than arc or laser sources

13 Comparison with conventional arc Single unnecessarily large, high current conductive pathSingle unnecessarily large, high current conductive path 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

14 Experimental apparatus for corona ignition (constant volume)

15 Experimental apparatus for corona ignition

16 USC corona discharge generator "Inductive adder" circuit "Inductive adder" circuit Pulse shaping to minimize duration, maximize peak power Pulse shaping to minimize duration, maximize peak power Parallel placement of multiple MOSFETs (thyratron replacement) all referenced to ground potential Parallel placement of multiple MOSFETs (thyratron replacement) all referenced to ground potential > 40kV, 40kV, < 100 ns pulse

17 Images of corona discharge & flame Axial (left) and radial (right) views of discharge Axial view of discharge & flame (6.5% CH 4 -air, 33 ms between images)

18 Characteristics of corona discharge Arc leads to much higher energy consumption with little increase in energy deposited in gasArc 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 depositionCorona has very low noise & light emission compared to arc with same energy deposition Corona only Corona + arc

19 Characteristics of corona discharges “Optimal” energy above which ignition properties are nearly constant

20 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 ignitionBoth ignition delay time (0 - 10% of peak P) & rise time (10% - 90% of peak 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 minimal increase in surface area (  drag, heat loss)“Brush” electrode provides localized field strength enhancement with minimal increase in surface area (  drag, heat loss)

21 Peak 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 quenching distance)Corona: more combustion occurs at higher pressure (smaller quenching distance) Corona: lower fraction of unburned fuelCorona: lower fraction of unburned fuel Consistent with measurements of residual pressure (need GC verification)Consistent with measurements of residual pressure (need GC verification)

22 Modified electrode “Brush” electrode provides localized field strength enhancement with minimal increase in surface area (  drag, heat loss)“Brush” electrode provides localized field strength enhancement with minimal increase in surface area (  drag, heat loss) ≈ 5x faster rise time than arc≈ 5x faster rise time than arc Stoichiometric CH 4 -air, 1 atm Ignition source Delay time (ms) Rise time (ms) Arc at end plate 1980 Arc at tip 1740 Arc at center 1941 Corona (plain electrode) Corona (modified electrode)

23 Pressure effects Results similar at reduced pressure - useful for high-altitude ignition

24 Pressure effects Results similar at higher pressure

25 Pressure & fuel effects - propane-air Results similar with other fuels (e.g. propane)

26 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?)

27 PDE 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 pressure than with arc ignition, much higher peak pressure (17 psig vs. ≈ 1 psig)Initial results promising - ≈ 3x shorter time to reach peak pressure than with arc ignition, much higher peak pressure (17 psig vs. ≈ 1 psig)

28 Prior work: Diesel Emission NO – Plasma Interactions Energy efficient: ≈ 10 eV/molecule or less possibleEnergy efficient: ≈ 10 eV/molecule or less possible Transient plasma provides dramatically improved energy efficiency - by 100x compared to prior approaches employing quasi-steady dischargesTransient 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 destroyed10 eV/molecule corresponds to 0.2 % of fuel energy input per 100 ppm NO destroyed Applicable to propulsion systems, unlike catalytic post- combustion treatmentsApplicable to propulsion systems, unlike catalytic post- combustion treatments

29 NO removal by corona discharge Diesel engine exhaustDiesel engine exhaust Needle/plane corona discharge (20 kV, 30 nsec pulse)Needle/plane corona discharge (20 kV, 30 nsec pulse) Lower left: before pulseLower left: before pulse Lower right: 10 ms after pulseLower right: 10 ms after pulse Upper: difference, showing single- pulse destruction of NO (≈ 40%)Upper: difference, showing single- pulse destruction of NO (≈ 40%)

30 Conclusions Corona ignition is promising for ignition delay reductionCorona ignition is promising for ignition delay reduction 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? (Radical ignition courses similar minimum ignition energies to thermal sources, but shorter ignition delays)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 electronicsEnabling technology: corona generators - require sophisticated approach to electronics

31 Potential applications PDE-relatedPDE-related Integration into PDE test facilityIntegration into PDE test facility »NPS (Brophy) »WPAFB (Schauer) »Coaxial 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 & NO x remediationPipe dream: integration of electrostatic fuel dispersion, ignition & NO x remediation OthersOthers FlameholdingFlameholding »Quasi-steady, constant pressure jet flames - USC »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

32 Future work - science-related Transient plasmas are a new area for applicationsTransient plasmas are a new area for applications Quantitative understanding of physics needed for applications, but theory almost nonexistentQuantitative understanding of physics needed for applications, but 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, radical generator, spatial distribution of energetic electrons relative to streamer head)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!)Modeling of chemical reactions between ions / electrons / neutrals (no “GRI Mech” for ionized species!)


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