EP1706672B1 - Premixing burner arrangement and method for operating a combustion chamber - Google Patents

Abstract

A premixing burner is disclosed for operating a combustion chamber with a liquid and/or gaseous fuel, with a swirl generator for a combustion inflow air stream for forming a swirl flow, and with injection of fuel into the swirl flow. The swirl generator is adjacent to the combustion chamber indirectly via a mixing zone or directly, in each case via a burner outlet, a cross-sectional widening at the burner outlet being provided which, is discontinuous in the flow direction of the swirl flow and through which the swirl flow bursts open so as to form a backflow zone. A contour locally narrowing the flow cross section of the swirl generator or of the mixing zone in the flow direction can be provided upstream of the burner outlet.

Description

Technical area

The invention relates to a premix burner and a method for operating a combustion chamber with a liquid and / or gaseous fuel with a swirl generator for a combustion supply air to form a swirl flow and means for injecting fuel into the swirl flow, wherein the swirl generator indirectly via a mixing zone or directly adjacent to the combustion chamber in each case via a burner outlet, wherein at the burner outlet an unsteady in the flow direction of the swirl flow cross-sectional enlargement is provided, bursts through the swirl flow to form a return flow zone.

State of the art

Premix burners of the aforementioned type are known from a variety of prepublished publications, such as. From the EP A1 0 210 462 as well as the EP B1 0 321 809 , to name just a few. Premix burners of this type is the general principle of action, within a mostly designed as a conical swirl generator, which provides at least two with corresponding mutual overlap Teilkegelschalen to produce a consisting of a fuel-air mixture swirl flow, which within a is brought in the flow direction of the premix burner combustion chamber to form a spatially stable as possible premix flame for ignition. Here, the spatial position of the premix flame is determined by the aerodynamic behavior of the swirl flow whose swirl increases with increasing propagation along the burner axis, thus becomes unstable and ultimately bursts through an unsteady cross-sectional transition between the burner and combustion chamber in an annular swirling flow to form a return flow in the front area, the premix flame forms

However, it has been shown that the vortex backflow zone has only limited stability properties, which is why a large number of proposals have been made to improve the stability properties of such backflow zones. For a stable vortex return zone, the axial profile of the swirl flow generated by the swirl body essentially applies Center, ie in the area of the burner axis, should be low in spin and, moreover, there should be an axial velocity excess. These considerations have become a burner according to the EP 0 321 809 B1 guided.

The double-cone burner described in this publication is in FIG. 2 shown in a schematic manner in the form of a longitudinal sectional view and has a conical swirl generator 1, whose two interposed Teilkegelschalen each include two air inlet slots 2. The swirl generator 1 opens at the burner outlet 3 via an unsteady cross-sectional widening directly into the combustion chamber 4. By the tangential feed of the combustion air supply along the air inlet slots 2, a swirl flow is generated, which propagates in the axial flow direction with increasing swirl about the axial direction of the swirl generator. Due to the increasing swirl along the axial flow direction, the instability of the swirling flow increases and turns into an annular swirling flow with backflow. It forms substantially within the combustion chamber 4 in the region of the burner outlet 3, a return flow zone 5 with a forward front in the flow direction or a front stagnation point 6, the axial position relative to the premix burner 1 essentially by the cone angle 2y and the slot width of the air inlet slots 2 is determined. By selecting the size of the above geometry values, the size and appearance of the backflow zone 5 can be substantially determined.

Within the backflow zone 5, the premix flame 7 forms, which stabilizes at the front region of the inner backflow zone 5.

Investigations of the stability of such a flame 7 have shown that the aerodynamic stability of the inner recirculation or backflow zone 5 have a decisive influence on the position, shape and size of the premix flame 7.

If the premix burner described above is used to generate hot gases for driving a gas turbine plant, then for reasons of optimizing the efficiency of the gas turbine plant, it is necessary to keep the pressure loss across the burner as low as possible. Since swirl number and pressure loss are in direct proportionality to each other, the lowest possible swirl number within the swirl flow is desired, which should be just chosen so large that forms an inner backflow zone.

On the other hand, it is desirable to keep the front stagnation point 6 of the backflow zone 5 as aerodynamically stable as possible in order to prevent the premix flame front anchored to the front stagnation point 6 from causing strong thermoacoustic instabilities due to strong variation in the flame position, which not only has a lasting effect on the efficiency of a gas turbine plant but also cause significant stress on almost all components of the gas turbine plant which come into direct contact with the hot gases, which ultimately reduces the overall service life of the plant. However, the desire for the highest possible aerodynamic stability of the front flame front within the backflow zone contradicts the efficiency-induced swirl reduction, which leads to smaller swirl gradients in the burner, especially at the location of the front stagnation point 6. However, a smaller swirl gradient implies a greater deflection of the stagnation point in Direction of flow in case of possible interference and supports the above-described formation of thermoacoustic instabilities.

Presentation of the invention

The invention has the object of developing a premix burner according to the features of the preamble of claim 1 such that on the one hand the aerodynamic stability of the inner Rückströmzone should be increased in particular in the region of the front stagnation point, without having to take a significant additional burner pressure loss in purchasing. Furthermore, it is appropriate to provide a corresponding method for operating a combustion chamber, which is intended both to avoid the occurrence of thermoacoustic vibrations, and the effort to achieve the lowest possible burner pressure loss.

The solution of the problem underlying the invention is specified in claim 1. A method according to the invention is the subject matter of claim 8. The features which further develop the invention in an advantageous manner can be found in the subject matter of the subclaims and in the description in particular with reference to the exemplary embodiments.

The premix burner according to the invention is based on the idea that the aerodynamic stability of the free inner backflow zone can be increased by locally increasing the swirl gradient of the swirl flow upstream of the forming backflow zone in the flow direction. Due to the merely local enlargement of the swirl gradient, ie along the axially propagating swirl flow within the premix burner, it is necessary to raise the swirl number in the axial flow direction spatially limited from an initial swirl number to a larger swirl number and then immediately lower it to the output swirl number or one compared to this smaller swirl number. It turns out that with the measure according to the invention, the overall burner pressure loss is insignificant is increased, resulting in no or very little impact on the overall efficiency of a gas turbine.

Thus, according to the invention, a premix burner with the features of the preamble of claim 1 is characterized in that upstream of the burner outlet, a contour that locally locally tapers the flow cross section of the swirl generator or, if present, the mixing zone in the flow direction is provided.

This, the flow cross-section locally tapered contour is arranged in the flow direction of a first, the flow cross-section steadily decreasing gate section, which merges continuously into a second gate section with a smallest flow cross-section, in which in the flow direction third, the flow cross section again steadily increasing the gate section connects. By providing a contour locally tapering the flow cross section in the flow direction, the swirl or burner flow in the region of the first slotted section is accelerated due to the steady flow cross-section reduction according to Bemoulli's flow relationships and correspondingly delayed after passing through the region with the smallest flow cross section.

Because of this convergent-divergent flow guidance along the burner axis in the flow direction through the contour which narrows the flow cross-section, the swirl gradient is locally increased, which in turn improves the aerodynamic stability of the forming front face of the return flow zone. It is essential that due to an unchanged burner contour at the burner outlet, especially since the contour is upstream of the burner outlet, the burner pressure loss is only insignificantly affected. An impairment of the overall efficiency of a gas turbine can thereby be largely avoided.

If the premix burner is a double-cone burner whose swirl generator consists essentially of two sub-cone shells, and no further mixing tube is provided between the double-cone burner and the combustion chamber, so that the swirl generator with its burner outlet directly into the combustion chamber via an unsteady cross-sectional widening opens, then the provision of the invention, which can be added as an additional form at a suitable axial location along the inner peripheral edge of both Teilkegelschalen addition, whereby the possibility of Retrovistierbarkeit is given or the shaping is already integrally incorporated into the innwandige side of both Teilkegelschalen for one elliptical cross-step shaping at the location of the narrowest or smallest flow cross section caused by the contour. Of course, the inventive measure is also applicable to Vormischbrennersystemen whose swirl generator are assembled from more than two Teilkegelschalen or provide a mixing tube between the swirl generator and combustion chamber as an additional mixing zone. In the case of the provision of mixing tubes, the flow cross-section tapering contour in the inner wall region of the mixing tube is provided near the burner outlet at the transition to the combustion chamber.

The inventive concept of the local flow cross-sectional tapering for the purpose of aerodynamic stabilization of the forming Rückströmzone within a premix burner, which is preferably used to operate a combustion chamber, which is used for firing a gas turbine plant, is based on the procedural idea, aerodynamic conditions at the location of the foremost stagnation point of Rückströmzone to create, which prevent an axial emigration of the stagnation point. For this purpose, the swirl flow oriented in the axial flow direction is determined by the contour-related nozzle effect accelerated within the vortex burner, for example within the swirl generator axially before the foremost stagnation point of Rückströmzone and also delayed in the flow direction before the stagnation point of Rückströmzone such that at the axial location of the stagnation point as large a speed gradient prevails with flow direction reversal. This can be achieved by means of a convergent and divergent flow guidance that is specifically located in front of the location of the stagnation point. Further details can be found in the description of the embodiments in the further.

Brief description of the invention

Fig. 1

schematisierte Teil-Längsschnittdarstellung durch einen Drallerzeuger,

Fig. 2

schematisierte Längsschnittdarstellung durch einen Vormischbrenner mit Brennkammer,

Fig. 3

Querschnittsdarstellung durch einen Drallerzeuger am Ort des geringsten Strömungsquerschnittes,

Fig. 4

Diagrammdarstellung des Geschwindigkeitsgradienten in Strömungsrichtung längs der den Strömungsquerschnitt verjüngenden Kulisse,

Fig. 5

Diagramm zur Darstellung der Druckfluktuationen bei niedrigen Temperaturen sowie

Fig. 6

Diagrammdarstellung mit Emissionswerten.

The invention will now be described by way of example without limitation of the general inventive idea by means of embodiments with reference to the drawings. Show it:

Fig. 1

schematic partial longitudinal section through a swirl generator,

Fig. 2

schematic longitudinal section through a premix burner with combustion chamber,

Fig. 3

Cross-sectional view through a swirl generator at the location of the smallest flow cross-section,

Fig. 4

Diagram representation of the velocity gradient in the flow direction along the flow cross section tapering backdrop,

Fig. 5

Diagram showing the pressure fluctuations at low temperatures as well as

Fig. 6

Diagram representation with emission values.

Ways to carry out the invention, industrial usability

FIG. 1 shows a schematic section of a longitudinal section through a swirl generator of a double cone premix burner with a burner wall 8 with the burner axis A includes a half cone angle γ. In front of the burner outlet 3, a contour 9 which narrows the axial flow cross-section is provided on the inner wall of the burner wall 8. The contour 9 reduces the flow cross-section along the burner axis A within a local area 10 such that the shape and size of the burner outlet 3 are not impaired by the contour 9. The contour 9 has a first gate section 91, through which the flow cross section is steadily reduced. Immediately adjoining the first gate section 91 is a second gate section 92 which predetermines the smallest flow cross section. Preferably, the second gate section 92 is formed only point or linear. At the region of the smallest flow cross-section is followed downstream by a third link portion 93, through which the flow cross-section is widened again, preferably to a level which is predetermined by the burner wall 8 on the outlet side.

In the case of a double-cone burner, the contour 9 tapering in the flow cross-section runs in the circumferential direction to both partial cone shells annularly closed, so that through the interaction of the respective contours 9 attached to both partial cone shells, a flow slot is formed which corresponds to that of a Venturi nozzle.

$$\mathrm{0}\mathrm{,}\mathrm{5}\mathrm{\le }\mathrm{R}\mathrm{2}\left(\mathrm{x}\right)\mathrm{/}\mathrm{RB}\left(\mathrm{x}\right)\mathrm{\le }2$$

und $$\mathrm{y}\mathrm{<}\mathrm{\alpha }\mathrm{<}\mathrm{40}\mathrm{°}$$

mit

x:

Orts-Koordinate längs der Mittelachse einer Teilkegelschale

R1:

radialer Abstand zwischen der Mittelachse einer Teilkegelschale und der Oberfläche der Kontur am Ort x längs der Mittelachse

RB:

radialer Abstand zwischen der Mittelachse einer Teilkegelschale und der Oberfläche der ursprünglichen Teilkegelschale am Ort x längs der Mittelachse

R2:

Überhöhung der Kontur gemessen von der Oberfläche der Teilkegelschale am Ort x längs der Mittelachse

α:

Winkel zwischen einer Tangentialoberfläche an der Kontur und der Mittelachse der Teilkegelschale am Ort x längs der Mittelachse

γ:

halber Kegelwinkel.

More detailed information regarding the design and arrangement of the contour 9 within the premix burner are derived from theoretical considerations and experimental observations. Going in relation to FIG. 1 assuming that the burner axis A is viewed in the flow direction as the x-axis, the following exemplary design parameter requirements result with respect to the x-axis: $$\mathrm{0}\mathrm{.}\mathrm{5}\mathrm{\le }\mathrm{R}\mathrm{1}\left(\mathrm{x}\right)\mathrm{/}\mathrm{RB}\left(\mathrm{x}\right)\mathrm{\le }\mathrm{1}$$

$$\mathrm{0}\mathrm{.}\mathrm{5}\mathrm{\le }\mathrm{R}\mathrm{2}\left(\mathrm{x}\right)\mathrm{/}\mathrm{RB}\left(\mathrm{x}\right)\mathrm{\le }2$$

and $$\mathrm{y}\mathrm{<}\mathrm{\alpha }\mathrm{<}\mathrm{40}\mathrm{°}$$

With

x:

Local coordinate along the central axis of a cone cone shell

R1:

radial distance between the central axis of a partial cone shell and the surface of the contour at location x along the central axis

RB:

radial distance between the central axis of a partial cone shell and the surface of the original partial cone shell at location x along the central axis

R2:

Elevation of the contour measured from the surface of the sub-cone shell at location x along the central axis

α:

Angle between a tangential surface on the contour and the central axis of the subcone cup at location x along the central axis

γ:

half cone angle.

For the terms "burner axis" and center axis of the respective cone shells, it should be noted that, for reasons of a simplified description with respect to the flow behavior within the swirl generator, only one burner axis A is spoken of. Due to the multi-part of the swirl generator, which provides two or more intermeshing partial cone shells, however, each individual sub-cone shell has a sub-conical central axis assigned to it, in short the central axis of the respective sub-cone shell. Due to the spatial arrangement of the subcone shells these corresponding central axes do not coincide. However, for the above design parameter requirements, it is necessary to raise to the corresponding center axes of the subcone cups.

On the description of FIG. 2 has already been dealt with in detail in the introduction, so that a further description is omitted here.

FIG. 3 shows a schematic cross-section through a double-cone burner in the region of the contour-related narrowest flow cross-section 92. Both partial cone shells 10, 11 each have their respective central axes M11, M 12 and are interlocked so that they include two opposing tangential air inlet slots 2 together. Due to the contours 9, the entire flow cross-section is narrowed by the swirl generator in the manner of an ellipse shape (dashed line). Such an elliptical flow cross section advantageously has aerodynamically stabilizing effects on the burner behavior over a wide operating range. To avoid an impairment of the inflow to the air inlet slots 2, the contours 9 are thinned in accordance with aerodynamics accordingly in these areas, so as not to reduce the Schlüzweite ultimately.

In FIG. 4 is a diagram for illustrating the axial velocity profile by the premix burner or swirl generator shown. The x-axis corresponds to the burner axis, the y-axis to the flow direction oriented in the axial flow direction u of the burner flow. In the case of a conventional burner arrangement, ie without the use of the present invention, the flow cross-section locally tapered contour 9 (see solid line) increases the axial flow velocity within the premix burner, is slowed down due to increasing flow instability and it is not least due to the unsteady cross-sectional widening at the burner outlet a local flow reversal (see location of the stagnation point 6), whereby the above-mentioned backflow zone (5) is formed.

In order to stabilize the foremost stagnation point 6 of the backflow zone 5, ie to leave unchanged as possible relative to the x-axis, it has been shown that a greater velocity gradient can be achieved by a local increase in flow velocity and a clearer speed delay at the location of the stagnation point 6, which improves the positional stability of the Stagnation point 6 improved significantly. The contour 9, which is likewise schematically illustrated above the diagram and tapers the flow cross-section, serves for this purpose due to the Bernoullieffektes first to an acceleration of the flow velocity in the x-direction and after exceeding the range with the smallest flow cross section leads to an efficient flow delay, whereby the velocity profile undergoes a larger gradient, especially in the front stagnation point 6 (see dotted line). Due to this local increase of the velocity gradient or also of the swirl gradient due to the convergent divergent flow guidance, the aerodynamic stability of the stagnation point 6 increases without having to accept appreciable burner pressure losses.

The performance of atmospheric combustion tests, each with two identically constructed burners with and without contouring, has shown that premix burners with the contouring according to the invention have significantly lower pressure fluctuations than correspondingly conventionally designed premix burners. FIG. 5 shows a diagram representation along the x-axis of the flame temperature and along the y-axis, the strength of pressure fluctuations in a normalized representation is indicated. The polyline with the square markings corresponds to the operation of a premix burner with contouring according to the invention, the count interspersed with the diamonds corresponds to a conventional premix burner. It is very clear that, especially at low flame temperatures, much lower pressure fluctuations occur in the premix burner designed according to the invention than in a conventional one.

It also appears that the provision according to the invention has almost no effects on an increased emission behavior with regard to nitrogen oxides. In FIG. 6 a diagram is shown along the x-axis and along the y-axis of the nitrogen oxide concentration is plotted in a normalized representation. Both the premix burner with contouring designed according to the invention (see line with rectangles) as well as a conventional premix burner (see line with diamonds) run largely parallel at a low level.

LIST OF REFERENCE NUMBERS

1

Premix burner, swirl generator

2

Air inlet slot

3

burner outlet

4

combustion chamber

5

Backflow zone

6

Front stagnation point or front front of the return flow zone

7

Premix flame, backflow bubble

8th

burner wall

9

contour

91,92,93

gate sections

10

Local, axial area

11, 12

Cone shell

Claims (11)

1. Premixing burner for operating a combustion chamber (4) by means of a liquid and/or gaseous fuel, with a swirl generator (1) for a combustion inflow air stream for forming a swirl flow, and with means for the injection of fuel into the swirl flow, the swirl generator (1) being adjacent to the combustion chamber (4) indirectly via a mixing zone or directly, in each case via a burner outlet (3), a cross-sectional widening at the burner outlet (3) being provided, which is discontinuous in the flow direction of the swirl flow and through which the swirl flow bursts open so as to form a backflow zone (5), the swirl generator (1) having at least two part-conical shells (11, 12) which complete one another to form a body and which jointly enclose a conically designed swirl space with a cone angle 2γ and air inlet slits (2) tangential in the cone longitudinal extent, and a mid-axis (M11, M12) being assigned in each case to each part-conical shell (11, 12), said mid-axes running separately from one another in spatial terms, characterized in that a contour (9) locally narrowing the flow cross section of the swirl generator (1) or of the mixing zone in the flow direction is provided upstream of the burner outlet (3), the contour is arranged in the flow direction in the region of the foremost front of the backflow zone and has in the flow direction a first segment (91) which continuously reduces the flow cross section, a second segment (92) with a smallest flow cross section and a third segment (93) adjoining the second segment in the flow direction and continuously increasing the flow cross section.
2. Premixing burner according to Claim 1, characterized in that the contour (9) is provided at the inner circumferential edge of the swirl generator (1) or of the mixing zone.
3. Premixing burner according to Claim 1 or 2, characterized in that the contour (9) encloses a flow duct which is designed in the manner of a Venturi tube.
4. Premixing burner according to one of Claims 1 to 3, characterized in that the third segment (93) possesses, within the premixing burner, an axial position which lies in the region of that front (6) of the forming backflow zone (5) which is foremost in the flow direction.
5. Premixing burner according to one of Claims 1 to 4, characterized in that the swirl generator (1) is directly adjacent to the combustion chamber (4) via the burner outlet (3), and the contour (9) locally narrowing the flow cross section of the swirl generator (1) can be described by the following geometric conditions: $$\mathrm{0.5}\mathrm{\le }\mathrm{R}\mathrm{1}\left(\mathrm{x}\right)\mathrm{/}\mathrm{RB}\left(\mathrm{x}\right)\mathrm{\le }1$$

   

   $$\mathrm{0.5}\mathrm{\le }\mathrm{R}\mathrm{2}\left(\mathrm{x}\right)\mathrm{/}\mathrm{RB}\left(\mathrm{x}\right)\mathrm{\le }2$$

   

   and $$\mathrm{y}\mathrm{<}\mathrm{\alpha }\mathrm{<}\mathrm{40}\mathrm{°}$$

   

   with

   x: locus coordinate along the mid-axis of a part-conical shell

   R1: radial distance between the mid-axis of a part-conical shell and the surface of the contour at the locus x along the mid-axis

   RB: radial distance between the mid-axis of a part-conical shell and the surface of the original part-conical shell at the locus x along the mid-axis

   R2: elevation of the contour, measured from the surface of the part-conical shell at the locus x along the mid-axis

   α: angle between a tangential surface of the contour and the mid-axis of the part-conical shell at the locus x along the mid-axis

   γ : cone half angle.
6. Premixing burner according to one of Claims 1 to 5, characterized in that the smallest flow cross section is shaped elliptically in the region of the contour (9).
7. Premixing burner according to one of Claims 1 to 6, characterized in that the combustion chamber (4) is followed by turbine stages of a gas turbine plant.
8. Method for operating a combustion chamber, using a premixing burner according to one of Claims 1 to 7, characterized in that the swirl flow is locally accelerated and decelerated in the axial flow direction within the swirl generator (1) or in a mixing zone adjoining the swirl generator (1).
9. Method according to Claim 8, characterized in that the axial acceleration of the swirl flow takes place upstream of the foremost front (6) of the forming backflow zone (5), and deceleration takes place at least partially upstream of the foremost front (6) of the forming backflow zone (5).
10. Method according to one of Claims 8 to 9, characterized in that the acceleration and deceleration take place, utilizing the Bernoulli effect.
11. Method according to one of Claims 8 to 10, characterized in that the swirl gradient of the swirl flow is increased locally in the flow direction.

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