fundamental mechanisms, predictive modeling, and novel … · 2019. 12. 12. · the ohio state...

Nonequilibrium Thermodynamics Laboratories The Ohio State University

Fundamental Mechanisms, Predictive Modeling,

and Novel Aerospace Applications

of Plasma Assisted Combustion

Overview of research results at OSU

Walter Lempert, Igor Adamovich, J. William Rich, and Jeffrey Sutton

Michael A. Chaszeyka Nonequilibrium Thermodynamics Laboratory

The Ohio State University

MURI 2nd Annual Review Meeting

November 9-10, 2011

Columbus, OH


Task 1: Low-to-Moderate (T=300-800 K) temperature, spatial and time-

dependent radical species concentration and temperature

measurements in nanosecond pulse plasmas in a variety of fuel-air

mixtures pressures (P=0.1 - 5 atm), and equivalence ratios (υ~0.1-3.0)

Goal: Generate an extensive set of experimental data on radical species

concentrations and temperature rise; elucidate kinetic mechanisms of

low-temperature plasma chemical fuel oxidation and ignition using

kinetic modeling. Bridge the gap between room-temperature data

(low-pressure gas discharges) and high-temperature data (shock tubes)

Thrust 1. Experimental studies of nonequilibrium air-fuel

plasma kinetics using advanced non-intrusive diagnostics


Test bed: High-temperature nsec pulse discharge

cell for ignition delay and species measurement

Discharge cell placed in a tube furnace (6 inch bore, up to T=12000 C)

Optical access (LIF, TALIF, CARS) on the sides, for species and temperature measurements

Fuels to test in near future: H2, CH4, C2H4, C3H8, methanol & ethanol vapor

Flow Direction

• H2-air at T0= 100-300 C, P=50-150 torr,

ϕ=0.5-1.2

• Flow velocity u=12 cm/sec; residence time

in the discharge ~0.5 sec

• Pulse repetition rate (20 kV peak) ν=10-

40 kHz

60 mm straight angle prism

provides optical access to entire discharge


10 Hz, 100 msec

Time between pulses: 25 μsec

Laser delay time after last pulse

time

Burst of Pulses

Laser pulse

• Pulser produces a rapid “Burst” of 1-1000 pulses,

typically with 25 – 100 μsec spacing (10 – 40 kHz).

• For laser diagnostics burst is repeated at 10 Hz to

match laser repetition rate.

• Fresh sample of gas with every burst.

High Voltage Nanosecond Pulsed Plasma Generation

80 100 120 140 160 180

-20

-10

0

10

20

Time, nsec

Voltage, kV

H2-air, P=40 torr

experiment

Gaussian fit

40 kHz Burst – 10 Hz Laser Timing


H2-air at T0= 200 C, P=104 torr,

ν=40 kHz, ϕ=1

Ignition moment defined as beginning of

ignition “footprint”; verified by ICCD images

Spontaneous OH Emission-Based (310 nm)

Ignition Measurements.

Air , T0=25C, P=60 torr, ν=40 kHz

-40 -20 0 20 40 60 80

-20

-15

-10

-5

0

5

10

15

Time, nsec

Voltage, kV

Experiment

Gaussian fit

Peak voltage: ~18 kV, FWHM: ~15 nsec

0 10 20 30 40 50 60 70

Time, sec

-20

-10

0

10

20

Voltage, kVGate 1

(plasma)

Gate 2

(flame)


YR 1: Ignition delay time reduction

due to plasma-generated radical chemistry

Camera gate for

“last pulse” temperature

Ignition

delay

Use minimum number of pulses in a burst needed for ignition

Measure “last pulse” temperature (by N2 1st Positive emission), T=700 ± 100 K.

Measure ignition delay after the burst, τplasma

Compare with thermal ignition delay at the same T and P, τthermal

τplasma= 0.5-2.0 msec << τthermal (no ignition at T=700 K, τthermal= 1 msec at T ~ 850 K)

Consistent with measurements in FIW plasmas in shock-preheated mixtures (MIPT)


Yr 2 Results: Plasma Uniformity and Ignition: H2/Air

(After Last Pulse in Burst)

ICCD Images after burst (Gate: 30μsec)

ICCD Single Pulse Images during burst (Gate: 2μsec)

Pulse #125Pulse #50Pulse #5

Region of Interest

Delay 1.05ms Delay 1.55ms Delay 2.55ms

Delay 0.05ms Delay 0.35ms Delay 0.55~0.95ms

Region of Interest

Emission in center lags by ~0.7ms compared to edge.Delay 4.00ms

Tinit = 500K , P = 80 Torr, 125pulses @ 20 kHz, FID Pulser (~10 nsec) , υ=0.5


Time Expansion of Images During Ignition0.55 to 0.95 msec AFTER Last Pulse in 125 Pulse Burst




Region of Interest

Ignition is Quite Uniform Spatially

Note: Each Image is an Average of 5 Shots.

Ignition is Extremely Reproducible in Time


Yr 1: Temperature Measurement by N2 2nd

Positive Emission vs Pulses in Burst

N2(C3Π→B3Π, v′=0→v″=0)

H2-air

Air

Time-Resolved Temperature in H2-air Plasma

Peak Temperature

and OH* emission

are nearly

coincident

Experimental T

is somewhat

lower than

prediction.


Yr 2: Ignition Delay Measurements: H2/Air(After Last Pulse in Burst)

Hydrogen-air

T= 200C ϕ=1 =40 kHz,

150 Pulses

120 Pulses

115 Pulses

114 Pulses

Using a different pulser (FID) and a new dielectric

(perfluoreastomer) results in higher coupled energy,

fewer number of pulses in burst to achieve ignition.

Measured ignition delay: Extremely good

reproducibility.

Ignition delay vs. burst duration consistent with kinetic

model predictions; quantitative comparison need

accurate “last pulse” temperature measurements and

time-resolved [OH] measurements.

Ignition Threshold

Only one extra pulse makes

a difference between

ignition / no ignition

Last pulse

temperature

difference

predicted by

model ≈2°C

H2-air, ϕ=1

Ignition Delay AFTER Laser Pulse in Burst

Time, sec


Yr 2: H2 /Air OH (LIF) Number Density Measurements in

Nanosecond Pulsed Discharge vs # Pulses in Burst (CPT Pulser – 25 nsec Pulse Duration)

υ = 1, P = 94 Torr, T = 200 oC.

OH: 40 kHz Burst.

υ = 1, P = 74 Torr, T = 100 oC.

Single Discharge Pulse.

- = Full Reaction Set(Popov – 22 Processes)

… = Reduced Reaction Set

H + O2 + M ↔ HO2 + M (1)

O + HO2 ↔ OH + O2 (2)

OH + H2 ↔ H + H2O (3)

H + HO2 ↔ H2O + O (4)

H + HO2 ↔ OH + OH (5)

H + HO2 ↔ H2 + O2 (6)

O + H2 ↔ H + OH (7)

H + O2 ↔ O + OH (8)

OH + HO2 ↔ H2O + O2 (9)

Modeling Input for Specific

Energy Coupling is the

same for all four cases.


Yr 2: OH LIF Temperature Diagnostic

Development – A Few Details

EXCITATION at ~283 nm: (1, 0)

COLLECTION at ~310 nm: Full (1,1) & (0,0)

OH X

OHAv’=1

v’=0

v’”=1

v’”=0

b01

A10

b10

Q10

V10

A00

b00

Q00

V01Nu1

Nu2

Nl1

Nl2

Excitation Spectra and Voigt Fits.

Q1(5)

Ф – fluorescence quantum yield

calculation (assumes steady-state

approximation)*

J

A A V A Q

V A Q10 00 10 00 00

10 10 10

Afi: spontaneous emission rate, s-1

Qfi : quenching rate coefficient, s-1

Vfi: V-V Transfer Rate (Assumed 0.58 Q)

Qfi for H2 , H2O, N2, and O2 in the flame

taken from literature – Assumed J-

independent.

A+Q in the plasma: measured directly

(includes small J dependence)!

(*Cathay, et al, Comb &Flame, 2008)


Hencken Flat-Flame Burner for Temperature Validation

and Absolute OH Number Density Calibration

C2H4

N2

Nitrogen flow

Measurement

Location – 10mm

Above Surface

(Lucht et al. 1997)

PAC B,

Flame B,

PAC

Flame

Flame F,

PAC F,

Flame ΟΗ,PAC ΟΗ,f

f

S

Snn

Q1(5)

Q2(11)

Two Line Temperature (Burner)

(Best-fit Temperature: 2260 K +/- ~125 K)Hencken Burner (1 Bar) Calibration

Burner TOH vs υ


T0=500 K, P=80 torr, ϕ=0.5

Experiment: 125 pulses (burst duration 6.25 msec)

Model prediction at P=const: 139 pulses (burst duration 6.95 msec)

Model prediction at V=const: 118 pulses (burst duration 5.90 msec)

Estimated pressure overshoot decay time: ~10 msec

Problem: model considerably over predicts peak OH number density

(Measure OH is close to equilibrium whereas model predicts significant super-equilibrium).

Yr 2: H2 /Air OH (LIF) Number Density & T

Measurements vs Time after 125 Pulse Burst @ 20 kHz (FID Pulser – 10 nsec Pulse Duration)


T0=500 K, P=80 torr, ϕ=0.5

Quenching Model Dependence of OH LIF Temperature

0

500

1000

1500

2000

0 2 4 6 8 10 12

Tem

per

atu

re, K

Time after the last pulse, msec

J-independent

J-dependent


Yr 2: OH LIF and Emission Temporal Profiles

H2 – Air (FID Pulser)

20 kHz, 125 pulses, phi=0.5 10 kHz, 125 pulses, phi=0.5

20 kHz, 125 pulses, phi=0.4

Key chemiluminescent processes at

low temperature.

H + O + M ↔ OH* + M

OH* + M ↔ OH + M

M = N2, O2, H2, H2O, OH, O, H


Kinetic modeling of OH* formation

H + O + M ↔ OH* + M

OH* + M ↔ OH + M

M = N2, O2, H2, H2O, OH, O, H

Peak OH predicted to precede peak temperature, peak OH


YR 2: Effect of O2 (1D) (SDO) on Ignition Delay

in C2H4 – O2 Mixtures (RF side discharge)

*NOTE: O2(1D) yield measurements were taken directly after their formation and also

downstream. The locations of these measurements are marked by:

Test

Cell

Pump

Gas

Out

PMT

NO

2

cyli

nder

H2

fuel

cyli

nder

Syn

thet

ic

air

mix

ture

Pulser

O2(1D) Cell

RF Discharge

Gas In

Synthetic Air Mixtures

20% O2 in Helium

50% O2 in Helium

20% O2 in Argon

-NO2 is used to titrate out possible effects due to O3 and atomic oxygen.

This is done downstream of O2(1D) formation.

NO2 + O → NO + O2

NO + O3 → NO2 + O2


The Ohio State University Nonequilibrium Thermodynamics Laboratory

O2(a1Δ) IR Emission Spectra

Without NO2 titration With NO2 titration

20% O2/Ar, P=60 Torr

*At these conditions, temperature inferred from O2 b→X spectra is 400K.


Example Results – OH Emission Traces

(a) Baseline: No O2(a1Δg) (RF

discharge turned off), no NO2

added.

Δt = 17 msec

(b) O2(a1Δg) (RF discharge turned

on) and NO2 added.

Δt = 8 msec

(c) O2(a1Δg) but no NO2

Δt = 15 msec

(d) NO2 but no O2(a1Δg).

Δt = 11 msec

(a)

(c)

(b)

(d)

υ = 0.75 (C2H4), 50% O2-Ar (“Oxidizer),

P=65 Torr, υ = 50 kHz.


The Whole Story is More Complicated

20% O2 in Argon with stoichiometric (f=1.0) C2H4 fuel.

Suggests:

i. NOx Chemistry is

complicated.

ii. Three body processes

increasingly important as

pressure increases.

iii. Above 65 Torr discharge

becomes somewhat

filamentary.

iv. Energy loading per

molecule likely drops with

increasing pressure.

O TALIF measurements

on-going.


O TALIF with Xenon Optical Calibration*(Doebele Group – U. Duisburg-Essen)

2221o o

o ND Xe2Xe Xe21 o

a Xe XeS 1N g N

S a O F TO

5p6 1S06p’[3/2]2By obtaining ratio of spectrally integrated

O atom TALIF signal to that from

xenon with identical:

i. beam focusing and spatial mode

structure.

ii. collection optics and geometry

iii. spectral filtering

iv. photomultiplier gain

Then relative signal can easily be converted

to absolute O atom number density.

109837

88631

76795

227

1590

1

2

0

Oxygen

3P

= 225.7 nm

Laser

3S° 1

= 844.6 nm

3P 2, 1, 0

O+(4S° )3/2

G-4582

2p 3P 3p 3P

3p 3P 3s 3S

Bamford, et al., 1986

dt)t(IN)T(FGg)h(

agVS)(

)(

ND

0

2

00

2

2

2

21

*(Niemi, et al, 2005,

Grinstead, et al., 2000)


O atom TALIF Measurements

Nd:YagSHG

THG

1064

Dump

Dye

Laser

BBO

Boxcar

SRS272

PMT

UV

Separator

DELAY LINE

Gas Out

Gas In

Pulser

Quartz Test

Cell

Filters

619nm Mirrors

355nm Mirrors

225nm Mirrors

840nm

Collection lens

355nm Mirror

619nm Transmit

UV focusing lens

Photodiode


Preliminary Results: Ar/O2 with/without added NO2.

P=40 Torr

~0.07% NO2 by Volume

20% O2

80% Ar


Preliminary Ar/O2/C2H4 Results

P=40 Torr

Ar/O2 : 4:1


Oscilloscope

Dye Laser +

Mixer

Pulse

Generator

Q-switched

Nd:Yag Laser

(Second

Harmonic)

Spectral

Filter + Slit

Photodiode

Delay

Generator

PMT

Flow Controller

Burn

er

O Atom TALIF Measurements in Atmospheric Pressure

Pin – to – Plane nsec Pulsed Discharge*

(*w. S. Pendleton, M. Gunderson: USC

Cam Carter: AFRL – Propulsion

Directorate)

Filament dimensions:

~0.25 mm x 0.8 cm.


Some Results: Fuel/Air – Time After Single Pulse

(No Ignition)

28kV pulse amplitude used for all data, each data point is 3000 shots averaged

1 10 100 1000

1E16

1E17

1E18

Ethylene/Air Mixtures Ethylene phi=0

Ethylene phi=0.25

Ethylene phi=0.5

Ethylene phi=2.4

O a

tom

nu

mb

er

de

nsity (

cm

-3)

Time (s)

10 100 1000

1E16

1E17

1E18 Methane phi=0

Methane phi=0.3

Methane phi=0.6

Methane phi=1.2

O a

tom

num

ber

density (

cm

-3)

Time (s)

Methane/Air Mixtures

The addition of fuel radically alters the discharge afterglow chemistry, even outside the

limits of combustion. Propane/Air was also measured, yielding O atom behavior similar to

methane/air.


Task 8: Development and validation of a predictive kinetic model of non-

equilibrium plasma fuel oxidation and ignition, using

experimental results of Thrust 1

Goal: Identify key mechanisms, reaction, and rates of plasma chemical

fuel oxidation processes for a wide range of fuels, pressures,

temperatures, and equivalence ratios. This is absolutely essential

to predictive capability of the model.

Thrust 2. Kinetic model development and validation


Low-temperature hydrogen-air

plasma chemistry kinetic model

• Air plasma model: equations for ground state species (N, N2, O, O2, O3, NO,NO2, N2O), charged species (electrons and ions), and excited species(N2(A

3Σ), N2(B3Π), N2(C

3Π), N2(a'1Σ), O2(a1Δ), O2(b

1Σ), O2(c1Σ), N(2D),

N(2P), O(1D)) produced in the plasma.

• Two-term expansion Boltzmann equation for plasma electrons

• Fuel-air plasma: model combined with H2-air chemistry model by Popov (22reactions), supplemented with H2 dissociation by electron impact and inreactions with electronically excited nitrogen

• Peak E/N chosen to match coupled pulse energy to value predicted by OSUnanosecond pulse discharge model (Phys. Plasmas, 2009)

• CxHy-air model previously validated by comparing with experiments inethylene-air plasmas (time-resolved O atom number density, temperature)

• H2-air validation: model predictions compared to the experimental data ofThrust 1 (ignition delay time, time-resolved OH number density, andtemperature in H2-air plasmas)

• Reduced plasma chemistry kinetic mechanism is identified; effect of radicalson ignition delay time is demonstrated


Reduced mechanism: dominant radical generation

processes and chemical reactions in H2-air plasma

Key radical generation processes

N2 + e- → N2(A3Σ, B3Π, C3Π, a'1Σ) + e-

N2(C3Π, B3Π, a'1Σ) + M → N2(A

3Σ) + M

N2(A3Σ) + O2 → N2 + O + O

N2(A3Σ) + H2 → N2 + H + H

O2 + e- → O(3P) + O(3P,1D) + e-

H2 + e- → H + H + e-

O(1D) + H2 → H + OH

H2-O2 reduced reaction mechanism

H + O2 + M = HO2 + M

O + HO2 = OH + O2

OH + H2 = H + H2O

O + H2 = H + OH

H + HO2 = H2O + O

H + HO2 = OH + OH

H + HO2 = H2 + O2

H + O2 = O + OH

OH + HO2 = H2O + O2

Model predicts significant significant additional energy release from fuel

species reacting with radicals produced by the plasma (O, H, and OH)


Model predictions for full and reduced reaction sets

(H2-air f=1, P=40 Torr – CFT Pulser – NO Oven)

0 5 10 15 20 25 30

300

600

900

1200

0 200 400 600 800 1000 1200

Time, msec

Number of pulses

T, K

H2-air, P=40 torr

f=1

model, full set

model, reduced set

Temperature predicted by

full and reduced kinetic models

Full mechanism

0 5 10 15 20 25 30

1.0E-6

1.0E-5

1.0E-4

1.0E-3

1.0E-2

1.0E-1

1.0E+0

Time, sec

Species mole fractions

H2

O

H2O

OH

H

HO2

0 5 10 15 20 25 30

1.0E-6

1.0E-5

1.0E-4

1.0E-3

1.0E-2

1.0E-1

1.0E+0

Time, sec

Species mole fractions

H2

O

H2O

OH

H

HO2

Reduced mechanism

Data Unobtainable Between 500

and 600 Pulses (Beam Steering?)


Effect of O and H generation by the pulsed

discharge on ignition delay

• Reduced reaction sets with and

without plasma chemical O and H

atom generation processes

• Ignition is NOT predicted in absence

of generation of O and H atoms by

nanosecond discharge, for the same

discharge input power

0 5 10 15 20 25 30 35

300

600

900

1200

0 200 400 600 800 1000 1200

Time, msec

Number of pulses

T, K

f=1

full set

no O and H generationby plasma

(H2-air υ = 1, P = 40 Torr)


Thrust 3. Experimental and modeling studies

of fundamental nonequilibrium discharge processes

Task 10: Characterization and modeling of nsec pulse discharges

Goal: Prediction of E/N and electron density in the plasma, individual

pulse energy coupled to the plasma, and their scaling with

pressure, temperature, pulse waveform, and mixture

composition


CARS Measurements of Vibrational Energy Loading

and Relaxation in Pulser-Sustainer Discharge of Mach 5

Wind Tunnel.

Simplified Schematic Illustrating Discharge, CARS Measurement

Location, and Relaxing Gas Injection Point.

Pulser Electrodes: 4 x 4 x 0.5 (gap) cm (dielectric barrier).

Sustainer Electrodes: 4 x 1 x 4 (gap) cm (copper).

Pulser Repetition Rate: 100 kHz (~5 nsec duration, 20 kV).

DC Sustainer: ~1-4.5 kV (~1 Amps maximum)

Flow velocity in plenum ~40 m/sec (~.4 msec residence time in discharge).


Nitrogen, P=650 torr, ν=100 kHz, no DC electrodes

Nitrogen, P=350 torr,

ν=100 kHz, no DC electrodes

0.3 seconds after start (pulse # 30,000)

Pulse energy 5.2 mJ/pulse

Average discharge power 520 W (100 kHz)

Nitrogen, P=300 torr, ν=100 kHz, no DC electrodes

Flow into the page

Pulsed electrodes

Sustainer DC electrodes

(removable)

Alumina ceramic plates

Repetitive nsec pulse discharge

sustained in a high-pressure N2 flow


Schematic of Psec CARS diagnostic*:

for N2 Rotational and Vibrational Temperature

Ekspla Nd:YAG laser

- 10 Hz, ~150 psec pulses

- 125 mJ per pulse max @ 532 nm

Modeless Psec Dye Laser

- Broadband ~602-608 nm FWHM

-~ 7% conversion efficiency

Spectral Resolution ~0.5 cm-1

Nd:YAGDelay Path

Wind Tunnel

Flow

¾ m

Spectrometer Camera

R 473nm / T 532-

607nm Dichroic

Mirrors

Broadband

Dye Laser

250mm

Lenses

476nm Band pass Filter

100mm Lens

Beam Dump

532 nm “Pump / Probe”

607 nm “Stokes”

R 532nm

/ T

607nm

Dichroic

Mirror

Relay Lens

Magnification

System

Unstable Resonator Spatially

Enhanced Detection (USED) CARS

Spatial Resolution ~1-2 mm.

(* Patterned after S. Roy, et al.)


Portable, broadband psec CARS diagnostic system

Spectrometer

Dye Laser

YAG Power

Supply

Dye Laser in operation

(view from opposite

angle)

Computer


Single Shot Precision ~ 20 K (2σ) at T = 300 K,

P = 370 Torr

Trot = 316.7 K 300 K single shot spectrum,

P = 300 Torr N2.


Psec CARS spectra: N2(X,v) populationsNsec pulse discharge vs. pulser / DC sustainer discharge

Nitrogen, P=300 Torr

TV ~760 K, Trot =~380 K

Discharge Average Power = 520 W

Significant vibrational disequilibrium in a nsec pulse discharge without DC sustainer

as measured in “afterglow” ~2 msec downstream from discharge section.

20 shot average

20 shot average

(Averaging helps with dye laser spectral fluctuations).


How Can the Pulsed Discharge Alone Excite

N2 Vibration?

20 kV/ 0.5 cm / 300 Torr

~ 300 Td

Tvib = 760 K Corresponds to v=1 fraction ~0.012,

corresponding to 3.5 meV/molecule.

ΔTRot/Trans = 80 K corresponding to 23.9 meV/molecule

If all energy goes to rot, trans, or vib then ~13% of total

discharge power loads vibration!

Measured energy loading is 28.4 meV/molecule as compared

to 27.4 meV into rot/trans/vib from CARS spectra.

(1 Td = 10-17 V-cm2)

4: N2 vibration

5: N2 electronic states


Air, P=100 torr, Upeak=25 kV, 1 cm gap

Top: 50 FWHM pulse, Bottom: 5 FWHM pulse

(~15-25% of Energy Coupled at LOW (<50 Td) – Pulse Duration Dependent

Prediction of Energy Coupling by Nsec Pulsed

Discharge Model


Some Pulser – Sustainer Spectra

Sustainer VPS = 4.5 kV (E/n ~ 10 Td), 300 Torr pure N2


Some Preliminary Results from S. Pendleton

Filamentary Discharge: N2 CARS

N2 (v=1) is present in discharge afterglow from ~10-200 us with Tv > 1500 K.

Rotational temperature analysis still underway on these measurements.

E/n ~ 200 Td at tip (~2x increase due to field concentration near tip)

473.0 473.5 474.0 474.5 475.0 475.5 476.0

105

106

Inte

nsity (

a.u

.)

Wavelength (nm)

v=0

v=1

N2 CARS Signal Intensity

Note: Streamer diameter ~0.25

mm whereas USED CARS

obtained from ~1-2 mm x 50

micron cylindrical volume

element.

Hence most of v=0 signal

originates outside of filament!


Sub-Nsec Electric Field Measurement by

CARS-like Four Wave Mixing

“E-Field CARS” is a 4 wave mixing process.

The typical CARS probe beam is replaced by an external electric field, which is at essentially zero frequency. This creates an IR “CARS” signal at the vibrational-rotational transition Q(1) (v=0, J=1 → v=1, J=1) frequency.

The physical origin of this signal is the dipole induced by the external field.

Energy Level Diagram

for E-Field CARS

ωS

ωp

IR CARS

ωAS

ωp

For “Psec” E-Field CARS the temporal resolution can be limited by

the decay time of the coherence (<~200 psec @ 100 Torr for H2).

(Roy, Appl. Phys. Lett 97, 2010)

E

IR IR Pump Stokes External

CARS CARS Pump Stokes Pump

ExternalIR IR

CARS CARS Pump

IR PumpCARS

External

IR CARS

E E E E

E E E E

EE*

E E

I IE

I


Generation of H2 Stokes Beam Using High

Pressure Stimulated Raman Shifting Cell.

Raman shifted Stokes beam required for CARS.

Stokes beam created in a high pressure Raman

cell.

532nm Pump beam used in Stimulated Raman

Scattering thus creating a Stokes beam with

λ=683nm.

Data taken for different input energies with

different pressures.0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 20 25 30

Ou

tpu

t E

ner

gy

(m

J)

Input Pump Energy (mJ)

8 bar H2 in Raman Cell

A.S.

Probe

Stokes

0

1

2

3

4

5

6

7

0 5 10 15 20 25

Ou

tpu

t E

ner

gy

(m

J)

Input Pump Energy (mJ)

11 Bar H2 in Raman Cell

Anti-Stokes

Stokes

Pump

0

1

2

3

4

5

6

7

8

9

0 5 10 15 20 25 30

Ou

tpu

t E

ner

gy

(m

J)

Input Energy (mJ)

5 bar H2 in Raman Cell

A.S.

Probe

Stokes


Some Recent Results - 760 Torr H2 Scope Traces(DC sub-Breakdown Field)

Signal vs Field Strength (100 Torr)

Electric Field (V/cm)

0 500 1000 1500 2000 2500 3000 3500 4000

Sq

rt (

IR*P

um

p/C

AR

S)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

(*Roy, Appl. Phys. Lett 97, 2010)

τCoh vs P*

-6 -4 -2 0 2 4 6

x 10-7

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

-6 -4 -2 0 2 4 6

x 10-7

6

6.5

7

7.5

8

8.5

9x 10

-3

-1 -0.5 0 0.5 1 1.5 2

x 10-7

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

-1 -0.5 0 0.5 1 1.5 2

x 10-7

0

0.05

0.1

0.15

0.2

0.25

-1 -0.5 0 0.5 1 1.5 2

x 10-7

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-1 -0.5 0 0.5 1 1.5 2

x 10-7

0

0.2

0.4

0.6

0.8

1

1.2

1.4

-1 -0.5 0 0.5 1 1.5 2

x 10-7

0

0.2

0.4

0.6

0.8

1

1.2

1.4

-6 -4 -2 0 2 4 6

x 10-7

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

360 V/cm

4 Averages

-1 -0.5 0 0.5 1 1.5 2

x 10-7

0

0.05

0.1

0.15

0.2

0.25

1780 V/cm

4 Averages3570 V/cm

4 Averages

~ 10 mJ @532 – 8 Bar Raman Cell


Raw IR nsec N2 E-Field Data*

(Electric field measured between two plane electrodes

in atmospheric air - 625 Torr in Albuquerque, NM)

75 mJ Pump / 35 mJ Stokes

(* w. S. Kearney and E. Barnat, SNL – Albuquerque)


IR/CARS Signal Ratio vs External Field

(Nsec N2)

SQRT (IR/CARS) vs External DC Field

External Field (kV/cm)

0 2 4 6 8 10

Sq

rt (

IR/C

AR

S)

(au

)

0.0

0.1

0.2

0.3

0.4

0.5

2

CARS CARS Pump Stokes Pump

IR IR Pump Stokes External

ExternalIR IR

CARS CARS Pump

ExternalIR IR

CARS CARS Pump

E E E E

E E E E

EE*

E E

EI*

I E


Thrust 4. Studies of diffusion and transport of active

species in representative 2-D reacting flow geometries

Task 12: Ignition and flameholding in nonequilibrium plasma cavity flows

at low static temperatures

Goal: Determine viable approaches to flameholding in high-speed flows

using low-temperature plasmas. We simply cannot process the

entire flow with the plasma.


Cavity injection flow ignition and flameholding:

experimental setup

• Static pressure P=150-200 torr (0.2-0.26 atm), flow velocity u=30-200 m/sec

• Plasma produced in the cavity by repetitively pulsed nsec discharge, U=25 kV, ν=40 kHz

• Fuel (ethylene or hydrogen) injected (a) into flow upstream of the cavity; (b) into the

cavity downstream of high voltage electrode

• Ignition detection: OH emission; plasma and flame development: ICCD emission

imaging and high frame rate NO PLIF imaging; temperature: N2 emission spectroscopy;

burned fuel fraction: FTIR absorption spectra (in premixed flows)

Cavity L/D=3

Air

Flow

Fuel injection

Air Flow


ICCD images of plasma and flame

in H2- air flows (injection into the cavity)

P=150 torr, u=60 m/sec, ν=40 kHz, υglobal =

Pulse# 180

#240 to #241

Pulse# 140

Pulse# 200

Pulse# 240

#200 to #201

#160 to #161

#140 to #141

Optical access windows

To

vacuum

system

High voltage

electrode block

Ceramic platesPressure tap

Fuel injection ports

Main flow


280 Hz frame rate schlieren images of injection flow5% precooled He injected into cavity, main flow is air

Transient injection (several msec after injection valve opens)

Injection jet penetration into cavity ~ 1 cm

Near steady state injection 220 ms after injection valve opens, global ϕ=0.05

He injection

Camera field of view

Main Flow Direction

u=150 m/sec

P=150 torr


10 kHz frame rate NO PLIF images of injection flow5% NO-He mixture injected into cavity, main flow is air

Transient injection (several msec after injection valve opens)

Injection flow directed upstream, toward HV electrode

Steady state, global ϕ=0.042 ([NO]~3∙1015 cm3)

Injection flow mixed with air flow in the entire cavity

NO-He injection

Camera field of view

Main Flow Direction

u=150 m/sec

P=200 torr


Air and H2-air plasma temperatures

Ignition and flameholding vs. flow velocity (P=150 torr)

• Air plasma temperature does not exceed T=1500 C

• H2-air plasma/flame temperature after ignition T=600-8500 C

• Ignition and stable flame detected in the entire velocity range tested,

up to u=140 m/sec, global equivalence ratio range ϕ=0.03-0.13

• At u≥90 m/sec, flameholding achieved at global ϕ=0.05-0.1

Open symbols: ignition

Closed symbols: flameholding

0

100

200

300

400

500

600

700

800

900

40 60 80 100 120 140 160

Velocity (m/s)

Air

Hydrogen-Air

Temperature (°C)

60

70

80

90

100

110

120

130

140

150

160

0 0.03 0.06 0.09 0.12 0.15

Global equivalence ratio

Velocity (m/sec)

No ignition Ignition


Air and H2-air plasma temperatures

Ignition and flameholding vs. flow velocity (P=200 torr)

• Air plasma temperature does not exceed T=1500 C

• H2-air plasma/flame temperature after ignition T=600-9000 C

• Ignition and stable flame detected in the entire velocity range tested,

up to u=190 m/sec, global equivalence ratio range ϕ=0.01-0.09

• At u≥80 m/sec, flameholding achieved at global ϕ=0.02-0.04

Open symbols: ignition

Closed symbols: flameholding

60

80

100

120

140

160

180

200

220

0 0.02 0.04 0.06 0.08 0.1

Global equivalance ratio

Velocity (m/sec)

No ignition Ignition

0

100

200

300

400

500

600

700

800

900

60 80 100 120 140 160 180 200

Velocity (m/sec)

Air

Hydrogen-Air

Temperature (°C)


Ongoing work: scaling down cavity size,

using flush mounted electrodes

• Reducing cavity length and depth while keeping L/D same, L/D=3

• Using non-metal test section (nylon plastic lined with alumina ceramic plates)

• Extending nsec pulse discharge cavity ignition to Mach 2 flows

Optical access windows

To

vacuum

system

High voltage

electrode block

Ceramic platesPressure tap

Fuel injection ports

Main flow


Questions??

fundamental mechanisms, predictive modeling, and novel … · 2019. 12. 12. · the ohio state...

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