Energy conversion in transient molecular plasmas:

Implications for plasma assisted combustion

Igor Adamovich

Department of Mechanical and Aerospace Engineering Ohio State University

KAUST Research Conference: New Combustion Concepts March 6-8, 2017

Faculty: Igor Adamovich and J. William Rich Research backgrounds of students and post-docs: Mechanical Engineering, Aerospace Engineering, Electrical Engineering, Chemical Physics, Physical Chemistry, Physics

OSU NETLab (since 1986)

Outline

I. Energy transfer in nonequilibrium, high-pressure molecular plasmas: dominant processes

II. Electric field: discharge energy loading and partition

III. Electron density and electron temperature: discharge energy loading and partition

IV. Dynamics of temperature rise: “rapid” heating and “slow” heating

V. Air and fuel-air plasma chemistry

VI. Coupling between N2 vibrational relaxation and radical chemistry?

VII.Summary and future outlook

I. Molecular Energy Transfer: Dominant Processes

Energy Partition in Air Plasma vs. Electric Field

Energy fraction

Reduced electric field

(4) N2 vibrational excitation: Quasi-steady-state discharges,

low E/N

(1) O2 vibrational excitation: Non-self-sustained discharges,

very low E/N (5,6) electronic excitation,

dissociation, (7) ionization: Pulsed discharges, high E/N

Yu. Raizer, Gas Discharge Physics, Springer, 1991

• Reduced electric field, E/N, controls input energy partition in the discharge

• Rates of electron impact processes: depend on E/N exponentially

Energy conversion in molecular plasmas: Key processes and parameters

• Energy coupled to electrons and ions by applied electric field

• Electric field in the plasma: controlled by electron / ion transport, surface charge accumulation

• Energy partition (vibrational and electronic excitation, dissociation, ionization): controlled by electron density and electric field (or electron temperature)

• Temperature rise in discharge afterglow: controlled by quenching of excited electronic states, vibrational relaxation

• Plasma chemical reactions, rates of radical species generation: controlled by populations of excited electronic states, e.g. N2*, O*, Ar*

• Approach: time-resolved measurements of 𝑬𝑬, ne , Te , T, N2(v), N2*, Ar*, radical species (O, H, OH, NO, HO2) in ns pulse discharges, reproducible shot-to-shot

• Objective: quantitative insight into energy conversion mechanisms dominating plasma-assisted combustion

II. Electric field in transient plasmas: insight into discharge energy loading and partition

Diagnostics: CARS-like 4-wave mixing

(with E-field acting as a “zero-frequency” probe beam)

2-D Ns Pulse Discharge in Atmospheric Air

• Ns pulse discharge between a high-voltage electrode and a thin quartz plate

• Discharge gap 0.6 - 1.0 mm, two-dimensional geometry, diffuse plasma

• Time-resolved electric field measured at multiple locations in the discharge gap

-100 -50 0 50 100 150 200 250-7.5

-5.0

-2.5

0.0

2.5

Time [ns]

-7.5

-5.0

-2.5

0.0

2.5

0

1

2

3 Voltage [kV] Current [A] Coupled energy [mJ]

Laser beam locations

Front view, 100 ns gate

Side view, 2 ns gate Top view, 2 ns gate

• Surface ionization wave plasma ~ 200 μm thick, wave speed ~ 0.03 mm/ns

• Electric field measured by picosecond four-wave mixing (calibration by electrostatic field)

• Time resolution 2 ns, spatial resolution across laser beam ~ 100 μm

• Objective: electric field mapping in ns pulse discharges in high-pressure fuel-air mixtures

“Curtain Plasma” Images, Negative Polarity Pulse

Laser beam locations

Electric Field Vector Components in a Surface Ionization Wave Discharge

• Initial field offset (at t < 0): charge accumulation on dielectric from previous pulse

• Field follows applied voltage rise, increases until “forward breakdown”

• After breakdown, field reduced due to charge accumulation on dielectric

• Field is reversed after applied voltage starts decreasing

• Away from HV electrode, field peaks later (Ey before Ex): surface ionization wave

• Measurements in a hydrogen-air diffusion flame underway

Laser beam locations -100 0 100 200 300 400

-6

-4

-2

0

2

4

6

8 Voltage

Current Absolute field Actual field

Time [ns]

-U [k

V], I

[A]

-30

-20

-10

0

10

20

30

40

Elec

tric

field

[kV/

cm]

HV electrode

Forward breakdown

Reverse breakdown

-100 -50 0 50 100 150 2000

5

10

15

20

25

30

- Ex Ey (Ex

2 + Ey2)1/2

Elec

tric

field

[kV/

cm]

Time [ns]150 μm from surface

III. Electron density and electron temperature in transient plasmas:

insight into discharge energy loading and partition

Diagnostics: Thomson scattering

• Electron density: area under Thomson scattering spectrum

• Electron temperature: spectral linewidth

• Gaussian scattering lineshape: Maxwellian EEDF

• Raman scattering rotational transitions in N2 used for absolute calibration

Filtered Thomson Scattering: ne , Te, and EEDF inference

0.0E+001.0E+042.0E+043.0E+044.0E+045.0E+046.0E+047.0E+048.0E+04

525 530 535 540Wavelength (nm)

Gaussian Fit

Rayleigh scattering blocked

528 532 5360

10000

20000

30000

40000

50000

60000

Inte

nsity

[a.u

.]

Wavelength [nm]

N2 Raman scattering

10 mm

He, 200 Torr

Thomson Scattering Spectra Ns pulse discharge in H2-He and O2-He, P=100 Torr

10% O2-He ne= 1.7·1013 cm-3, Te= 1.6 eV, T=350 K

5% H2-He, ne = 1.5∙1014 cm-3, Te = 2.0 eV

Electron Density and Electron Temperature Ns pulse discharge in O2-He

• “Double maxima” in ne, Te : two discharge pulses ≈ 400 ns apart

• Electron temperature in the afterglow Te ≈ 0.3 eV (controlled by superelastic collisions)

• Modeling predictions in good agreement with data

• Measurements in air are more challenging (strong interference from N2, O2 Raman scattering)

0 200 400 600 8000

1

2

3

4 Experimental ne

Predicted ne Experimental Te Predicted Te

Time [ns]

n e [1

014 c

m-3]

0

1

2

3

4

T e [e

V]

10% O2-He

IV. Dynamics of temperature rise in transient plasmas: “rapid” heating and “slow” heating

Diagnostics: vibrational and pure rotational ps CARS

Temperature rise in ns pulse discharge and afterglow: Air vs. nitrogen, P=100 Torr

• Compression waves formed by “rapid” heating, on sub-acoustic time scale, τacoustic ~ r / a ~ 2 μs

• What processes control other features of temperature rise (e.g. “slow” heating”)?

t= 1-10 μs (frames are 1 μs apart)

Air, P=100 Torr

10 mm

2 mm

• Strong vibrational excitation in the discharge, N2(v=0-8)

• Tv(N2) rise in early afterglow: V-V exchange, N2(v) + N2(v=0) → N2(v-1) + N2(v=1)

• Tv(N2) decay in late afterglow: V-T relaxation, N2(v) + O → N2(v-1) + O , radial diffusion

• “Rapid” heating: quenching of N2 electronic states, N2(C,B,A,a) + O2 → N2(X) + O + O

• “Slow” heating: V-T relaxation, N2(X,v) + O → N2(X,v-1) + O

• “Rapid” heating: pressure overshoot , compression wave formation

• NO formation: dominated by reactions of N2 electronic states, N2* + O → NO + N

Comparison with modeling predictions in air: vibrational kinetics and temperature rise

V. Fuel-air chemistry in transient plasmas: kinetics of plasma assisted combustion

Diagnostics: Rayleigh scattering, OH LIF, H and O TALIF, CARS

Hot central region: OH production dominated by chain branching H + O2 → OH + O ; O + H2 → OH + H

Colder peripheral region: OH accumulation in HO2 reactions

H + O2 + M → HO2 ; H + HO2 → OH + OH

2-D kinetic modeling is challenging

Plasma chemical reactions and transport: ns pulse discharge in H2 - O2 - Ar, P=40 torr

Burst of ns discharge pulses to couple sufficient energy

Good shot-to-shot reproducibility

Large-volume, preheated “0-D” plasmas: Plasma chemical reactions result in ignition

H2 – air, ϕ=0.3 T0=500 K, P=100 torr

C2H4 – air, ϕ=0.3 T0=500 K, P=100 torr

Pulse #10 Pulse #100

End View

Long burst: plasma assisted ignition, Tignition ≈ 700 K < Tauto-ignition ≈ 900 K

H2-air, ϕ=0.4

50 pulses

Short burst: OH transient rise and decay

[H], [O] atom kinetics in large-volume plasmas (H2-O2-Ar, CH4-O2-Ar, P=300 Torr, T0=500 K)

• No adjustable parameters in the model: experimental coupled power waveforms are used

• Model reproduces absolute [H], [O] quite well (up to 35% of all O2 molecules dissociate)

• Most dominant reactions are well understood

• Measurements in heavier hydrocarbon mixtures (C2H4, C3H8, C4H10 , and C5H12) are underway

VI. Is there coupling between N2 vibrational relaxation and radical chemistry?

Diagnostics: OH LIF and CARS

[OH] in fuel-air plasmas after a ns pulse discharge at 1 atm, φ=0.1

Hypothesis: Energy transfer from N2 breaks up HO2 radical, extending H and OH lifetimes:

N2(v=1) + HO2 → N2(v=0) +HO2(v2+v3) → H + O2

[OH] rise at long time delays after discharge pulse is not explained by kinetic modeling

Ns pulse discharge filament in pin-to-pin geometry

Previous results (Drexel U.) hypothesized effect of N2 vibrational relaxation on OH kinetics

Or is this purely thermal effect? (temperature was not measured)

Combined OH LIF / CARS experiment H2-air, P=100 Torr

air, Negative polarity 100 shots average

• LIF: time-resolved, absolute [OH]

• CARS: time-resolved temperature, N vibrational level populations

• Determine correlation between [OH] and Tv(N2)

3% H2- Air 100-shot OH LIF images

Results so far: inconclusive

• [OH] in H2-air appears to follow Tv(N2) (measured in air)

• Not clear if OH is due to vibrational-chemistry coupling or “conventional” chemistry

• Comparison with kinetic modeling predictions (N2 vibrational kinetics coupled with H2-air chemistry, N. Popov, Moscow State U.) will help provide insight into kinetics

Summary: What Have We Learned?

• Growing body of time-resolved, spatially-resolved data characterizing transient, high-pressure air and fuel-air plasmas

• Measurements of electric field, electron density, and electron temperature necessary for insight into discharge energy coupling and partition

• Measurements of temperature, N2(v) populations, and excited electronic states of N2* necessary for insight into temperature dynamics

• Measurements of N2* and key radicals (O, H, OH, and NO) critical for quantifying their effect on fuel-air plasma chemistry

• Comparing measurement results with kinetic modeling predictions helps understand what is going on – and provides confidence in the models

• Parameters used for conventional combustion mechanism validation (e.g. ignition delay time or laminar flame speed) are not sufficient for PAC mechanism

• “Sufficient” data set for validation of PAC chemistry mechanism: time-resolved temperature, N2 vibrational temperature, and key radical concentrations during a repetitively pulsed plasma-enhanced ignition process

AFOSR MURI “Fundamental Mechanisms, Predictive Modeling, and Novel Aerospace Applications of Plasma Assisted Combustion” AFOSR BRI “Nonequilibrium Molecular Energy Coupling and Conversion Mechanisms for Efficient Control of High-Speed Flow Fields” DOE PSAAP-2 Center “Exascale Simulation of Plasma-Coupled Combustion” (under U. Illinois at Urbana-Champaign prime) US DOE Plasma Science Center “Predictive Control of Plasma Kinetics: Multi-Phase and Bounded Systems” NSF “Fundamental Studies of Accelerated Low Temperature Combustion Kinetics by Nonequilibrium Plasmas”

Acknowledgments

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