EP0675322A2 - Premix burner - Google Patents

Abstract

A gaseous and/or liquid fuel is blown as a secondary stream into a gaseous main stream and then into the main burner (52) which has a circular channel (20). The main stream is first guided over vortex generators (9) arranged side by side around the circumference of the channel. A Venturi nozzle (50) is set downstream of the vortex generators. The secondary stream is introduced into the channel in the area where the Venturi nozzle has its greatest restriction. The pilot burner works on the double-cone principle with two hollow conical parts boxed inside each other in the flow direction with their relevant centre axes off-set from each other. The adjoining walls of the two parts form in the longitudinal extension tangential channels (119) for the combustion air with gas inlets set in the area of the tangential gaps. <IMAGE>

Description

Technical field

The invention relates to a premix burner, consisting essentially of a pilot burner and a plurality of main burners arranged around the pilot burner.

State of the art

Both in oil operation at very high pressure and in gas operation with gases that contain a lot of hydrogen, it can happen with premix burners that the ignition delay times are so short that flame-retardant burners can no longer be used as so-called low-NOx burners.

The mixing of fuel into a combustion air flow flowing in a premixing duct is usually done by radial injection of the fuel into the duct by means of cross jet mixers. However, the momentum of the fuel is so low that an almost complete mixing takes place only after a distance of approximately 100 channel heights. Venturi mixers are also used. The injection of the fuel via grid arrangements is also known. Finally, spraying in front of special swirl bodies is also used.

The devices operating on the basis of transverse jets or stratified flows either result in very long mixing distances or require high injection pulses. With premixing under high pressure and substoichiometric mixing ratios, there is a risk of the flame flashing back or even of self-ignition of the mixture. Flow separations and dead water zones in the premixing tube, thick boundary layers on the walls or possibly extreme speed profiles over the cross-section through which the flow is flowing can be the cause of auto-ignition in the tube or form paths through which the flame can strike back from the downstream combustion zone into the premixing tube. The geometry of the premixing section must therefore be given the greatest attention.

The so-called premixing burners of the double-cone type can be designated as flame-holding burners. Such double-cone burners are known, for example, from EP-B1-0 321 809 and are described later in relation to FIGS. 1 and 3. The fuel, there natural gas, is injected into the combustion air flowing in from the compressor through a series of injector nozzles. As a rule, these are evenly distributed over the entire gap.

In order to achieve a reliable ignition of the mixture in the downstream combustion chamber and a sufficient burnout, an intimate mixture of the fuel with the air is required. A good mixing also helps to avoid so-called "hot spots" in the combustion chamber, which among other things lead to the formation of the undesired NO _x .

The above-mentioned injection of the fuel using conventional means, such as, for example, a cross-jet mixer, is difficult since the fuel itself has an insufficient impulse to achieve the required large-scale distribution and the fine-scale mixture.

Presentation of the invention

The invention is therefore based on the object of providing a measure for a premix burner of the type mentioned at the outset, with which intimate mixing of combustion air and fuel is achieved within a very short distance while at the same time distributing the speed uniformly in the mixing zone. Furthermore, with such a burner, the flame should certainly be prevented from kicking back without using a mechanical flame holder. The measure should also be suitable for retrofitting existing premix combustion chambers.

• dass in die, einen kreisförmigen Kanal aufweisenden Hauptbrenner ein gasförmiger und/oder flüssiger Brennstoff als Sekundärströmung in eine gasförmige Hauptströmung eingedüst wird,
• dass die Hauptströmung zunächst über Wirbel-Generatoren geführt wird, von denen über dem Umfang des durchströmten Kanals mehrere nebeneinander angeordnet sind,
• dass stromabwärts der Wirbel-Generatoren eine Venturidüse angeordnet ist,
• und dass die Sekundärströmung im Bereich der grössten Einschnürung der Venturidüse in den Kanal eingeleitet wird.

According to the invention, this is achieved by

• that a gaseous and / or liquid fuel is injected as a secondary flow into a gaseous main flow into the main burner, which has a circular channel,
• that the main flow is initially conducted via vortex generators, of which several are arranged next to one another over the circumference of the flowed-through channel,
• that a Venturi nozzle is arranged downstream of the vortex generators,
• and that the secondary flow in the area of the largest constriction of the Venturi nozzle is introduced into the channel.

With the new static mixer, which is represented by the 3-dimensional vortex generators, it is possible to achieve extremely short mixing distances in the burner with a low pressure drop. Due to the generation of longitudinal vortices without a recirculation area, a coarse mixing of the two streams takes place after a full vortex revolution, while fine mixing as a result of turbulence Flow and molecular diffusion processes already exist after a distance that corresponds to a few channel heights.

This type of mixture is particularly suitable for mixing the fuel into the combustion air at a relatively low admission pressure and with great dilution. A low admission pressure of the fuel is particularly advantageous when using medium and low calorific fuel gases. The energy required for mixing is largely drawn from the flow energy of the fluid with the higher volume flow, namely the combustion air.

The downstream arrangement of a Venturi nozzle behind the vortex generators has the advantage that, with the largest constriction of the Venturi nozzle, one has a simple means in hand to introduce the fuel into the swirled flow with the least counter pressure. The correct dimensioning of the Venturi nozzle also has the advantage that the flow velocity therein exceeds the flame velocity, so that the flame cannot strike back into the injection plane of the fuel.

The vortex generators upstream of the Venturi nozzle are characterized by a roof surface and two side surfaces, the side surfaces being flush with the same duct wall and including an arrow angle α with one another, and the longitudinal edges of the roof surface being flush with the longitudinal edges of the roof protruding into the flow duct Side surfaces and at an angle of attack Θ to the duct wall.

The advantage of such vortex generators can be seen in their particular simplicity in every respect. Manufacturing technology the element consisting of three flow-around walls is completely problem-free. The roof surface can be joined with the two side surfaces in a variety of ways. The element can also be fixed to flat or curved channel walls in the case of weldable materials by simple weld seams. From a fluidic point of view, the element has a very low pressure drop when flowing around and it creates vortices without a dead water area. Finally, due to its generally hollow interior, the element can be cooled in a variety of ways and with various means.

It is appropriate to choose the ratio height h of the connecting edge of the two side surfaces to the channel height H so that the vortex generated fills the full channel height or the full height of the channel part assigned to the vortex generator immediately downstream of the vortex generator.

It makes sense if the two side surfaces including the arrow angle α are arranged symmetrically about an axis of symmetry. This creates swirls of equal swirl.

If the two side surfaces enclosing the arrow angle α form an at least approximately sharp connecting edge with one another, which together with the longitudinal edges of the roof surface forms a tip, the flow cross-section is hardly impaired by blocking.

If the sharp connecting edge is the exit-side edge of the vortex generator and it runs perpendicular to the channel wall with which the side surfaces are flush, then the non-formation of a wake area is advantageous.

If the axis of symmetry runs parallel to the channel axis, and the connecting edge of the two side surfaces forms the downstream edge of the vortex generator, whereas consequently the edge of the roof surface which runs transversely to the channel through which the flow is flowing is the edge which is first acted upon by the channel flow, two identical but opposite vortices are generated on a vortex generator. There is a swirl-neutral flow pattern in which the direction of rotation of the two vortices is ascending in the area of the connecting edge.

Further advantages of the invention, in particular in connection with the arrangement of the vortex generators and the introduction of the fuel, result from the subclaims.

Brief description of the drawing

Fig. 1

einen Teillängsschnitt eines Brenners;

Fig. 2

einen Querschnitt durch den Brenner

Fig. 3A

einen Querschnitt durch einen Vormischbrenner der Doppelkegel-Bauart im Bereich seines Austritts;

Fig. 3B

einen Querschnitt durch denselben Vormischbrenner im Bereich der Kegelspitze;

Fig. 4

eine perspektivische Darstellung eines Wirbel-Generators;

Fig. 5

eine Ausführungsvariante des Wirbel-Generators;

Fig. 6

eine Anordnungsvariante des Wirbel-Generators nach Fig. 4;

Fig. 7

einen Wirbel-Generator in einem Kanal;

Fig. 8

eine weitere Ausführungsvariante des Wirbel-Generators;

Fig. 9

eine Anordnungsvariante des Wirbel-Generators nach Fig 8.

Several exemplary embodiments of the invention are shown schematically in the drawing.
Show it:

Fig. 1

a partial longitudinal section of a burner;

Fig. 2

a cross section through the burner

Figure 3A

a cross section through a premix burner of the double cone type in the region of its outlet;

Figure 3B

a cross section through the same premix burner in the region of the cone tip;

Fig. 4

a perspective view of a vortex generator;

Fig. 5

a variant of the vortex generator;

Fig. 6

a variant of the arrangement of the vortex generator according to FIG. 4;

Fig. 7

a vortex generator in a channel;

Fig. 8

a further embodiment of the vortex generator;

Fig. 9

a variant of the arrangement of the vortex generator according to FIG. 8.

Only the elements essential for understanding the invention are shown. The direction of flow of the work equipment is indicated by arrows. In the various figures, the same elements are provided with the same reference symbols. Elements which are not essential to the invention, such as housings, fastenings, line bushings, the provision of fuel, the regulating devices and the like have been omitted.

Way of carrying out the invention

1 and 2, 53 denotes a cylindrical burner wall. It is connected on the outlet side to the front wall 100 of the combustion chamber (not shown) by suitable means. This combustion chamber can be either an annular combustion chamber or a silo combustion chamber, with several such burners being arranged on the front wall 100 in each case.

In the interior of the burner wall, the inlet-side end of which is shown in broken lines in FIG. 1, six main burners 52 are grouped around a centrally arranged pilot burner 101. In the example, the pilot burner is a premix burner of the double-cone type, although this is not mandatory. It is important that this pilot burner should have the smallest possible geometry. About 10-30% of the fuel should be burned in it. The main burners 52 are cylindrical in shape. On their tubular wall 54, vortex generators 9 are initially arranged in the direction of flow, the outlet of which opens into a Venturi nozzle 50. The fuel is supplied to the pilot burner and the main burners via fuel lances 120 and 51, respectively. The combustion air flows from a plenum, not shown, into the housing interior 103, from where it flows into the burners 101, 52 in the direction of the arrow.

The schematically illustrated premix burner 101 according to FIGS. 1, 3A and 3B is a so-called double-cone burner, as is known for example from EP-B1-0 321 809. It essentially consists of two hollow, conical partial bodies 111, 112 which are nested one inside the other in the direction of flow. The respective central axes 113, 114 of the two partial bodies are offset from one another. The adjacent walls of the two partial bodies in their longitudinal extent form tangential slots 119 for the combustion air, which in this way reaches the interior of the burner. A first fuel nozzle 116 for liquid fuel is arranged there. The fuel is injected into the hollow cone at an acute angle. The resulting conical fuel profile is enclosed by the combustion air flowing in tangentially. The concentration of the fuel is continuously reduced in the axial direction due to the mixing with the combustion air. In the example, the burner is also operated with gaseous fuel. For this purpose, gas inflow openings 117 distributed in the longitudinal direction are provided in the region of the tangential slots 119 in the walls of the two partial bodies. In gas operation, the mixture formation with the combustion air thus begins in the zone of the inlet slots 20. It goes without saying that mixed operation with both types of fuel is also possible in this way.

At the burner outlet 118, a fuel concentration that is as homogeneous as possible is established over the applied annular cross section. A defined dome-shaped backflow zone is created at the burner outlet Peak the ignition occurs. So far, double-cone burners are known from EP-B1-0 321 809 mentioned at the beginning.

Before going into the installation of the new mixing device in the main burners 52, the vortex generator 9 which is essential for the mode of operation of the invention will first be described.

The actual channel, through which a main flow symbolized by a large arrow flows, is not shown in FIGS. 4, 5 and 6. According to these figures, a vortex generator essentially consists of three free-flowing triangular surfaces. These are a roof surface 10 and two side surfaces 11 and 13. In their longitudinal extent, these surfaces run at certain angles in the direction of flow.

The side walls of the vortex generator, which consist of right-angled triangles, are fixed with their long sides on a channel wall 21, preferably gas-tight. They are oriented so that they form a joint on their narrow sides, including an arrow angle α. The joint is designed as a sharp connecting edge 16 and is perpendicular to that channel wall 21 with which the side surfaces are flush. The two side surfaces 11, 13 including the arrow angle α are symmetrical in shape, size and orientation in FIG. 4 and are arranged on both sides of an axis of symmetry 17. This axis of symmetry 17 is rectified like the channel axis.

The roof surface 10 lies with a very narrow edge 15 running transversely to the flow through the channel on the same channel wall 21 as the side walls 11, 13. Its longitudinal edges 12, 14 are flush with the longitudinal edges of the side surfaces projecting into the flow channel. The roof area runs under one Angle of attack Θ to the channel wall 21. Their longitudinal edges 12, 14 together with the connecting edge 16 form a tip 18.

Of course, the vortex generator can also be provided with a bottom surface with which it is fastened in a suitable manner to the channel wall 21. However, such a floor area is not related to the mode of operation of the element.

In Fig. 4, the connecting edge 16 of the two side surfaces 11, 13 forms the downstream edge of the vortex generator. The edge 15 of the roof surface 10 which runs transversely to the flow through the channel is thus the edge which is first acted upon by the channel flow.

The vortex generator works as follows: When flowing around edges 12 and 14, the main flow is converted into a pair of opposing vortices. Their vortex axes lie in the axis of the main flow. The number of swirls and the location of the vortex breakdown (if the latter is desired at all) are determined by appropriate selection of the angle of attack Θ and the arrow angle α. With increasing angles, the vortex strength or the number of swirls is increased and the location of the vortex burst moves upstream into the area of the vortex generator itself. Depending on the application, these two angles Θ and α are predetermined by the structural conditions and by the process itself. Then only the length L of the element and the height h of the connecting edge 16 need to be adjusted (FIG. 7).

FIG. 5 shows a so-called half "vortex generator" based on a vortex generator according to FIG. 1, in which only one of the two side surfaces of the vortex generator 9a is provided with the arrow angle α / 2. The other side surface is straight and oriented in the direction of flow. In contrast to the symmetrical vortex generator, only one vortex is generated on the arrowed side. Accordingly, there is no vortex-neutral field downstream of the vortex generator, but a swirl is forced on the flow.

In contrast to FIG. 4, the sharp connecting edge 16 of the vortex generator 9 in FIG. 6 is the point which is first acted upon by the channel flow. The element is rotated by 180 °. As can be seen from the illustration, the two opposite vortices have changed their sense of rotation.

7, the vortex generators are installed in a channel 20. As a rule, the height h of the connecting edge 16 will be coordinated with the channel height H - or the height of the channel part which is assigned to the vortex generator - in such a way that the vortex generated immediately downstream of the vortex generator already reaches such a size that the full channel height H is filled. This leads to a uniform speed distribution in the cross-section applied. Another criterion that can influence the ratio h / H to be selected is the pressure drop that occurs when the vortex generator flows around. It goes without saying that the pressure loss coefficient also increases with a larger ratio h / H.

In the example shown, four vortex generators 9 are distributed at a distance over the circumference of the circular cross section according to FIG. The above-mentioned height of the channel part, which is assigned to the individual vortex generator, corresponds in this case to the circle radius. Of course, the four vortex generators 9 could also be strung together on their respective wall segments 21 in the circumferential direction in such a way that no gaps are left free on the channel wall. Ultimately, the vortex to be generated is decisive here.

The vortex generators 9 are mainly used for mixing two flows. The main flow in the form of combustion air attacks the transverse inlet edges 15 in the direction of the arrow. The secondary flow in the form of a gaseous and / or liquid fuel has a substantially smaller mass flow than the main flow. In the present case, it is introduced into the main flow downstream of the vortex generators.

1, the fuel is injected here via a central fuel lance 51, the mouth of which is located downstream of the vortex generators. This lance is dimensioned for about 10% of the total volume flow through channel 20. A longitudinal injection of the fuel in the flow direction is shown. In this case, the injection pulse corresponds approximately to that of the main flow pulse. A cross-jet injection could be provided just as well, the fuel pulse then having to be approximately twice that of the main flow.

The injected fuel is dragged along by the vortices and mixed with the main flow. It follows the helical course of the vortices and is evenly and finely distributed in the chamber downstream of the vortices. This reduces the risk of impinging jets on the opposite wall and the formation of so-called "hot spots" - in the case of the radial injection of fuel into an undisturbed flow mentioned at the beginning.

Since the main mixing process takes place in the vortices and is largely insensitive to the injection pulse of the secondary flow, the fuel injection can be kept flexible and adapted to other boundary conditions. In this way, the same injection pulse can be maintained throughout the load range. Since mixing through the The geometry of the vortex generators is determined, and not by the machine load, in the example the gas turbine output, the burner configured in this way works optimally even under partial load conditions. The combustion process is optimized by adjusting the ignition delay time of the fuel and mixing time of the vortices, which ensures a minimization of emissions.

Furthermore, the intensive mixing results in a good temperature profile over the cross section through which the flow is flowing and also reduces the possibility of the occurrence of thermoacoustic instability. Due to their presence alone, the vortex generators act as a damping measure against thermoacoustic vibrations.

In order to prevent the flame from reigniting in the burner, a Venturi nozzle 52 is provided downstream of the vortex generators. This is dimensioned so that at an exit speed of approximately 80-150 m / sec the flow velocity in the narrowest cross section is approximately 150-180 m / sec. The distance of the narrowest cross section to the trailing edges 16 of the vortex generators will be chosen so that the generated vertebrae are already fully formed in the narrowest cross section. The location of the fuel injection is at the level of the largest constriction of the Venturi nozzle.

FIGS. 8 and 9 show a top view of an embodiment variant of the vortex generator and a front view of its arrangement in a circular channel. The two side surfaces 11 and 13 enclosing the arrow angle α have a different length. This means that the roof surface 10 rests with an edge 15a running obliquely to the flow through the channel on the same channel wall as the side walls. The vortex generator then naturally has a different width Angle of attack Θ on. Such a variant has the effect that vortices with different strengths are generated. For example, this can act on a swirl adhering to the main flow. Or, due to the different vortices, a swirl is forced on the originally swirl-free main flow downstream of the vortex generators, as is indicated in FIG. 9. Such a configuration works well as an independent, compact burner unit. When using several such units, for example in a gas turbine ring combustor, the swirl imposed on the main flow can be used to improve the cross-ignition behavior of the burner configuration, for example at partial load.

Of course, the invention is not limited to the examples described and shown. With regard to the arrangement of the vortex generators in the network, many combinations are possible without leaving the scope of the invention. The secondary flow can also be introduced into the main flow in a variety of ways, for example only or additionally via wall bores in the venturi tube

Reference list

9, 9a

Vortex generator

10th

Roof area

11

Side surface

12th

Long edge

13

Side surface

14

Long edge

15

transverse edge of 10

16

Connecting edge

17th

Line of symmetry

18th

top

20, a

channel

21, a, b

Canal wall

Θ

Angle of attack

α, α / 2

Arrow angle

H

Height of 16

H

Channel height

L

Length of the vortex generator

50

Venturi nozzle

51

Fuel lance

52

Main burner

53

Brenner wall

54

Main burner wall

100

Front wall of the combustion chamber

101

Double cone burner

102

Air intake

103

Interior of the housing

111

Partial body

112

Partial body

113

Central axis

114

Central axis

116

Fuel nozzle

117

Gas inflow opening

118

Burner outlet = combustion chamber

119

tangential gap

120

Fuel lance

Claims (9)

Premix burner, essentially consisting of a pilot burner and a plurality of main burners arranged around the pilot burner,
characterized, that a gaseous and / or liquid fuel is injected as a secondary flow into a gaseous main flow into the main burner (52) having a circular channel (20), - That the main flow is initially conducted via vortex generators (9), of which several are arranged next to one another over the circumference of the channel (20) through which flow flows, - that a Venturi nozzle (50) is arranged downstream of the vortex generators, - And that the secondary flow in the area of the largest constriction of the Venturi nozzle is introduced into the channel (20). Premix burner according to claim 1, characterized in that the pilot burner works according to the double-cone principle with essentially two hollow, conical, part-bodies (111, 112) nested one inside the other in the flow direction, the respective central axes (113, 114) of which are offset from one another, the adjacent walls of the form two tangential channels (119) in the longitudinal extension for the combustion air, and in the region of the tangential column in the walls of the two partial bodies, gas inflow openings (117) distributed in the longitudinal direction are provided. Premix burner according to claim 1, characterized in that - That a vortex generator (9) has three freely flowing surfaces which extend in the direction of flow and one of which forms the roof surface (10) and the other two the side surfaces (11, 13), - that the side surfaces (11, 13) are flush with the same wall segment (21) of the channel and enclose the arrow angle (α, αh) with one another, - That the roof surface (10) with an edge (15) running transversely to the channel (20) through which it flows lies on the same wall segment (21) as the side walls, - And that the longitudinal edges (12, 14) of the roof surface, which are flush with the longitudinal edges of the side surfaces projecting into the flow channel, extend at an angle of attack (Θ) to the wall segment (21). Premix burner according to claim 3, characterized in that the two side surfaces (11, 13) of the vortex generator (9) including the arrow angle (α) are arranged symmetrically about an axis of symmetry (17). Premix burner according to claim 3, characterized in that the two side surfaces (11, 13) including the arrow angle (α, αh) comprise a connecting edge (16) which together with the longitudinal edges (12, 14) of the roof surface (10) Form tip (18), and that the connecting edge lies in the radial of the circular channel (20). Premix burner according to claim 5, characterized in that the connecting edge (16) and / or the longitudinal edges (12, 14) of the roof surface are at least approximately sharp. Premix burner according to claim 1, characterized in that the axis of symmetry (17) of the vortex generator (9) runs parallel to the channel axis, the connecting edge (16) of the two side surfaces (11, 13) the downstream edge of the vortex generator (9) forms, and wherein the edge (15) of the roof surface (10) running transversely to the channel (20) through which flow is the edge first acted upon by the main flow. Premix burner according to claim 1, characterized in that the ratio height (h) of the vortex generator to the channel height (H) is selected so that the vortex generated immediately downstream of the vortex generator (9) the full channel height or the full height of the Fills the channel part assigned to the vortex generator. Premix burner according to claim 1, characterized in that the secondary flow is introduced via a fuel lance (51) arranged centrally in the channel (20) by means of longitudinal injection or transverse jet injection.

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