EP0987491A1 - Method for preventing flow instabilities in a burner - Google Patents

Abstract

Disturbance air (22) is injected into the combustion air flow (15) in order to prevent the formation of coherent flow instabilities of the combustion air flow after outlet in the combustion chamber (28). The burner (26) has no premixing stretches and is operated with liquid or gaseous fuel.

Description

TECHNICAL AREA

The present invention relates to the field of burners, in particular the burner for use in gas turbines. It relates to a device and a Method for operating a burner, in which a combustion air flow Fuel is transported to a combustion chamber, where the fuel is burned.

STATE OF THE ART

With modern burners, especially burners such as those used in gas turbines, it is becoming increasingly important to keep the combustion as efficient as possible and as free as possible from pollutants. Pollutant limit values are prescribed by the authorities, among others, and the guidelines regarding CO and NO _x emissions are becoming ever stricter. The corresponding optimization of the combustion can be done in a variety of ways, for example by adding additives such as water to the fuel, by using catalysts, or also by ensuring ideal fuel-air mixtures for the combustion. Optimal fuel-air ratios can be created by premixing fuel and combustion air (so-called premix burners) or by injecting fuel and combustion air together into the combustion chamber in a special way.

eingehüllt", so dass sich zwischen den Halbkonussen ein kegeliges Flüssigbrennstoffprofil ausbildet, welches sich in Richtung der Brennkammer ausweitet und dort verbrennt. Gasförmiger Brennstoff wird aus Brennstoffzufuhrrohren, die den Lufteintrittsschlitzen entlang verlaufen, durch Bohrungsreihen quer in die eintretende Luft eingedüst.From EP-B1-0 321 809 a burner for liquid and gaseous fuels without a premixing section has become known, in which combustion air supplied from the outside enters tangentially between at least two inlet slots between displaced, arranged hollow half-cones and flows there in the direction of the combustion chamber, and at which on the tapered side of the half-cones facing away from the combustion chamber, the liquid fuel is injected centrally. To a certain extent, the fuel is captured by the combustion air and

enveloped ", so that a conical liquid fuel profile is formed between the half-cones, which expands in the direction of the combustion chamber and burns there. Gaseous fuel is injected transversely into the incoming air through rows of holes from fuel supply pipes which run along the air inlet slots.

Problematic with such burners, and generally with burners where one Combustion airflow flowing into a combustion chamber in a similar manner is the Combustion air escapes into the combustion chamber. While the combustion air in the Brenner strokes along the walls of the half-cones and is guided by them is formed in the flow direction of the combustion air behind the front edge the half-cones immediately a shear layer. This shear layer lies between the located in the combustion chamber, essentially stationary and hot Combustion gases, and the emerging, flowing mixture of fuel and combustion air. It is now in the nature of such shear layers that at some point these roll up and swirl result. This rolling up runs in such a way that so-called Kelvin-Helmholtz waves initially appear on the shear layers form, the wave crests of which run transversely to the flow direction, and which then generate in vortex.

It turns out that these instabilities on shear layers in combination with the incineration process are primarily responsible for an important class of thermoacoustic oscillations triggered by fluctuations in reaction rates. These largely coherent waves lead to a burner of the above Kind in typical operating conditions for vibrations with frequencies of about 100 Hz. Since this frequency with typical fundamental eigenmodes of of many ring burners of gas turbines coincide, the thermoacoustic ones Oscillations are a problem.

PRESENTATION OF THE INVENTION

The invention is therefore based on the object, a device or a To provide burner, as well as a method, which the training of coherent flow instabilities of the combustion air flow after exiting the Prevents combustion chamber.

This object is achieved in a device or a method of the aforementioned Type solved by injecting disturbance air into the combustion air flow. The essence of the invention therefore lies in the fact that the injected fault air specifically targets this Rocking of thermoacoustic oscillations already at their causal Education prevented.

A first preferred embodiment of the invention is characterized in that that the coherent flow instabilities after the combustion air flow exits into the combustion chamber due to shear layers between the combustion air flow and essentially stationary hot gases occur in the combustion chamber, and that the fault air attacks these shear layers. Then is preferred the fault air is substantially perpendicular to a main flow direction of the Combustion air flow and substantially parallel to the shear layers, preferred even injected into the shear layers, into the combustion air flow. Thereby the formation of Kelvin-Helmholz waves in the direction of flow is targeted in the Core suffocated.

Another embodiment of the invention is characterized in that it the burner is a double cone burner that the injection of the Fault air occurs through fault nozzles, and that the fault air is immediately at the front edges of the half-cones take place where the shear layers form. Furthermore, the fault nozzles are preferred evenly in certain Distances distributed on the circumferences of the front edges of the half-cones, so the periodicity of the waves on the shear layers is disturbed, which prevents of the thermoacoustic oscillations in the seeds of their formation.

Further embodiments of the method and the device result from the dependent claims.

BRIEF EXPLANATION OF THE FIGURES

Fig. 1

zeigt eine schematische Darstellung einer Scherschicht und die in der Beschreibung verwendeten Grössen inkl. Koordinatensystem;

Fig. 2

zeigt den dimensionslosen Wachstumskoeffizienten als Funktion der dimensionslosen Wellenlänge;

Fig. 3

zeigt die dimensionslose Wellenzahl mit maximalem Wachstum als Funktion der dimensionslosen transversalen Komponente des Wellenvektors;

Fig. 4

zeigt den dimensionslosen Wachstumsfaktor als Funktion der dimensionslosen transversalen Komponente des Wellenvektors; und in

Fig. 5

sind schematische Darstellungen eines Doppelkegelbrenners mit Störungsdüsen abgebildet. a) perspektivische Ansicht, b) Schnitt senkrecht zur Hauptströmungsrichtung durch Störungsdüse, c) Ansicht von oben, d) Ansicht von vorn.

The invention will be explained in more detail below using exemplary embodiments in conjunction with the drawings.

Fig. 1

shows a schematic representation of a shear layer and the sizes used in the description including the coordinate system;

Fig. 2

shows the dimensionless growth coefficient as a function of the dimensionless wavelength;

Fig. 3

shows the dimensionless wave number with maximum growth as a function of the dimensionless transverse component of the wave vector;

Fig. 4

shows the dimensionless growth factor as a function of the dimensionless transverse component of the wave vector; and in

Fig. 5

are schematic representations of a double-cone burner with fault nozzles. a) perspective view, b) section perpendicular to the main flow direction through the disturbance nozzle, c) view from above, d) view from the front.

WAYS OF CARRYING OUT THE INVENTION

The principle of action of the described approach should initially be based on some theoretical ones Considerations are rationalized and explained, then the technical embodiments described.

v = 0, w = 0

wobei H die folgende Heaviside-Funktion ist

und u, v, und w die Geschwindigkeiten entlang x,y, und z sind.FIG. 1 shows a section through an idealized shear layer 10, as is required for the subsequent calculations. The shear layer 10 is of a thickness h, and the coordinate system is laid out such that the axes x and z lie in the shear layer, the axis y is perpendicular thereto, and the main direction of flow (longitudinal) is along x. To simplify the calculations, the origin of the coordinate system is such that the thickness of the shear layers 10 extends along y from -h / 2 to + h / 2, and that the layer located at the top in the figure extends along x at a speed U _o moved to the right, while the layer shown in FIG. 1 below moves to the left at a speed -U _o along x. Applied to the situation when leaving a burner 26, this means that the upper layer represents the emerging combustion air 15 at a speed 2 U _o along x to the right, and that the lower layer represents the idealized stationary air in the combustion chamber. In the shear layer 10, a linear velocity profile along y is assumed, which has the following mathematical form: u = u 0 ( y ) = H ( y - h / 2 ) U 0 + H ( y + h / 2 ) H (- y + h / 2 ) 2yU 0 /H - H (- y - h / 2 ) U 0

v = 0, w = 0

where H is the following Heaviside function

and u, v, and w are the velocities along x, y, and z.

If one assumes varicose disturbances along the shear layer 10 and now uses the equations for flow at constant volume (valid for low Mach number values), as well as mass and torque conservation, a system of equations with the following results at the points y = ± H / 2nd steady solution: αh U 0 2nd = 1- k 2nd e.g. H 2nd k 2nd H 2nd {exp (-2 kh )-[1- kh ] 2nd }.

Here, α is the growth exponent of the disturbance in 1 / s, U _{0 is} the edge speed at the shear layer 10, k is the wave number along x and z, defined as k 2nd = k 2nd x + k 2nd e.g. , and k _z is the component of the wave vector along z, ie in the transverse direction.

The above solution is reduced for the case k _z → 0 to the case of the two-dimensional Kelvin-Helmholtz waves. If one carries the dimensionless growth exponent (left side of the above equation) for the two-dimensional case as a function of the dimensionless wavelength of the Kelvin-Helmholtz waves, defined as λ H = 2π k x H the functional relationship shown in FIG. 2 is obtained. Interestingly, it turns out that the interference is stable for wavelengths λ <4.91 h (range 13), while it increases for λ> 4.91 h (range 12). Maximum growth is obtained for λ = 7.89 h (11).

The remarkable result of the general, three-dimensional case of the above solution is that the shear layer 10 is stable for all values of the x component of the wave vector k _x (in the direction of flow), provided that: k _z h | > 1,278! In other words, a sufficiently strong transverse ripple with a transverse wavelength λ _z , which satisfies the condition λ _z <4.91 h , can prevent the formation of Kelvin-Helmholtz waves. Figure 3 accordingly shows the norm of the wave vector for greatest growth as a function of the dimensionless transverse component of the wave vector. The associated relationship between the dimensionless growth coefficient and the dimensionless transverse component of the wave vector is shown in Figure 4. As mentioned above, it turns out that for | k _z h | > 1,278 any growth of the longitudinal ripple is prevented.

The idea now is to induce a suitable transverse disturbance in the shear layer to prevent the Kelvin-Helmholtz waves. To calculate the ideal type of this disturbance, the thickness of the shear layer 10 should actually be calculated at the point of the wave refraction. However, it is easier to orientate yourself immediately to the existing conditions in practice and to include the frequency of the vertebral detachment actually occurring, here designated f , in the calculation. Since the vortices propagate in the main flow direction x with half the main flow velocity, the following relationship can be established: λ = U 2nd f where U is the absolute flow velocity right next to the shear layer 10. If one now assumes that the frequency f corresponds to the wavelength with maximum growth, the stability condition results λ e.g. <0.312 U f .

If one now assumes a flow rate of U = 20 m / s, which is rather low for double-cone burners and a conservatively high frequency of f = 125 Hz, the distance between the disturbances is obtained λ e.g. = 0.312 20th m / s 125 Hz ≈ 5 cm .

In practice, this means the following: If you disturb, for example by means of injection of fault air 22 in the transverse direction, i.e. perpendicular to the main flow direction and in the shear layer 10 with a spacing of the perturbation nozzles 16 of about 5 cm in the x direction, the formation of Kelvin-Helmholtz waves in Direction of flow, so no thermoacoustic oscillations of the frequency assumed above from 125Hz.

FIG. 5 shows various views of a double-cone burner, on the basis of which the technical implementation of the principle described above is to be shown. Figure 5a shows a perspective view of a double-cone burner. The combustion air 14 passes laterally through the entry slots 23 with slightly shifted Hollow half-cones 18 and 21 arranged in axes flow to the front end of the burner under the description of a slight curve, and occurs after passing the front edges 24 of the cones from the burner 26 into the combustion chamber. At the tapered end of the halyards 18 and 21 is a cylindrical part 20, in which is arranged a fuel nozzle which in this case liquid Fuel is injected centrally between the two half cones 18 and 21. The combustion air flow 14 envelops the injected fuel and a fuel cone is formed, which widens towards the front, and which after leaving the Combustion chamber 28 burns in the flame 17 at the burner mouth 27.

In the half cones 18 and 21 there are now interference nozzles at regular intervals 16 arranged directly at the front edges 24. They jet, each for themselves a fault air flow 22 perpendicular to the combustion air flow direction 15 in the combustion air flow 15. This is done as indicated in Figure 5b): The Fault nozzles 16, which are supplied by lines 25, spray the fault air 22 at a flat angle under the half cones. This is directly at the front edges 24, so that the fault air 22 essentially in the behind the Edge-forming shear layer 10 between the combustion air flow 15 and that in the substantially stationary air flows into the combustion chamber 28. The injection takes place perpendicular to the direction of the combustion air flow 15 (circle with point in the Middle stands for an arrow that looks upwards out of the paper plane) and therefore generates the disturbance in the shear layer required by the theory 10th

Figure 5c) shows a view from above of the double-cone burner 26. Here, the spacing of the interference nozzles 16 is easy to understand. The transversal disturbance, so that the 5 cm mentioned in the numerical example above results in wavelength λ _z , must take place in such a way that the disturbance nozzles 16 generate disturbance air streams 22 which are 5 cm apart in the x direction, ie in the flow direction of the combustion air stream 15. FIG. 5d) shows a schematic front view of a double-cone burner 26. The orthogonal flow of the two air flows 15 and 22 can again be seen. It is important that the injected air 22 does not have any strong, inward-facing components so that the main air flow 15 is not disturbed. In addition, the total pressure of the injection of the disturbance air 22 must be at least as high as the total pressure of the combustion air 15 flowing past, so that significant transverse disturbances can form at all.

LIST OF DESIGNATIONS

10th

Shear layer

11

Maximum of the dimensionless growth coefficient

12th

Area without growth

13

Area with growth

14

Combustion airflow upon entry

15

Mixture of combustion air and fuel after discharge

16

Interference nozzles

17th

flame

18th

first half cone

19th

Fuel nozzle

20th

cylindrical part of the burner

21

second half cone

22

Fault air

23

Entry slot

24th

Front edge of the semi-cone

25th

Line to 16

26

burner

27

Burner mouth

28

Combustion chamber

Claims (20)

Method for operating a burner (26), in which a combustion air stream (14) transports fuel into a combustion chamber (28), where the fuel is burned,
characterized in that
Disturbance air (22) is injected into the combustion air flow (15) in order to prevent the formation of coherent flow instabilities of the combustion air flow (15) after exiting into the combustion chamber (28). A method according to claim 1, characterized in that the burner (26) is a burner (26) without a premixing section. Method according to one of claims 1 or 2, characterized in that that the burner (26) with liquid or gaseous fuel is operated. Method according to one of claims 1 to 3, characterized in that that the coherent flow instabilities change after the Combustion air flow (15) into the combustion chamber (28) due to shear layers (10) between the emerging mixture of combustion air and fuel (15) and substantially stationary hot gases form in the combustion chamber (28). A method according to claim 4, characterized in that the flow instabilities as a result of Kelvin-Helmholtz waves on the shear layers (10) occur. Method according to one of claims 4 or 5, characterized in that that the fault air (22) is substantially perpendicular to a Main flow direction of the combustion air flow (15) and substantially parallel to the shear layers (10) in the flow of the mixture of combustion air and fuel (15) is injected. A method according to claim 6, characterized in that the fault air essentially in the shear layers (10) between the combustion air flow (15) and essentially stationary hot gases is injected into the combustion chamber (28). Method according to one of claims 6 or 7, characterized in that that the fault air (22) essentially shortly before the outlet of the combustion air-fuel mixture (15) in the combustion chamber (28) is injected into the mixture (15). A method according to claim 8, characterized in that the burner (26) is a double cone burner in which combustion air (14) through at least two inlet slots (23) tangentially between displaced, arranged hollow half-cones (18, 21) enters and there flows in the direction of the combustion chamber (28) that on that of the combustion chamber facing away, tapered side of the half cones (18,21) of the Fuel is injected centrally, and / or that gaseous fuel from two gas supply pipes running along the air inlet slots through rows of holes across the incoming air flow it is injected that the half cones (18, 21) on the combustion chamber side of Front edges (24) are limited, and that injection of fault air (22) through fault nozzles (16). A method according to claim 9, characterized in that the interference nozzles (16) essentially immediately before the leading edges (24) are embedded in the half cones (18, 21) and that the fault nozzles (16) the fault air (22) in the combustion air flow (15) and essentially in the immediately behind the leading edges (24) inject the resulting shear layers (10). A method according to claim 10, characterized in that a plurality of interference nozzles (16) are arranged, and that the interference nozzles (16) evenly on the circumference of the half-cones (18,21) distribute the fault air (22). A method according to claim 11, characterized in that the spacing which were evenly distributed on the half cones (18, 21) Perturbation nozzles (16) produces perturbations which increase the coherent Flow instabilities of the combustion air flow (15) prevent by a dimensionless component of the wave vector perpendicular to the main flow direction of the combustion air (22) is generated which is larger than a critical value. A method according to claim 12, characterized in that the critical Value is 1,278, and accordingly the spacing of the Fault nozzles (16) as a function of one that occurs without fault nozzles (16) Frequency of the coherent flow instabilities of the combustion air flow (15) is selected. A method according to claim 13, characterized in that the total pressure the injection of the fault air (22) is at least as large, like the total pressure of the combustion air flowing past (14, 15). Burner (26) for performing a method according to claim 6, characterized characterized in that the burner (26) as a double-cone burner is formed, in which combustion air (14) by at least two entry slots (23) tangentially between displaced, hollow half cones (18,21) enters and there towards the combustion chamber (28) flows that the half-cones (18,21) on the combustion chamber side be limited by leading edges (24) and that the injection of Fault air (22) through fault nozzles (16), which is immediate in front of the front edge (24) of the half cones (18, 21) in the half cones (18, 21) are recessed in such a way that they create interference air (22) vertically to the main flow direction of the combustion air (15) from the Outside of the half cones (18, 21) into the combustion chamber (28) inject flowing combustion air (15). Burner according to claim 15, characterized in that the interference nozzles (16) are arranged in the half-cones (18, 21) in this way, so that they are essentially the fault air (22) behind the leading edge inject shear layers (10). Burner according to claim 16, characterized in that a plurality of interference nozzles (16) are arranged, and that the interference nozzles (16) evenly on the circumference of the half-cones (18,21) are distributed. Burner according to claim 17, characterized in that the uniform Spacing of the interference nozzles (16) is selected such that it is equal to or less than a critical value, and that the critical value from the flow velocity of the combustion air and that which occurs in the burner without interference nozzles (16) Frequency of the coherent flow instabilities of the combustion air flow (15) results. Burner (26) according to claim 18, characterized in that the critical value as the quotient of the flow velocity multiplied by 0.312 the combustion air (15) and the burner without Interference nozzles (16) occurring frequency of the coherent flow instabilities of the combustion air flow (15) results. Burner (26) according to one of claims 18 or 19, characterized in that that with a burner (26) without fault nozzles (16) frequency of the coherent flow instabilities of the Combustion air flow (15) in the range of 100 to 125 Hz and one Flow rate of the combustion air (15) in the range of 20 to 30 m / s, the interference nozzles (16) on the cones (18, 21) Spacing in the range of 3 to 5 cm, especially in the area from 4.5 to 5cm.

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