FIELD

The present disclosure describes technology related to the field of the prevention of combustion instabilities, especially in swirl stabilized combustion chambers of gas turbine engines.

BACKGROUND

Combustion technologies are probably the highest energy density sources of energy used conveniently by man. Even with the emergence of so-called clean, alternative energy sources, combustion is necessary to keep up with energy demands, and is irreplaceable in some fields of application, especially in the fields of gas turbine engines, whether for electric power generation, or in gas turbine engines for aircraft propulsion.

Some chemical products of such combustion are undesirable, being implicated in everything from local health and environmental damage to global climate change. One of the major classes of pollutants emitted by combustors our oxides of nitrogen, especially NO and NO2, collectively known as NO _X . Such nitrogen oxides are formed from the high temperature combustion of inert atmospheric.

By using lean premixed combustion, lower flame temperatures are achieved which results in the advantageous lowering of NO _X emissions. However, lean premixed combustion has the disadvantage that it is particularly perceptible to combustion instabilities, which are pressure and heat release oscillations originating from the coupling between the combustor acoustics, fluid dynamics and combustion. The acoustic oscillations in these combustors can reach unacceptably high levels. Such oscillations may increase noise pollution but also pose a danger to the operation of combustors in the premixed mode. Strong, discrete pressure oscillations are capable of causing mechanical damage to the combustor or nearby mechanical parts. These pressure oscillations are accompanied by strong oscillations in the flow velocity, which can grow to sufficiently large amplitudes to cause flame blow-off or flashback. Significant research has been carried out on the driving mechanisms of combustion instabilities in premixed swirl stabilized combustors, which are commonly used in gas turbines. The most prominent instability mechanisms in premixed swirl stabilized combustion are flow-flame interaction, acoustic waves-heat release coupling and equivalence ratio oscillations. Though the different mechanisms are recognized, the exact pathway by which the instabilities are triggered in different combustion systems are not always clear. Moreover, operating parameters such as flow velocity, swirl number, equivalence ratio, mixture inlet temperature and fuel composition significantly impact the combustion dynamics in swirl stabilized combustors.

It is well established that premixed swirl stabilized flames exhibit unique flame shapes depending on the operation conditions. Observance of different flame structures have been linked to design parameters such as exhaust contraction ratio, inlet confinement ratio, central body geometry, Reynolds number, Swirl number, and other parameters. Moreover, flame shapes are closely related to changes in operating conditions such as equivalence ratio, inlet temperature and mixture composition. Four distinct flame shapes have been observed in a swirl combustor followed by a sudden expansion: a columnar-jet flame, a Vortex-breakdown bubble stabilized columnar flame, a V-shaped flame stabilized along the inner shear layer (ISL) and an M-shaped flame stabilized on both the inner recirculation zone (IRZ) and the outer recirculation zone (ORZ). Among the four different flame shapes, the V-shaped flame and the M-shaped flame are of practical interest. The strong impact of the mean flame shape on vortex-flame interactions and combustion instability in premixed flames has been shown by a number of research groups.

The transition between the flame shapes can lead to unstable combustion. The intermittent ignition and extinction in the ORZ sometimes becomes violent enough to cause blow-off. Moreover, recent studies on the transition between stable and unstable combustion have been linked to the flame propagating into the outer recirculation zone (ORZ) of the combustion zone. As shown in the article by S. Taamallah et al, titled “Turbulent Flame Stabilization modes in Premixed Swirl Combustion: Physical Mechanism and Karlovitz number-based Criterion”, published in Combustion and Flame, 166 (2016) 19-33, as the equivalence ratio increases, the transition to a flame in the ORZ results from intermittent ignition of the reactant mixture in the ORZ by a flame kernel having sufficient energy to travel from the inner recirculation zone (IRZ), and to cross the shear layer (ISL) to the outer recirculation zone (ORZ), where it ignites the mixture. Because of the intermittent nature of the transition flame, this transition is characterized by a low frequency instability of about 10Hz. This low frequency may be related to structural changes within the recirculation zone during transition, which causes periodic expansion and contraction of the IRZ bubble. By comparing acoustically coupled and uncoupled combustors, previous experimental measurements on a swirl stabilized dump combustor with CH4/H2 fuel mixtures, have demonstrated that the mechanism of instability during transition between an IRZ and ORZ stabilized flame is due to fluid dynamic -heat release interaction. The intermittent existence of the flame in the ORZ induces large fluctuations in heat release thereby causing macro-scale changes in flame structure. Intermittent ignition of the reactants in the ORZ causes large amplitude heat release oscillations. Heat release fluctuations observed for this transition mechanism is a low frequency mode, which when coupled with the combustor acoustics can lead to thermoacoustic instabilities, causing large amplitude pressure oscillations at about 5-10Hz. The transitional flame can be referred to as “ORZ flickering” flame.

Currently used suppression techniques for combustion instability are based on the coupling between heat release of the combustion fluid dynamics and acoustics. Commonly used technologies for control and suppression of such combustion dynamics include:

(i) acoustic dampers, and acoustic actuators, including loudspeakers,

(ii) fuel staging, pilot flames, improved fuel/air mixing strategies, and fuel injector geometry.

(iii) modification of burner geometries.

These approaches generally focus on controlling the fuel distribution and acoustic environment. In practice, several techniques for suppressing combustion instabilities are used in every combustor, in order to overcome the instabilities which may be due to different sources.

There have been studies on the potential application of gas injection as a method to control instabilities. Studies on steady air injection in a backward facing step combustor have been published by an MIT research group, such as in the paper by H.M.Altay et al., entitled “Mitigation of thermoacoustic instability utilizing steady air injection near the flame anchoring zone” in Combustion and Flame, 157(4):686 - 700, (2010). The effects on combustion instability were studied by forcing air through a slot and micro-jet array, both having the same effective area. Effective reduction of overall sound pressure level (OASPL) was achieved by injecting the air stream, with the micro-jets exhibiting far more effectiveness than the slot injection. In an article from the same group, by Z. LaBry et al, entitled “Flow Structures in a Lean-Premixed Swirl-Stabilized Combustor with Microjet Air Injection” published in Proceedings of the Combustion Institute 33 (2011) 1575-1581, there is described the use of micro-jet injectors at the dump plane of a swirl stabilized combustor to suppress combustion instabilities. High momentum jets of air, or fuel and air mixtures, are injected into the ORZ through small holes arranged in different angular configurations to the main fuel/air injection direction, and it was found that counter swirl injection is most useful for suppressing instabilities. Use of such microjet air or air/fuel injection is additionally described in US patent No. 8,708,696 to A. Ghoniem et al, for “Swirl-Counter-swirl Microjets for Thermoacoustic Instability Suppression”. This reference specifically states that the microjets work through the modification of the flowfield in the flame anchoring zone. The counter swirl of the high speed air injection maintains a more compact combustion configuration. Use was made of high velocity air injection, at the speed of sound, and mass flow rates ranging from 12% to 18% of the total mass flow into the combustion chamber. The gas generally used was air, though fuel/air mixtures were also studied.

The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.

SUMMARY

The present disclosure attempts to provide novel systems and methods that overcome at least some of the disadvantages of prior art systems and methods. In the present application, there are described new exemplary systems and methods for controlling the stability of combustion, by eliminating or reducing the transitional “ORZ flickering” mode, recognized as a macro-scale repeatable mode of instability for combustion chamber configurations. In this mode, the flame alternates unstably between a V-shaped flame shape and an M-shaped flame shape. This instability mode usually occurs above lean blow-off limits, but still at reasonably lean conditions. The methods and systems described in this application suppress combustion instabilities induced by "ORZ flickering" mode. The method involves the injection of small quantities of dilution gases, preferably of inert gases, at low velocities, into the ORZ region of combustion. The diluent gas injections are performed at low flow rates, generally of the order of only 1 to 3% of the overall flow rate, but with a possibility of up to 5% of the overall flow rate. This dilutes the fuel/air composition close to the dump plane in the ORZ region, and it is believed that this mechanism suppresses the combustion flame in the peripheral regions of the combustion chambers, close to the outer walls, decreasing the temperature of the flame in that region, thereby assisting in the maintenance of the V-flame combustion, and hence suppressing transition to an M-flame configuration. Moreover, the injection of the diluent gas was experimentally observed to suppress thermo-acoustic oscillations at higher equivalence ratio, apparently by reducing the combustion intensity in the region near the side walls, due to the combustion inhibiting effect of the diluent gas on the mixture in the ORZ.

Two features of the gas injection distinguish the presently described configurations and methods from previously described configurations, where the injected flow is much faster and of much greater mass. The lower flow speed and lower mass of the present configurations, unlike some prior configurations, is not intended to cool the walls of the combustion chamber, and furthermore, does not affect the V-shaped mode, since the effect of the slow injection flow speed will not reach the parts of the V-flame more distant from the dump plane. The effect of the slow-speed gas injection can thus generate a significant effect on the shape of the flame in the combustion chamber, since the injector flows affect only the outer regions of the ORZ mode, thereby strongly inhibiting propagation of the flame into the ORZ region, and delaying any combustion instabilities associated with transit to that mode. Thus, only a very small stream of the diluent gas can generate a significant change in the combustion scheme.

In more detail now, gas turbine systems generally use swirling flows and a sudden expansion region in order to provide a compact combustion flame, rather than a long flame which would have low efficiency. The geometry of the main fuel/air injectors provides a recirculation zone in which the flame can stabilize. The flame can stabilize on an inner recirculation zone, or inner-and-outer recirculation zones, depending on combustion chamber conditions, and the fuel/air mixture being used. As previously mentioned in the Background section, it is known that the change in flame shape leads to combustion instabilities. Although both V-flame and M-flame combustion provide good efficiency and acceptably low noise combustion, the transitions between the V-flame and the M-flame configurations result in combustion instability, and such a transition instability is disadvantageous for both of the characteristics of efficiency and noise. This source of combustion instability is reduced according to the methods and combustion chamber configurations of the present disclosure, by reducing the likelihood that the combustion undergoes transitions between the V-flame and the M-flame. Typical diluents that can be used include air, nitrogen and carbon dioxide, with carbon dioxide being the most effective because besides being a diluent, it acts as a chemical kinetic inhibitor.

The flame shape of the combustion for a given combustion chamber is a strong function of the equivalence ration of the fuel/air mixture used. An increase in equivalence ratio results in the transition from a V-shaped flame to an M- shaped flame, where the transition region itself is marked by the low frequency flame instabilities. Use of the diluent injection methods and systems of the present disclosure, enables a shift in these low frequency instabilities to higher equivalence ratios, and hence enables stable V-shaped flame use up to a higher equivalence ratio. A first overall effect of this is that use of these methods and systems enables stable combustion to be maintained up to a higher equivalence ratio, thereby enabling more efficient combustion. However, a second advantage of these methods and systems is that the equivalence ratio up to which it is possible to operate a stable combustion mode in a lean premixed-fuel engine, can be changed by change in the level and characteristics of the injection characteristics, or by their cessation altogether. This has important advantages in the quest for the provision of fuel flexibility for gas turbine engines. This is an important issue, as outlined in the review article by S. Taamallah et al, entitled “Fuel flexibility, stability and emissions in premixed hydrogenrich gas turbine combustion; ....” published in Applied Energy, 154, (2015) pp. 1020 - 1047. There is a movement towards the use of synthetic gases, or hydrogen rich gases instead of natural gas, which is largely composed of methane, or other types of fuel. A change in the fuel used, results in a change in the equivalence ratio of the fuel/air mixture, and hence also in the likelihood of flame transition instabilities. Furthermore, a change in the power output required of the gas turbine engine, may also amend the level of equivalence ratio at which the transition instability occurs. Similarly, the supply of gas for the operation of the engine may change from time to time, according to market availability. As a result of any of these situations, a prior art engine may not be operated at its optimum efficiency without transition instability effects, since each different fuel will have different such characteristics.

It is not generally possible to change in real time, the physical characteristics of the combustion chambers of an engine, to match a change in the fuel gas used, or to ensure optimum efficiency under varying power requirements. The methods and systems of the present disclosure enable the equivalence ratios at which transition instabilities occur to be moved, thus enabling flexible fuel operation to be achieved, by the simple action of changing at least one of the type of dilution gas used, the gas flow rate, the gas concentration, or even the position of injection of the dilution gas. This can be accomplished by means of controlled valves in the gas supply to the injection jets.

One important field in which this advantage is important is in the use of hydrogen enhanced synthetic fuels, as mentioned above, which are currently coming into more widespread use, because of their cleaner burning characteristics and specifically, their lower carbon dioxide combustion products. Though there is at least one manufacturer who currently offers a gas turbine engine with combustion chambers specifically designed for the faster kinetics of high hydrogen content gases, most gas turbine manufacturers, including the previously mentioned manufacturer, limit the allowed level of hydrogen in synthetic gases for use in standard gas turbine engines, to 10%, and this severely limits the options of fuel flexibility for the vast majority of currently installed gas turbine engines, whether in power plants, aircraft or ships. The problem with the use of high hydrogen content gases is that they may generate combustion instabilities and flashback problems at comparatively low equivalence ratios, thereby limiting the power output and efficiency of the engine, and thus nullifying some of the advantages of the use of hydrogen enhanced gases. The use of the methods and systems of the present disclosure, thus allow the moving of the equivalence ratio at which transition instabilities occur using hydrogen enhanced syngases, to a higher equivalence ratio, and hence to higher power and efficiency, merely by a simple adjustment of the feed valve or valves of the gas dilution jets of the systems of the present disclosure. The presently described systems therefore have the advantage that they enable different fuels to be used, and can shift the equivalence ratio at which instabilities occur to higher or lower positions, according to the fuel used and to the operating conditions of the combustion chamber.

Additionally, the use of diluent gas flows as described in the present disclosure, can also result in the elimination of the high frequency thermo -acoustic instability, using appropriate diluents and conditions, up to an equivalence ratio of 0.75, thereby enabling a wider range of fuel mixture conditions to be used.

There is thus provided in accordance with an exemplary implementation of the devices described in this disclosure, a combustion chamber of a gas turbine unit, comprising:

(i) at least one inlet opening adapted to supply a fuel/air mixture for combustion in the chamber, and

(ii) a plurality of second openings disposed in positions radially external to the at least one inlet opening, in positions between the at least one input opening and side walls of the combustion chamber, the second openings having cross sectional dimensions substantially smaller than those of the at least one inlet opening, wherein the plurality of second openings are adapted to inject a diluent gas into the combustion chamber, in the region where the outer recirculation zone is determined to be located, and at a flow rate substantially smaller than the mass flow rate of the air/fuel mixture.

Any of the above described combustion chambers may further comprise a swirl generating element adapted to apply a rotational swirl motion to the fuel/air mixture entering the combustion chamber. The cross sectional dimension of the plurality of second openings may be at least one order of magnitude smaller than the cross sectional dimension of the at least one inlet opening. Also, the total injection rate of the diluent gas injected into the combustion chamber, should be no more than 5% of the total mass input rate of the fuel/air mixture, or in other implementations of such combustion chambers, should be no more than 3% of the total mass input rate of the fuel/air mixture. Furthermore, the momentum of the flow of the injected diluent gas may be less than the momentum of the fuel/air mixture. According to further implementations of the combustion chambers of the present disclosure, the diluent gas should be an inert gas, and it may optionally comprise at least one of air, nitrogen or carbon dioxide.

In yet other implementations of the above described combustion chambers, at least some of the plurality of second openings may be disposed upstream of regions of the combustion chamber where it is desired to curtail the flame combustion. In such cases, the regions of the combustion chamber where it is desired to curtail the flame combustion may be associated with combustion instabilities.

The plurality of second openings of any of the above described combustion chambers may comprise openings disposed at different radial positions, such that different parts of the outer recirculation zone can be selected for injection of the diluent gas.

Furthermore, any of the above described combustion chambers may have an associated controlled valve in the flow path to at least one of the second openings, such that the injection of diluent gas through the at least one second opening can be controlled. In such a situation, the combustion chamber may further be associated with a feedback control system using an input from a sensor adapted to provide information related to the combustion configuration in the combustion chamber.

According to additional embodiments of the present disclosure, the flow of the diluent gas into the combustion chamber may be adapted to suppress combustion instabilities. Additionally, an increase in the flow of the diluent gas into the combustion chamber is adapted to increase the equivalence ratio of the fuel/air mixture at which combustion instabilities occur in the chamber. In such a case, selection of the conditions of the injection of the diluent gas into the combustion chamber should enable the efficient use of fuels having different combustion properties in the combustion chamber. The flow of the diluent gas into the combustion chamber then enables use of fuels with high hydrogen content, without generating combustion instabilities in the combustion chamber.

There is also provided, in accordance with further exemplary implementation of the devices described in this disclosure, an improved gas turbine engine, in which the use of any of the above described combustion chamber implementations enables an engine having improved characteristics in at least some of the realms of efficiency, fuel flexibility, combustion stability, environmental friendliness, and longer maintenance-free combustion chamber operation.

There is further provided in accordance with other aspects of the present application, a method of reducing combustion instabilities in a combustion chamber, comprising:

(i) inputting into the combustion chamber, through at least one input opening, a fuel/air mixture, and

(ii) injecting into the combustion chamber, through a plurality of openings located between the at least one input opening and a side wall of the combustion chamber, a flow of a diluent gas, the flow rate being substantially smaller than the mass flow rate of the fuel/air mixture input to the combustion chamber, wherein at least some of the plurality of openings are located such that the injected diluent gas impinges on regions within the combustion chamber where it is determined that the outer recirculation zone is located.

In such a method, injection of the diluent gas may quench the flame combustion in the regions on which it impinges, such that the flame shape is changed. In such a case, the regions of the combustion chamber where it is desired to quench the flame combustion, may be associated with the generation of combustion instabilities. In these methods, the momentum of the injected diluent gas is less than the momentum of the fuel/air mixture. The diluent gas may be an inert gas, and may comprise at least one of air, nitrogen or carbon dioxide.

According to other aspects of the above described methods of this disclosure, the injecting of the diluent gas into the combustion chamber is intended to suppress combustion instabilities. In addition, an increase in the flow of the diluent gas into the combustion chamber may increases the equivalence ratio of the fuel/air mixture at which combustion instabilities occur in the combustion chamber. In that case, the conditions of the injecting of the diluent gas into the combustion chamber may enable the efficient use of fuels having different combustion properties. Additionally, the flow of the diluent gas into the combustion chamber may enable use of fuels with high hydrogen content, without generating combustion instabilities in the combustion chamber.

There is further provided in this disclosure, a method of amending the flame shape in the combustion chamber of a gas turbine unit, comprising the steps of:

(i) inputting into the combustion chamber, through at least one input opening, a fuel/air mixture, and

(ii) generating a local change in the fuel/air composition by injecting into the combustion chamber, through a plurality of openings located between the at least one input opening and a side wall of the combustion chamber, a flow of a diluent gas, the flow rate being substantially smaller than the mass flow rate of the fuel/air mixture input to the combustion chamber, wherein at least some of the plurality of openings may be located such as to cause a reduction in the extent of the flame in the regions within the combustion chamber where it is determined that the outer recirculation zone is located.

According to such a further method, the reduction in the extent of the flame in the regions within the combustion chamber where it is determined that the outer recirculation zone is located, suppresses combustion instabilities. Furthermore, an increase in the flow of the diluent gas into the combustion chamber may increase the equivalence ratio of the fuel/air mixture at which combustion instabilities occur in the combustion chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

Fig.l illustrates schematically a laboratory scale atmospheric pressure swirl stabilized combustor, used to test and confirm the results of the presently described invention;

Fig. 2 shows the face of the combustor through which the diluent jets are introduced into the ORZ region, with micro-diameter holes distributed evenly on the face;

Figs. 3 A, 3B and 3C show schematic representations of the combustion chamber shown in Fig. 1, showing the two types of combustion flame shapes of practical interest; Figs. 4A to 4D show high-speed chemiluminescence images of the flame dynamics in the combustion chamber of Fig. 1, without any diluent injection gas applied, the four images being for increasing equivalence ratios of from 0.58 to 0.75;

Fig. 5 shows a series of macroscopic shapes of the baseline methane/air flames for a range of equivalence ratios of from 0.52 to 0.75;

Fig. 6 shows an example of the Power Spectral Intensity (PSD) as function of equivalence ratio for the baseline configuration without any diluent air jet injection;

Figs. 7A to 7D show high-speed chemiluminescence images of the flame dynamics of the combustion chamber shown in Fig. 1, similar to those of Figs. 4A to 4D, but with injection diluent gases applied; and

Fig.8(a) and 8(c) show representations of the normalized standard deviation of the chemiluminescence intensity of various combustion conditions, with equivalence ratios of from 0.58 to 0.75, comparing the baseline conditions with those using the diluent injection flows of the present disclosure, while Figs. 8(b) and 8(d) show the PSD plots for the conditions with and without diluent gas flows.

DETAILED DESCRIPTION

Reference is now made to Fig. 1, which illustrates schematically an exemplary laboratory scale atmospheric pressure swirl stabilized combustor, used to test and confirm the results of the presently described invention, using methane premixed with air as the fuel for combustion. Air is introduced 11 through a choked inlet whose predetermined mass flow rate can be validated with a mass flow-meter 12, such as an Alicat MCR-1000 mass flow meter, as provided by Alicat Scientific Inc. of Tucson, AZ 85743 USA. Methane 13 is introduced using a mass flow controller 14, such as the Bronkhorst EL-Flow, as provided by Bronkhorst High-Tech B.V., of Ruurlo, NL-7261 AK, Netherlands, achieving a resolution in equivalence ratio of (p=0.01. Fluctuations in equivalence ratio is decoupled from the acoustics by using a choked nozzle 15 placed 350 mm from the dump plane 18. A flow straightener 16 is used to reduce turbulence intensity by breaking up large vortices.

The exemplary system of Fig. 1 uses an 8-vane swirler 17, shown in more detail in Fig. 2, placed 40 mm upstream of the dump plane 18 for stabilizing the flame. The swirling flow has a mean swirl number of about 0.7 at the dump plane created by a swirl blade angle of 45°. The flow passes through a 39.5 mm cylindrical inlet section expanding into an 80 mm square section combustor 19. A ceramic center-body with a diameter of 10 mm extends from the swirler 17 to the dump plane 18. The square section combustor 19 is a 3mm thick fused quartz tube to handle the heat loads, while being cooled by bypass air on its periphery. The combustor has a viewing area of 120x80 mm provided by 30 mm thick second quartz windows 21, intended to hold the chamber pressure. The exit 22 of the combustor expands into the atmosphere and acts as a non-reflective pressure outlet acoustic boundary condition. Based on linear acoustic analysis, the fundamental acoustic mode is -250 Hz and second mode is -650 Hz. An acoustic pressure sensor (not shown in Fig. 1) such as the model PCB 103B02 supplied by PCB Piezotronics of Depew, NY, is mounted near the exit. It was not possible to mount it close to the dump plane 18 due to the diluent gas plenum and windows. It is to be understood that the dimensions and materials of the experimental set-up shown in Fig. 1, are only those used in the studies leading to the presently described results, and that alternative arrangements and dimensions could also be used, without detracting from the claimed invention of this application.

As shown by the diluent injection lines 23, schematically indicated in Fig. 1, the diluent gas is injected into the combustion chamber in the region where the ORZ is located, and at the radially outer edge of the ORZ, close to the walls of the combustion chamber. The required flow rates are regulated by using a choked nozzle (not shown in Fig. 1) upstream of the diluent injection holes. This choked nozzle could be implemented as an electronically controlled valve, such that the extent of the gas flow can be changed according to need, as was expounded in the Summary section hereinabove. A feedback controller may be used in order to adjust the diluent gas flow, according to any changes in the combustion detected by sensors, or according to the power demands of the engine, to ensure optimum efficiency and minimal pollution under different operating conditions. The diluent flow line may be split into two or more before injection into the plenum to ensure homogeneous distribution through all injection holes. In addition, there could be several different sets of injection holes, of different diameters, to inject the diluent gases at slightly different radial positions, thereby increasing the flexibility of the control of the effects of the microjets on the combustion configuration. Nitrogen, air or CO2 were used as dilution gases, though other gases may be equally suitable. A k-type thermocouple 14 placed at a comer protruding 5mm from the combustor face records the temperature in the ORZ. The mass flow rate of diluent gas injected during experiment may be between 1% and 3% with respect to the premixed inlet flow, though it is to be understood that the flow rates of a combustor constructed according to the present application could be somewhat more, even up to approximately 5% of the mass input fuel/air flow.

Reference is now made to Fig. 2 which shows a view of the face of the combustor at the dump plane, with 24 micro-diameter holes 23 of size 0.3 mm distributed evenly, through which the diluent jets are introduced into the ORZ. A spark plug 27 for ignition, and the thermocouple 14 for temperature measurement are shown on the face drawing.

It is to be understood that the system and components shown in Figs. 1 and 2 are only examples which illustrate the operation and apparatus used to investigate the implementation and effect of the methods of the present disclosure, and the improvements thereby achieved, and that the details therein are not intended to limit the form or shape of combustion chambers and their associated elements in which or with which the methods of the disclosure may be applied. In particular, the combustion chambers could be of the combustion can geometry, or a single silo combustor, as are commonly used. In both of those cases, the geometry is similar to the cylindrical shape arrangement shown in Fig. 1, such that the analyses suggested in the following drawings can be readily applied to such combustors. Annular combustion chambers have a more complex geometry, and the optimum location of the diluent injection holes may be determined either by analysis of chemiluminescence imaging of the combustion dynamics using a combustion chamber with a section of the wall replaced with a transparent window, or by numerical analysis for calculating the extinction strain rate of the mixture in the outer recirculation zone. This numerical analysis included two steps. Firstly, the time averaged gas composition in the ORZ is calculated using Reynolds-Averaged Navier-Stokes (RANS) simulations, and subsequently, the extinction strain rate in the ORZ is calculated using a premixed opposed jet model.

Reference is now made to Figs. 3(a), 3(b) and 3(c), which show a schematic representation of a combustion chamber such as that shown in Fig. 1, as shown in the article by T. F. Guiberti, et al., entitled “Analysis of Topology Transitions of Swirl Flames Interacting with the Combustor Side Wall”, published in Combustion and Flame, 165, (2015), 4342-4357. These drawings show the two types of combustion flame shapes of practical interest, namely the V-shaped flame and the M-shaped flame. In Fig. 3(a), the different regions of the combustion are shown, with the outer recirculation zones (ORZ) being the region into which the micro jets of diluent gas are injected into the combustion chamber, according to the methods and systems of the present disclosure. In order to promote combustion of the V-flame shape, the injection positions should be in the outer portion of the ORZ region, closer to the combustion chamber wall, in order to quench combustion in those outer areas, so that the flame conforms more to the V-shape. This would be in distinction to what is thought to be required by the system and methods shown in the above-mentioned article by Z. LaBray, and in the MIT Doctoral Dissertation by Z. LaBray entitled “Suppression of thermoacoustic instabilities in a swirl combustor through microjet air injection”, where it is presumed that the stable flame with the injectors is stabilized on the IRZ. In those publications, the microjets are used to modify the fluid dynamics of the ISL and IRZ, in contrast to the presently described process of impacting the ORZ.

Fig. 3(b) shows the shape of the V-flame combustion, with the solid line showing the shape of the region of combustion, while Fig. 3(c) shows the shape of the M-flame combustion, with the solid line showing the characteristic M-shape of the region of combustion.

The diluent gas injected can advantageously be either air, nitrogen, or carbon dioxide. The least effective is air since it contains approximately 20% oxygen. Nitrogen injection has a better effect in increasing the flame stability, while carbon dioxide has the best effect. It is believed that the CO2 effect is best of the gases investigated, because of the reaction of the CO2 with the fuel, in a reaction similar to that which is active in CO2 fire extinguishers, besides the smothering effect that is present in open air fire extinguisher use. In that reaction, the carbon dioxide inhibits the combustion by combining with H* radicals in the high temperature burning hydrocarbon fuel, to generate carbon monoxide and OH radicals. This reaction competes with the dominant chain branching reaction in conventional combustion process in which the oxygen of the air mixture reacts with hydrogen atom to form OH and O radicals.

In addition, a low flow rate should be used, typically of between 1% and 3% of the fuel flow rate. Axial injection is most conveniently used, and the slow input speed of the injected gas keeps the quenching effect close to the dump plane and therefore does not affect the V-flame combustion further away from the dump plane. All of these features are in stark distinction to the characteristics of the micro injector gas flows described in the above-mentioned LaBry article and in the US patent No. 8,708,696, where very high speed flow was used, with the flow being a significant fraction of the main fuel/air flow, generally between 15 and 20% thereof, and in which the gas used, whether air or a fuel/air mixture, was selected to contain a positive combustion effect in controlled regions.

Reference is now made to Figs. 4 A to 4D which show high-speed chemiluminescence images of the flame dynamics of the combustion chamber shown in Fig. 1, without any diluent injection gas applied, i.e. the baseline images. The four images are taken for increasing equivalence ratios of from 0.58 to 0.75. Fig.4A shows an image taken from a video stream of the combustion with an equivalence ratio of 0.58, showing what is regarded as being representative of a stable V-flame. It should be emphasized that the term stable refers to the overall spatial condition of the combustion, since the single frame does not show the turbulent nature of the combustion itself, but the significance is that the combustion is all contained within a reasonable well-defined spatial region, as described by the “V-flame” nomenclature. Fig. 4B shows an image for (|) = 0.61, at which there are signs of unstable flickering in the ORZ region, as will be explained hereinbelow. Fig. 4C shows an image for (|) = 0.65, at which the combustion has now transitioned to what is considered to be a dynamically stable M-flame, and Fig. 4D shows an image for (|) = 0.75, at which the M-flame has become unstable due to high frequency heat release oscillations, as will be explained hereinbelow.

Fig. 5 now shows a series of macroscopic shapes of the baseline methane/air flames for a range of equivalence ratios of from 0.52 to 0.75. The drawings exhibit the following flame shapes, going from the lowest to the highest equivalence ratios shown: a columnar-jet flame, a V-shaped flame stabilized along the ISL and M shaped flame stabilized on both the IRZ and the ORZ. Among the flame shapes, the V and M shaped flames, occurring above an equivalence ratio of 0.56, have higher practical significance. As the equivalence ratio is increased, the flame exhibits different macro scale structures, which have a significant impact on combustion dynamics. At an equivalence ratio of 0.56 (though this ratio is not shown specifically in Fig. 5), a stable V-shaped flame is present. Increasing the fuel flow rate leads to increasingly large heat release oscillations in the ORZ, at equivalence ratios of 0.59 to 0.62. Intermittent combustion of reactants at the ORZ causes oscillatory heat release, thereby making this condition "dynamically unstable", and which will be referred to as an "ORZ flickering" flame. Analysis of overall chemiluminescence intensity for an equivalence ratio of 0.61 during this transition, shows heat release oscillations at about 10 Hz.

Further increase in equivalence ratio to 0.63 and higher values, causes the flame to stabilize on both the inner and outer recirculation zones, characterized by an M-shaped flame. It is observed at these conditions, the flame zone close to the confinement walls attempt to propagate upstream, as seen at (p =0.65 in Fig. 5. Flames at these conditions are still dynamically stable as seen from low overall chemiluminescence fluctuations. However, further increase in equivalence ratio to (p=0.7 and above leads to heat release oscillations at a frequency of 250-275 Hz. The oscillation amplitude increases with equivalence ratio up to the maximum investigated fuel flow rate corresponding to (p=0.80. The frequency of oscillation corresponds to the first longitudinal acoustic mode based on a 1-D acoustic model of the combustor. Hence, the excited mode at these conditions is irrevocably a thermo-acoustic instability.

Reference is now made to Fig. 6, which shows an example of the Power Spectral Density PSD as a function of equivalence ratio for the baseline configuration, without any diluent air jet injection. From the graph, there is shown that a stable M shaped flame exists only in the region of equivalence ratios of approximately 0.63 < (p <0.7, between the 10 Hz instability due to ORZ flickering at the transition from the V flame to the M flame, as mentioned above in connection with the series of images of Fig. 5, and the 250 Hz instability at the first thermo-acoustic mode.

Reference is now made to Figs. 7 A to 7D which show high-speed chemiluminescence images of the flame dynamics of the combustion chamber shown in Fig. 1, similar to those of Figs. 4A to 4D, but with injection diluent gases applied. The images on the left hand side of the page show combustion without the addition of diluent injection gases. The images on the right-hand side show the effect of applying diluent gases according to the methods of the present application. The four images are taken for equivalence ratios of 0.61 and 0.75. Fig. 7A is similar to Fig. 4B, and shows an image taken of combustion without any diluent gas injection, with an equivalence ratio of 0.61, showing unstable heat release flickering in the ORZ region. Fig. 7B shows an image for the same equivalence ratio (|) = 0.61, but in this case with an injected flow of CO2 at a level of 1% of the total mass flow. In contrast to the baseline image of Fig. 7A, Fig. 7B shows that the combustion now has the form of a stable V-flame, thus illustrating the effectiveness of the inert gas injection of the present disclosure. Fig. 7C now shows an image for (|) = 0.75, without any diluent flow, and the combustion is shown as having transitioned to an unstable M-flame, because of the first longitudinal acoustic mode instability at 250 Hz, similar to that shown in Fig. 4D. Fig. 7D shows an image for the same fuel ratio, (|) = 0.75, but in this case with an injected flow of CO2 at a level of 3% of the total mass flow, resulting in a stable M- flame shape, thus illustrating the effectiveness of the inert gas injection of the present disclosure.

Reference is now made to Fig.8(a) and 8(c) which show representations of the normalized standard deviation of the chemiluminescence intensity of various combustion conditions, with equivalence ratios of from 0.58 to 0.75. Fig 8(a) shows the baseline situation, without any injection gas, and is thus the same as the equivalent images of Fig. 5. Fig. 8(c), on the other hand, shows combustion using the same fuel ratios as in Fig. 8(a) but in this case, with injections of N2 at the ORZ region with a local dilution at 3% m/m of the bulk flow. This image was obtained with the gas injected at low momentum through 24 radially spaced micro-diameter holes, whose position is shown in the drawings by the two outer tubes in positions corresponding to the ORZ region. The introduction of the nitrogen diluent shifts the low frequency ORZ flickering mode at 8-12 Hz, to higher equivalence ratios as seen from the flame images in Fig. 8(c). Comparing the baseline series - (a), with the series with 3% injection of N2 - (c), clearly shows that the flame flickering behavior has been shifted from (p =0.59 in the baseline case to q>=0.65 with 3% addition of N2.

Figs. 8(b) and 8(d) show the power spectral density of the heat release fluctuations as a function of equivalence ratios for the combustion cases of Figs. 8(a) and 8(c) respectively. These two plots show that the intensity of the heat release fluctuations with the diluent gas injections is two orders of magnitude lower than in the baseline cases without diluent. The "ORZ flickering" excited frequency is also shifted to higher values of (p. Thus, the use of the diluent gas injection extends the occurrence of "ORZ flickering" to higher cp, and its intensity is quelled. Furthermore, the 250 Hz thermoacoustic instability mode is also significantly damped with the use of dilution gases. Comparing the contour plots (b) and (d) of Fig. 8, it can be clearly seen that the high frequency mode at 250Hz appears to be completely damped with the introduction of 3% N2. The mode of damping can be attributed to dilution of the composition in the ORZ, thus preventing flame flashback along the walls. Comparing Figs. 8(a) and 8(c) at (p =0.75, it is evident that the flashback of high intense combustion products along the combustion chamber walls in the ORZ is damped when 3% N2 is introduced.

One consequence of the results shown in Figs. 8(a) to 8(d) is that the control of the combustion instabilities by the use of diluent gas injection enables the use of different fuel compositions whose acoustic noise emission can be controlled by simply changing the diluent injection gas conditions. Similarly, for aircraft gas turbine engines, the control of acoustic emissions under different flight conditions for instance, can be controlled by use of different diluent injection conditions. This can be done dynamically according to the conditions of the engine requirements during different phases of the flight. When using prior art techniques for controlling combustion instabilities, such as acoustic dampers, such dynamic control is not possible.

Furthermore, the use of combustion chambers incorporating the methods and control procedures described in this disclosure, enables the development of new gas turbine engines, having the ability to use a wider range of fuels, including syngas and hydrogenrich fuels with their concomitant advantages relating to environmental friendliness, and without causing degenerative effects from the use of such fuels at equivalence ratios which may cause combustion instabilities. These advantages can also be used for controlling the combustion conditions for any other fuels used in their respective engines. Such gas turbine engines may incorporate, inter alia, control systems to adjust one or more of the percentage of the diluent gases, the injection location of the diluent gases, the type of diluent gas, and even other parameters related to the injection of the diluent gases, in order to enable the engine to operate efficiently with different types of fuel at their optimum equivalence ratios without significant combustion instabilities, and even in order to make such control adjustments in real time, in accordance with the operating conditions and load on the engine.

The postulated mechanisms by which the methods of the present application have been described as operating, are not intended to be considered as confirmed mechanisms, and the claimed invention is intended to be operative regardless of the mechanisms in fact operating in the described systems and providing the exemplary results. The measurements performed appear to provide support for the suggested mechanisms, but are used as a basis for any claimed method or system irrespective of whether that is the correct physical or chemical process operating.

The combustion chamber used for the experimental support for the claimed invention is a laboratory configuration of simple geometrical structure. A real life combustion chamber, such as that of a commercial gas turbine engine, may have a significantly more complex shape than those shown in the present application, and the claimed elements should not therefore be limited to the geometric configurations described herewithin, but are to be understood to cover the geometric situation of a variety of shapes and sizes used in various applications.

Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. Furthermore, it is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.