DISSOCIATIVE RECOMBINATION KINETICS RELATED TO PLASMA ENHANCED COMBUSTION July 15 2004

1
DISSOCIATIVE RECOMBINATION KINETICS RELATED TO
PLASMA ENHANCED COMBUSTION July 15 2004

• PIs
• Albert Viggiano
• Space Vehicles Directorate
• Air Force Research Laboratory
• Hanscom AFB
• Mats Larsson
• University of Stockholm

2
Outline

• Motivation
• Plasma enhanced combustion
• Technique
• Results
• C2Hn n 1-5
• C3H7 , C4H9 , C3H4
• Acknowledgements

3
Combustion for Hypersonic Vehicles (gt Mach 4-5)

• AF has an interest in high-speed airbreathing
  propulsion
• There are claims that plasmas speed combustion
• It is known that trace additives affect
  combustion chemistry
• ? It is reasonable to expect plasmas will
• May be an enabling technology for hypersonic
• airbreathing propulsion using hydrocarbon fuels
• Plasma ignition and flameholding
• MHD mixing

4
Technical Approach
Lab Results (VS, CRYRING)
Models and Codes (VS/PR)
System Impacts (PR)
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Create Reaction Database for Plasma Enhanced
Combustion

• In house laboratory study ion-molecule
  reactions
• Starting materials mainly studied
• Fuels C1-C12 Straight-Chain Alkanes
• C4-C8 Branched-Chain Alkanes
• C6-C10 Aromatics (Alkylbenzenes, Naphthalene
• Positive Ions - NO, O2, N2, O, N, H3O,
  H3O(H2O)
• Negative Ions - O-, O2-, CO3-, NO3-,
• Byproducts just starting
• Secondary Ions - CnHm CnHmO etc.
• Fuel components CO, O, O3, O2, CO2 etc.
• High pressure chemistry thermal dissociation
  and stabilizaiton (Troe)
• University of Stockholm
• Termination dissociation recombination
• e- AB ? A B
• Important question is how many radicals are
  produced

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Problem in Scramjet Engines
Ignition Delay, tig L / v
t
Ignition Delay Effect,
t
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Plasma Enhanced Combustion Model Results Assumes
CnHm e ? CnHm-1 H

• CH3, H, O, and OH are equally effective at
  reducing computed ignition delay time
• H3O C are more effective
• NO/e- is the most effective for isooctane and
  the mixture whereas H3O/e- is more effective for
  ethylbenzene
• Key effects
• Production of atomic and radical species
• -in large part by dissociative recombination
• -standard assumption is H atom ejected
• Heat Release
• Fuel breakdown
• Models due to Skip Williams

Computed Ignition Delay Time (ms)
Mixture
8
C
7
CH
3
H
6
O
OH
5

-
NO
/e

-
H
0
/e
4
3
Ionization levels gt 10-6 are effective in
reducing the ignition delay time.
-6
-5
-4
-3
10
10
10
10
Species Mole Fraction
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In-House Laboratory

• Initially measured air plasma ions with fuel
  components
• Starting to measure oxidation reactions (O3, O2,
  O, O2 (1D))
• Need to measure HC - HC,
• e.g. hydride transfer can lead to catalytic
  cycles
• State-of-the-art, unique instruments
• Selected Ion Flow Drift Tube
• High Temperature Flowing Afterglow
• Turbulent Ion Flow Tube
• Flowing Afterglow Langmuir Probe

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CRYRING SchematicUniversity of Stockholm, Manne
Siegbahn Laboratory
Swedish National Facility
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(No Transcript)
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Raw Data for Rate Measurements
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C2Hn Cross Section vs. Energy
Noise is from low signals and background
subtractions Many orders of magnitude No
resonances
All have similar Tn dependence C2H lt C2H3
C2H4 lt C3H7
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Kinetics

• k(300) (300/T)n Lehfaoui et. al.
• n FALP ratio
• C2H 2.70E-07 0.76
• C2H2
• C2H3 5.00E-07 0.84
• C2H4 5.60E-07 0.76
• C2H5 2.80E-07 0.81 6.0E-07 0.47
• C3H7 1.90E-06 0.68 8.3E-06 2.2
• C3D7 5.80E-07 0.73 8.3E-07 0.7
• C4D9 5.80E-07 0.59 8.3E-07 0.7

• Small differences between ions, T dependences
  within error
• 30 systematic difference between CRYRING and
  FALP
• C3H7 has unknown error

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C2H3 Products Raw DataSmall Detector, Grid In
Background Signal
Background
Signal
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C2H3 Products and ProbabilitiesTransmission
Probability T

• C2H3 e Mass(Probability)
• Note mass 1 is not used in determining
  branching ratios (detector noise)
• ?C2H2 H 6.74 eV 27 26 1
• T2 T(T-1) T(T-1)
• ? C2H H2 5.59 eV 27 25 2
• T2 T(T-1) T(T-1)
• ? C2H 2H 1.07 eV 27 26 25 2 1
• T3 2T2(1-T) T(1-T)2 T2(1-T) 2T(1-T)2
• ? C2 H H2 0.4 eV 27 26 25 24 2 1
• T3 T2(1-T) T2(1-T) T(1-T)2 T(1-T)2
  T(1-T)2
• ? CH3 C 2.39 eV 27 15 12
• T2 T(T-1) T(T-1)
• ? CH2 CH 1.17 eV 27 14 13
• T2 T(T-1) T(T-1)

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C2H3 Product Matrix
T transmission probability N2C3H signal at
mass (energy) 27 amu Na branching at channel
a Solve matrix below to obtain branching ratios
 
Mass/Energy Probability
Matrix
BR
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C2H3 Branching Ratios

• C2H3 e ? C2H2 H 6.74 eV 29
• ? C2H H2 5.59 eV 6
• ? C2H H H 1.07 eV 59
• ? C2 H H2 0.4 eV 3
• ? CH3 C 2.39 eV 0.6
• ? CH2 CH 1.17 eV 3

It is the large exothermicites that lead to
neutrals missing the detector since in that case
large perpendicular velocities are possible
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Comparison to Models
 
Models
lt radicals producedgt 2.28
2.24
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C2Hn Channel Comparison
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C3H(D)7 Branching Ratios
H D C3H6 H 0.42 0.13 difference between H
and D is C3H5 H2 0.11 0.12 part
real part due to H loss C3H5 H H 0.22 C3H4
H2 H 0.09 0.09 C2H4 CH3 0.04 0.03 C2H4
CH2 H C2H3 CH4 0.19 0.02 C2H3 CH3
H 0.15 C2H2 CH4 H 0.11 0.03 C2H2 CH3
H2 0.21 Total C-C 0.35 (0.33 Astrid) 0.44 real
difference C3H3 H2 H2 lt0.05 0 C2H6
CH 0 C2H5 CH2 0





of radicals 1.68 (H) 1.74 (D) assumes 2H
vs. H2 etc.
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Other Results
ASTRID
C4H9 e ? C3 C 0.38 0.41
C4
0.62 0.57 C2 0.015
C3H4 e ? C2 C 0.08 0.10
C3 0.87 0.90
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Summary

• Database for DR of organic ions is growing
• Entire C2Hn series done, larger systems started
• Rates have moderate agreement between FALP and
  CRYRING
• Good agreement between ASTRID and CRYRING
  branching
• CRYRING has H(D) resolution for larger systems
• D effect in C3H7 branching
• First four body channel for covalently bonded
  species
• More radicals produced than originally believed
• Larger effect in plasma enhanced combustion

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Acknowledgement

• Sue Arnold, Skip Williams, Tony Midey, Tom
  Miller AFRL
• M. Larsson - University of Stockholm PI for this
  work
• University of Stockholm Students and Postdocs
• S. Kalhori R. Thomas V. Zhaunerchyk
• S. Rosen A. Derkatch W. D. Geppert
• A. Ehlerding F. Hellberg F. Österdahl
• M. af. Ugglas CRYRING MS
• J. Semaniak Swietokrzyska Academy, Kielce, Poland
• Entire Staff at CRYRING
• Funding AFOSR Molecular Dynamics (Berman)
  International Research Initiative (EOARD)

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Future Work

• Fast moving vehicles create plasmas mainly
  alkali ion emission
• Plasmas interfere with radio frequency
  communications
• Alkali ions are lost by three-body recombination
• Na e M(e) ? Na (M)e
• Little is known about this process
• Alternative mechanism is
• Na H2O M ?? Na(H2O) M
• Na(H2O) e ? neutral products
• Thermodynamics are known so study DR step
• will probably be studied in the next year or
  so

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Rates of Reactivity

• Neutral-neutral reactions often immeasurably slow
• Neutral-radical reactions 10-11 - 10-16 cm3 s-1
• RadicalRadical reactions 10-10 - 10-13 cm3 s-1
• Ion- molecule reactions gt 10-9 cm3 s-1
• Ion-electron reactions gt 10-7 cm3 s-1
• ? small concentrations
• of plasma can have a large
• influence on combustion

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Interstellar Chemistry

• Most molecules in interstellar clouds are formed
  by ion-molecule chemistry followed by DR (e A
  ? products)
• Until recently, little was know about products of
  DR
• Data base is growing due to storage rings
• Early models included DR as only two body
  channels
• - Before measurements
• H3O e ? H2O H
• - After measurements
• H3O e ? H2O H (18)
• ? OH H H (67)
• ? OH H2 (11)
• ? O H2 H (4)

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Scramjet Test Facility WPAFB
Plasma Technology Breakthrough Enabling
Hypersonic Airbreathing Propulsion
WPAFB Scramjet Test Facility
Polytechnic University Plasma Ignitor
Our Role (1) Determine Mechanisms for
Plasma-Enhanced Combustion Ignition (2) Guide
Combustor Development
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C6D6 Pettersson Andersson (U. of Gottenberg)
Larsson and van der Zande
Indicates ring breakup
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C2H Branching Ratios

• C2H e ? C2 H 6.53 eV, 45
• ? CH C 3.89 eV, 38
• ? C C H 0.35 17
• Large difference in exothermicity but similar
  branching ratio for the 2 body channels
• Average number of radicals is 2.17

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Approach to UnderstandingPlasma Enhances
Combustion

• Our part
• Measure ion and e- chemistry
• Simple models
• At WPAFB
• Better models
• Test stands

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Raw Data Small Detector, No Grid
Peak 850
Not on same scale
Background is from collisions with rest gas
(Plt10-11 Torr)
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C2H4 Small Detector Grid In
Absent without grid
Peak with no grid
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Raw Data Large Detector, No Grid
Not on same scale
Sit on this peak
No side peak
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C2H4 Large Detector Grid In
Unresolved, shifted to lower mass/energy
Partially resolved
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Scramjet Combustor Technology Development
AFRL/VS/PR Polytechnic Univ - Plasma Augmented
Scramjet Combustor
Spencer Kuo
Polytechnic Univ Plasma Ignitor
AFRL/PR Scramjet Test Facility
Mach 2 Test

• Developed a plasma combustion model
• Performed computations to guide ignitor and
  combustor design
• Designed to ignite JP-7

• Goals
• Eliminate air throttle for ignition
• Remove cavity
• Enable aeroramp

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Kinetics Run
Wait 4 s to cool by hn and e-