The present invention concerns a multistage hybrid system for the induction, anchorage and stabilization of distributed flame in advanced combustors for gas turbine.

The invention relates to the field of combustors for gas turbines, and falls within the scope of interventions aimed to induce and sustain distributed reaction zones within combustors for gas turbines, in order to exploit the favorable prerogatives of the distributed flames in terms of more efficient combustion, leveling of peak temperatures of the flame, reduced polluting emissions, potential insensitivity to thermoacoustic oscillations. In particular, the invention proposes to solve the problem of inducing and supporting distributed reaction zones within combustors for gas turbines, even in conditions of full load operation, when it is not possible to take advantage of the promoting effect that the shortages of fuel exerts on volumetric distribution of the flame (slower chemical kinetics than the turbulent mixing).

More particularly, the invention proposes to transfer to the field of gas turbine the combustion technology known as "Flameless" (or "Colorless Distributed Combustion"), created and progressed in the field of furnaces, proposing a burner specifically designed for this purpose.

It is known that in the field of furnaces, to promote the volumetric distribution of a flame, it is proposed to delay the chemical kinetics (of the reactive processes of oxidation of fuel) and to enhance the turbulent mixing between the reaction mixture and the products (Low Damkoeler Number Combustion).

To this end , it is arranged to provide a high rate of recirculation of the flue gases, to dilute the reactants, reduce the percentage of oxygen and slow down the reactions; the same result can be achieved by slowing down the chemical kinetics by subtracting fuel and by reducing the preheating temperature of the reactants. In this way the development of a flame front can be inhibited even in an ultra lean preformed mixture [Manabu Fuchihata, Masashi Katsuki, Yukio Mizutani, Tamio Ida, Observation of the flame structures emerging at low Damkohler number fields", Proceedings of the Combustion Institute, Volume 31 , Issue 1 , January 2007, Pages 1353-1359].

With the same objective, in the Flameless combustion developed for the furnaces, the high-temperature of preheating of the reactants is exploited to increase the viscosity, the volumetric flow rate and the speed of the spouts, enhancing the turbulent mixing. The same objective can be achieved even with a smaller preheating, reducing the passage areas and increasing the swirl numbers (angular momentum), at least within limits compatible with the growth of the pressure drops introduced by the combustor (in fact the penalty associated with the throttling and the generation of a motion of swirl is offset by the lower volumetric expansion of cooler reagents).

There is then to consider that the "conventional" Flameless combustion [A. Knight, . de Joannon, "MILD Combustion", Progress in Energy and Combustion Science 30 (2004) 329-366] is intended to deal with two conflicting demands: raising energy efficiency by realizing heat recovery (preheating of the reactants with the exhaust fumes) and lower the maximum combustion temperature to reduce NO _x emissions. This second aim was achieved by homogenizing the temperature increase associated with the combustion, by making it volumetrically distributed. In this perspective, the preheating of the reactants is not a requirement imposed by the physical-chemical mechanism of the Flameless combustion, but fixed by the need of heat recovery. Thus, the overcoming of the temperature of self-ignition simultaneously in a large volume is essential only inside the chamber, as a result of a rapid mixing with the flue gases, which will provide to inhibit the development of a flame front

[Wunning J. A., Wunning J. G., "Flameless Oxidation to Reduce Thermal NO Formation", Prog. Energy Combust. Sci. Vol 23, pp. 81-94, 1997]. 000284

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Therefore, the Flameless combustion without any preheating, and thus without risk even in premixed form, is not only possible, but sometimes desirable. Successful experiments with premixed Flameless combustion with reagents at room temperature have recently been conducted [LI P. F., Ml J. C, DALLY B., WANG F. F., WANG L, LIU Z. H., CHEN S., ZHENG C. G., "Progress and recent trends in MILD combustion", SCIENCE CHINA, February 2011 , Vol.54 No.2: 255-269].

The possibility to obtain a premixed Flameless combustion, even with reagents to room temperature, makes evident the convenience of exploiting this technology in all those cases in which heat recovery is not needed, if not counterproductive, such as in gas turbines for large combined plants. In such a case, in fact, the thermal regeneration would subtract heat to the underlying steam cycle penalizing it in terms of efficiency (Carnot effect) and of producible steam flow, ie in power output. Overall, therefore, in spite of the benefit on the gas cycle, the overall performance of the plant it would be decreased.

In this context, the application of Flameless combustion in gas turbines appears very promising, even more so when considering that burning at a ratio of equivalence far below the flammable range of a preformed mixture is at the limit conceivable in only two distinct cases, ie in the presence of a pilot diffusive flame, which renders the reaction zone (also considerably) richer, with the disadvantage of increasing the harmful emissions and, if it create the conditions, of hindering the transition to the state of Colorless Distributed Flame, causing instability by "forced localization"; or under regime of volumetric (distributed) combustion, "cleaner" and potentially not responsible for other forms of instability if not the most delicate thermo-diffusive ones, not affected by pulses of heat release [Chiocchini S., Giulietti E., E. Giacomazzi, Stringola C, S. Cassani, Palgiari L., "Combustion Stability Improvement of a Full Premixed Dry Low NOx Gas Turbine Combustor Near Lean Blow Out", XXXVI Italian

Section of Combustion Institute Meeting, 13 to 15 June 2013 Procida Island, Naples]. In the first case, that is when the main premixed flame is forced to maintain a circumscribed front originated from a restricted anchorage region, the instability preceding the LBO (Lean Blow Out) can arise in advance, ie at higher equivalence ratios, and in more energetic form. This is in perfect agreement with the vulnerability of the flames locally attached to fluctuations in fluid dynamics of the root zone. The whole is summarized in Figure 1 [Overman N., "Flameless Combustion and Application for Gas Turbine Engines in the Aerospace Industry", Master of Science Thesis, Department of Aerospace Engineering and Engineering Mechanics, Graduate Faculty of the University of Cincinnati, 2007], from which it is possible to appreciate that all curves fully internal to the field of distributed combustion (in the specific case obtained for Φ<0.4÷0.45), lie below the Lean Extinction Limit. The substantial overlap between the ranges of extinction due to excessive thinness and of transition to the Colorless regime confirms the inhibitory effect of the second phenomenon over the first, demonstrating that the onset and persistence of the final "Colorless Flame", already incipient for equivalence ratios Φ-0.45, go together with the leaning of the mixture; ie that the distributed combustion extends the scope of existence of premixed flames towards the poorest dosages.

Ultimately, therefore, the promoting effect that the leaning of the mixture exerts on volumetric shedding of the flame can be exploited to burn in premixed and homogeneous form even in the operation under adjustment, at the lower partial loads, without the need to move back to the diffusive regime (usually provided at the start-up transient and at load), responsible of increased pollutant emissions. This quality of the distributed combustion assumes a fundamental role in the current approaches to managing large combined systems. In fact, when renewable energy, the exploitation of which can not be deferred, come to also cover the base load, gas-steam groups should be operated for several hours at the minimum power compatible with the respect of environmental constraints.

More difficult will be to promote and support distributed flames when the effect of "deprivation" of fuel is insufficient. In other words, the P T/IT2015/000284

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current challenge is to make the "Colorless Distributed Combustion" compatible with the specific power characteristics of the gas turbine in the operation at full load. Compared to this purpose none of the most modem and sophisticated solutions managed entirely satisfactory.

The main cause of the partial effectiveness of the techniques currently in use for achieving Flameiess combustion in burners for gas turbine resides in the difficulty of reconciling intense power densities (compactness), contained pressure drops introduced by the burner and "high" temperatures of turbine inlet (thermodynamic efficiency), so to prevent to go too far below the nominal relations of equivalence (ie up to the minimum threshold of stability of the flame allowed by the distributed combustion, as shown in Figure 1). In other words, once the thermal load required at the design point (fuel flow rate) is set, it can not be reconciled with an air flow rate still high, although compatible with the continuity of combustion. A limit is thus imposed on the first identified mechanism of alternative induction of the "Colorless Distributed Combustion": the slowdown in chemical kinetics for lack of fuel with respect to the stoichiometric dosage. Consequently, the most recent "Dry Low NO _x " combustors _, while operating with mixtures classified as ultra lean (Φ < 0.6), consistent with a suitable containment of the pressure drops, are not able to ensure a level of turbulent mixing sufficient to promote the Flameiess combustion. To overcome this limitation, while also enhancing the dilution effect of the flue gases, in the recent past it has been used to so-called "Trapped Vortex Combustor" (TVC) [K. Hsu, C. Carter, Katta V., W. Roquemore, "Characteristics of Combustion Instability Associated with

Trapped-Vortex Burner", 37th AIAA Aerospace Sciences Meeting and Exhibit, AIAA 99-0488, January 11-14 / 1999, Reno, NV] [Haynes J., Micka D., Hojnacki B., Russell C, Lipinski J., Shome B., Huffman M., "Trapped Vortex Combustor Performance for Heavy-Duty Gas Turbines", ASME GT2008-50134, June 9 -13, 2008 ]. In them the homogenization and dilution are promoted by the "mixing" of a whirlwind of reaction mixture 1 within a cavity expressly designed to receive it (Figure 2). Nevertheless, the adoption of a TVC pilot stage [indicated with reference numeral 2 in Figure 2], if on the one hand favors the volumetric spreading of the flame 3 (Figure 3, US7621132B2), on the other hand, because of its positioning, which is inevitably peripheral, implies that the heat and active radicals (intermediates promoters of chemical reactions), conveyed together with the flue gases, should flow back into the reaction zone, stabilized within the shear layer between the central 4 and the peripheral recirculation zone 5, until the rear stagnation point 6 before coming into contact with the fresh mixture coming from the axial pre-mixer 7 (note that the pilot using a nested TVC stage, Figure 2, can be seen as exaltation of the peripheral recirculation zones of Figure 3, in addition made seats of additional injections of fuel). Thus, especially in the highly pressurized combustors of the gas turbine, where the effect of "collisional quenching" of the radicals is more marked, the time required for the described recirculation may exceed the period of relaxation of the radicals themselves, compromising their ability to trigger, promote and support the various branches of the chain reaction of the fuel [US2011/0016867A1]. Therefore it can be understood that the most appropriate site to adduce excited radical and the activation heat is the rear stagnation point 6. For the same reasons, in the same point should be located the igniter, which, in the form of the energy emitted by an electric arc, administers from the outside the heat of activation when the flame 3 is still turned off and no recirculation of flue gases may exist.

When, however, the ignition electrode is placed in the outer recirculation zones 5, often the mixture introduced therein should be rich in order to stably support the combustion, ie to generate an excess of active radicals to compensate the described relaxation, as well as to overcome the small exchange of ignition flow between the peripheral recirculation zones and the fresh mixture (eventually if the fluid flow field is stationary and axisymmetric there would be no exchange, nor any possibility of ignition: the transfer would be left solely to instabilities of the shear layer 8). So, often, the flame can not propagate towards the central recirculation zone 4 until the main stream of the mixture does not become quite "ready to ignite", ie rich, preheated and provided with an abundant amount of active radicals.

In other words, the stabilization of the main flame 3 can not happen if not in step with the growth of the thermal load, the output power, the regime of rotation of the gas turbine and ultimately the pressure that the compressor develops and supports in the combustion chamber. This deferred extension of the area of reaction from the pilot flame, relegated within the peripheral shear layer [indicated with reference numeral 8 in Figure 4], to the inner layer of sliding 9, only after the pressure in the combustor has taken to rise, has the effect of accelerating the relaxation of the active radicals, counterproductive for the ignition of the main flow of mixture arriving at the rear stagnation point 6. Moreover, the process leading the flame 3 to extend from the outer recirculation zones 5 to the main one 4, not only can lead to important losses (at the chimney) of unburned mixture in the stage of starting and loading, but, occurring at different times for the various burners of a same annular combustion chamber, is placed at the origin of an thermal inhomogeneity (burner to burner) of gases sent to the turbine, risky for the integrity of the first stages of the machine.

To avoid at least this second drawback third generation TVC have been conceived, provided with continuous cavity, closed around the axis of the machine, along the entire annular combustion chamber and no longer limited to the individual burner (as if the entire combustion chamber was generated by revolution of Figure 2 around a horizontal axis below and not more for arrangement of multiple tubular burners to form a circular ring similar to the cylinder of a revolver). However, precisely because of the rich dosage required to stabilize the flame driving it from the cavity, in such TVC is not possible the achievement of the Colorless regime.

The problem of sequential ignition, first in the cavity and then in the inner shear layer 9, has been addressed by developing fourth generation TVC burners (Figure 5), operating solely on the principle of the trapped vortex [Levy Y., Sherbaum V., Arfi P., "Basic thermodynamics of FLOXCOM, the low-ΝΟχ gas turbines adiabatic combustor", Applied Thermal Engineering, 24 (2004) 1593-1605], However, except in some simplified implementation architecture and still at laboratory scale [Gupta A. K., Khalil A. E. E., "Swirling Distributed Combustion for Clean Energy Conversion in Gas Turbine Applications", Applied Energy, 88 (201 1) 3685- 3693], the achieved power density have been unsatisfactory (minimum size limited by the actual development of the vortex), the pressure drop has proved excessive, the levels of pollutant emissions ineligible and the obtaining of distributed flames doubtful [FLOXCOM Final Report, "Low ΝΟχ FLameless OXidation COMbustor for High Efficiency Gas Turbines", Israel Institute of Technology, December 21 , 2004].

Another drawback comes from having transferred in toto the use of diffusive pilot flames, or partially premixed, from conventional burners (stabilized by "bluff bodies" - aerodynamic obstacles - or vanes) to the TVC. And in fact, a pilot of this kind in combustors based on the principle of "cavity flameholding" has often proved responsible for not being able to make them free from thermoacoustic pulsations and to inhibit the full flameless regime [Xuan Lv, Yufeng Cui, Aibing Fang, Gang Xu, Bin Yu,

Chaoqun Nie, "Experimental Test on a Syngas Model Combustor With Flameless Technology", Proceedings of ASME Turbo Expo 2010: Power for Land, Sea and Air, June 14-18 2010, Glasgow, UK]. This even in the after-burners, where, in a sense, the dilution effect is greatest, being the oxygen only that residual in the flue gases of the primary burner (operating in excess air) [Gutmark E., Paschereit O, Guyot D., Lacarelle A., Moeck J., Schimek S., Faustmann T., Bothien M., "Combustion Noise in a Flameless Trapped Vortex Reheat Burner (FTVRB)", 13th AIAA/CEAS Aeroacousics Conference, 21-23 May 2007, AIAA 2007-3697]. Still, it has been proposed a system that, by recognizing the external recirculation zones 5 of a conventional burner (Figure 3) as "embryos" of TVC cavities, provides for injecting fuel 10 precisely in a IT2015/000284

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peripheral position, as shown in Figure 6 [Levy Y., Rao G. A., Sherbaum V., "Preliminary Analysis of a New Methodology for Flameless Combustion in Gas Turbine Combustors", ASME Turbo Expo 2007, Paper GT2007- 27766, May 14-17, 2007, Montreal, Canada]. Consequently the reaction zone assumes an annular shape and, touching the wall of the liner (flame tube), once the latter is equipped with special flaps 11 , can be chilled. It is therefore possible to: reduce the temperature of the flame; limit harmful emissions, by counteracting the genesis of thermal NO _x and favoring the regression of the equilibrium of dissociation of CO2 to CO; reduce the oxygen content of the flue gases, limiting the percentage of recirculation sufficient to promote, by dilution of the reactants, the Flameless combustion. There remain, however, the problems associated with the trigger and the loading already described for the TVC of second and third generation.

Moreover, since it is not possible to subtract any part of the flame to cooling, at lowest thermal loads the stabilization may be even more critical. Ultimately, each of the solutions above relies primarily on only one of the possible arrangements today known to promote and stabilize the Flameless regime. Moreover, even in the presence of more complex and composite strategies, the solutions aimed at making the effects of various functional elements cooperating with each other are still insufficient. So much so that, as in the last example cited with reference to Figure 6, the same authors, in an attempt to remove the drawbacks exhibited by a previous embodiment (excessive pressure drop, high emissions, scarcely distributed combustion, example mentioned with reference to Figure 5), regress by a fourth generation TVC to a design (Figure 6) again potentially affected by late and sequential stabilization of the flame (precisely inhibited by combustors based only on the principle of "Cavity Flamehoding").

Finally, it remains to mention the fluid architecture that can be considered the base of the present invention [US8322142B2] [Lee J. G., Armstrong J. P., Santavicca D. A., "Experiments on Lean Blowout and OX Emission of a Premixed Trapped Vortex Combustor With High G- Loading", ASME Paper Gt 2011-46396, June 6-10, 2011], in the sense that, by applying to this realization of reference (Figure 7) all the specific and unusual functional measures forming the object of claims 1-10, it is possible to generate a possible, but not complete implementation of the invention. As can be seen (Figures 7A, 7B, 7C, 7D), in the course of the maneuver of shutdown for progressive closure of the valve of the fuel, the addition of an annular cavity 12, between the swirler 13 and the combustion chamber 14, allowed to interpose between the stable starting arrangement ("Type I Flame" 15', the same as in original tubular burner, devoid of the cavity) and the final blowing of the flame out of the combustor (Lean Blow Out, LBO), a second morphology 15, centrifuged and "trapped", of the reagent area. This made it possible to make the mixtures even poorer, at the limit of turning off, without picking up the level of unburned. The whole as a result of the extension of the residence times, and then the time granted to the completion of the chemical reactions, along the entrapment path. Also in this case, however, the result is limited solely to ultra lean mixtures, ie to the minimum thermal loads, incompatible with the concentrations of power necessary to the design point.

It is the opinion of the proposers to the present invention that the limits of the results of the prior art is often attributable to having confused the flameless combustion with its original creation (furnaces), so trying to transfer it more or less forcibly to gas turbines. All this without taking into account that each specific form of "Colorles Distributed Combustion" is characterized by a corresponding mechanism of interaction between turbulence and combustion, for obtaining and controlling which (subordination to the prefixed purposes) it is required a "dedicated" design strategy [C. Duwig, B. Li, Z.S. Li, M. Alden, "High resolution imaging of flameless and distributed turbulent combustion", Combustion and Flame,

159 (2012) 306-316]. In this context it is included the solution according to the present invention, which aims to contribute to the complete and definitive (adaptive) transfer of the "Flameless" combustion technology (or "Colorless Distributed Combustion") from the furnaces, where it is born and progressed, to gas turbines.

These and other results are obtained according to the present invention by proposing a multistage hybrid system for the induction, anchorage and stabilization of distributed flame in the advanced combustors for gas turbines in which the "volumetric" flame that the burner generates is, on the one hand, induced:

- by stirring the reaction mixture in a trapped vortex;

- with the help of multiple, distributed and sequential fuel injection;

- by means of pre-dilution of the combustion air with flue gases in self recirculation;

by the other stabilized:

- aerodynamically, by means of swirlers;

- statically, with the aid of retention cavity;

- thermo-acousticaily, through perforated attenuating plates.

The object of the present invention is therefore to provide a multistage hybrid system for the induction, anchorage and stabilization of distributed flame in the advanced combustors for gas turbines which allows to overcome the limitations of combustors designed according to the prior art, in order to stably allocate Flameless combustion schemes within the flame tubes (combustion chambers) of gas turbines.

A further object of the invention is that said multistage hybrid system for the induction, anchorage and stabilization of distributed flame in advanced combustors for gas turbines can be achieved with costs equal to or lower than the solutions of the prior art [US8387394B2], while ensuring at the same time substantially lower operating costs, both for the upper combustion efficiency and for the larger stabilizing capacity, which minimizes the interventions of active control systems and/or service interruptions.

Another object of the invention is to propose a multistage hybrid system for the induction, anchorage and stabilization of distributed flame in advanced combustors for gas turbines that is robust, durable, safe and reliable.

It is therefore a specific object of the present invention a multistage hybrid system for the induction, anchorage and stabilization of distributed flame in advanced combustors for gas turbines as defined in claim 1.

Further characteristics of the multistage hybrid system for the induction, anchorage and stabilization of distributed flame in the advanced combustors for gas turbines according to the present invention are defined in the subsequent dependent claims 2-9.

The present invention will be now described, for illustrative but not limitative purposes, according a preferred embodiment thereof, with particular reference to the figures of the accompanying drawings, in which:

- Figure 1 shows a diagram representative of the effect that the preheating temperature and the internal pressure drop have on the Lean Blow Out equivalence ratio of a first device according to the prior art [Overman N., cit],

- Figure 2 shows the principle diagram of a second generation Trapped Vortex burner,

- Figure 3 shows the location of the flame in a burner stabilized by means of a swirler [US7621132B2],

- Figure 4 shows in detail the structure of the flame stabilizing zone visible in Figure 3 [J. O'Connor, T. Lieuwen, "Recirculation zone dynamics of a transversely excited swirl flow and flame", Physics Of Fluids, vol. 24 (2012), 075107], - Figure 5 shows the principle diagram of the fourth generation TVC burners, operating solely according to the principle of the trapped vortex [Levy Y., Sherbaum V., Arfi P., cil],

- Figure 6 shows the scheme of implementation relating to one of the possible criteria for the obtaining of the flameless combustion in the burners of gas turbines according to the prior art [Levy Y., Rao G. A., Sherbaum V., cit.],

- Figures 7A-7D, show the constitution diagram, the arrangement of the internal flow and the performance improvements, compared to a conventional burner stabilized by means of a swirler ( "Type I Flame" generator 15' in Figure 7B), secured by a combustor according to the prior art [US8322142B2] [Lee J. G., et a!., cit.], in which it appears the solution closest to the present invention,

- Figures 8A and 8B show the results of numerical simulations based on the architecture illustrated with reference to Figures 7A-7D, by identifying the major causes of deviation between the predicted flow morphology and the expected one,

- Figure 9 exhibits the principle diagram of the multistage hybrid system for the induction, anchorage and stabilization of distributed flame in the advanced combustors for gas turbines according to a first embodiment of the present invention,

- Figure 10 shows the swirler 13 of the combustor of Figure 9, viewed frontally in a first perspective view and, for reasons of visibility, deprived of five of the six helical blades 23,

- Figure 11 shows a second perspective view of the swirler 13 of

Figure 10, in its complete structure, as it would appear, making transparent the shell that forms the annular entrapment cavity and the downstream cyindrical chamber,

- Figure 12 shows the arrangement of the flux generated by optimizing the coupling between the swirler 13 and the trapping cavity (32, 33), within which, for clarity of representation, the flow 26 of recirculation of the flue gases, which constitutes the arc of return of each helical branch 27, has been arranged separately, as if it belonged to a second jet generated by a interblade compartment behind,

- Figures 13A and 13B respectively show the cross section of the swirler, conducted to the center plane of the radial cylindrical manifolds 30, and the section according to a median helix of a blade 23,

- Figure 14 shows some sections and views of the trapping cavity (in the bilobed form of Figure 12) and the swirler, with particular regard to the mutual arrangement between the helical rows of injection holes, on the periphery of the chamber (32, 33), and the internal path for recirculation of the flue gases,

- Figure 15 shows a dual cavity blade section for pre-dilution of the combustion air under driven operation with a pre-combustor (41) of the type shown in the subsequent Figure 16 or under self sustaining (ignition) by internal recirculation of the flue gases,

- Figure 16 exhibits the principle diagram of the multistage hybrid system for the induction, anchorage and stabilization of distributed flame in the advanced combustors for gas turbines according to a second embodiment of the present invention, sectioned in correspondence of one of the cooling fins in the reaction zone,

- Figures 17A, 17B and 17C show the equivalence between a combustor designed to work exclusively according to the principle of the trapped vortices [Levy Y. et al., cit], and the single lobe of a flame centrifuged within a trapping cavity [Lee J. G., et al., cit.], when use is made to one retrograde drawing 31 of the flue gases (according to the present invention).

Starting from the solutions of the prior art previously described, with particular reference to the "High G-Loading TVC" (Figure 7A), it is firstly noted that the desired flow configuration, including a large central recirculation zone (wedged into the funnel at the base of the combustion P T/IT2015/000284

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chamber, down to below the trapping cavity and represented in the detail 4 of Figure 7A) as a matter of facts does not happen. It follows an impediment to force the flow coming from the swirler toward a path pushed against the walls and thereby ready to open towards the annular chamber with the highest penetration degree (ratio of the "trapped" 16 and "untrapped" 17 flow rates, Figure 7B). The rate of recirculation of the flue gases and of stirring with reagents, promoters of the distributed flames, therefore has found a considerable limitation.

The performed simulations (Figure 8A) show in fact the existence of two separate regions of slowed-retrograde flow, a first flow region 18, just outlined, below the cavity and a second flow region 19, much more extensive, within the main combustion chamber.

As a consequence, the swirler-cavity exchange mechanism, even though occurring in the only possible way, according to an alternation of azimuth inputs and outputs (in Figure 8B the particular 20 shows the region of the radial velocities entering in the cavity; the rest of cylindrical opening being engaged by the outgoing flow), allowed degrees of penetration at most between 40% and 60% (referred to the total flow rate that accesses from the swirler 13).

According to the proponents of the present invention, this limit has intervened due to the lack of optimization in the coupling between swirler and cavity . And in fact, the exchange of flow, far more copious, that the desired melting of the zones of recirculation would impose, can not be separated by the use of a chamber closed around a conical diffuser (Figure 9, detail 21) already proved instrumental in controlling shape and arrangement of the central recirculation zone [US2011/0016867A1]. Referring to Figure 9, according to the present invention, to achieve a rate of stirring comparable with that of the fourth generation TVC (Figure 5), it was therefore resorted to a configuration according to which the trapping cavity 12 is inserted between the swirler 13 and the not interrupted part of the conical diffuser 21 , opening towards the inside where the flux lines lapping the central recirculation zone 4 are still in full centrifugation, before converging, receding, towards the axis of the burner 22. From this point of view it can be assumed that the axial length of the recirculation zone 4 is equal to about 2÷2.5 times the outer diameter (of outflow) of the swirler 13 (if the angle of semi-opening of the conical diffuser is comprised within the optimal range between 20 and 25°) [US2011/0016867A1]. In this lies the first aspect that, with the stated purpose, distinguishes the present invention over the prior art [US8322142B2].

Still on the subject of interaction between swirler 13 and trapping cavity 12, as mentioned, the regular exchange of flow between the two can only be established for the alternation between the interblade spouts (in Figure 8B is shown their section 20 in the cylindrical surface of acces to the cavity 12) and the slowed trails generated by the swirler 13. Eventually, if the field of fluid motion was axisymmetric, in the stationary solution there could not be any exchange and a minimum transfer of fluid flow would in fact be guaranteed by the only unstable motions of the shear layer (8 in Figures 3 and 4) between trapped vortices 5 and central swirl (wedge between the shear layer 8 and 9 in Figure 4). Thus, to enhance the first and predominant effect of azimuthal periodicity, it is proposed according to the present invention of having recourse to an axial swirler 13

(Figure 10) and no more to a centripetal one, by practicing on the end 24 of the helical blades 23, from the side of the trailing edge (Figure 11), profiled recesses 25, apt to create "discharges", ie the preferential outlet pathways for the flow inside the cavity.

The whole to circumscribe the choking operated by these return fluid "blades" (the flow of which is indicated by the reference number 26 in Figure 12) on the underlying vortex, precisely where it escapes already slowed down (blade trail) from the swirler 13 and certainly not in correspondence of the interblade spouts (indicated with 27 in Figure 12), which are responsible for the momentum necessary for centrifugation. The artifice described, allowing also to "fill" the trail of each blade 23 with the stream 26 coming back from cavity , allows to realize stubby blades, as 2015/000284

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required to enhance the azimuthal periodicity and the exchange of flow between swirler and cavity, but without increasing excessively the pressure drops caused by a truncated trailing edge not aerodynamically profiled. The large thickness of the blades is also appropriate to generously proportionate the channels 30 of the circuit of internal recirculation of the flue gases 31, visible in Figures 9, 10, 13A and 13B.

In addition, it is proposed according to the present invention an additional modification of the base scheme. According to the prior art [US8322142B2], it is believed that the central swirled spout can not enter into an annular cavity seat of peripheral injections, since the introduced flow, which has to escape, would cause an excessive impediment. However, assuming that in some conditions the flame should reside throughout and only in the trapping cavity (15, Figure 7B), if this does not have peripheral openings, all incoming mixture will enter from the only internal access and emerging from it in the form of flue gases (input and output in alternate areas in azimuth direction, as shown in Figures 7B and 8B, parts 16 and 20; the same alternation is visible in Figure 12, where a branch is similar to 27, but behind it and not shown for clarity, constitutes the entrance arch of the recirculation flow 26). The mutual obstruction of the flow of fresh reagents and products will then be maximum.

Without prejudice to the global reach, adding on the one hand the peripheral feeding, through the holes 28 arranged on a wall 29, on the other the channels 30 of the internal recirculation of the flue gases 31 (Figures 9, 10, 11 and 12), both fluxes in question are reduced and with them the competition in dividing the area of the main access to the cavity 12. Moreover, to mitigate the impediment caused by the distributed peripheral injection, it will be organized with spouts that do not imply more obstruction than would be created if the relative flow rates were maintained through the only original access (internal opening of the cavity). Still, in the final architecture of the burner, it will be provided for the cooling of the cavity 12, with the result of reducing the volumetric expansion of the flue gases and so mitigate the action of blocking caused by the same.

Therefore, even for the foregoing reasons, namely for:

- subtracting the outgoing flow from the main opening of the cavity 12;

- limiting the amount of motion of the flow coming back from the annular chamber, preserving the swirled central spout by suffocation;

- avoiding destructive interference between the centrifuged vortex and peripheral injections;

it was tried:

- to allocate transit areas as wide as possible in the path of internal recirculation 30 of the flue gases (Figures 10 and 13), also overfeeding it with the dynamic pressure of the fluid threads common to the two trapped vortex (31 in Figures 9, 10 and 12 ) in as many corresponding lobes of the cavity (32 and 33 in Figures 11 and 12);

- to expand the annular chamber 12 (and consequently the capsule pre-mixer 34 of Figure 9), within the limits allowed by the maximum transverse dimensions of the original burner (Figure 7A), without reducing the overall value of the power density (the volume of which the cavity increases is withdrawn from the cylindrical chamber in origin located downstream, so it is not added, but reallocated keeping with the need to promote the development of a vortex of reacting mixture);

- to distribute the pseudo tangential axes of the injection holes 28 on the surfaces 29 of the two semi-toroidal rooms according to points of helices (Figure 14) with a pitch equal to the azimuthal periodicity of the flow (60°). In particular, in Figure 14 is shown the distribution of the injection holes on the surface of the multiple trapping cavity. The construction of the helices 36, for which points pass the axes of the holes, is approximated: the profile of the longitudinal sections 37 (of the bilobed cavity 32-33 of Figure 12), strictly speaking, should be divided into arcs of equal length, while, for simplicity, arcs are taken corresponding to equal angles centered on the circle of greatest diameter among those that make up the perimeter. The radial view of the helices 36 is also approximated: each radius (from the axis of the burner) corresponds to a different pitch, while only one, "p", has been considered, measured on the outside diameter. Strictly speaking, in the plain development 36, on each segment parallel to the axis of the helix (starting from the points of division of the profiles 37), one should measure the pitch relative to the corresponding radius, divide it into many equal parts as the number of arches taken on the perimeter of the section and then locate the point with the number of the division concerned. With these approximations, the axial views of the helices on which the axes of the injection holes fall appear as in the details 38 and 40 of Figure 14. For completeness, the figure also shows the straight section 39 of a helical blade, conducted through the radial axis of a channel of internal recirculation 30 (Figures 10, 13A and 13B).

However, it is worth pointing out that the selected functional elements play multiple roles, also providing additional results of performance optimisation. For example, the circuit of the internal recirculation of the flue gases intervenes in the overcoming of residues impediments to the full extension of the flameless technique to combustors for gas turbine engines. This according to three synergistic effects. First through a structure sufficiently compact to find accommodation in the architecture of a compact burner for gas turbines; in addition without having to introduce special baffle plates toward backward peripheral paths [US7168949B2], always because of pressure drops due to fluid impact and additional heat losses. These superior characteristics of compactness and energy efficiency are derived from a spontaneous effect of dynamic overfeeding, aimed at exploiting the kinetic energy of the fluid threads 31 common to the two counterrotating trapped vortices to create an overpressure of inertia at the entrance of the circuit (Figures 10, 12 , 13A and 13B). A second contribution is then provided by the fact that the withdrawal of the flue gases is reintroduced in the interbiade compartments 35 (Figure 13A and 13B), and in points such as to perform the combustion of any residual diffusive driving (Figure 15) in diluted air, thus inhibiting the effects of increased harmful emissions and constriction of the flame to a localised anchor.

Finally, the spontaneous overfeeding of the recirculation circuit is enhanced with the increase of flow; so at full load self-reliance of the main flame (generated sequentially from the mixtures: secondary air 45 - diffusion gas 44 and primary air 52 - gas premix 53, figures 9 and 12) via the flue gases, recirculated thermal activators, it will be complete without need to be supplemented by a diffusive pre-combustor. Accordingly, contrary to what happens in the systems with sequential injection [Hayashi S., Yamada H., Makida M., "Extending Low-NOx Operating Range of a Lean Premixed-Prevaporized Gas Turbine Combustor by Reaction of Secondary Mixtures Injected Into Primary Stage Burned Gas", Proceedings of the Combustion Institute 30 (2005) 2903-2911], where the morphology of the reagent zone is diversified in stages and reaches the flameless regime only in the last, according to the present invention, the volumetric distribution of heat release will be guaranteed for the most part of the flame.

Another task performed by the circuit of internal recirculation of the flue gases is to improve the allocation and retention of the flame 15 (Figure 9) within the cavity 12, through a mechanism of "fluidic feedback action" that sucks the flame 15 from the rear, forcing it to back-bend towards the trapping cavity 12.

In addition, the circuit of internal recirculation of the flue gases 31 intervenes in exalting the resistance to the triggering of thermoacoustic instabilities, expounding a Venturi effect (suction from the cavity towards the interbiade compartments 35, Figure 13A and 13B), ie feeding the root of flame with flows of flue gases, thermal activators of combustion, as 2015/000284

21

greater as the flame tends to stretch downstream under the action of the dynamics of the recirculation zone 4 (Figure 9).

Similarly, the array of injection holes 28 is both "breaking grid" of instabilities, as it dampens acoustic oscillations due to dissipative effect and breaks up the turbulent paths, preserving the flame 15 from periodic distortions. Still, the strategy, already introduced optimizing the coupling swirler 13 - cavity 12, to provide a distributed, "oriented" and premixed injection, remove the diffusion pilot of the chamber serving as a localised anchor of the flame 15, which prevents to spread volumetrically, to escape the fluctuations of fluid of a restricted root zone and consequently to become immune to the thermoacoustic pulsations [Xuan Lv, Yufeng Cui, Aibing Fang, Gang Xu, Bin Yu, Chaoqun Nie, "Experimental Test on a Syngas Model Combustor With Flameless Technology", Proceedings of ASME Turbo Expo 2010: Power for Land, Sea and Air, June 14-18 2010, Glasgow, UK].

In the present invention is then proposed a multistage hybrid system for the induction, anchorage and stabilization of distributed flame in advanced combustors for gas turbines, characterized by the combination of three essential characteristics: mating swirler - cavity, internal recirculation of the flue gases and oriented and distributed injection of the mixture within the trapping cavity. The specific embodiment described with reference to Figures 9-15 constitutes a first embodiment of the present invention, while figure 16 which follows will show a second and particularly preferred embodiment, aimed at promoting the maximum degree the development and the sustenance of distributed flames.

Therefore it is now illustrated, with reference to Figure 16, the role of the additional technical measures, that the preferred embodiment of the present invention contemplates, in remedying the additional operational problems that today afflict the burners for gas turbine classified as prior art. With regard to the problems connected with the start-up, merely contemplating the conventional and consolidated architectures of the burners for gas turbines, the only way to retain the trigger in a central location would be the one commonly adopted, of first turning on a diffusive pilot flame, subsequently taken from the main flame which it supports. In doing so, however, there is a risk of blocking in whole or in part the effect of volumetric distribution of the reagent zone promoted by the coupling between the swirler 13 and the cavity 12. An obstacle to the transition to the regime of Colorless Distributed Combustion would in fact be restored and the flame would be left at the mercy of fluctuations in fluid dynamics of a restricted anchorage area, no longer able to distribute and escape the thermoacoustic oscillations.

According to the preferred embodiment of the present invention, shown with reference to Figure 16, it is proposed to decouple the dynamics of the two flames, resorting to a kind of pilot "pre-combustor" 41 , for the generation of hot gases, rich in active radicals, axially injected, in the optimal site, upstream of the rear stagnation point 6. The device, already described in US2011/0016867A1 , here takes an unusual arrangement of synergistic cooperation with the measures of its realization already described with reference to FIGS 9- 15, to the objective of promoting and supporting the distributed flames. The pre-combustor is therefore for the first time coupled and optimized in mutual interaction, not with a traditional burner stabilized by means of the swirler [US2011/0016867A1], but with a High-G Loading TVC [US8322142B2], which is also totally renewed according to the already exposed description of the first embodiment (figures 9 -15).

Although it is apparently possible to believe that this arrangement does not lead to substantial benefits compared to a traditional driving, as will be discussed, it is precisely interrupting the continuity of the reaction zone 42 between the pilot flame and the main flame 15 that arises the prerequisite to be able to realize a proper flameless combustion zone, immediately in front of the axial swirler 13 (Figure 16). In fact, by physically separating the two flames 15, 42 (eventually as if the flue gases for driving were generated by an outer combustor and therefore adduct to the main 4

23

combustion chamber 43 through a dedicated line), they are leaved coupled only via pressure waves, which sweep the entire combustor, and of the activating flow of energy and chemical species (ejected from the nozzle of the pre-combustor 41). On the contrary, the feedback, chaining by means of a global dynamic heat release, tendentially ceases.

Naturally, for the injection of the flue gases to take place with a suitable margin of time compared to the relaxation of the radicals, the pre- combustor 41 must be as short as possible, compatibly with the autonomous and complete confinement of its flame 42, which, extending beyond it, would return to make a conventional diffusive "not decoupled" piloting, rejoining to the premixed main flame 15.

Shape and position of the pilot stage are used to optimize the exploitation of not only radicals, but also of the flue gases, such as vectors of the heat of activation for the main mixture. Thus, on the whole, compared to a traditional piloting, the higher efficiency of the pre- combustor 41 enables to minimize size and power, limiting the percentage of gas that burns in conditions of rich mixture and partially premixed, to the full advantage of the quality of the fumes.

In the light of the exposed description a further favorable prerogative of the described system is revealed: to promote a true "conventional" MILD combustion zone, immediately downstream of the pilot stage. And in fact, the flue gases (with a reduced oxygen content) that the latter generates go immediately to dilute the swirled flow produced by the axial swirler 13, where only in the vicinity the output the diffusive gas 44 (Figure 16) is introduced, dosing globally rich. Thus, although the air 45 that flows around and cools the wall of the pilot pre-combustor 41 (secondary air) is strongly preheated, the separation from methane that is maintained almost to the contact with the flue gases, the defect of oxygen already before dilution and the energetic mixing with the products of the prechamber, should ensure that the temperature of self- ignition is simultaneously achieved in a large volume, without development of a flame front.

From the point of view of the contribution to the more energetic promotion and more effective livelihood of distributed flames, in addition to the considerations already explained previously, the device according to the present invention contributes in accordance with the arrangements described in the following.

First, centrifugation of the central swirled spouts and the Venturi effect explicated by the nozzle 46 of the pre-combustor 41 , thus bearing responsibility for an overpressure between cavity 12 and nozzle 46, have suggested to open a path between the two (which would also result in the interblade compartments 35, Figures 13A and 13B), useful for purposes previously discussed with reference to the channels 30 of recirculation and which will be subsequently incorporated. At each branch of the path, obtained by digging internally each blade of the swirler13 (as already shown with reference to Figures 10, 13A, 13B and 14), it is then conferred the direction of the fluid thread 31 common to the two counterrotating vortices trapped, so as to take advantage of the inertia to give life to a spontaneous effect of "dynamic overfeeding". Considering then that both this inertial push on the inlet side (exerted by the lines of current 31), and the centrifugation, the suction at the outlet side (towards the nozzle 46), and overall the autonomous tendency of the flame to be trapped bending back towards the cavity within which it is drawn, they intensified with the flow, it is reasonable to expect the following.

First, each lobe of the flame will remain trapped in the corresponding "hollow", without "impacting" its leading edge, in the broader field of regimes (deletion of the "Typel Flame" 15' of Figure 7B and 7D and of the anticipated correspondent Lean Blow Out), resulting in "transforming" a second generation TVC in a fourth. In fact, were it not for the separate "withdrawal" 31 of the flue gases (Figure 17A and 17B) , the same flow chart (Figure 17C) would apply equally to the two schemes in question. But this is precisely the additional and separate recirculation that, by allowing pre-dilute the "stirring air" (Figure 17B) with an injection in co-swirl immediately before entering the combustion chamber, would seal the superior ability of the system in promoting Flameless combustion. Secondly, the occurrence of oscillations of the "Inner Recirculation

Zone" 4 (Figure 16) and of the thermoacoustic instability that may result for the periodic distortion of the surface of the flame will be automatically thwarted. And in fact such a shift forward of the "Rear Stagnation Point" 6, causing further depression in the nozzle 46 of the pre-combustor 41 , will also determine an additional effect of retrieval on the flue gases in recirculation through the channel 30, with the result to "force" the anchoring of the flame 15 (produced, in the root zone, from the mixture secondary air 45 - diffusion gas 44 in progressive mixing). Further contribute to this desirable antagonistic action will be provided by cooling, and then the tendency to recontract after expansion (due to the sliding of the recirculation zones 4), of the gas enclosed between the two branches of the flame (15) which almost join up around the right side trapped vortex. The opposite will happen in case of backward movement, ie it will rise up an autonomous action of contrast to the "flame flash back" (ascent of the flame upstream into the swirler 13).

Properly adjusting the geometric details, in full power operation the only recirculation will play the role already assumed at partial loads by the pilot stage: activate and support the main combustion, while diluting the reactants to form a distributed flame. At the point of design, or when overfeeded, it is estimated, therefore, to be able to disconnect the feeding from the pre-combustor 41 , with the advantage of extending the regime of "Colorless Distributed Combustion" to the whole flame, without the need to get it (as in the current "fuel staged" systems) using the action of the diluent gas produced upstream by a diffusive (or just partially premixed) pilot stage, responsible for the increased emissions of ΝΟχ . Even in this possibility of alternation between the pre-combustor and the internal recirculation of the flue gases in the role of sustenance of the main flame consiste the optimization of the coupling between the pilot stage (now in behavior on/off) and the redesigned High-G Loading TVC .

From the point of view of obtaining a totally distributed flame, to avoid the preventive, although in a minimum extent anticipated, contact between the "diffusion gas" 44 (Figure 16) and the "secondary air" 45, ie a diffusive piloting, it will be possible to provide the blades 23 (Figure 10) of the swirler 13 with two consecutive cavities, respectively, a first cavity 47 (Figures 3A, 13B and 14) for flue gases and a second cavity 48 for the diffusion gas (Figure 15), so the fuel to be injected into an already diluted air vein. Substantially, the second cavity would in this way replace the nozzles of the "Diffusion Gas" 44 of Figure 16.

The constant entrapment of each lobe of the flame 15 would then avoid dangerous "stretching" towards the output section of the combustor and, especially in the more compact chambers for aeronautical use, the trigger of the insidious "Acoustic-Entropy Combustion Instabilities" [E. Motheau, L. Selle, Y. Mery, T. Poinsot, F. Nicoud, "A mixed acoustic- entropy combustion instability in a realistic gas turbine combustor", Center for Turbulence Research, Stanford University, Proceedings of the Summer Program 2012], [Goh S. S., Morgans A. S., "The Influence of Entropy Waves on the Thermoacoustic Stability of a Model Combustor", Combust. Sci. Technol., 185: 249-268, 2013].

Regarding the effect of the antagonist diffusive pilot flames on the establishment of the full Flameless regime and on the acoustic decoupling, the particular and unusual system of multiple tangential injection of the preformed mixture within the cavity 12 has been specially designed in order to remove this obstacle.

In order to achieve the "optimal" rate of pre-mix, ie the one that, without compromising ignition and combustion stability, enables the flame to distribute, it is decided to increase the geometric-functional degrees of freedom of the system. Precisely it has been assumed the division of the annular capsule pre-mixer 34 (Figure 9) in multiple segmented sectors, each fed from the corresponding branch of a premix injector 49 (Figures 9 and 16) possibly adjustable in axial position and geometry. The major approaches to cavity 12 correspond to more energetic and less partially premixed pilotings, that is, to favor the maintenance of the flame 15 rather than its volumetric shedding. Thus, according to the results of subsequent numerical simulations, by varying the geometric parameters of the nozzle 49 (including the distance of the injection point from the perforated plate 29), it will be possible to reach the desired morphology of the flame 15. The described measure also allows to maintain a separate access for the dilution air 50 (Figure 16), intended for the cooling of the cavity.

Compared to the solutions according to the prior art [US6826912B2, US4151709B1 , US7086854B2], provided with a distributor of the mixture similar to the one proposed according to the present invention, use is made additionally both of the sustenance that the injection organized as in Figure 14 provides to the trapped helical vortex, and of the multiplication of holes 28 up to create a out-and-out grid. This sort of honeycomb, which is used to stop possible instabilities [Eldrege J., Dowling A., "The Absorption of Axial Acoustics Wave by a perforated Liner with Bias Flow", Journal of Fluid Mechanics, 485, 307-335 (2003)], and that in the previous embodiments is inserted in the form of a plate upstream of the swirler 13, in the present invention realizes at the same time the distributor of the mixture (perforated wall 29 for feeding the cavity) and the environment of entrapment of the flame. The possible resonance of the interspace forming the capsule pre-mixer 34 can be countered by suitably varying the thickness of the perforated wall 29.

One could argue that the mixture in progressive formation within the capsule pre-mixer 34, receiving heat through the walls 29 of the cavity 12, can ignite before entering the cavity itself. The risk that this can happen is restricted by laying in the design of the entire system of the present invention a plurality of cooling fins 51 (Figure 16), traversed by cooling air, and disposed in the vicinity of the injection holes 28; or by varying the relative position of cavity 12 - premix fuel injectors 49, and/or the number and the arrangement of the injectors 49 themselves, so as to regulate the residence time in the cavity of the pre-mixer 34, and thus the amount of heat received by the reactants. Furthermore, the mixture is in fact only partially preformed and poor enough in dosage that only in the turbulent regime, "frontally clashing" with the activating flue gases in recirculation (Figure 3) , can stably support a flame. Finally, it is possible to vary the equivalence ratios of the mixtures that feed the cavity 12 and the swirler 13, by making the first below the limit of flammability (this also favors the development of a distributed flame within the entrapment chambers as an effect of deprivation of fuel). All things considered, therefore, the matter is to adjust things so that the incubation time does not end before entering the combustion chamber.

Still, the cooling of the reaction zone, already suggested in the prior art, is here implemented with superior efficacy (Figure 16). In fact, not only the centrifugation of the main flame 15, towards the walls of the cavity 2 that accommodates it, ensures greater penetration of the reagent mixture between the cooling fins 51 , but moreover the heat exchange surfaces in turn are chilled from the inside, according to a scheme devoid of any anticipation. Cooling is delegated to the convective action of the secondary dilution air 50, which does not take part in the combustion process and which therefore do not reintroduces the heat received as the effect of preheating of the reactants. Moreover, the proposed scheme extends the possibility of cooling from conventional combustors to those based on the principle of "cavity flameholding", moreover in the presence of a finely divided injection. This results, with respect to the objective of inducing and supporting distributed flames, in reconciled effects of stirring between reactants and products (TVC), absence of diffusive piloting and lower oxygen content in the flue gases in recirculation. Also the risk of destabilization of the main flame for excessive cooling to the minimum thermal loads can be effectively countered: it is precisely in the phase of the load that a (pilot) flame is available, which is not subjected to refrigeration (the "secondary air" 45 that laps the walls of the pre- combustor 41 is then used as preheated combustion air immediately downstream).

Finally, following a construction scheme similar to that of the return channels of the multistage centrifugal compressors, an oblique centerline could be given to the cooling fins 50, so as to satisfy, at the entrance, the motion of swirl of the flow outgoing from the pre-combustor 41. If this is deemed constructively too heavy, or too short for the variability of the angle of incidence with the operating conditions, the fins 50 could be replaced by a continuous toroidal interspace, crossed by the extensions of the injection holes (more effective also to avoid the excessive heating of the mixture formed in 34 before it enters the cavity 12).

In conclusion, even the only joint and conspiring use of the artifices above, in the forms already known and less improved (when present), would not find to now any anticipation in a single specimen of a burner for gas turbine engines. It follows that the invention presented here is a candidate to: 1) increase the efficiency of combustion; 2) bring down the flame temperature and pollutant emissions; 3) achieve a combustion that although ultra lean and premixed, is less prone to the triggering of dangerous thermoacoustic oscillations; 4) inhibit the interaction of the flame with the field of acceleration out of the combustor (Acoustic-entropy combustion instabilities); 5) mitigate the stress from differential thermal expansion on the terminal nozzle of the combustor and on the blades of the first stages of the turbine. All through the synergistic integration of the most up to date, effective and sophisticated measures on the one hand and the introduction of other innovative measures. Overall, it is expected, therefore, the obtaining of a higher degree in the multiparameter performance optimization defined by the previous list.

The present invention has been described for illustrative but not limitative purposes according to its preferred embodiments, but it is to be understood that variations and/or modifications can be made by those skilled in the art without departing from the related scope of protection, as defined by the appended claims.