FLAME CONTROL IN THE FLAME-HOLDING REGION

Abstract

A combustion system includes a fuel nozzle, a charge source, a discharge electrode, and a voltage supply coupled to the charge source and discharge electrode. The charge source is configured to apply a polarized charge to a flame supported by the nozzle, and the discharge electrode is configured to attract a flame-front portion of the flame to hold the flame for flame stability. The discharge electrode can be toroidal in shape, positioned coaxially with the nozzle downstream from the nozzle. The voltage supply is configured to hold the charge source at a charge potential and the discharge electrode at the discharge potential. The nozzle can be configured to apply the polarized charge to a fuel stream emitted by the nozzle, whereafter the charge is passed to the flame upon combustion of the fuel.

Description

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority benefit from U.S. Provisional Patent Application No. 61/803,080, entitled “FLAME CONTROL IN THE FLAME-HOLDING REGION”, filed Mar. 18, 2013; which, to the extent not inconsistent with the disclosure herein, is incorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to combustion systems, and more particularly, to methods and devices used to affect flame shape for improving flame stabilization.

BACKGROUND

Combustion systems are employed in a vast number of applications, in industry, commerce, and private residences. Tightening government regulations, increasing costs of fuel, and public opinion all contribute to a continual pressure to reduce emissions and improve efficiency of such combustion systems.

Flame stability is an important element in most combustion systems. A flame can be said to be stable when flame propagation speed, i.e., the rate at which a flame front travels upstream in a flow of fuel, and the velocity of the fuel flow are in equilibrium. Some combustion systems employ fuel nozzles from which fuel is ejected at velocities that exceed the flame propagation speed for the particular fuel and conditions, and in which the velocity does not drop below the propagation speed before the fuel stream is too dilute to support combustion. Without flame-holding structures designed for the purpose, such systems could not support a stable flame. The flame front would be swept downstream and extinguished. A typical flame holder includes a bluff body, a V-gutter, or some other form of flow blockage that is positioned at the periphery of the fuel stream. The obstruction of the flame holder causes turbulence in the aerodynamic wake of the flame holder, including vortices in which hot gases from the combustion reaction are recirculated and mixed with uncombusted reactants, which then ignite, maintaining a continuous ignition position in the lee of the flame holder, thus sustaining a continuous combustion reaction, and a stable flame.

SUMMARY

According to an embodiment, a combustion system is provided with a mechanism for holding a flame at a selected location. The combustion system includes a fuel nozzle, a charge source, a discharge electrode, and a voltage supply that is coupled to the charge source and discharge electrode. The charge source is configured to apply a polarized charge to a flame supported by the nozzle, and the discharge electrode is configured to attract a flame-front portion of the flame to hold the flame for flame stability. The discharge electrode can be toroidal in shape, positioned coaxially with the nozzle downstream from the nozzle. The voltage supply is configured to hold the charge source at a charge potential and the discharge electrode at the discharge potential. The nozzle can be configured to apply the polarized charge to a fuel stream emitted by the nozzle, whereafter the charge is passed to the flame upon combustion of the fuel. The discharge potential is preferably at a circuit ground potential or at a polarity that is opposite a polarity of the polarized charge. Because of the electrical difference between the charge potential and the discharge potential, a flame-front portion of the flame is attracted toward the discharge electrode, which serves to hold the flame at a location near the discharge electrode.

In some embodiments in which the discharge electrode is toroidal in shape, the electrode is positioned concentric with a longitudinal axis of the nozzle. During operation, the flame front is attracted to the discharge electrode at a multitude of points on the surface of the electrode surrounding the flame.

According to an embodiment, the flame front makes intermittent or continuous contact with the discharge electrode.

According to an embodiment, the discharge electrode is one of a plurality of electrodes arranged in radial symmetry around the nozzle. Each of the plurality of electrodes is configured to be held at the discharge potential. According to an embodiment, the voltage supply is configured to selectively apply the discharge potential to some of the plurality of electrodes, while decoupling others of the plurality of electrodes from the charge/discharge circuit. In this way, a position and size of the flame can be at least influenced, if not fully controlled, by selection of the positions in which the electrodes are arranged, and selection of the electrodes to be held at the discharge potential and those to be decoupled from the charge/discharge circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present disclosure are described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. Unless indicated as representing the background art, the figures represent aspects of the disclosure.

FIG. 1 illustrates regions that may be identified in a flame within a combustion volume, according to an embodiment of the present disclosure.

FIGS. 2-4 are schematic diagrams of combustion systems, according to respective embodiments.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, which are not necessarily to scale or proportion, similar reference characters typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description and drawings are not intended to limit the scope of the claims. Other embodiments and/or other changes can be made without departing from the spirit or scope of the present disclosure.

In the embodiments disclosed below, various electrodes are described as being configured to apply a charge to a flame, while other electrodes are configured to apply electrical energy to the flame, or to discharge the flame, etc. An element can be said to be charged when it has either a surplus of electrons, in which case it is negatively charged, or a shortage of electrons, in which case it is positively charged. Thus, an electrical charge inherently includes a polarity. The element can be discharged by enabling electrons to flow to or from the element until an equilibrium is reached. This equilibrium can be an absolute equilibrium, in which each of the atoms that comprise the element has its nominal complement of electrons, or, more commonly, a relative equilibrium, in which electrons are permitted to flow freely between two or more elements until each element is at a same electrical potential. It will be recognized, therefore, that for the most part, a charge is a relative value that is characterized by a difference in electrical potential between two elements, and that the absolute polarity of an element is largely ignored in favor of the relative polarity. In other words, even if each of a pair of elements has a shortage of electrons and is thus positively charged, in an absolute sense, if they are not at the same electrical potential, one will be negatively charged, relative to the other. If these elements are placed in electrical contact, electrons will flow from the “negatively” charged element to the other (and by convention current will flow from the more “positively” charged element to the other), until the difference in potentials is discharged.

With respect to the present disclosure, although a flame can be described as being charged via a charge source or electrode, and discharged via a discharge electrode, it can be just as correct to describe the process oppositely, because each electrode acts to discharge the flame with respect to a potential difference between the respective electrode and the flame, and in the process—inasmuch as the electrodes are held at different potentials—acts to charge the flame, with respect to a potential difference between the opposite electrode and the flame.

In view of the discussion above, where the specification or claims refer to charge and discharge, these terms are interchangeable. Furthermore, because an actual physical attraction can be formed between elements without resulting in a discharge of a difference in potentials—the well known phenomenon of static attraction being an example—some desired results can be obtained without an actual discharge occurring.

Various burner systems are disclosed herein as embodiments. Many elements are omitted from the embodiments described, particularly where such elements are not necessary for an understanding of the principles disclosed. In practice, these and other embodiments typically include more extensive combustion systems used in industry and commerce as parts of, for example, electrical power generation, boilers, refineries, smelters, foundries, commercial and residential HVAC systems, etc.

FIG. 1 is a schematic diagram of a portion of a burner 100 including a burner nozzle 106 configured to emit a fuel stream 102 along a longitudinal axis A of the nozzle and to support a flame 104, according to various embodiments. Relative particle velocity within the flame 104 is represented by arrows V, of lengths corresponding to relative velocity. For the purposes of the present disclosure, a flame can be divided into three general regions or portions. The first region R₁, closest to the nozzle 106, is a flame-holding region. Adjacent to, and downstream from the flame-holding region R₁ is the second region R₂, a momentum-dominated fluid dynamics region, and furthest downstream, the third region R₃ is a buoyancy-dominated fluid dynamics region.

The term flame particle refers primarily to gaseous atoms and/or molecules (e.g., gaseous fuel, fuel vapor, nitrogen, oxygen, argon, and reaction intermediates) that comprise the fluid within a flame, as well as the small solid (e.g., soot and/or ash) particles that may be entrained within the flame.

A flame front 108 of the flame 104 is located in the flame-holding region R₁. As fuel flows from the nozzle 106 in a downstream direction in the fuel stream 102, the flame front 108 is continually moving upstream. The velocity of the fuel stream 102 is a function of a number of factors, including the geometry of the nozzle 106 and the pressure of the fuel within the nozzle 106. As the fuel stream 102 moves downstream, it slows as the flow diverges and entrains air from the surrounding atmosphere. Meanwhile, the flame propagation rate, i.e., the speed at which the flame front 108 moves upstream in the flow of fuel, depends upon factors that include the type of fuel, the amount of entrained oxygen, ambient temperature, etc. When the flame propagation rate and the fuel stream velocity are at equilibrium, the flame 104 remains substantially stationary relative to the nozzle 106, and the flame 104 is said to be stable.

Within the momentum-dominated fluid dynamics region R₂ of the flame 104, the velocity and vector of flame particles within the flame 104 are substantially determined by the velocity and vector associated with the fuel stream 102. In this region, the velocity of the flame particles is sufficiently high that other common factors, including buoyancy, have little influence on their vectors. However, as the flame particles move downstream, they lose velocity, and the buoyancy of the flame 104, relative to the cooler and denser surrounding gases, tends to push the flame upward. As the flame particles move further downstream and continue to lose velocity, the direction of movement is increasingly dominated by flame buoyancy.

The shape of the flame 104, and the relative dimensions of the three regions R₁-R₃, can vary significantly, according to many factors. For example, in some cases, the buoyancy-dominated fluid dynamics region R₃ is nonexistent, or very nearly so, as in, e.g., some welding torch flames. In these types of flames, the fuel is substantially consumed before the velocity has dropped to a level where buoyancy can exert a significant influence. In other cases, the momentum-dominated fluid dynamics region R₂ is substantially nonexistent, as in, for example, the case of a candle flame or other flame in which little or no velocity is imposed on the flame 104 by the fuel, so that the velocity and vector of the flame particles are entirely controlled by other factors, including buoyancy.

A flame can stabilize very close to the nozzle 106 or some distance downstream, depending upon the various factors mentioned above. However, in some systems, the initial fuel stream velocity is sufficiently high, that by the time the stream has slowed to below the flame front propagation speed, the fuel has entrained enough air that it is too dilute, and is no longer flammable. Typically, such systems employ some type of flame holder configured to generate turbulence, as discussed in the background above.

As described herein, the inventors have recognized that the interaction of the flame 104 with one or more electrodes can affect flame stability. In particular, it is noted that the interaction of a charged flame (or charged fuel stream) with a flame holding electrode can maintain combustion, even when current flow is controlled to prevent spark discharge. It is further noted that actuation of one or more electrodes (e.g., mechanically or otherwise causing selective positioning of an electrode) can also interact with combustion fluid flow adjacent thereto and thereby affect flame stability. Moreover, the inventors note interaction between electrode actuation and electrode voltage (and/or current flow).

Controlling an electrical and/or fluid dynamic interaction between the flame 104 and/or fuel flow 102 can thus affect flame stability and produce other desired effects. The other desired effects can include reducing pollutant emissions, reducing sooting, increasing blackbody emissivity of the flame, increasing maximum heat output, and/or expanding turn-down (reducing minimum heat output) of the flame 104.

FIG. 2 shows a schematic view of a combustion system 200, according to an embodiment. The system 200 includes a toroidal electrode 202 and a voltage supply 204. The nozzle 106 is configured to act as a charge electrode, i.e., a portion of the nozzle 106, preferably at least the upper rim, is electrically conductive, and is electrically coupled to the voltage supply 204. The toroidal electrode 202 is centered on the longitudinal axis A of the nozzle 106 in a position that falls within the flame-holding region R₁ of a flame 104 supported by the nozzle 106, and is coupled to circuit ground 206.

During operation, a charge is imparted to the fuel stream 102 as it leaves the nozzle 106. The charged fuel stream 102 is ignited substantially as it passes through the toroidal electrode 202, and its charge is passed to the flame 104. The inventors have discovered that in an electrically charged flame, the flame seeks a discharge opportunity, and moves toward such an opportunity, to the extent possible. In the present case, the flame front 108 of the charged flame 104 is attracted to the grounded toroidal electrode 202, where a portion of the charge can be released. Consequently, the flame front 108 remains in contact with the toroidal electrode 202 even when the velocity of the fuel stream 102 far exceeds a normal flame propagation speed for the fuel and conditions present. The inventors have found that the flame front 108 does not usually contact the toroidal electrode 202 around its entire circumference, especially at higher fuel stream velocities, but instead will maintain contact at one or two points on the electrode 202, which points of contact move from place to place on the electrode during operation.

Because the toroidal electrode 202 is larger in diameter than the cone defined by the diverging fuel stream 102, the electrode 202 does not introduce a significant amount of turbulence or drag into the fuel stream 102 or flame 104, which reduces or eliminates the energy and efficiency losses normally associated with flame holders in conventional combustion systems.

In the embodiment shown, the toroidal electrode 202 is coupled to circuit ground 206. This can be advantageous, because it means that the charge electrode—in this case, the nozzle 106—can apply either a positive or negative charge to the fuel stream 102, and in either case, the flame 104 will seek to discharge on the toroidal electrode. According to other embodiments, the toroidal electrode 202 is coupled to the voltage supply 204, which is configured to apply a voltage potential having an absolute value greater than zero to the electrode 202. Where the voltage applied to the toroidal electrode 202 is greater than zero, the applied voltage preferably has a polarity that is opposite a polarity of the charge applied to the flame 104. One possible advantage of applying voltages greater than zero to both the nozzle 106 and the toroidal electrode 202 is that a greater voltage difference can be achieved for a given available maximum absolute value.

The appropriate magnitude of the voltage difference between a charge electrode and a discharge electrode will vary according to a number of factors, including, for example, the velocity of the fuel stream 102 relative to the normal flame propagation speed, the difference in diameters of the toroidal electrode 202 and the cone of the fuel stream 102 at the point where it passes through the toroidal electrode 202, the ambient temperature and humidity, the availability of oxygen, etc.

According to various embodiments, the voltages applied to the nozzle 106 and/or the toroidal electrode 202 can have a positive polarity, a negative polarity, or can alternate, in a regular time-based signal. In particular, the inventors note somewhat enhanced effects are produced by a relatively positively charged flame interacting with a relatively negative voltage (e.g., grounded) toroidal electrode 202 compared to inverted DC voltages. Similarly, the inventors note that increasing a portion of a regular time-based signal (e.g., a voltage-biased AC signal) during which the flame is relatively positively charged relative to the toroidal electrode (compared to an inverted polarity relationship) tends to pull the base of the flame downward toward the toroidal electrode 202. Accordingly, varying a bias voltage or a temporal duty cycle of a time-varying toroidal electrode voltage and/or charge electrode signal can be used to select a flame base position across a range of fuel flow rates.

Interestingly, the inventors note that at least under some conditions, pulling the base of the flame downward simultaneously moves the tip of the flame upward. Accordingly, controlling voltage/current relationships and/or toroidal electrode 202 geometry can be used to control flame length.

Potential differences investigated by the inventors range from about 10 kV to about 80 kV. Effects were found to be a function of fuel pressure (and hence velocity) and to be relatively independent of burner scale.

According to an embodiment, the voltage supply 204 is configured to vary the voltage difference between the nozzle 106 and the toroidal electrode 202 as conditions within the combustion volume change. According to another embodiment, the voltage supply 204 is configured to release the flame front 108 from the toroidal electrode 202 under selected conditions. For example, in the case where the velocity of the fuel stream 102 is reduced, the flame 104 may be capable of stabilizing without a flame holder. In another embodiment, the fuel composition and fuel stream velocity are selected to be such that without a flame holder, the flame 104 will stabilize some distance from the nozzle 106, so that the position of the flame 104 is selectable between the toroidal electrode 202 and the more distant location.

The flame front 108 can be released from the toroidal electrode 202, for example, by removing the voltage signal from the nozzle 106 so that no charge is applied to the fuel stream 102, or by decoupling the toroidal electrode 202 from ground so that there is no discharge path.

FIG. 3 shows a schematic view of a combustion system 300, according to an embodiment. The combustion system 300 is substantially similar to the system 200 of FIG. 2, except that the toroidal electrode 202 of the system 300 is movable, by operation of an actuator 304. The toroidal electrode 202 is shown coupled to the voltage supply 204 rather than to ground 206, as shown in FIG. 2, but as previously indicated, the electrical circuit can be arranged in any of a number of configurations, only some of which are shown or described herein. Furthermore, where both the voltage supply 204 and the toroidal electrode 202 are coupled to circuit ground 206, they can be considered to be coupled together, as well.

In the embodiment shown, the actuator 304 is configured to rotate the toroidal electrode 202 about an axis that lies transverse to the longitudinal axis A of the nozzle 106. By changing the angular position of the toroidal electrode 202 relative to the axis A, the apparent aperture size of the electrode can be reduced. According to an embodiment, the actuator is configured to change the angular position of the toroidal electrode 202 when conditions within the combustion volume are such that the flame 104 cannot hold to the toroidal electrode 202 at its normal distance from the axis A. For example, in some cases, when the flow rate of the fuel stream 102 is reduced, the diameter of the fuel cone can also reduce, increasing the distance that the flame 104 must bridge to contact the electrode 202.

Angular adjustment of the toroidal electrode 202 enables an increased turndown ratio, as compared to the combustion system 200 of FIG. 2.

According to another embodiment, the actuator 304 is configured to move the toroidal electrode 202 axially along the longitudinal axis A. Accordingly, when the fuel flow rate is reduced, thereby reducing the diameter of the cone of the fuel stream 102, the actuator 304 is configured to move the toroidal electrode 202 in a downstream direction, i.e., away from the nozzle 106, to a position where the diameter of the cone of the fuel stream 102 is once again only slightly smaller than the inner diameter of the toroidal electrode 202. Conversely, as fuel flow increases, the actuator 304 is configured to move the toroidal electrode 202 in an upstream direction to prevent the fuel stream 102 from impinging on the electrode 202 and producing undesirable turbulence.

FIG. 4 is a schematic view of a combustion system 400, according to an embodiment. The system 400 includes a plurality of electrodes 402 arranged in radial symmetry around the longitudinal axis A of the nozzle. Each of the electrodes 402 is coupled to the voltage supply 204 and configured to be held at a discharge potential, relative to a charge applied to the flame 104 via the nozzle 106. The discharge potential can be ground potential, or an absolute value greater than zero and having a polarity opposite that of the charge applied to the flame 104. The electrodes 402 can have any appropriate shape, such as, for example, cylindrical or rectangular rods or posts, with or without rounded, conical or sharp tips, amongst others. The position of the electrodes 402 along the axis A, i.e., the distance downstream from the nozzle 106, is a design choice that will be based on a number of factors, including, for example, air flow rate, fuel flow rate, type of fuel, different atmospheric conditions in and around the combustion volume, etc.

In operation, a charge is applied to the flame 104, as previously described, which then seeks to discharge via one or more of the plurality of electrodes 402. The inventors have found that typically, the flame 104 will hold to one of the plurality of electrodes 402 at a time, but will jump from one to another in an apparently random manner. Of course, if one of the electrodes 402 is decoupled from its connection to the voltage supply 204 or ground, the flame 104 will not hold to that electrode 402. Thus, by selectively decoupling one or more of the plurality of electrodes 402, the location at which the flame 104 holds can be influenced or selected.

According to an embodiment, the plurality of electrodes 402 can be arranged at various distances radially from the axis A, such as in concentric circles, for example. Selected groups of the electrodes 402 can be energized, according to the particular requirements of the moment. For example, when fuel flow is reduced, an inner ring of electrodes 402 can be energized in order to accommodate a flame 104 with a relatively small diameter. Conversely, when fuel flow is increased, rings of electrodes 402 positioned further from the axis A can be energized, at a distance consistent with the reach of the flame 104.

According to another embodiment, the plurality of electrodes 402 are arranged at various distances axially, downstream from the nozzle. For example, some of the electrodes 402 can be positioned near or within the momentum-dominated fluid dynamics region R₂. When the more distant electrodes 402 are energized, the flame 104 is drawn outward toward those electrodes 402. This can result in an increased flame diameter—when the additional electrodes 402 are evenly spaced around the flame 104, and are simultaneously activated—or can cause the flame to shift in the direction of one or another electrode 402 that is separately energized.

In addition to the discharge electrodes 402 disclosed herein, various other electrode shapes and configurations can be employed, particularly for the purpose of holding a flame in a combustion system. For example, a linear conductive body can be positioned extending near to, or through the flame 104, in order to stabilize the flame 104.

According to some embodiments, at a higher fuel flow rate in combustion volume it may not be desired that the flame 104 contact heat transfer surfaces. Accordingly, the flame 104 can be contained at a particular, predefined location, by selection of a particular discharge electrode 402, for example, to allow for a high turn-down ratio and a large range of fuel flow to provide a high rate to the combustion reaction while also preventing flame 104 from greatly increasing in length.

The flame 104 can be held at a certain location. This location can be changed to move the flame 104 to the left or to the right, up or down, to change flame-holding characteristics without difficulty and to enable defining the borders of momentum-dominated fluid dynamics region R₂. The shape of the flame 104 can be affected in the momentum-dominated fluid dynamics region R₂ independently of other actions, such as holding the flame 104, in the flame-holding region R₁.

High voltages or ground can be employed to hold the flame 104 by applying a charge to the flame 104. The ability to apply a charge to the flame 104 may require the use of charge injection. In order to get a charge injection, high voltage is required to create a field curvature over an electrode employed for charge injection. For this, an ionizer can be used. Ionizers are typically operated at voltages having an absolute value of greater than 1000VDC or VAC. According to some embodiments, the ionizer is positioned some distance from the flame 104, and is used to ionize the air feeding the flame 104 with a relatively high ion density, such that a significant volume of ions are entrained by the flame. The relatively high ion density in the flame 104 then interacts with ground or relatively low voltages on the electrodes 402.

According to an embodiment, resistance is added to the circuit of one or more of the plurality of electrodes 402 to modify the voltage at which the electrodes 402 are placed. This voltage can be calculated to regulate voltages on the electrodes 402 to allow differences in the way in which and/or the location where the flame 104 is held.

While various aspects and embodiments have been disclosed, other aspects and embodiments may be contemplated. The various aspects and embodiments disclosed here are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims

1. A combustion system, comprising:

a fuel nozzle configured to emit a fuel stream;

a charge source configured to apply a polarized charge to a flame supported by the nozzle; and

a first discharge electrode positioned and configured to attract a flame-front portion of the flame when energized at a discharge potential.

2. The system of claim 1, comprising a voltage source electrically coupled to the charge source and the first discharge electrode, and configured to hold the charge source at a charge potential and the first discharge electrode at the discharge potential.

3. The system of claim 2, wherein the voltage source is configured to hold the first discharge electrode at the discharge potential having a polarity opposite a polarity of the polarized charge.

4. The system of claim 1, comprising a voltage source electrically coupled to the charge source and the first discharge electrode, configured to hold the charge source at a charge potential, and wherein the first discharge electrode is electrically coupled to a circuit ground in common with the voltage source.

5. The system of claim 1, wherein the first discharge electrode is positioned and configured to discharge a portion of the polarized charge when contacted by the flame-front portion of the flame.

6. The system of claim 1, wherein the fuel nozzle comprises a charge electrode, and is configured to apply the polarizing charge to the fuel stream as it is emitted from the nozzle.

7. The system of claim 1, wherein the charge source is configured to generate ions.

8. The system of claim 1, wherein the first discharge electrode has a toroidal shape, and is positioned coaxially with the fuel nozzle and downstream therefrom, with reference to a direction of flow of a stream of fuel emitted by the nozzle.

9. The system of claim 8, wherein the fuel nozzle is configured to emit the fuel stream in a divergent cone, and wherein the toroidally-shaped first discharge electrode is sized and positioned such that an inner diameter of the first discharge electrode is at least equal to a diameter of the divergent cone at a point at which the fuel stream passes the first discharge electrode.

10. The system of claim 8, comprising an actuator coupled to the first discharge electrode and configured to move the electrode relative to the nozzle.

11. The system of claim 10, wherein the actuator is configured to rotate the first discharge electrode about an axis lying perpendicular to a longitudinal axis of the nozzle.

12. The system of claim 11, wherein the actuator is configured to adjust an angular position of the first discharge electrode relative to the longitudinal axis of the nozzle.

13. The system of claim 10, wherein the actuator is configured to translate the first discharge electrode along a line parallel to a longitudinal axis of the nozzle.

14. The system of claim 1, comprising:

a plurality of discharge electrodes, including the first discharge electrode;

a voltage source electrically coupled to the charge source and to each of the plurality of discharge electrodes.

15. The system of claim 14, wherein the plurality of discharge electrodes are positioned in radial symmetry around a longitudinal axis of the fuel nozzle.

16. The system of claim 14, wherein the voltage source is configured to hold each of the plurality of discharge electrodes at the discharge potential.

17. The system of claim 16, wherein the voltage source is configured to selectively hold some of the plurality of discharge electrodes at the discharge potential, while decoupling others of the plurality of discharge electrodes from a circuit including the voltage source and the charge source.

18. The system of claim 15, wherein each of the plurality of discharge electrodes is positioned at one of a plurality of distances radially from the longitudinal axis of the fuel nozzle.

19. The system of claim 15, wherein each of the plurality of discharge electrodes is positioned at one of a plurality of distances axially from the fuel nozzle.

20. A method, comprising:

emitting a fuel stream from a burner nozzle of a combustion system;

applying an electrical charge to a flame supported by the fuel stream;

attracting a flame front of the flame toward a discharge electrode by holding the discharge electrode at a discharge potential, relative to the electrical charge.

21. The method of claim 20, wherein the holding the discharge electrode at a discharge potential includes holding the discharge electrode at a potential having a polarity that is opposite a polarity of the electrical charge.

22. The method of claim 20, wherein the holding the discharge electrode at a discharge potential includes the discharge electrode at a ground potential that is common to a circuit coupled to charge source configured to apply the electrical charge.

23. The method of claim 20, wherein the attracting a flame front of the flame toward a discharge electrode includes discharging a portion of the electrical charge upon contact of the flame front with the discharge electrode.

24. The method of claim 20, wherein the applying an electrical charge to a flame includes applying the electrical charge to the fuel stream as it exits the burner nozzle.

25. The method of claim 24, wherein the applying the electrical charge to the fuel stream includes applying the electrical charge to an electrically conductive portion of the burner nozzle.

26. The method of claim 20, wherein the applying an electrical charge to a flame includes generating ions and introducing the ions to the flame.

27. The method of claim 20, wherein the holding the discharge electrode at a discharge potential includes holding a toroidally-shaped discharge electrode at the discharge potential.

28. The method of claim 20, wherein the holding the discharge electrode at a discharge potential includes holding a plurality of discharge electrodes at the discharge potential.