Download Electric Field Effect on Flame

Impact of Electric Fields on
Combustion Related
Phenomena
Aviation Fire Dynamics
April 3, 2013
Arda Cakmakci
[1]
Contents
• Motivation for studying E-fields on
flames
• Flame composition
• E-field background
• Theory of E-field effects on flames
• E-field generation
• Observed effects on flames
• Soot reduction study
• Conclusion
Motivation for Electric Fields
• No adverse effects on materials that are
already on fire
• Virtually endless supply compared to liquid
suppressants (assuming a huge power supply
present)
• Electric fields interrupt the chemical chain
reaction needed for combustion
• Applications for flame stability have been
shown
Flame Composition
• Flames are plasma containing charged ions
(atoms that have lost an electron)
• Flames have mostly positive ions and high
concentration of free electrons
• In all hydrocarbon flames the positive ions
CH3+. H3O+ CHO+ and C3H3+ are present. [2]
• The rate of ion generation depends on flame
temperature and mixture composition but is
independent of pressure
Chain Reactions in Flames
Reactant: RE
Molecule: M
Radical: R
Intermediate Product: I
Reaction Product: P
Not necessary: ( )
Excited int. energy: *
RE + (M)
R+M
R+M
R+M




R
I + (P)
αR + M*
P+M
Chain initiation
Chain propagation
Chain branching
Radical destruction, chain termination
• The chain initiation reaction is usually highly endothermic (heat-absorbing).
This is why a high activation energy is needed to start the combustion process.
• Chain propagation reactions are slightly exothermic or endothermic, but there is not
much energy difference between the products and reactants in this reaction.
• The chain branching reaction is slightly endothermic with low activation energy.
• The energy to drive the overall reaction comes from the chain termination reactions
which are highly exothermic.
Ion Production in Flames
•
There is no agreement as to the abundance and identity of negative ions formed in a flame,
or as to the process by which they would be formed.
•
The primary mechanism for ion formation in a flame is chemi-ionization.
•
Chemi-ionization is the formation of an ion through the reaction of a gas phase atom or
molecule with an atom or molecule in an excited state.
1. Ionization by collision
A+B  A+ + B + e2. Electron transfer
A+B  A+ + B3. Ionization by transfer of excitation energy
A+B*  A+ + B + e4. Chemi-ionization
A+B  C+ + D + eA+B  C+ + D-
Positive ion concentrations vs. distance
• CHO+ and C3H3+ are the
primary chemi-ions, thus
being parent ions from which
all the other ions are formed
either by charge transfer
reaction or ionization by
electron collision.
• Ion production and
concentration greatest in
luminous zone near flame tip
[2]
Ion current (A), range 10-12 to 10-8
Distance from burner exit (cm), range 0 to 12
Flame Composition cont.
• A 100 K increase in flame temperature
could increase ion generation rate 2 to
4 times [2]
• In lean mixture 3x1013 ions/sec-cm2
• In near stoichiometric mixture
6x1014 ions/sec-cm2
Understanding the Electric
Field
• An electric field is the electric force per unit
charge
• The region in which forces are experienced
due to the presence of electric charges is
called the electric field
• Electric field is produced by applying a
voltage across electrodes (cathodes and
anodes)
• Electric field can produce an ion wind, or
also known as a corona wind
Basic E-Field Visualization
•
•
•
•
[3]
Equipotential lines (red dotted lines shown below) are like contour lines on a map
which trace lines of equal altitude. In this case the "altitude" is electric potential or
voltage.
Equipotential lines are always perpendicular to the electric field (EF shown as the
black lines with direction arrows).
In three dimensions, the lines form equipotential surfaces.
Movement along an equipotential surface requires no work because such
movement is always perpendicular to the electric field.
Electric Field Effect on Flame
•
•
•
•
•
Increase flammability limits
Reduce pollutants
Effect temperature (by entraining air)
Modify burning velocity
Increase or decrease the heating to surfaces
surrounding flame
• Note: All mentioned effects dependent on
electrode geometries
Electric Field Effect on Flame
• Forces acting on a “gas” particle can be described
by coulomb's law, and translated into terms of
density of negative/positive particles is[2]:
• F ≈ Eq±n±
• In terms of voltage, the electric field can be written
as:
• E ≈ ΔV / d
• In terms voltage dependence, the force on a gas is:
• F = ΔV (en±/d)
Field strength vs. electrode
distance
•
Breakdown current density occurs
when the electric field strength
reaches the breakdown strength in the
gas.[2]
•
The saturation current density occurs if
the electrodes are far enough from the
flame that ionization breakdown does
not occur first.[2]
•
The saturation current is reached when
the field is stripping the ions out of the
flame as fast as they are being
produced.[2]
•
The factors that affect this are flame
size, composition, electrode geometry,
and field strength.
[2]
Electric Field Effect on Flame
cont.
• Corona wind is the effect of ions imparting a momentum on
neutral gas particles, which in turn create a movement of air
with the potential of taking out a flame
• Effects on combustion chemistry, corona winds strip charged
radicals affecting sustainable combustion(accelerating
electrons to energies capable of exciting, dissociating, or
ionizing neutral species upon impact), and have been shown
to reduce flame temperature
• Ionic wind can occur within the flame, charged particles move
towards electrodes
• Arcing can also occur:
http://www.youtube.com/watch?v=a7_8Gc_Llr8
Suggested Effects on Chemistry 1[2]
•
•
•
The chain branching reaction controls the overall rate of reaction,
and therefore the flame speed & flame temperature. The rate of the
reaction is influenced by the vibrational energy level of O2, indicated
by v.
The free electrons in a flame pick up translational energy which
through the interaction is then passed on to the O2 population as an
increase in internal vibrational energy v. This then increases the rate
of the chain branching reaction and in turn flame temp. & speed
The electron remains after colliding with the O2, since there is only
transfer of mechanical vibrational energy
Chain branching reaction:
Vibrational energy exchange:
Suggested Effects on Chemistry 2[4]
• To verify previous study, N2 (N2 is a strong absorber of e-) was
removed/replaced with an Argon mixture to see expected
results of greater energy transfer efficiency of e- to O2 .
• The opposite effect was observed, as with greater energy
transfer implying greater reaction rate and increased flame
temperature this was not found. There was essentially no
change
• Study concluded there must be other modes of energy
transfer
Collisional energy absorption:
Suggested Effects on Chemistry 3[5]
•
•
•
There is vibrational energy transfer from N2 to O2 in addition to the
direct e- to O2 interaction
If the free electrons do not collide with O2 they will surely collide with
N2, either way the chain branching reaction rate will not significantly
change by removing N2
While this suggests that the absence of N2 will not necessarily
increase reaction rates, it does not imply it would decrease chain
branching rates
Collisional energy absorption:
Collisional energy transfer:
Electrode Configurations
• Plate: flame placed between two plates (or gauze)
acting as anode/cathode
• Cylindrical: usually in the form of a meshed grid
surface between an upper charged ring and a base
metal. The mesh grid acting as an electrode itself.
• Point: either cathode or anode, depending on polarity
of corona wind, is situated in front of a counter
electrode with a spacing which accelerated charged
particles pass through
• Note: For all configurations assume DC produced
fields, unless I say otherwise
Cylinder Electrode
[2]
[6]
Cylinder Electrode
Setup
EF Strength in Relation to Electrodes
[7]
Plate Electrode
[2]
[8]
Plate/Gauze Electrode
Example of gauze
electrode; fine
mesh grid of
conducting metal
[9]
Plate Electrode
Setup
EF Strength in Relation to Electrodes
[7]
Point Electrode
[11]
Point Electrode
Setup
EF Strength in Relation to Electrodes
[7]
Cool/Interesting Video
• https://www.youtube.com/watch?v=zk
AJQ0Ggh_Y
Electrode Comparisons
• Through one study, the plate electrodes
required greater applied voltage of around
15 kV to extinguish a laminar diffusion
flame. The cylinder setup required between
only 5 and 8 kV[2]
• Majority of studies have been on the
cylindrical mesh (gauze electrode) setup,
which also showed decrease in flame lift-off
height
• No comparative studies have been done for
point electrode configurations
Flame Extinction
[2]
Experimental Observations
• The luminous zone of flame is shown to be
attracted to the negative electrode, being
the CHO+, H3O+, C2H3O+, and CH5O+ ions
(fuel: methane)[2]
• The blue section of flame shown to be
attracted to positive electrode, being the
O2−, OH−, O−, CHO2−, CHO3−, and CO3−
ions (fuel: methane)[2]
• The accumulation of charge is once again
relatable to the electric force density, F = ρE
Flame Structure Effects
Images of stoichiometric CH4/air
flame:
a) without EF
b) with EF of positive polarity
c) with EF of negative polarity.
Distance between electrodes: 8
mm.
[10]
In the case of positive polarity, the EF forces
positively charged gas to move downward
causing deceleration of the gas flow and, as a
consequence of the integral flow continuity,
the observed flow divergence. Conversely,
the observed flow convergence in the case of
negative polarity can be attributed to the
upward acceleration of positively charged
gas by electrical force.[10]
Experimental Observations
cont.
• Flame instability or stability is increased
with increase in current, or voltage
• At near extinguishment, when most
instability observed, the luminous region of
flame disappears (possibly due to stripping
of carbon before it “gets hot enough to
glow”)
• Diffusion flames require lower
extinguishment voltage than premixed
flames
Positive effects of Electric
Fields
• Depending on electrode placement, energy is
added to combustion process by raising the
energy level of free electrons, increasing
reaction rate
• Electric field applied across burner exit shown
to reduce soot emission
• Electric fields effect on nitrogen molecules in
pre-flame zone shown to accelerate the
oxidation process
Control of Soot Emission
Study 1
•
Goal: To reduce the
concentration of growing
ions, i.e., soot precursors,
which directly relate to the
soot emission of the flame.[12]
•
The charged species and
electrons carried into the
flame may influence the state
of charging of incipient soot
particles and also reduce the
concentration of growing
ions, i.e., soot precursors,
which directly relate to the
soot emission of the flame.[12]
[12]
DC corona discharge on soot
emission
DC corona application:
additional air and
inorganic charged species
and electrons, produced
in the air near the tip of
the positive electrode, are
carried into the flame
mainly by corona
winds.[12]
[12]
AC corona discharge on soot
emission
AC corona application: The
inorganic charged species
and electrons are carried
into the flame by diffusion
processes.[12]
Cation is an ion with fewer
electrons than protons
[12]
AC vs. DC Electric Fields
• DC requires electrode placement very close
to flame surface to maximize ion currents
and gas velocities
• DC requires direct exposure of electrodes,
no electrically insulated material
• AC is time-oscillatory, and electric field is
localized at flame surface
• AC generated field does not depend on
location of electrodes, but rather E-field
strength at point
Control of Soot Emission
Study 2
[13]
Soot vs. Voltage
Application of EF
in luminous zone
near flame tip
showed greater
impact on soot
production.
[13]
Flame Temperature vs. Voltage
[13]
Study 2 Findings
• The soot emission was decreased increasing the
applied voltage, and the efficiency of the soot
suppression exceeded 90% in the region of applied
voltage over 7 kV.
• At the applied voltage over 8 kV, the flame
temperature reached above 2000 'C.
• It is considered that the ionic wind enhanced the
mixing of the fuel gas and surrounding gas,
consequently, the high flame temperature of flame
caused the oxidation of soot particles.
•
AC vs. DC with insulated
electrodes
Electrohydrodynamic response of the flame using
glass-insulated electrodes for DC (E0 = 75 kV/m; left)
and AC (E0 = 75 kVrms/m, ν = 800 Hz; right) fields
Schlieren Imaging
[14]
Summary
• Electric field use in large uncontrolled fires has not
been applied or studied
• Most research has been on the scientific side, used
only on small flames, i.e. bunsen burners and candles
• Flame stability has been studied. Electric fields effect
on nitrogen molecules in pre-flame zone shown to
accelerate the oxidation process.
• Not enough evidence of ionic wind having a very
impactful effect on combustion chemistry, mostly
theory
• Soot reduction applications may be worth further
studying in actual combustion systems, i.e. gas
turbines
DARPA
• http://www.youtube.com/watch?feature=
player_embedded&v=TXhdHeNUD1o
Personal Thoughts
• Possible “threshold” voltage, where at
a certain point the e-field changes
from more of a chemical effect to a
mechanical one
• Applications in gas turbines?
• Not a great idea for fire suppressant
Sources
1. http://nikemissile.org/Humor.shtml
2. Electric Fields for Flame Extinguishment, T.S Call, D.B Schwartz, March
1993 Air Force Civil Engineering Support Agency, HQ AFCESA/RACF,
Tyndall AFB FL
3. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/equipot.html#c2
4. Jaggers, H.C., and Von Engel, A., "The Effect of Electric Fields on the
Burning Velocity of Various Flames," Combustion and Flame 16, 275-285
(1971).
5. Shebeko, Y.N., "Effect of an AC Electric Field on Normal Combustion
Rate of Organic Compounds in Air," Fiz. Goreniva Vzrvva 18, No. 4, 48-50
(1982).
Sources
6. www.basinc.com
7. Electrical Modification of Combustion and the Affect of Electrode
Geometry on the Field Produced, Timothy J. C. Dolmansley, Christopher
W.Wilson, David A. Stone, Hindawi Publishing Corporation, Modeling
and Simulation in Engineering, Volume 2011, Article ID 676428
8. http://www.thunderbolts.info/wp/2011/10/17/essential-guide-to-theeu-chapter-2/candle_flame_plasma_in_e-field_450x337/
10.Towards the mechanism of DC electric field effect on flat premixed
flames, E.N. Volkov, A.V. Sepman, V.N. Kornilov, A.A. Konnov, Y.S.
Shoshin, L.P.H. de Goey, Department of Mechanical Engineering,
Eindhoven Technical University, Eindhoven, the Netherlands
11. http://www.electronics-cooling.com/2012/03/ionic-winds-a-newfrontier-for-air-cooling/
Sources
12. Control of soot emission of a turbulent diffusion flame by DC or AC
corona discharges, Hiromichi Ohisa, Itsuro Kimura, Hideyuki Horisawa,
Department of Aerospace Engineering (H. O., I. K.) and Department of
Precision Mechanics (H. H.), Tokai University, Hiratsuka, Kanagawa,
Japan
13.Variation of flame shape and soot emission by applying electric field,
Masahiro Saito, Masayuki Sato, Katsuhiro Sawada, Department of
Biological and Chemical Engineering, Gunma University, Kio,u, Gunmaken 376, Japan
14. AC electric fields drive steady flows in flames, Aaron M. Drews,1
Ludovico Cademartiri, Michael L. Chemama, Michael P. Brenner,
George M. Whitesides, and Kyle J. M. Bishop, Kavli Institute, Harvard
University, Cambridge, Massachusetts 02138, USA