Impact of Electric Fields on Combustion Related

Phenomena

Aviation Fire DynamicsApril 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 FlamesRE + (M) R Chain initiationR + M I + (P) Chain propagationR + M αR + M* Chain branchingR + M P + M 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 notmuch 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 reactionswhich are highly exothermic.

Reactant: REMolecule: MRadical: RIntermediate Product: IReaction Product: PNot necessary: ( )Excited int. energy: *

Ion Production in Flames

1. Ionization by collisionA+B A+ + B + e-

2. Electron transferA+B A+ + B-

3. Ionization by transfer of excitation energyA+B* A+ + B + e-

4. Chemi-ionizationA+B C+ + D + e-

A+B C+ + D-

• 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.

Positive ion concentrations vs. distance

[2]

• CHO+ and C3H3+ are theprimary chemi-ions, thus being parent ions from which all the other ions are formedeither by charge transfer reaction or ionization by electron collision.

• Ion production andconcentration greatest in luminous zone near flame tip

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

[2]

• 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.

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 transferCollisional 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

[7]

Setup EF Strength in Relation to Electrodes

Plate Electrode

[2][8]

Plate/Gauze Electrode

[9]

Example of gauze electrode; fine mesh grid of conducting metal

Plate Electrode

[7]

EF Strength in Relation to ElectrodesSetup

Point Electrode

[11]

Point Electrode

[7]

EF Strength in Relation to ElectrodesSetup

Cool/Interesting Video

• https://www.youtube.com/watch?v=zkAJQ0Ggh_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 EffectsImages of stoichiometric CH4/air flame: a) without EFb) 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

[12]

• 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]

DC corona discharge on soot emission

[12]

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]

AC corona discharge on soot emission

[12]

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

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

[13]

Application of EF in luminous zone near flame tip showed greater impact on soot production.

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

[14]

Schlieren Imaging

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

Sources1. http://nikemissile.org/Humor.shtml2. 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).

Sources6. www.basinc.com7. 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 6764288. http://www.thunderbolts.info/wp/2011/10/17/essential-guide-to-the-eu-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 Netherlands11. http://www.electronics-cooling.com/2012/03/ionic-winds-a-new-frontier-for-air-cooling/

Sources12. 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, Japan13.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, Gunma-ken 376, Japan14. 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