SYSTEMS AND METHODS FOR SUPPRESSING THERMOACOUSTIC INSTABILITIES OF FLAMES

TECHNICAL FIELD

The present disclosure in general relates to manipulating flames and, more particularly, to applying an electric field and suppressing thermoacoustic instabilities of a flame 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 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 made it to widespread adoption due to their great expense or their 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 electrical potentials to the combustion reaction to improve efficiency and reduce harmful emissions. To that end, the present disclosure includes aspects which can include an apparatus having combustion burner configured to output a flame and a gas flow from a face of the burner, the gas flow of which can define a gas flow path in a direction away from the burner. The apparatus can include a first conductive element positioned within the flame, a second conductive element positioned across the face of the burner, and a power source. A positive electrode and a negative electrode can each be coupled with the power source such that the positive electrode can be electrically coupled with the first conductive element and the negative electrode can be electrically coupled with the second conductive element. The positive electrode and the negative electrode can be selectively operable to apply a first electric signal to the flame to form a modified flame having a modified flame characteristic relative to the flame. The positive electrode and the negative electrode can be selectively operable to apply a second electric signal, simultaneous with the first electric signal, to the flame to affect a characteristic of the modified flame.

In some embodiments, the first electric signal can define a first signal frequency, the second electric signal can define a second signal frequency, and the second signal frequency can be greater than the first signal frequency. In certain embodiments, the second electric signal includes a periodic voltage signal, such as a sinusoidal signal, a pulse-width modulation (PWM) signal, or a pulse train signal. In still other embodiments, the first electric signal can define a first signal amplitude, the second electric signal can define a second signal amplitude, and the second signal amplitude can be greater than the first signal amplitude.

Additional embodiments can include a sensor and a data processor. The sensor can be configured to determine an acoustic characteristic of the combustion burner and output a data signal based upon the acoustic characteristic. The data processor can be communicatively coupled with the sensor and the power source. The data processor can be configured to receive the data signal and selectively operate the power source to modify at least one of the first electric signal and the second electric signal based upon the data signal. In some embodiments, the data processor can be configured to compare the acoustic characteristic to a pre-determined acoustic characteristic and modify at least one of the first electric signal and the second electric signal to thereby achieve an improved acoustic characteristic. In some embodiments, the data processor can be configured to compare the acoustic characteristic to a pre-determined acoustic characteristic and modify the signal frequency of the second electric signal to thereby achieve an improved acoustic characteristic.

In other aspects, a method of operating a combustion burner to affect a flame output from the combustion burner is described. The method can include various acts such as generating a flame from the burner, generating a first electric field between the positive electrode and the negative electrode, thereby generating a modified flame, and combining a second electric field with the first electric field to affect the modified flame. The second electric field can selectively generate pinch-off and pocket formation of the modified flame.

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 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. 11B depicts a graphical chart showing the heat-release amplitude as it relates to the electric field frequency;

FIG. 12A depicts a system schematic and resultant flame shape according to a Rijke tube experiment;

FIG. 12B 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. 13A depicts a block diagram showing a thermoacoustic system that is unstable and produces oscillations of heat-release and pressure;

FIG. 13B depicts a block diagram showing that the electric field driven heat-release interacts with the thermoacoustically-driven component;

FIG. 13C 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. 14 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. 15 depicts a series of images illustrating flame pinch-off and pocket formation during a thermoacoustic instability;

FIG. 16 depicts a series of images illustrating the feedback control stabilized flame experiencing pinch-off and pocket formation;

FIG. 17 depicts a series of images illustrating an electric pulse on the flame surface, showing the creation of a surface retraction by applying a positive voltage pulse, 1 millisecond in width and 2000 volts in amplitude;

FIG. 18 depicts images illustrating surface distortions added to the unstable flame using secondary forcing methods, illustrating a first image (left) of the thermoacoustic instability flame at 142 Hz showing the retracted regions where pinch-off will occur, and illustrating a second image (right) of the thermoacoustic instability flame also being forced by the secondary forcing signal of 214 Hz, with the arrows indicating the surface retracted regions caused either by the instability forcing or the secondary forcing signal;

FIG. 19A depicts a graphical representation of a measured period between pocket formation events, showing the measured period between pocket formation events for the unstable flame oscillating at 142 Hz;

FIG. 19B depicts a graphical representation of a measured period between pocket formation events, showing the measured period for the case of the 142 Hz instability and forcing the flame with the secondary forcing voltage at 214 Hz;

FIG. 20 depicts a schematic showing a thermoacoustic instability cycle;

FIG. 21A depicts a graphical representation of the effect on the pressure oscillation from adding secondary forcing, showing the pressure time-series of the thermoacoustic instability;

FIG. 21B depicts a graphical representation of the effect on the pressure oscillation from adding secondary forcing, showing the pressure time-series after adding the secondary forcing signal;

FIG. 22 depicts a schematic diagram showing secondary forcing combined with a feedback controller to manipulate a flame;

FIG. 23A depicts a graphical representation of the improvement made to a flame using by secondary forcing, showing the pressure time-series with only the feedback controller

FIG. 23B depicts a graphical representation of the improvement made to a flame using by secondary forcing, showing the pressure time-series with the feedback controller and secondary forcing at 200 Hz;

FIG. 24 depicts a graphical representation of acoustic pressure spectra improvement with secondary forcing, showing the addition of a 200 Hz secondary forcing signal to produce a further 14 dB reduction in the peak pressure amplitude compared to just the feedback controller;

FIG. 25 depicts a schematic showing the trade-off between pocket formation pressure and secondary forcing pressure;

FIG. 26A depicts a graphical representation of the impact of secondary forcing frequency on heat release and pressure, showing the magnitude response of forcing the unconfined flame (i.e., no acoustics) with the electric field and measuring the fluctuating component of heat-release, where the frequencies where the magnitude approaches zero (circled) are due to the regions of retracted and expanded surface distortion mostly offsetting one another;

FIG. 26B depicts a graphical representation of the impact of secondary forcing frequency on heat release and pressure, showing the pressure standard deviation relative to the secondary forcing frequency;

FIG. 27 depicts a graphical representation of sound pressure spectra with the feedback controller only (gray) and with the secondary forcing at 214 Hz (black);

FIG. 28 depicts a flowchart representation of one exemplary method of affecting a flame using a secondary voltage signal to disrupt pinch-off and pocket formations; and

FIG. 29 depicts a flowchart representation of one exemplary method of combining a secondary voltage signal with a primary voltage signal to affect a flame via a feedback controller.

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 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. This technology may work for non-gaseous fuels or non-continuous combustion but tests of these have not yet been performed.

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 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 Methods for Electric Field Induced Flame Heat-Release Modulation Using a Bluff-Body

The systems and methods include three general parts: (i) creation of an EHD bluff-body, (ii) the accompanying heat-release change, and (iii) the use of 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 FIGS. 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 Error! Reference source not found. 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 Error! Reference source not found., is used is derive the relationship between the flame heat-release and changing flame volume

$q (t) = {\dot{E}}_{In} (t) - \frac{d E_{CV} (f)}{dt},$

(“Equation 1”) where Ė_In(t) is the rate of energy into the control volume and E_CV(t) is the chemical potential energy associated with the control volume. These two terms can be expressed in terms of the reactant properties

${\dot{E}}_{In} (t) = h_{c} ρ A v (t),$

(“Equation 2”) and

$E_{CV} (t) = h_{c} ρ V (t),$

(“Equation 3”) where h_cis 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

$q (t) = h_{c} ρ A v (t) - h_{c} ρ \frac{dV (t)}{dt},$

(“Equation 4”). 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

$q^{'} (t) = q (t) - \bar{q},$

(“Equation 5”). The heat-release deviation, q′(t), in terms of Equation. 4 is then

$q^{'} (t) = - h_{C} ρ \frac{dV (t)}{dt},$

(“Equation 6”).

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 Error! Reference source not found., 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. 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. 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. 11A. 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. 11B. More particularly, FIG. 11B 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.

C. Suppression of Thermoacoustic Instabilities

As described above, the electric field actuation of heat-release suppresses thermoacoustic instabilities. To test this, a multi-element (“honeycomb” style) cathode and burner were placed inside of a round quartz tube in a configuration known as a Rijke tube, as shown in FIG. 12A. 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. 12B. A feature of a thermoacoustic instability is the pressure and heat-release oscillations will be in phase, shown by the traces in FIG. 12B.

A key question with suppressing thermoacoustic instabilities is how to affect the instability, which depends on the actuator type. As shown in FIG. 13A, 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. 13B, where a block for the electric field has been added. The heat release driven by the electric field is q_E(t) while the thermoacoustically driven heat-release is q_A(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; q_E−q_A. This was accomplished with a feedback control system, represented by the cycle diagram of FIG. 13C. 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. 14. The upper graphical plot of FIG. 14 shows the acoustic pressure and flame heat release while the lower graphical plot of FIG. 14 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.

Demonstrated and described above are improved systems and methods configured to suppress instabilities using an electric field and feedback control loop. This electric field method is significantly less expensive compared to previous actuator types, has no moving parts to wear out, and is significantly more power efficient.

D. Exemplary Methods for Electric Field Induced Flame Heat-Release Modulation Using a Secondary Control Signal

In addition to the electric field stabilization systems and methods described above, a secondary forcing procedure may be provided via an additional control signal that significantly improves the instability reduction. Particularly, the secondary forcing method, which will be described in greater detail below, is an additional control signal configured to improve the instability reduction already provided by the electric field stabilization methods described above. For example, experiments on a laboratory scale instability showed that the electric field stabilization method reduced the peak pressure amplitude by 22 times, and the secondary forcing method then provided a further eight times reduction. This is a significant improvement and even more so when considering it adds no cost to the electric field stabilization method above.

i. Flame Pinch-Off and Pocket Formations

As described above, a thermoacoustic instability is an undesired oscillation of the combustor pressure and flame heat-release. Small oscillations of pressure and heat-release decrease combustion efficiency and increase the production of harmful emissions. Large pressure oscillations can permanently damage the device.

FIG. 15 shows a series of images of the uncontrolled flame experiencing a thermoacoustic instability. The flame surface is heavily distorted by the fluctuating acoustic velocity, producing both expanded and retracted regions of the surface. The distorted features convect downstream and when retracted portions get close to the flame tip, the flame surface pinches off and an unburned reactant pocket forms (labeled in image), causing unwanted heat release fluctuation and sound. This process increases the amount of flame surface area variation and as the heat-release is proportional to the surface area for laminar flames, the pocket formation process increases the heat release fluctuation. The process of pinch off and pocket formation will be referred to herein as “pocket” formation.

Further amplifying the problem is the pocket formation process is driven by the acoustic fluctuations of the thermoacoustic instability, therefore the pocket formation process has the same frequency as the instability. If the pocket formation process and its accompanying heat release fluctuation occur in phase with the acoustic pressure, the pocket formation process amplifies the instability pressure oscillation. As discussed above, it was found that this pocket formation process occurred for the controlled flame, as shown in the images in FIG. 16. The flame surface in the images is much smoother than the unstable flame of FIG. 15, but expanded and retracted regions of distortion still exist, and pocket formation can be seen in the last image of FIG. 16. As this increases the heat release fluctuation, it would inevitably be a source of unwanted pressure which needed to be eliminated. As previously discussed, no feedback controller was found that could eliminate this so the method of secondary forcing was developed to address it.

ii. Systems and Methods for Adding Breaking Points in the Flame Surface (“Secondary Forcing” Method)

The secondary forcing method can be summarized as reducing pocket formation by intentionally causing more of it. The flame surface distortions shown above in FIGS. 15 and 16 were caused by flow velocity fluctuations driven by the thermoacoustic instability. However, it was found that the electric field could create surface distortions, as shown in Error! Reference source not found. Here, a short 1 millisecond electric field pulse was applied to the flame. The images show the electric field pulse creates an inward surface distortion that originates at the base of the flame and then convects downstream at the bulk flow velocity. When the inward distortion feature gets close to the flame tip around 20 milliseconds, it causes pinch-off and pocket formation. While this might seem unwanted, controlling when this pocket formation occurs at is highly useful. If multiple pulses are created with a constant period, those pulses will create pinch-off and pocket formation around the same constant period.

The first part of the secondary forcing method is to use the electric field to cause pinch-off and pocket formation, but at a different frequency than the instability. FIG. 18 shows an example of this. The left image shows the original flame distorted by only the thermoacoustic instability. The right image shows the flame distorted by the instability and the secondary forcing signal. The secondary forcing voltage applied to the anode was a sine wave with a DC offset:

U
_S.F.(t)A_S.F.sin(2π·f_S.F.t)+U_DC

(“Equation 7”). The instability was forcing the flame at 142 Hz while the secondary forcing signal was set to 214 Hz. Setting the secondary forcing signal to a higher frequency than the 142 Hz of the instability causes more frequent pocket formation events. The image on the right clearly shows a smaller spacing between distortions and more retracted regions on the flame surface. Each of these retracted regions will cause a flame pinch-off point (indicated by arrows) when they reach the flame tip. Thus, the effect of secondary forcing on the flame at a frequency greater than the instability frequency is to cause more frequent pinch-off and pocket formation events.

The period between successive pocket formation events was measured from high-speed images of the flames. FIG. 19A shows the measured period between pocket formation events for the flame experiencing the thermoacoustic instability at 142 Hz. The period is consistently around a mean value of 7.1 milliseconds, which matches the instability oscillation period (1/142 Hz=7.04 ms). FIG. 19B shows the measured period for the secondary forcing at 214 Hz. The trace shows a complicated pattern with periods ranging from as low as 2 milliseconds to as high as 11 milliseconds, demonstrating how the secondary forcing signal spreads the pocket formation timing away from the instability oscillation period

The benefit of disrupting the regular timing of the pinch-off process is shown by the thermoacoustic instability cycle diagram of FIG. 20. A thermoacoustic instability has two main components: an acoustic resonance formed by the acoustic modes of the combustion chamber and the combustion source. The diagram shows how one forces the other, with the acoustic pressure (p′) forcing the combustion into oscillation and the combustion heat-release (q′) in turn forcing the acoustic resonance. When the pocket formation occurs near the acoustic resonance of 142 Hz, the pressure oscillation is driven to a maximum amplitude which in turn forces the combustion heat-release to be even larger. Using the secondary forcing to move the pocket formation process away from the acoustic resonance frequency lowers the amplitudes of the acoustic pressure and the combustion heat-release. This effect is captured by measuring the acoustic pressure. FIG. 21A shows the acoustic pressure during the instability and shows large amplitudes ranging from 100 to 145 Pa. FIG. 21B shows the acoustic pressure while forcing with the secondary forcing and shows a clear reduction in amplitude compared to the original instability, and a beating pattern due to the mixing of the 142 Hz oscillation and 214 Hz secondary forcing signal. The overall reduction demonstrates that using the secondary forcing signal to disrupt the pocket formation process and shift it away from the acoustic resonant frequency is beneficial.

ii. Combining Secondary Forcing with the Feedback Controller

A significant benefit of the secondary forcing method stems from combining it with the feedback controller and electric field stabilization methods described in sections II(A) through II(C) above. A diagram of the combination of the two methods is shown in FIG. 22. The secondary forcing method is implemented on a microprocessor and the computed secondary forcing voltage (U_S.F.) added to the computed voltage for the feedback controller (U_C). The secondary forcing signal is:

$U_{S . F .} (t) = A_{S . F .} \sin (2 π f_{S . F .} t)$

(“Equation 8”) where A_S.F.is the amplitude of the secondary signal and f_S.F.is the secondary forcing frequency. Although only sinusoidal signals were used for the secondary forcing, conceptually any periodic signal could be used to create the pinch-off and pocket formation.

This combination reduces the total sound level in separate and complimentary ways. The feedback controller reduces the majority of the heat-release and pressure oscillation amplitudes caused by the instability, and the secondary forcing disrupts the periodic pocket formations. The net effect of the secondary forcing disruption on pocket formation is a reduction of the gain between pressure perturbations and the heat-release, represented in the “Combustion” block of FIG. 22. The improvement this makes is shown by the pressure traces in FIGS. 23A and 23B. FIG. 23A shows the pressure time-series with just feedback control while FIG. 23B shows the pressure time-series with the added secondary forcing. The reduction is more clearly seen by comparing the acoustic pressure spectra, shown in FIG. 24. The spectra show that secondary forcing reduces the peak amplitude by a further 14 dB, or a factor of 5 times. This is a significant reduction of the pressure amplitude, especially given the simplicity of the secondary forcing signal.

iii. Optimizing the Secondary Forcing Signal Frequency

Optimizing the secondary forcing frequency can further reduce the pressure amplitude of the flame. Using the secondary forcing method reduces the acoustic pressure at some frequencies while increasing it others. There are two sources of acoustic pressure affected by the secondary forcing, as shown in the diagram of FIG. 25. Along the “Secondary Forcing Path,” the secondary forcing voltage drives a heat-release oscillation (q_E(t)) at the secondary forcing (f_S.F.), which forces an acoustic pressure amplitude at the same frequency. The previous example used a secondary forcing frequency at 200 Hz and the pressure this creates can be seen as a local peak at 200 Hz in the spectrum of FIG. 24. This acoustic pressure is undesired and a side-effect of the secondary forcing method. The beneficial part of the secondary forcing method occurs in the path “Pocket Formation Path.” This path is the pressure created by external disturbances (d(t)) being amplified by the thermoacoustic components. The secondary forcing effects this path by reducing the gain of the “Combustion” block. Thus, the secondary forcing creates undesired pressure at the forcing frequency but reduces pressure over a range of frequencies by disrupting pocket formation.

The trade-off between these two sources of pressure can be optimized because the heat-release (q_E(t)) forced by the secondary forcing signal is strongly frequency dependent while the pocket formation effect is not. This frequency dependence is evident in FIG. 26A which shows the resulting heat-release amplitude when forcing an unconfined flame (i.e., no acoustics) with the secondary forcing. The series of regularly spaced local minima are the result of laminar flame dynamics and the simultaneous presence of inward and outward distortions on the flame surface. This is a well-known effect when forcing laminar flames. The useful part of this effect is the flame can be forced with the secondary forcing at the frequencies corresponding to a local minimum (circles) and the heat-release amplitude will be minimum. Thus, the locations of local minima are the optimal frequencies for the secondary forcing because they'll generate the lowest amount of unwanted heat-release fluctuation and therefore also pressure fluctuation.

This idea was tested by stabilizing the instability with the feedback controller and then varying the frequency of the secondary forcing voltage while measuring the standard deviation of the pressure. The measured standard deviation of the pressure is plotted versus the secondary forcing as shown in FIG. 26B. For reference, the lowest pressure standard deviation achieved with the feedback controller is included on the plot (dashed line). The circles of the plot show where the local minima were expected to occur at, based on the unconfined flame magnitude response in FIG. 26A. Points 1, 2, and 4 show clear local minima while points 3 and 5 do not. Point 3 is likely not due to its proximity to the acoustic resonance peak at 142 Hz. The local maximum of point 5 cannot be explained at this time. The pressure standard deviation at points 1 and 2 is higher than the feedback controller reference (dashed line) because the secondary forcing frequency is lower than the pocket formation frequency (˜140 Hz), so most pocket formations would still occur around the instability frequency. The plot indicates that only secondary forcing frequencies greater than the instability frequency are beneficial.

The pressure spectra for the feedback controller and the feedback controller with secondary forcing at 214 Hz is shown in FIG. 27. The optimized secondary forcing at 214 Hz reduces the peak amplitude by 18 dB. Considering that the electric field stabilization and feedback controller reduced the peak amplitude by 27 dB, reducing the peak by another 18 dB is quite significant. Additionally, the pressure standard deviation was reduced by 52%, from 6.9 Pa to 3.3 Pa. Also, there is a clear reduction in the pressure component at the secondary forcing frequency. The 200 Hz secondary forcing frequency had an amplitude of 87 dB while the 214 Hz forcing amplitude is 79 dB. This 8 dB reduction comes from forcing at the local minimal and contributes to the overall pressure standard deviation reduction.

Accordingly, the use of a secondary forcing signal with an optimized forcing frequency reduced the peak pressure amplitude by a further 18 dB compared to the feedback only controller. This resulted in a total peak pressure reduction of 45 dB from the original thermoacoustic instability condition. The secondary forcing method is simple to implement as it only needs to be a periodic signal, such as the sinusoidal signal discuss here. Other periodic signals like a pulse-width modulation (PWM) or a pulse train would work as well. The simple secondary forcing signal improves upon the limit observed with the feedback only controller because it disrupts the physical phenomenon of pinch off and pocket formation. Lastly, since the secondary forcing signal is simple, the optimal parameters (e.g., amplitude and frequency) for it could be found or adapted in real-time, using the microprocessor, so that no prior knowledge of the specific application would be required. This makes the method well suited for wider applications without requiring significant engineering time for each application to optimize the effects.

FIG. 28 depicts a flowchart showing one exemplary method (200) for utilizing a secondary forcing signal to cause pinch-off and pocket formation that is at a different frequency than the same being forced by the instability. At step (202), a flame is generated from a burner. At step (204), a first electrode is positioned within or adjacent to the flame, and a second electrode is positioned within or adjacent to the flame. At step (206), a first voltage signal is applied between the electrodes to form an EHD bluff-body and therefore affect the flame. At step (208), a second voltage signal is simultaneously applied to the electrodes along with the first voltage signal, whereby the first and second voltage signals are combined. Accordingly, this method (200) disrupts the timing of the pocket formation of the flame and shifts its frequency away from the acoustic resonant frequency where it causes the highest pressures. The addition of the secondary forcing signal and subsequent disruption of the repetitive pocket formation reduces the heat release and pressure oscillations.

FIG. 29 depicts a flowchart showing one exemplary method (300) for combining the secondary forcing signal with the EHD stabilization method being implemented with a feedback controller. At step (302), the flame is generated from a burner, whereby a first electrode is positioned within or adjacent to the flame, and a second electrode is positioned within or adjacent to the flame. At step (304), a secondary forcing voltage signal is generated. At step (306), the secondary forcing voltage signal is combined with the feedback controller-generated voltage potential being applied to the flame. At step (308), the feedback controller (e.g., the microcontroller) applies the combined signal to the flame to counteract the thermoacoustic instability and disrupt the pinch-off and pocket formations withing the flame. Next, at step (310), the feedback controller monitors the flame stability. Finally, at step (312), the feedback controller determines whether adjustments need to be made to the secondary forcing voltage signal to further affect the flame and makes the adjustments accordingly.

Optimizing the secondary forcing voltage potential at step (312) can include one or more additional sub-steps. For example, the secondary forcing voltage potential is generated having an amplitude and a frequency. The frequencies of local minima in surface area amplitude exist because simultaneous inward and outward distortions present on the flame surface largely cancel one another. The heat release is therefore proportional to the surface area, so heat release amplitudes also experience minima at the same frequency. As such, the feedback controller determines a frequency of local minima by prior frequency response, a priori model based on flame geometry, or through real-time adaptive methods such as gradient descent, LMS, extremum seeking, or other known methods.

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.

SYSTEMS AND METHODS FOR SUPPRESSING THERMOACOUSTIC INSTABILITIES OF FLAMES

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