TECHNICAL FIELD
The present disclosure in general relates to manipulating flames and, more particularly, to applying an electric field to a flame via electrodes to affect combustion.
BACKGROUND
This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Fossil fuel combustion is currently the largest source of energy with the U.S., accounting for 79% of energy produced in 2021. Combustion processes are employed in many residential, commercial, and industrial systems for a wide range of purposes, such as, e.g., for environmental heating, power generation, smelting, refining, propulsion, etc. In response to global warming and air quality concerns, stricter legislation is being created worldwide to reduce the harmful emissions that are a product of combustion, including carbon monoxide (CO), carbon dioxide (CO2), nitrous oxides (NOX), sulfur oxides (SOX), and unburned hydrocarbons (UHC). These restrictions apply to all market sectors including power generation, residential, commercial, and industrial.
An effective method for reducing NOX emissions is to use extra air beyond what is needed for complete combustion, referred to as “burning leaner.” Burning leaner aims to lower the flame temperature and the formation of thermal NOX. However, these leaner conditions are often more prone to the combustion phenomenon known as “thermoacoustic instabilities” and they severely limit the optimization of the combustor design. Thermoacoustic instabilities are a common combustion problem studied since the 1850's and were a significant problem in the F1 engines during the Apollo program. The combustion community has long sought actuators which could affect the combustion process in real-time and suppress the instabilities. Several potential actuation methods have received considerable research attention including loudspeaker forcing and pulsed fuel injection, but none have been widely adopted due to their expense, lack of reliability, or inability to scale from laboratory to large scale thermal power.
SUMMARY
Described herein are systems and methods related to the manipulation of flames and the area of combustion. The combustion process can be modified by applying an electrical potential to the combustion reaction to improve efficiency and reduce harmful emissions. To that end, the present disclosure includes aspects which can include a combustion burner, a first conductive element positioned across the face of the combustion burner, a second conductive element positioned within a flame, and a power source. The combustion burner can be configured to output the flame and a gas flow from a face of the burner. In some embodiments, the gas flow can define a gas flow path in a direction away from the burner. The power source can be coupled with a positive electrode and a negative electrode. The positive electrode can be electrically coupled with the second conductive element and the negative electrode can be electrically coupled with the first conductive element. The positive electrode and the negative electrode can be configured to generate an electric field oriented parallel to the gas flow path. In some embodiments, the electric field can be oriented in an opposite direction to the gas flow path, while in other embodiments the electric field can be oriented in the same direction to the gas flow path. The power source can be further configured to generate the electric field to form at least one flame root defined by the flame and can be configured to selectively modify the electric field to increase or decrease a quantity of the at least one flame root.
In some aspects, the apparatus can further include a sensor and a data processor. The sensor can be configured to determine an acoustic characteristic of the combustion burner and can output a data signal based upon the acoustic characteristic. The data processor can be communicatively coupled with the sensor and the power source and can be configured to receive the data signal and selectively operate the power source to modify the electric field based upon the data signal. In some versions, the data processor can be configured to compare the acoustic characteristic to a pre-determined acoustic characteristic and to modify the electric field to achieve an improved acoustic characteristic.
Additional aspects of the present disclosure provide methods of operating a combustion burner to affect a heat-release of the combustion burner. In one embodiment, the method can include generating a flame from the burner, generating an electric field between the positive electrode and the negative electrode, forming an electrohydrodynamic bluff-body via the conductive element based upon the electric field, generating a first flame root based upon the electrohydrodynamic bluff-body, and increasing a strength of the electric field to generate a second flame root based upon the electrohydrodynamic bluff-body.
This summary is provided to introduce a selection of the concepts that are described in further detail in the detailed description and drawings contained herein. This summary is not intended to identify any primary or essential features of the claimed subject matter. Some or all of the described features may be present in the corresponding independent or dependent claims, but should not be construed to be a limitation unless expressly recited in a particular claim. Each embodiment described herein does not necessarily address every object described herein, and each embodiment does not necessarily include each feature described. Other forms, embodiments, objects, advantages, benefits, features, and aspects of the present disclosure will become apparent to one of skill in the art from the detailed description and drawings contained herein. Moreover, the various apparatuses and methods described in this summary section, as well as elsewhere in this application, can be expressed as a large number of different combinations and subcombinations. All such useful, novel, and inventive combinations and subcombinations are contemplated herein, it being recognized that the explicit expression of each of these combinations is unnecessary.
BRIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims which particularly point out and distinctly claim this technology, it is believed this technology will be better understood from the following description of certain examples taken in conjunction with the accompanying drawings, in which like reference numerals identify the same elements and in which:
FIG. 1 depicts a chart illustrating several commercial markets utilizing natural gas burners;
FIG. 2A depicts a schematic of a jet burner having a ring annulus bluff-body;
FIG. 2B depicts a conical flame structure created by the ring annulus bluff-body of FIG. 2A;
FIG. 2C depicts a conical flame structure upon inserting a bluff-body rod within the flame root, showing a resultant “V” in the flame shape that is formed by the creation of a new flame root on the backside of the bluff-body rod;
FIG. 3 depicts a schematic representation of the modification of FIG. 2C;
FIG. 4A depicts a schematic of a jet burner having a single copper wire positioned across the burner face, showing certain dimensions of the single copper wire according to one example embodiment;
FIG. 4B depicts the flame shape of a jet burner according to the system of FIG. 4A, showing the flame shape while the electric field is disabled;
FIG. 4C depicts the flame shape of a jet burner according to the system of FIG. 4A, showing the flame shape while the electric field is enabled;
FIG. 5 depicts a schematic representation of the resultant flame shape of FIG. 4C;
FIG. 6 depicts a schematic diagram showing example interval volumes defined by the conical and “V” flame surfaces;
FIG. 7 depicts a schematic diagram showing a rate of energy into the control volume, the control volume defined by the flame surface, and a rate of energy out of the flame surface (commonly referred to as the flame heat-release);
FIG. 8 depicts a graphical chart showing the measured heat-release deviation obtained when applying a slowly varying ramp of electric field magnitude to a flame;
FIG. 9A depicts a graphical chart showing the single-wire cathode heat-release obtained while applying a ramp waveform in the applied electric field magnitude;
FIG. 9B depicts a graphical chart showing the control volume energy reduction as it relates to the electric field magnitude for the single-wire cathode, showing both axes being normalized;
FIG. 10 depicts a graphical chart showing one example of a desired linear relationship approximated by a quasilinear relationship by adding N cathode elements, showing N transitions of the flame geometry;
FIG. 11A depicts a schematic of a jet burner having three copper wires positioned across the burner face;
FIG. 11B depicts the flame shape of a jet burner according to the system of FIG. 11A, showing progression of flame shapes as the electric field is increased;
FIG. 12A depicts a graphical chart showing the three-wire cathode heat-release obtained while applying a ramp waveform in the applied electric field magnitude;
FIG. 12B depicts a graphical chart showing the control volume energy reduction as it relates to the electric field magnitude for the three-wire cathode, showing both axes being normalized;
FIG. 13 depicts a schematic of a jet burner having a multi-element “honeycomb” structure positioned across the burner face, showing certain dimensions of the structure according to one example embodiment;
FIG. 14 depicts the flame shape of a jet burner according when utilizing the system of FIG. 13, showing progression of flame shapes as the electric field is increased;
FIG. 15A depicts a graphical chart showing the honeycomb structure cathode heat-release obtained while applying a ramp waveform in the applied electric field magnitude;
FIG. 15B depicts a graphical chart showing the control volume energy reduction as it relates to the electric field magnitude for the honeycomb structure, showing both axes being normalized;
FIG. 16 depicts a schematic of a jet burner having a multi-element circumferential structure positioned on the burner face, showing an enlarged portion of the structure for clarity;
FIG. 17A depicts a pair of graphical charts showing forced flame and heat-release with an electric field, the top plot showing the applied sinusoidal electric field with a frequency of 186 Hz, the bottom plot showing the corresponding resulting heat-release;
FIG. 17B depicts a graphical chart showing the heat-release amplitude as it relates to the electric field frequency;
FIG. 18A depicts a system schematic and resultant flame shape according to a Rijke tube experiment;
FIG. 18B depicts a graphical chart showing the oscillations in the pressure and flame heat-release during the thermoacoustic instability of the system of FIG. 17A;
FIG. 19A depicts a block diagram showing a thermoacoustic system that is unstable and produces oscillations of heat-release and pressure;
FIG. 19B depicts a block diagram showing that the electric field driven heat-release interacts with the thermoacoustically-driven component;
FIG. 19C depicts a block diagram showing one exemplary feedback control system using measurement of the acoustic pressure to modulate the electric field and suppress the thermoacoustic instability;
FIG. 20 depicts a pair of graphical charts showing one example of suppressing a thermoacoustic instability using the electric field and feedback control, showing the controller initially disabled before being enabled at t=0 milliseconds;
FIG. 21 depicts a flowchart of one exemplary method of affecting a burner flame shape and heat release using an electrohydrodynamic (EHD) bluff-body; and
FIG. 22 depicts a flowchart of one exemplary method of suppressing thermoacoustic instability of a burner flame using feedback control and electric fields.
The drawings are not intended to be limiting in any way, and it is contemplated that various embodiments of the technology may be carried out in a variety of other ways, including those not necessarily depicted in the drawings. The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present technology, and together with the description serve to explain the principles of the technology; it being understood, however, that this technology is not limited to the precise arrangements shown, or the precise experimental arrangements used to arrive at the various graphical results shown in the drawings.
DETAILED DESCRIPTION
The following description of certain examples of the technology should not be used to limit its scope. Other examples, features, aspects, embodiments, and advantages of the technology will become apparent to those skilled in the art from the following description, which is by way of illustration, one of the best modes contemplated for carrying out the technology. As will be realized, the technology described herein is capable of other different and obvious aspects, all without departing from the technology. Accordingly, the drawings and descriptions should be regarded as illustrative in nature and not restrictive.
It is further understood that any one or more of the teachings, expressions, embodiments, examples, etc. described herein may be combined with any one or more of the other teachings, expressions, embodiments, examples, etc. that are described herein. The following-described teachings, expressions, embodiments, examples, etc. should therefore not be viewed in isolation relative to each other. Various suitable ways in which the teachings herein may be combined will be readily apparent to those of ordinary skill in the art in view of the teachings herein. Such modifications and variations are intended to be included within the scope of the claims.
Reference systems that may be used herein can refer generally to various directions (for example, upper, lower, forward and rearward), which are merely offered to assist the reader in understanding the various embodiments of the disclosure and are not to be interpreted as limiting. Other reference systems may be used to describe various embodiments, such as those where directions are referenced to the portions of the device, for example, toward or away from a particular element, or in relations to the structure generally (for example, inwardly or outwardly).
I. Overview
One potential solution to improve fossil fuel-based energy generation involves the use of electric fields to affect combustion. Many fuel-air chemistries produce charged particles during combustion. Applying an electric field to the flame accelerates these particles. Their resulting collisions with the bulk gas molecules create appreciable pressure and velocity effects, commonly referred to as the “ionic wind.”
Accordingly, described herein are systems and methods for significantly enhancing the electric field effect on a flame to modify the flow field, flame shape, and heat-release of the flame using the electric field. In some embodiments, heat-release modifications can be utilized to provide further improvements, such as suppressing thermoacoustic instabilities in the combustor. The systems and methods presented have no consumables or moving parts, is relatively inexpensive due to its use of simple materials and cheap electronics, and is efficient (e.g., in one example, the system only consumes a mere 40 mW of electrical power to control a 3 kW thermal power flame). Further, the systems and methods are applicable for gaseous fuel types and continuous combustion. Examples of gaseous fuels include natural gas, butane, methane, propane. Non-gaseous fuels are solid and liquid types, such as coal, gasoline, kerosene, and diesel. Continuous combustion is where the flame is constantly present, and examples include furnaces, stoves, and lighters. A non-continuous example is the automotive internal combustion engine.
The most common example of a gaseous fuel type is natural gas, which is the largest segment of U.S. energy consumption. Natural gas is used for electrical power generation, heating homes, cooking food, and many industrial activities. Various U.S. markets for natural gas are illustrated in FIG. 1, along with approximate new equipment costs associated with these markets (in USD).
II. Exemplary Systems and Methods for Affecting Flame Combustion
This section will discuss the improved systems and methods that allow modulation of burner flame heat-release with an electric field. Further, this section provides systems and methods for suppressing thermoacoustic instabilities.
A. Exemplary Systems and Methods for Electric Field Induced Flame Heat-Release Modulation
The systems and methods include three general parts: (i) creating an EHD bluff-body, (ii) affecting the heat-release, and (iii) using multi-element cathodes to improve the effects on the flame. Each part will be discussed in greater detail below.
i. Creation of an EHD Bluff-Body
An EHD bluff-body acts similar to an aerodynamic bluff-body-a classic tool in combustion used to stabilize flames. An example of an introduction of an aerodynamic bluff-body is shown in FIGS. 2A-2C. Particularly, FIG. 2A shows a burner (100) having a stabilization ring (102) around the circumference of the burner face. FIG. 2B shows an initial flame shape before inserting an aerodynamic bluff-body across the burner face, and FIG. 2C shows the change in flame shape after inserting a bluff-body (104) at the base of the flame on the burner face. In this example application, the bluff-body (104) is a 1.59-millimeter (mm) diameter metal rod. Its effect is to block the fluid flow and cause flow recirculation on the backside of the rod, as depicted in the diagram of FIG. 3. The flow recirculation reduces the velocity enough that a new flame root is created in this region, seen as a “V” in the flame surface. Flame geometries are largely defined by their flame roots and thus creating a new one causes a significant change in the flame shape, as seen when comparing FIG. 2B and 2C.
The process of creating an EHD bluff-body is shown in FIGS. 4A-4C. The process starts at FIG. 4A by adding a small diameter copper wire (204) stretched across the face (202) of the burner (200). The 0.127 mm diameter of this wire is small enough that it causes minimal interference to the flow field and the flame. Thus, it does not act as an aerodynamic bluff-body. The image of the flame (see, FIG. 4B) shows that it is has the original conical shape and no flame root or “V” is present.
The flame shape of FIG. 4C is formed by creating an EHD bluff-body using the copper wire (204) along with an electric field. As depicted in the diagram of FIG. 5, the electric field accelerates the positive ions present in the flow towards the wire (204). Their upstream movement causes collisions with the other gas molecules and creates a local reduction of the gas velocity near the wire (204). The gas velocity profile is depicted by the white line in the figure. Increasing the electric field magnitude causes a greater velocity reduction near the wire (204). This reduction can be great enough to reduce gas velocity all the way to the laminar flame speed of the gas mixture, SU. If this occurs, a flame root forms just downstream of the wire resulting in the observed “V” shape. This flame structure is the same as that seen with the previous aerodynamic bluff-body. Thus, the electric field reduces the flow velocity near the wire (204) and the flame responds to this velocity reduction the same way it does when the flow is blocked with an aerodynamic bluff-body. Due to the similarity, this effect is referred to as an EHD bluff-body.
The primary benefit of the EHD bluff-body compared to a traditional aerodynamic one is the EHD version can be switched on and off very quickly with the electric field, much like turning a light on and off with a switch. This switching can be up to a few hundred cycles per second. Essentially, the flame shape is now electrically controlled.
ii. Heat-Release Change
A heat-release change occurs when the flame shape transitions. The conical and “V” flame surfaces have internal volumes, as illustrated in FIG. 6. A key feature is that the internal volumes between the two shapes are different, with the volume of the conical flame being larger than the volume of the “V” flame. When the flame shape is changed with the electric field, the internal volume of the flame reduces. The volume reduces by the difference between the two internal volumes, ΔV. During the transition from the conical to “V” shape, this volume difference (ΔV) is consumed by the flame and adds to the thermal power of the flame.
The thermal power is more commonly referred to as “heat-release,” q(t). A control volume analysis, defined in FIG. 7, is used is derive the relationship between the flame heat-release and changing flame volume:
where ĖIn(t) is the rate of energy into the control volume and ECV(t) is the chemical potential energy associated with the control volume. These two terms can be expressed in terms of the reactant properties
Ė
In(t)=hcρAv(t), (“Equation 2”)
and
E
CV(t)=hcρV(t), (“Equation 3”)
where hc is the heat of combustion of the mixture, ρ is the reactant mixture density, A is the burner outlet area, v(t) is the gas velocity into the control volume, and V(t) is the control volume. The heat-release with these substitutions is
To simplify this expression, a constant incoming gas velocity is assumed, v(t)=v. Additionally, the change in heat-release can be created by changing the control volume, so the heat-release deviation can be viewed from the mean value, defined as
q′(t)=q(t)−q, (“Equation 5”).
The heat-release deviation, q′(t), in terms of Equation. 4 is then
This equation shows that changing the control volume, V(t), by the electric field will cause a deviation in the flame heat-release, q(t). This mechanism constitutes an actuator which can be used to suppress the thermoacoustic instability. An experimental example of this effect is shown in FIG. 8, which shows the measured heat-release deviation when applying a slowly varying ramp waveform in electric field magnitude. The response shows there is a 12 to 14% brief increase in heat-release when the flame transitions from the conical to the “V”-shape. When the electric field is then decreased and the flame makes the reverse transition from the “V” back to the conical shape, there is a 5 to 7% decrease.
iii. Multi-Element Cathode
Adding more cathode elements can improve the actuator relationship. The single wire heat-release response of FIG. 8 shows that the electric field changes the heat-release by causing the flame to transition between shapes. However, the changes in heat-release were only brief impulses, which may not be ideal for an actuator. To better characterize this actuator, an alternative representation is used. If the heat-release deviation, q′ (t), is integrated, it gives the energy reduction of the control volume, ΔQ. This is illustrated in FIG. 9A, which shows the heat-release versus time, and FIG. 9B, which shows the normalized control volume energy reduction (ΔQ) versus electric field strength (E). The control volume energy reduction is the area under the heat-release curve. Comparison of the two plots of FIGS. 9A-9B shows that the positive heat-release impulse (see, FIG. 9A) corresponds to the step increase in ΔQ (see, FIG. 9B). This occurs when the flame shape transitions from the conical to the “V” shape (see, FIG. 9B). Similarly, the negative heat-release impulse corresponds to the step decrease in ΔQ, occurring when the flame transitions back from the “V” to the conical shape (see, FIG. 9B). The step-like shape of the ΔQ vs. E plot is characteristic of affecting the flame with an EHD bluff-body. With a goal of making an actuator of heat-release using the electric field, the ideal relationship between the two instead would be linear and continuous, represented by the linear line in FIG. 9B. To better approximate the linear relationship, more cathode elements can be added. For each cathode element added, an additional EHD bluff-body is therefore created and another step change occurs in the ΔQ vs. E profile. This concept is illustrated in FIG. 10, which shows the approximation if N cathode elements are added. Particularly, FIG. 10 shows that the desired linear relationship can be approximated by a quasilinear relationship by adding N cathode elements, creating N transitions of the flame geometry. Each flame transition between two shapes creates a step change in the control volume energy reduction.
B. Exemplary Electrodes Configured for Electric Field Induced Flame Heat-Release Modulation
i. Three-Wire Cathode
To test the above-described concepts, a burner (300) with three wires (302) across the face (304) of the burner (300) was constructed as shown in FIG. 11A. Additionally, a power source (306) is configured to provide an electric voltage potential difference between a positive electrode (308) within the flame (312) and a negative electrode (310) on the burner face (304). In this example, the burner face (304), and therefore the three wires (302), are coupled with the negative terminal of the power source (306). Accordingly, the electric field is arranged primarily parallel to the gas flow direction (e.g., parallel to axis (318)) but is oriented to oppose the gas flow direction. Optionally, a data processor (314) may be communicatively coupled with the voltage source and the burner and configured to continuously measure the acoustic pressure via a sensor (316) to determine an acoustic instability, and to provide data signals to selectively operate the voltage source (306) to maintain combustion stability (see, the method of FIG. 21). FIG. 11B shows the progression of flame shapes as the electric field is increased. As the images of FIG. 11B show, there are four possible flame shapes because the three wires create three shape transitions as the electric field is increased. The flame images of FIG. 11B show that the flame becomes more compact as the electric field increases due to the increase of additional EHD bluff-bodies with increasing field strength. The heat-release response for this three-wire cathode, shown in FIG. 12A, illustrates that adding more cathode elements will cause more heat-release events. During the increasing portion of the electric field strength, there are now three separate positive impulses (310, 312, 314) corresponding to when flame shape transitions occur. Similarly, when decreasing the electric field strength, there are now three separate negative impulses (316, 318, 320). This demonstrates the concept that the number of individual heat-release events scales with the number of cathode elements.
In the control volume energy reduction plot, shown in FIG. 12B, the three positive impulses of heat-release create three positive steps, and the three negative impulses create three negative steps. The overall shape of the ΔQ vs. Ē plot for the three-wire cathode of FIG. 11A has several improvements over the single wire system of FIG. 4A. It shows multiple changes due to the additional cathode elements and this creates a closer encirclement of the desired linear profile. However, the response has significant hysteresis which may not be ideal for an actuator and three heat-release events over the entire electric field range is still a very coarse actuator. To further improve this profile, even more cathode elements may be added, as will be presented below.
ii. “Honeycomb” Cathode
To add more cathode elements across the burner face, a hexagonal metal material can be used which is referred to herein a “honeycomb” structure. The honeycomb may define hexagonally shaped cells. In one example embodiment, shown in FIG. 13, the cells may define a two-millimeter cell width and 0.127-millimeter wall thickness, therefore providing an approximately 87% open area. Electrically, the cell walls can act as the cathode elements and the high cell density can provide many potential conducting edges for EHD bluff-bodies to form. Additionally, a power source and pair of electrodes may be configured and positioned similar to power source (306) and electrodes (308, 310) as described with regard to FIG. 11A. When the electric field is applied and slowly increased, the flame distorts in small increments and in a nearly continuous manner as shown by the progression photos illustrated in FIG. 14. These smaller changes between successive flame shapes result in a more even heat-release and control volume energy reduction, as shown in the plots of FIGS. 15A-15B. Instead of a few discrete heat-release impulses as seen with the wire cathodes, the heat-release for the honeycomb cathode (see, FIG. 15A) is relatively flat for large portions. There are a few larger events corresponding to larger changes of the flame collapsing but this is only at the upper end of the electric field magnitude.
The constant portions of heat-release shown in FIG. 15A translate to linearly increasing portions in the control volume energy reduction plot as shown in FIG. 15B. Comparison of the ΔQ vs. Ē profile to the linear trendline shows this profile is the closest of the three cathodes to a quasilinear and semi-continuous relationship. There are still some jumps corresponding to the large structure changes of the flame, but this response illustrates an advancement in controlling heat-release compared to any previous methods currently known.
iii. Multi-Element Circumferential Cathode
As another alternative cathode arrangement, FIG. 16 provides a cathode (350) having a plurality of elements (352) arranged around the circumference of the burner face (354). In one embodiment, a metal or other conductive material can form the elements (352). Particularly, the elements (352) may be arranged each having a two-dimensional linear body aimed toward the central position (356) defined by a central axis (358) of the burner face (354). Additionally, a power source and pair of electrodes may be configured and positioned similar to power source (306) and electrodes (308, 310) as described with regard to FIG. 11A. When the electric field is applied and slowly increased, the flame distorts in small increments and in a nearly continuous manner, similar to the flame effects described above.
C. Heat-Release vs. Electric Field Forcing Frequency
The above description identifies the mechanism for how an electric field can distort a flame shape and create a change in heat-release. As described below, heat-release may be forced at the higher frequencies where thermoacoustic instabilities occur, which can be as low as 50 Hz and greater than 1 kHz. An example of forcing the flame and heat-release with the electric field is shown in FIG. 17A. The upper graphical plot shows a sinusoidal electric field with a frequency of 186 Hz while the lower graphical plot shows the resulting heat-release. The heat-release is largely sinusoidal and at the same frequency as the electric field, along with some distortion and a phase shift. The resulting amplitude of the heat-release is relatively small at slightly less than 1% of the mean value, but this is more than adequate to suppress thermoacoustic instabilities, as we demonstrate in the next section. The heat-release amplitude that can be created by the electric field is a strong function of the forcing frequency and diminishes with higher frequencies. An example of this is shown in the plot of FIG. 17B. More particularly, FIG. 17B shows how the heat-release amplitude varies with the frequency of the electric field. The plot shows that a peak amplitude of 7% occurs around 25 Hz and the magnitude reduces after that, however the magnitude is above 1% almost to 200 Hz.
D. Suppression of Thermoacoustic Instabilities
As described above, the electric field actuation of heat-release suppresses thermoacoustic instabilities. To test this, the honeycomb cathode and burner were placed inside of a round quartz tube in a configuration known as a Rijke tube, as shown in FIG. 18A. At the conditions tested, the flame had a thermal power of approximately 3 kW, which is similar to the size of a single burner in a residential furnace. The Rijke tube configuration creates the right conditions for a thermoacoustic instability to exist. When it occurs, the acoustic pressure and flame heat-release develop into a self-sustaining oscillation at the instability frequency (˜140 Hz). Examples of the oscillating pressure and heat-release are shown in FIG. 18B. A feature of a thermoacoustic instability is the pressure and heat-release oscillations will be in phase, shown by the traces in FIG. 18B.
A key question with suppressing thermoacoustic instabilities is how to affect the instability, which depends on the actuator type. As shown in FIG. 19A, the thermoacoustic instability is represented by a cycle diagram where the two cycle elements are a block representing the chamber acoustics and one representing the heat-release and combustion process. The two input/output variables of the cycle are the acoustic pressure and flame heat-release. With the electric field method developed here, it was identified that the main effect of the actuator was to modify the flame heat-release. This is represented in the cycle diagram of FIG. 19B, where a block for the electric field has been added. The heat release driven by the electric field is qE(t) while the thermoacoustically driven heat-release is qA(t).
These two sources add together to create the total heat-release of the flame, q(t). To suppress the instability, the concept is simply to use the electric field driven heat-release to cancel the thermoacoustic component; qE=−qA. This was accomplished with a feedback control system, represented by the cycle diagram of FIG. 19C. A feedback path sends the acoustic pressure (p(t)) to a controller and the controller sets the value of the electric field. If the controller is property tuned, the thermoacoustic instability can be suppressed.
An example of the controller turning on and suppressing a thermoacoustic instability is shown in FIG. 20. The upper graphical plot of FIG. 20 shows the acoustic pressure and flame heat release while the lower graphical plot of FIG. 20 shows the electric field. Just prior to the controller turning on at t=0 milliseconds, the pressure and heat release are in phase. Immediately after turning the controller on, the synchronization between the two is cancelled and the pressure starts to diminish. Shortly later, at around 25 milliseconds, the heat release is out-of-phase with the pressure which is the fastest way to suppress an instability and a sign that the controller and electric field are working effectively. It takes less than 60 milliseconds to suppress the fully instability.
FIG. 21 illustrates a flowchart showing a method (400) of affecting a burner flame shape and heat release using an electrohydrodynamic (EHD) bluff-body according to the description above. At step (402), a flame is generated from a burner. The burner includes a cathode which may be built into the face of the burner. In some embodiments, the cathode may include one or more narrow wires, while in other embodiments the cathode may include a multi-element structure which resembles a honeycomb structure. At step (404), a first electrode is positioned within the flame, and a second electrode is positioned adjacent to (i.e., outside of) the flame (see, for example, FIG. 11A) or, in some embodiments, also within the flame. In some embodiments, the first electrode is a positive terminal electrode, and the second electrode is a negative terminal electrode. Further, the second electrode may in some instances be built into the burner face. At step (406), a voltage potential is applied between the first and second electrodes to therefore form an electric field. The electric field may have a vertical orientation (i.e., in line with/in parallel to the gas flow direction), with the electric field opposing the gas flow direction. In some embodiments, such as for certain fuel and air mixtures, the electric field can be configured in the same direction as the gas flow direction. Accordingly, the cathode shape is configured to focus the electric field around it, such that positive ions which are naturally present in the flame are accelerated by the electric field toward the cathode. As the ions collide with the bulk gas molecules, the bulk gas velocity is reduced in the local area immediately downstream from the cathode. Accordingly, an EHD bluff-body is formed (as opposed to an aerodynamic bluff-body), which alters the flame shape and results in a change in the heat-release of the flame. Thereafter, at step (408), the voltage potential provided by the electrodes can be adjusted to modify the flame root again, such as by forming one or more additional EHD bluff-bodies and therefore to modify the heat release of the flame again. Particularly, increasing the voltage potential (and therefore the electric field strength) is operable to reduce the gas velocity even more. Once the gas velocity is reduced to the laminar speed of the gas mixture, a new flame root forms in the area immediately downstream from the cathode. This new flame root is operable to significantly modify the flame shape. Further, reducing the voltage potential reverses the affects.
With further regard to step (408), adjusting the electric field to therefore modify the number of flame roots is operable to modify the heat release of the flame as well. Particularly, a modified flame root by increasing the electric field effectively makes the flame more compact (i.e., shorter). For complex flame shapes where it is difficult to define a flame length, the mean axial distance to the surface is used and is defined as the flame centroid. Making the flame more compact moves the flame centroid upstream. An overall movement of the flame upstream therefore increases the flame heat-release. Decreasing the voltage and electric field strength to the point where the flame root diminishes then causes the shape to expand back to the original. This causes a brief decrease in heat-release for the same but opposite reasons as does increasing the electric field strength. In another sense, making the flame more compact means the reactions takes less volume to occur in such that the internal volume bounded by the flame shape is reduced. As the flame root forms and the flame dynamically transitions to the more compact form, the flame burns through the volume difference between the two shapes. This adds to the heat-release while the shape is in transition.
Further, when the cathode has multiple physical elements, each become a site for an EHD bluff-body and flame root to form. Increasing the voltage and electric field causes the first EHD bluff-body and flame root to form at one of the cathode elements, along with the accompanying brief increase of heat-release. Increasing the voltage and electric field further eventually causes another EHD bluff-body and flame root to form at a different cathode element, along with another brief increase in heat-release. If the cathode has N elements, then N independent EHD bluff-bodies and flame roots can form as the voltage and electric field are increased, which will result in N heat-release changes. Therefore, if a large number of cathode elements are used, the relationship between the applied voltage and resulting flame shape change becomes semi-continuous and quasilinear, which is ideal for an actuator. Given the systems and methods described above, the electric field may be selectively controlled to adjust the number of flame-roots in real time to therefore adjust the heat release in real-time.
FIG. 22 depicts a flowchart of one exemplary method (500) of suppressing thermoacoustic instability of a burner flame using feedback control and electric fields. At step (502), a flame is generated from a burner. The burner system may be configured similar to the burner described above with regard to method (400) and/or with regard to FIG. 11A. At step (504), a data processor coupled with a sensor is configured to measure an acoustic instability of the burner. It should be understood that thermoacoustic instabilities can be detected by a variety of methods and sensors. For example, an acoustic pressure sensor may be used. In one alternative example, the heat-release which fluctuates during the instability can be optically measured by a photomultiplier tube or photodiode. In another alternative example, the fluctuating heat-release can be measured by a conduction measurement, such as by using a Langmuir probe. Based on the measurement, the data processor is configured to selectively provide data signals to the voltage source to adjust the electric potential to thereby cause a heat-release change in the flame. The electric field induced heat-release change counteracts the thermoacoustically driven heat-release change to reduce total heat-release oscillation and the resulting pressure oscillation. Accordingly, the thermoacoustic instability in the system begins to diminish. Going forward, the system is configured to continue this measurement and adjustment procedure in real-time to therefore determine when acoustic instability arises and to adjust accordingly to maintain improved acoustic stability. In some embodiments, the data processor is configured to compare an acoustic characteristic, such as an acoustic pressure of the burner, to a pre-determined unacceptable acoustic pressure to determine whether acoustic instability is present in the system. Once the pre-determined unacceptable acoustic characteristic has been achieved (e.g., an acoustic oscillation magnitude), the data processor selectively adjusts the electric field to lower the oscillation magnitude to an acceptable level.
III. Discussion
Illustrated and described herein are systems and methods capable of modifying flow fields of flames, altering flame shapes, modifying the flame heat-release, and suppressing thermoacoustic instabilities. Certain portions of those systems and methods are described in additional detail below.
In some embodiments for modifying the flow fields of flames, a positive electrode (i.e., anode) is positioned downstream of the burner within the flame. A negative electrode (i.e., cathode) is placed near the burner face. The negative electrode may be thin in cross section so as not to cause flow recirculation and to act as an aerodynamic bluff body, causing a flame root to form downstream of it. A voltage difference may then be applied between the electrodes to create an electric field that is oriented mostly against the gas flow path. The positive ions (i.e., cations) present in the flame are accelerated by the electric field, particularly in the same direction as the electric field, thus they are accelerated against the bulk gas. The elastic collisions between the ions and bulk gas molecules transfers momentum to the flow field, which is an effect commonly referred to as “ionic wind.” Since the ions are accelerated against the bulk gas, the net effect of the ionic wind is to reduce the flow velocity. The velocity reduction is localized to the region immediately around the cathode due to unique combination of high electric field magnitude and high cation density there. The amount the velocity is reduced increases with the applied anode voltage and resulting electric field magnitude, and the anode voltage and resulting electric field strength can be adjusted to vary the velocity reduction amount.
In some embodiments for modifying the flame shape, it has been noted that the shape of a laminar flame is determined by stabilization points or roots, among other factors. A stabilization point is a region of space where the local flow velocity is less than or equal to the laminar flame speed of the gas mixture, SU. The laminar flame speed is an inherent combustion property of a fuel-oxidizer gas mixture. Adjusting the critical condition of the velocity to be less than or equal to the laminar flame speed creates a stabilization point, and a new flame root will form there. The flame root appears as a “V” or fold in the flame surface, centered immediately downstream of the negative electrode. The process of reducing the flow velocity to the laminar flame speed with the electric field is referred to herein as an electrohydrodynamic bluff body. If the negative electrode is located too close to a pre-existing flame root, creating an EHD bluff-body will have little impact on the flame shape. Therefore, the negative electrodes may be positioned away from pre-existing flame roots, including those created by aerodynamic or other EHD bluff bodies. An EHD bluff body and root can form over each cathode element added, and adding additional cathode elements proportionately increases the number of flame roots created as the electric field is increased. Creating multiple bluff bodies and roots leads to better control of the heat-release as the flame can be incrementally distorted in discrete steps.
In some embodiments for modifying the flame heat release, creating flame roots by the EHD bluff body can be used to modify the heat release and to fix combustion issues related to thermoacoustic instability. A laminar flame has an internal volume defined by the flame surface. When the flame shape changes due to creating an EHD bluff body and new flame root, the internal volume is reduced. Essentially, each flame root and fold in the flame surface makes the flame more compact, thus reducing its internal volume. As such, deploying multiple cathode elements causes incremental changes of the flame shape and thus heat release. Further, the multiple cathode elements improve the relationship between the electric field and resulting heat release change, which provides better control for suppressing thermoacoustic instabilities.
While examples, one or more representative embodiments and specific forms of the disclosure have been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive or limiting. The description of particular features in one embodiment does not imply that those particular features are necessarily limited to that one embodiment. Some or all of the features of one embodiment can be used in combination with some or all of the features of other embodiments as would be understood by one of ordinary skill in the art, whether or not explicitly described as such. One or more exemplary embodiments have been shown and described, and all changes and modifications that come within the spirit of the disclosure are desired to be protected.
Patent Application
20240175578
