EP1182398A1 - Process for increasing the fluidic stability of a premix-burner as well as premix-burner for carrying out said process - Google Patents

Abstract

The premix burner, for a gas turbine assembly, has a combustion air flow fed into the hollow combustion zone (6) at a tangent (3) to be mixed with the injected spiral flow of gas and/or liquid fuel coaxial to the burner axis (4). It induces a backflow zone at a cross section jump at the burner opening to stabilize the flame. The spiral flow is increasingly distorted towards the burner opening, and it enters the combustion zone with a flow cross section with a non-rotation symmetry and acts on the free flow cross section.

Description

Technical field

The invention relates to a method for fluid-mechanical stabilization of a premix burner, into which a combustion air flow is introduced tangentially into a burner interior, mixes with a coaxially oriented swirl flow with an injected gaseous and / or liquid fuel and induces a backflow zone at a burner outlet, which serves to stabilize the flame during operation of the burner. Furthermore, the invention relates to a premix burner for carrying out the method.
A preferred field of application of the invention is the operation of a gas turbine plant.

State of the art

EP 0 321 809 and EP 0 780 629 disclose premix burners of the type discussed here known. Such burners, which are characterized by very low pollutant emissions, are widely used in combustion chambers of gas turbine plants Hot gas generation used.

When operating gas turbine systems, thermoacoustic vibrations often occur in the combustion chambers. Fluidic instability waves arising at the burner lead to the formation of flow vortices, which strongly influence the entire combustion process and lead to undesired periodic heat releases within the combustion chamber, which are associated with strong pressure fluctuations. The high pressure fluctuations are associated with high vibration amplitudes, which can lead to undesirable effects, such as a high mechanical load on the combustion chamber housing, an increased NO _x emission due to inhomogeneous combustion and even an extinguishing of the flame within the combustion chamber.

Thermoacoustic vibrations are based, at least in part, on flow instabilities the burner flow, which is expressed in coherent flow structures, and that affect the mixing processes between air and fuel. With conventional Combustion chambers become cooling air in the manner of a cooling air film over the combustion chamber walls directed. In addition to the cooling effect, the cooling air film also has a sound-absorbing effect and contributes to the reduction of thermoacoustic vibrations. In modern gas turbine combustion chambers with high efficiency, low emissions and a constant temperature distribution at the turbine inlet is the cooling air flow into the combustion chamber significantly reduced, and almost all of the air is passed through the burner. However, this also reduces the noise damping Cooling air film, which reduces the sound-absorbing effect and exacerbates the problems associated with the unwanted vibrations occur.

One way of sound absorption is to connect so-called Helmholtz dampers in the area of the combustion chamber or the cooling air supply. However, with modern Combustion chamber designs the provision of such Helmholtz dampers associated with great difficulties due to limited space.

In addition, it is known that the fluid mechanical instabilities occurring in the burner and the pressure fluctuations associated therewith can be counteracted by stabilizing the fuel flame by additional injection of fuel. Such injection of additional fuel takes place via the head stage of the burner, in which a nozzle is provided on the burner axis for the pilot fuel gas supply, but this leads to an enrichment of the central flame stabilization zone. However, this method of reducing thermoacoustic vibration amplitudes has the disadvantage that the injection of fuel at the head stage can be accompanied by an increase in the emission of NO _x .

Have more detailed studies on the formation of thermoacoustic vibrations demonstrated that such undesirable coherent structures arise during mixing processes. Of particular importance are those that mix between two Shear currents forming currents within the coherent structures be formed. More detailed information on this can be found in the following publications: Oster & Wygnanski 1982, "The forced mixing layer between parallel streams ", Journal of Fluid mechanics, vol. 123, 91-130; Paschereit et al. 1995," Experimental investigation of subharmonic resonance in an axisymmetric jet ", Journal of Fluid Mechanics, vol. 283, 365-407).

As can be seen from the above reports, it is possible to look inside the coherent structures forming the shear layers by targeted introduction to influence an acoustic excitation in such a way that it does not arise becomes. Another method is to introduce an acoustic counter-sound field, so that the existing unwanted sound field through a specifically introduced phase-shifted sound field is literally canceled. This anti-noise technique however, requires a relatively large amount of energy, which is either external to the Burner system must be made available or the entire system to be branched off at another point, but this leads to a, albeit small, but still leads to a loss of efficiency.

In addition to the above-mentioned active options for targeted influence to reduce coherent structures forming inside the burner can alternatively with passive measures such disturbances in the burner flow be opposed. Passive measures are predominant constructive design features of the burner covering the operating area of a burner expand in terms of pulsations and emissions are particularly attractive since once installed, they require no further maintenance.

Presentation of the invention

The invention has for its object a method for increasing the fluid mechanical Provide stability of a premix burner, which the undesirable flow vortices, which are known as coherent pressure fluctuation structures educate, suppressed efficiently and without additional energy expenditure. The for this necessary measures on a premix burner should have a low design Cause effort and be inexpensive to implement. The The measures used should also be completely maintenance-free.

According to the invention, the object is achieved by a method for increasing the fluid mechanical stability of a premix burner and by a premix burner of the type mentioned in the independent claims.
Features which advantageously further develop the inventive concept are the subject of the dependent claims and of the description and the exemplary embodiments.

The method according to the invention is based on the basic idea of fluid mechanics Stabilization of a premix burner in which at least one Combustion air flow is introduced tangentially into a burner cavity and itself forming a swirl flow oriented coaxially to the burner axis with a injected gaseous and / or liquid fuel mixed and at one Cross-sectional jump at the burner mouth induces a backflow zone, which during burner operation to stabilize the flame, the swirl flow inside of the burner cavity towards the burner mouth at least increasingly radially deform a peripheral portion and in a non-rotationally symmetrical To let the flow cross-section enter the combustion chamber, this deformation at the expense of the free flow cross section of the burner cavity is produced.

The formation of coherent vortex structures is due to a rotational symmetry different shape of the flow cross section in the burner cavity and disturbed when entering the combustion chamber. At certain operating points is at Premix burners according to the prior art, the time delay of the fuel from Injection site constant up to the flame. From the deformation according to the invention of the flow cross section results in a wide distribution of the delay time. By the prevention of the formation of vortex structures at the burner outlet and one smeared time delay also suppresses periodic heat release, which in turn is responsible for the occurrence of thermoacoustic vibrations. By deforming the swirl flow through constricting portions of the cavity contour is forced, as will be explained elsewhere, there is also an acceleration of the flow, which is stabilizing affects the backflow zone.

A premix burner according to the invention is based on a premix burner for use in a heat generator, essentially consisting of a swirl generator with means for the tangential introduction of a combustion air flow in a cavity of the swirl generator and means for introducing at least a gaseous and / or liquid fuel in the combustion air flow below Formation of a swirl flow with an axial movement component towards Burner mouth at which the swirl flow induces a backflow zone bursts. A generic burner based on at least two hollow ones Flow direction of the hot gases nested conically widening Partial bodies whose central axes are offset from one another is described in EP 0321809 described, which is an integral part of this protection request. Such burner types, also known as cone burners or double-cone burners, have a tear-off edge at their burner outlet, the edge course of which there are two staggered semicircles with closed edges however almost circular and therefore approximately rotationally symmetrical to the burner axis is trained. The fuel / air mixture that forms in the burner cavity spreads in the form of a rotationally symmetrical swirl flow an axial component towards the burner mouth, with all its known Disadvantages with regard to the formation of coherent structures and the associated ones thermoacoustic pressure fluctuations.

On the other hand, one ensures that targeted radial deformations in the flow flow of the fuel / air mixture are introduced so that the flow cross-section differs from that of a rotationally symmetrical flow, so can in this way form coherent structures and a constant Delays in fuel can be effectively countered.

Such influence on the flow geometry can be at least take a section of the cavity wall, which wall section in one downstream end portion of the burner cavity has a smaller slope than in an upstream area. This at least one section leads with it compared to those wall sections at the same axis height that this property do not have a radial deviation from the circular shape towards the Brenner axis. Any partial contours in the burner cavity that deviate from the circular shape and at the trailing edge, e.g. straight or aspherically curved Wall sections across the circumference help to reduce eddies at.

As a basic design rule for the design of the burner outlet edge it should be noted that the geometric deviation from a round geometry at least is to be chosen so large that the resulting distance between the two geometries is greater than the boundary layer thickness of the flow caused by the exit geometry flowing.

Brief description of the invention

Fig. 1a

perspektivische Darstellung eines Vormischbrenners nach dem Stand der Technik, von dem die Erfindung ausgeht

Fig.1b

weitere Darstellung eines solchen Brenners in vereinfachter Form

Fig. 2a-d

stark schematisierte Wiedergabe des Erfindungsgedankens anhand verschiedener Formen von Drallerzeugern

Fig. 3

eine Ausführungsform eines erfindungsgemäss abgeänderten Brenners

Fig. 4-7

alternative Ausführungsformen der Erfindung

Fig. 8

Darstellung der Unterdrückung von Verbrennungsschwingungen durch Unterdrückung von Strömungswirbeln in einem Brenner.

The invention is illustrated below by way of example with reference to the drawing, without restricting the general inventive concept. Show it:

Fig. 1a

Perspective view of a premix burner according to the prior art, from which the invention is based

1b shows

further representation of such a burner in a simplified form

2a-d

highly schematic representation of the inventive concept based on various forms of swirl generators

Fig. 3

an embodiment of a burner modified according to the invention

Fig. 4-7

alternative embodiments of the invention

Fig. 8

Representation of the suppression of combustion vibrations by suppression of flow eddies in a burner.

WAYS OF CARRYING OUT THE INVENTION, INDUSTRIAL APPLICABILITY

1a and 1b show the structure and mode of operation of a premix burner in a highly schematic form, as is the starting point of the invention presented here.
The premix burner consists of two hollow, conically widening partial bodies (1) and (2) which are arranged so as to be axially parallel and offset from one another in such a way that they form tangential gaps (3) in two overlapping areas opposite each other in mirror image. Although two conically widening partial bodies (1) and (2) are shown as examples in FIGS. 1a and 1b, other configurations are also conceivable. So these burners are neither limited to the arrangement of two partial bodies (1) and (2), nor is their conical configuration absolutely necessary. This is familiar to the person skilled in the art. The gaps (3) resulting from the displacement of the longitudinal axes serve as inlet channels through which the combustion air (5) flows tangentially into the burner cavity (6) during burner operation. Along the tangential inlet channels (3) there are injection openings (7) through which a preferably gaseous fuel is injected into the combustion air (5) flowing past. In the interest of thorough mixing, the fuel is preferably injected within the column (3) immediately before entering the burner cavity (6). In the initial area of the burner, which can also be cylindrical (not shown), there is a central nozzle (8) for atomizing a liquid fuel, the capacity and operating mode of which depend on the burner parameters. The liquid fuel emerges from the nozzle (8) at an acute angle and forms a conical fuel profile in the burner cavity (6), which is enclosed by the combustion air (5) entering tangentially and passing into a swirl flow (9) and continuously forms a mixture which process can be supported by preheated combustion air or admixture of recirculated exhaust gas. Alternatively, it is also possible to apply gaseous fuel to the nozzle (8). On the combustion chamber side, the premix burner has a front plate (10) which acts as anchoring of the partial bodies (1) and (2) and which has a number of bores (11) for introducing air into the combustion chamber (12). The fuel / air mixture crossing the burner cavity (6) in a swirl flow (9) reaches the optimum fuel concentration over the cross section at the downstream end of the premixing section (13) at the burner mouth (14). When exiting the burner, the swirl flow (9) bursts, forming a backflow zone (15) with a stabilizing effect with respect to the flame front (17) acting there. This aerodynamic flame stabilization takes over the function of a flame holder. The dreaded failure of mechanical flame holders due to overheating with possible subsequent severe accidents on machine sets is therefore excluded. Furthermore, the flame loses no heat to cold walls except through radiation. This also contributes to the uniformity of the flame temperature and thus to low pollutant emissions and good combustion stability.
According to the invention, measures are now provided to radially deform the swirl flow (9) within the premixing section (13). This deformation should preferably take place symmetrically. However, this is not mandatory. It is an essential characteristic to achieve this deformation at the expense of the free flow cross section (18). The wall (21) of the cavity (6) has at least one section (22) in a downstream area (20), which has a smaller gradient with respect to the burner axis (4) compared to an upstream area (19). This means that, viewed over the cross section, has an approximately circular contour (21) of the burner cavity (6), distributed over the circumference, deviating from the circular contour of the cavity contour (21) in the direction of the central axis (4), i.e. constricting the cavity (6) Sections (22), as shown schematically in Fig. 2a - 2d in longitudinal section. In this context, it has proven to be advantageous to have the deformation of the flow accompanied by an acceleration of the flow. This measure has a particularly favorable effect on the stability of the burner. On the one hand, the cross-sectional shape of the flow (9) emerging from the burner, which deviates from the rotational symmetry, has a disruptive effect on the formation of coherent vortex structures and ultimately inhibits the formation of thermoacoustic vibrations. On the other hand, the acceleration of the swirl flow (9) at the burner outlet (14) resulting from the absolute or relative narrowing of the flow cross section (18) leads to a stabilization of the return flow zone (15), which means fluctuations in the return flow zone (15), the associated periodic heat release and this in turn inhibits the generation of thermoacoustic vibrations. The combination of these effects, which have the same effect, results in synergy effects which, in a particularly advantageous manner, allow the fluid-mechanical stability of a premix burner to be increased with very little technical effort. 2a-2d are intended to explain the inventive concept on the basis of highly schematic representations. Fig. 2a shows a known swirl generator geometry with which the invention can be implemented in a favorable manner, whereby - as mentioned elsewhere - the conical configuration of the swirl generator (13) is not mandatory.
2b-2d symbolize the idea of the invention, which consists in the wall (21) of the burner cavity (6) in at least one peripheral section (22) at the expense of the free flow cross section (18) in the direction of the burner axis for the purpose of deforming the flow profile (4) to bend. This can be done symmetrically or asymmetrically by at least one such section (22) constricting the flow cross section. Can insert into a downstream portion (20) of the cavity (6), which area (20), for example, at _^2/3 of the axial length, the cavity wall (21) bends in at least one peripheral portion (22) at an angle in the range of 2 ° up to 45 °, in particular 5 ° to 15 °, in the direction of the burner axis (4). From these schematic representations, a further advantage of the invention is evident to the person skilled in the art, namely the fact that existing burners can be retrofitted with little effort. The sections (22) constricting the flow cross section (18) can be realized by subsequently applied flow-guiding internals (28). FIGS. 3 to 7 show embodiments of burners designed according to the invention.
3 shows a preferred variant of the invention, according to which the burner mouth (14) has a polygonal outlet contour (16). As can be seen most clearly from the schematic representations of FIG. 2, the conically widening contour (23) of the burner cavity (6) is broken off in a downstream end region (20) and with a smaller gradient compared to the preceding region (19) Longitudinal axis (4) continued. The term reduced slope is also intended to include a course parallel to the longitudinal axis (4) or a convergent course, as can be seen from FIGS. A multitude of measures are available to the person skilled in the art to implement this suggestion. According to a preferred embodiment, correspondingly shaped plates (28) are welded into the bowl-shaped partial bodies (1) and (2) according to the state of the art, which - viewed planimetrically - represent tendons, the circle segments from the free flow cross section (18) of the burner cavity Cut out (6). One to four such plates (28) are preferably welded onto the inner wall (21) for each partial body (1) or (2). With new burners, the wall contour is shaped in the manufacturing process.
According to another embodiment shown in FIG. 4 in connection with FIG. 2c, the burner is formed in an upstream region (19) in a manner known per se from two staggered partial bodies (1) and (2) of essentially circular cross-section. In a transition region to about _^2/3 of the axial length of the inner wall (21) goes from its substantially circular contour over in a polygonal, the through increasingly expresses itself in the further course to the burner mouth (14). These sections (22) of the cavity wall (21) constricting the flow cross-section (18) like tendons have a lower divergence in relation to the longitudinal axis (4) compared to the upstream regions (19) of the cavity wall (6). The term smaller difference is also intended to include the possibility of a parallel or convergent course to the longitudinal axis (4). Considering the cross-section, the constricting sections (22) generally have a straight contour. A slightly convex or concave course is also possible. A convex course is particularly advantageous in the case of the arrangement of a small number or only one or two such sections (22).
Another embodiment, not shown in a figure, is not to provide the burner cavity (6) with a circular cross-section even in its upstream region (19), but rather to equip the burner with a cavity (6) which is contoured non-rotationally symmetrically. This embodiment is particularly suitable for polygonal contours (23) of the cavity cross section (18).
It is known per se from the prior art to equip burners, as defined above, with nozzles (24) or mixing tubes (25) downstream of the swirl generator (13) for the purpose of improved mixing and flame positioning in the case of difficult fuels , The invention is also suitable for such burner variants, by disturbing the flow instabilities and producing a smeared time delay of the fuel from the injection point to the flame, to increase the fluid-mechanical stability of such burners.
5 and 6 show a premix burner consisting of a swirl generator (13) for a combustion air stream (5) and means for injecting at least one fuel (7) and / or (8), with a mixing section downstream of the swirl generator (13) (25) is arranged. In the housing (26) bordering the mixing section (25), inlet openings (27) running in a uniform circumferential distribution to the longitudinal axis (4) can be arranged to inject an additional quantity of combustion air. Preferably in a region downstream of the inlet openings (27), the rotationally symmetrical flow cross section of the mixing section (25) is deflected and radially deformed by the free cross section (29) restricting sections (22). The outlet opening (16) takes on a polygonal cross-sectional shape, composed of a plurality of rectilinear sections (22). Exit contours (16) in the form of a regular or irregular (Fig. 5) polygon are promising. The individual rectilinear sections (22) of the outlet edge (27) span the outlet opening (16) of the burner. However, as already mentioned elsewhere, this straightness is not mandatory, and these sections (22) can also be convex or concave. Fig. 6 indicates a convex wall section (22) in an asymmetrical arrangement.
Finally, FIG. 7 shows an embodiment variant with a cylindrical or convergent nozzle section (24) at the downstream end of the burner. According to the prior art, these downstream nozzles (24) serve primarily to accelerate the flow at the burner outlet and thus to stabilize the backflow zone (15). According to one embodiment of the invention, this desirable acceleration is achieved by an increasing and decreasing cross-sectional area in the flow direction such that this nozzle section (24) is narrowed in the flow direction from an essentially circular cross-sectional shape to another cross-sectional shape, for example that of a regular or irregular polygon or one oval.

FIG. 8 shows a diagram that shows the combustion output along the abscissa shows the burner according to FIG. 3 and along the ordinate a scaling that the formation of thermoacoustic vibrations that as Result of coherent structures within the flow flow in the burner, quantified. Thermoacoustic vibrations in the 100 Hz range are considered. Comparing a burner with a conventional burner outlet according to the embodiment in Fig. 1 (see line traversed with squares) with an inventive trained burner outlet according to the embodiment in Fig. 3 (see line drawing interspersed with circles), it is clearly noticeable that in the latter case a significant lower proportion of thermoacoustic vibrations arises.

The exemplary embodiments set out above are in no way in one for the To understand the invention restricting meaning. They are instructive and as an outline of the Diversity of possible embodiments of those characterized in the claims Understanding invention.

LIST OF REFERENCE NUMBERS

1

partial body

2

partial body

3

tangential combustion air inlet duct

4

Brenner

5

combustion air

6

burner chamber

7

Injection openings for fuel

8th

central fuel nozzle

9

swirl flow

10

front panel

11

Cooling air holes

12

combustion chamber

13

Swirl generator, premixing section

14

burner mouth

15

backflow

16

Outlet cross section into the combustion chamber

17

flame front

18

Flow cross section of the burner cavity

19

upstream area of the burner cavity

20

downstream area of the burner cavity

21

Wall of the burner cavity

22

wall section

23

Inner contour of the burner cavity

24

burner

25

mixing section

26

Mixing section housing

27

trailing edge

28

fixtures

29

Flow cross-section in the mixing section

30

Wall of the mixing section

31

upstream area of the mixing section

32

downstream area of the mixing section

33

Flow cross section of the burner nozzle

34

Wall of the nozzle

35

upstream area of the nozzle

36

downstream area of the nozzle

Claims (20)

Method for fluid-mechanical stabilization of a premix burner, into which at least one combustion air stream (5) is introduced tangentially into a burner cavity (6) and which forms a swirl flow (9) oriented coaxially to the burner axis (4) with an injected gaseous and / or liquid fuel ( 7; 8) mixed and at a cross-sectional jump at the burner mouth (14) induces a backflow zone (15) which serves to stabilize the flame during operation of the burner, characterized in that the swirl flow (9) inside the burner cavity (6) in the direction is increasingly deformed onto the burner mouth (14) and enters the combustion chamber (12) in a non-rotationally symmetrical flow cross-section, this deformation being produced at the expense of the free flow cross-section (18). A method according to claim 1, characterized in that the deformation of the swirl flow (9) is accompanied by an increase in the flow speed. A method according to claim 2, characterized in that the deformation is accompanied by a reduction in the free flow cross section (18) of the burner cavity (6). Method according to Claim 1, characterized in that the deformation is achieved in such a way that at least one peripheral section (22) of the cavity wall (21) has a lower slope in a downstream region (20) of the burner cavity (6) than in an upstream region (19). A method according to claim 1, characterized in that the deformed flow profile is symmetrical in relation to at least one axis. A method according to claim 4, characterized in that the flow profile takes the contour of a polygon. A method according to claim 1, characterized in that it is used to operate lean premixed burners of a gas turbine plant. Premix burner for use in a heat generator, essentially consisting of a swirl generator (13) with means (3) for tangentially introducing a combustion air stream (5) into a cavity (6) of the swirl generator (13) and means (7; 8) for introducing at least one gaseous and / or liquid fuel in the combustion air flow (5) with the formation of a swirl flow (9) with an axial movement component towards the burner mouth (14), characterized in that the cavity contour (21) in the flow direction from a largely rotationally symmetrical to a non- Rotationally symmetrical cross-sectional shape passes over, when viewed over the circumference, at least a section (22) of the cavity wall (21) in a downstream end region (20) has a smaller gradient in relation to the upstream region (19) in relation to the longitudinal axis of the burner (4). Premix burner according to Claim 8, comprising at least two hollow, conically widening partial bodies (1) and (2) nested one inside the other coaxially to the longitudinal axis (4), the center axes of which are offset from one another and the walls (21) of which are tangential entry channels (3) in an overlap region Form combustion air (5) and at least one fuel nozzle (8) within the cavity (6) formed by the partial bodies (1) and (2), characterized in that the flow-limiting wall (21) at least one of the partial bodies (1) or ( 2) in a downstream end region (20) has at least one peripheral section (22) which has a smaller gradient in relation to an upstream region (19) in relation to the burner longitudinal axis (4). Premix burner according to Claim 8 or 9, characterized in that a plurality, preferably two to eight, of such sections (22) exist distributed over the circumference. Premix burner according to Claim 10, characterized in that the burner has a polygonal contour in an end region (20) including the burner mouth (14). Premix burner according to claim 11, characterized in that the burner has the contour of a regular polygon. Premix burner according to claim 11, characterized in that the burner has the contour of an irregular polygon. Premix burner according to Claim 9, characterized in that at least one of the partial bodies (1) or (2) delimits a convex outlet cross section (16) which deviates from the circular round. Premix burner according to claim 14, characterized by an at least approximately symmetrical outlet cross section (16). Premix burner according to claim 8, characterized in that the flow-limiting wall (21) of the burner cavity (6) between the upstream region (19) and the downstream end region (20) changes continuously or in one or more stages from one slope to the other. Premix burner according to claim 8, characterized in that the downstream end region (20) comprises approximately the last third of the length of the burner cavity (6). Premix burner according to claim 8, characterized in that in a downstream area (20) within the burner cavity (6) on the wall (21) the free flow cross-section (18) delimiting plates (28) are welded or fastened in another suitable manner. Premix burner according to Claim 9, comprising at least two hollow, conically widening partial bodies (1) and (2) nested one inside the other coaxially to the longitudinal axis (4), the center axes of which are offset from one another and the walls (21) of which are tangential entry channels (3) in an overlap region Form combustion air (5), and at least one fuel nozzle (8) within the cavity (6) formed by the partial bodies (1) and (2), further comprising a mixing section (25) downstream of that of the partial bodies (1) and (2) which mixing section has formed the swirl generator (13) (25) extending within a first starting region in the flow direction of the transition ducts for the formed in the cavity (6) swirl flow (9) and terminating at a trailing edge (27) into the combustion chamber (12), characterized that the free flow cross section (29) of the mixing section (25) in the flow direction with a simultaneous transition from a largely rotationally symmetrical is reduced to a non-rotationally symmetrical cross-sectional shape, in that at least one peripheral section (22) of the wall (30) delimiting the mixing section takes a smaller distance from the longitudinal axis (4) of the nozzle in a downstream area (32) compared to an upstream area (31). Premix burner according to claim 8, comprising a swirl generator (13) with means (3) for tangentially introducing a combustion air stream (5) into a cavity (6) of the swirl generator (13) and means (7; 8) for introducing at least one gaseous and / or or liquid fuel into the combustion air flow (5) with the formation of a swirl flow (9) with an axial movement component towards the burner mouth (14) and a burner nozzle (24) at the end on the combustion chamber side, characterized in that the free flow cross section (33) of the nozzle ( 24) in the flow direction with a simultaneous transition from a largely rotationally symmetrical to a non-rotationally symmetrical cross-sectional shape, in that at least one circumferential section (22) of the nozzle wall (34) in a downstream region (36) is less than the upstream region (35) from the longitudinal axis of the nozzle (4) occupies.

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