EP0987491B1 - Method for preventing flow instabilities in a burner - Google Patents

Description

TECHNICAL AREA

The present invention relates to Method for operating a burner, according to the preamble of claim 1. The invention relates also to a burner according to the preamble of claim 2.

STATE OF THE ART

In modern burners, especially in burners, such as those used in gas turbines, it is becoming increasingly important to keep combustion both as efficient as possible and as free of pollutants as possible. Pollution limits are imposed by the authorities, among others, and the directives on CO and NO _x emissions are becoming increasingly stringent. The corresponding optimization of the combustion can be done in many ways, for example by adding additives such as water to the fuel, by using catalysts, or by ensuring an ideal for combustion fuel-air mixture. Optimum fuel-air ratios can be generated by premixing fuel and combustion air (so-called premix burners) or by injecting fuel and combustion air together into the combustion chamber in a specific manner.

From EP-B1-0 321 809 is a burner for liquid and gaseous fuels known with premix, in which externally supplied combustion air moved tangentially between at least two entry slots arranged, hollow half-cones enters and there in the direction of the combustion chamber flows, and in which on the side facing away from the combustion chamber, the tapered side the half-cones of the liquid fuel is injected centrally. The fuel will so to speak, captured and "enveloped" by the combustion air, so that forms a conical liquid fuel profile between the half-cones, which expands in the direction of the combustion chamber and burns there. gaseous Fuel is taken from fuel supply pipes that run along the air intake slots run, injected through rows of holes across the incoming air.

Problematic in such burners, and generally in burners, in which a Combustion air flow flows in a similar manner in a combustion chamber, is the Outlet of the combustion air in the combustion chamber. While the combustion air in Burner along the walls of the semi-conical strokes and led by these is, forms in the flow direction of the combustion air behind the leading edge the half-cone immediately a shear layer. This shear layer is between the located in the combustion chamber, substantially stationary and hot Combustion gases, and the exiting, flowing mixture of fuel and combustion air. It is now in the nature of such shear layers that These eventually result in rolling up and swirling. This rolling up runs such that initially on the shear layers so-called Kelvin Helmholtz waves form whose wave crests run transversely to the flow direction, and which then lead to a vortex formation.

It turns out that these instabilities on shear layers in combination with the the incipient combustion process is primarily responsible for an important class 1 of reaction rate variations induced thermoacoustic oscillations are. These largely coherent waves result in a burner of the above Type under typical operating conditions to vibrations with frequencies of about 100 Hz. Since this frequency has typical fundamental eigenmodes of Of many torches of gas turbines coincide, represent the thermoacoustic Oscillations are a problem.

EP-A2-0 851 172 discloses a burner for operating a combustion chamber with a liquid and / or gaseous fuel. In this case, a burner for operating a combustion chamber with a liquid and / or gaseous fuel, the required combustion air passed through tangential air inlet channels in an interior of the burner. By This flow is created in the interior of a swirl flow, which at the output of Burner induces a backflow zone. To stabilize the flame front forming there, At least one zone is provided in each part body forming the burner, within which inlet openings for the injection of additional air into the swirl flow are provided. By this injection forms on the inner wall of the body part Film, which prevents the flame along the inner wall of the part body in the interior of the burner can strike back.

PRESENTATION OF THE INVENTION

The invention is therefore based on the object a procedure and to provide a burner for carrying out the method, which provides for the formation of coherent flow instabilities of the combustion air flow after exiting into the Combustion chamber prevented.

This object is achieved in a method of the aforementioned Art dissolved by the interference air is injected into the combustion air stream, wherein the interference air substantially perpendicular to a main flow direction of the Fuel-air mixture and substantially parallel to the shear layers, preferably even in the shear layers, into which the fuel-air mixture is injected. Thereby The training of Kelvin Helmholz waves in the direction of flow is aimed at Core smothered. The essence of the invention is therefore that the injected air pollution targeted the Swinging of thermoacoustic oscillations already at their causal Education prevented.

Another embodiment of the invention is characterized in that it the burner is a double - cone burner, that the injection of the Disturbance air through interference nozzles, and that the disturbance air directly at the leading edges of the half-cones takes place where the shear layers form. Furthermore, it is preferred that the disturbance nozzles be uniform in certain Distributes intervals on the sizes of leading edges of half-cones, becomes so the periodicity of the waves on the shear layers disrupted what the prevention the thermoacoustic oscillations targeted causes in the nucleus of their formation.

Further embodiments of the inventive method and the burner according to the invention emerge 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 with reference to embodiments in conjunction with the drawings.

Fig. 1

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

Fig. 2

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

Fig. 3

shows the dimensionless wavenumber 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 noise nozzles shown. a) perspective view, b) section perpendicular to the main flow direction through interference nozzle, c) top view, d) front view.

WAYS FOR CARRYING OUT THE INVENTION

The working principle of the described approach is initially due to some theoretical Considerations are rationalized and explained, then the technical embodiments described.

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 assumed for the subsequent calculations. The shear layer 10 is of a thickness h, and the coordinate system is laid so that the axes x and z are in the shear layer, the axis y is perpendicular thereto, and the main flow direction (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 at the top in the figure moves along x at a velocity U _o Moves to the right, while the layer shown in Figure 1 below moves at a speed -U _o along x to the left. Translated to the situation of exit from a combustor 26, this means that the upper layer represents the exiting combustion air 15 at a speed 2 U _o along x to the right, and that the lower layer represents the idealized steady state combustion gases 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) 2 yU 0 / hH (- 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.

Assuming varicose perturbations along shear layer 10 and now using the equations for constant volume flow (valid for low Mach numbers), as well as mass and torque retention, the result is a system of equations with the following at points y = ± h / 2 steady solution:

Where a is the growth exponent of the perturbation in 1 / s, U _{0 is} the edge velocity at the shear layer 10, k is the wavenumber along x and z, defined as k ² = k 2 / x + k 2 / z , and k _z is the Component of the wave vector along z, ie in the transverse direction.

The above solution reduces to the case of two-dimensional Kelvin-Helmholtz waves for the case k _z → 0. By plotting 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 on, we obtain the functional relationship shown in Figure 2. Interestingly, it turns out that for wavelengths λ <4.91 h (range 13) the perturbation is stable, 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), if: | k _z h |> 1.278! In other words, a sufficiently strong transverse ripple with a transverse wavelength λ _z that satisfies the condition λ _z <4.91 h can prevent the formation of Kelvin-Helmholtz waves. Figure 3 correspondingly 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 FIG. As mentioned above, it turns out that for | k _z h | > 1.278 any growth of the longitudinal ripple is prevented.

The idea is now to induce a suitable transverse perturbation in the shear layer 10 to prevent the Kelvin-Helmholtz waves. In fact, to calculate the ideal nature of this perturbation, the thickness of the shear layer 10 at the location of the wave refraction would have to be calculated. It is easier, however, to orientate oneself immediately to the present conditions of practice, and to include the actually occurring frequency of the separation of the vertebrae, here designated f , into the calculation. Since the vortices propagate in the main flow direction x at half the main flow velocity, the following relationship can be established: λ = U 2 f where U is the absolute flow velocity directly adjacent to the shear layer 10. Assuming now that the frequency f corresponds to the wavelength with maximum growth, the stability condition results λ z <0.312 U f ,

If one now assumes a rather low flow velocity of U = 20 m / s for double-cone burners 26 and a conservatively high frequency of f = 125 Hz, the result is a distance between the perturbations λ z = 0.312 20 m / s 125 Hz ≈ 5 cm ,

In practice, this now means the following: Disturbing, for example by means of injection of disturbance air 22 in the transverse direction, i. 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 x direction, the formation of Kelvin-Helmholtz waves in Direction of flow, so do not form thermoacoustic oscillations of the above assumed frequency of 125Hz.

Figure 5 shows various views of a double-cone burner 26, based on which the technical realization of the above-described principle is to be shown. figure 5a shows a perspective view of a double-cone burner 26. The combustion air 14 enters laterally through the entry slots 23 of the slightly shifted Axes arranged, hollow half-cones 18 and 21, flows to the front end the burner 26 with description of a slight arc, and occurs after passing the leading edge 24 of the half-cones 18, 21 from the burner 26 into the combustion chamber 28. Am tapered end of the half-cones 18, 21 is a cylindrical part 20, in which a fuel nozzle 19 is arranged, which in this case liquid Insert fuel centrally between the two half-cones 18 and 21. The combustion air 14 envelops the injected fuel, and it forms a fuel cone, which expands towards the front, and which as a mixture 15 of combustion air 14 and fuel and Greunstaff after exiting into the Combustion chamber 28 burns at the burner mouth 27 in a flame 17.

In the half-cones 18, 21 are now at regular intervals fault nozzles 16 arranged directly at the leading edges 24. They are jetting, each for themselves, Interference air 22 perpendicular to the flow direction of the mixture 15 in as of course 15 combustion air 14 and fuel. This happens as indicated in Figure 5b): The Noise nozzles 16, which are supplied by lines 25, the disturbance air 22 at a shallow angle under the half-cones 18, 21. This directly at the leading edges 24, so that the disturbance air 22 substantially parallel to or even behind the Edge forming shear layer 10 between the mixture 15 and in the flows substantially stationary combustion gases in the combustion chamber 28. The injection takes place perpendicular to the direction of the flow direction of the mixture 15 (circle with point in the Middle stands for an arrow, which looks upwards out of the plane of the paper) and therefore produces the theory-required perturbation in the shear layer 10th

FIG. 5c) shows a view from above onto the double-cone burner 26. Here, the spacing of the stub nozzles 16 can be easily traced. In order for the 5 cm mentioned in the above numerical _{example to} result in the wavelength λ _z , the transversal perturbation must be such that the perturbation nozzles 16 generate perturbation air streams which are 5 cm apart in the x-direction, ie in the flow direction of the mixture. FIG. 5d) shows a schematic front view of a double-cone burner 26. Again, the orthogonal intermingling of the mixture 15 and the interfering air 22 is recognizable. It is important that the injected air flow 22 has no strong, inwardly directed components, so that the mixture 15 is not disturbed , In addition, the total pressure of the injection of the disturbance air 22 must be at least as great as the total pressure of the mixture flowing past 15, so that significant transversal disturbances can form at all.

NAME LIST

10

shear layer

11

Maximum of the dimensionless growth coefficient

12

Area without growth

13

Area with growth

14

combustion air

15

Mixture of combustion air 14 and fuel

16

fault nozzles

17

flame

18

first half cone

19

fuel nozzle

20

cylindrical part of the burner 26

21

second half cone

22

fault air

23

entry slot

24

Leading edge of the half cone 18, 21

25

Line to fault nozzle 16

26

Burner, double cone burner

27

burner mouth

28

combustion chamber

Claims (5)

1. Method of operating a premix burner (26) which consists of hollow half cones (18, 21) arranged offset, in which

   a combustion-air flow (14) is injected tangentially via at least two inlet slots (23) formed by the half cones,

   a fuel is injected centrally and/or a gaseous fuel is injected into the combustion-air flow entering transversely,

   the combustion-air flow (14) is mixed with fuel inside the burner (26), and the forming fuel/air mixture (15) is transported into a combustion chamber (28) following the burner (26),

   in which combustion chamber (28) the fuel/air mixture (15) is burned, with hot gases being formed,

   coherent flow instabilities forming after discharge of the fuel/air mixture (15) into the combustion chamber (28) as a result of a shear layer (10) between the discharging mixture (15) and the hot gases in the combustion chamber (28), and

   disturbance air (22) being injected into the fuel/air mixture (15),

   characterized in that the disturbance air (22) is injected directly at the leading edges (24) of the half cones into the flow of the fuel/air mixture (15) essentially perpendicularly to the main flow direction of the fuel/air mixture (15) and essentially parallel to the shear layer (10).
2. Premix burner for carrying out the method,

   the premix burner (26) consisting of hollow half cones (18, 21) which are arranged offset and which form at least two inlet slots (23) for the combustion air (14),

   the half cones (18, 21) being defined on the combustion-chamber side by leading edges (24),

   a cylindrical part (20) with a fuel nozzle being arranged at the narrowed end of the half cones (18, 21),

   the half cones (18, 21) having disturbance nozzles (16) for injecting disturbance air,

   characterized in that

   the disturbance nozzles (16) are incorporated directly in front of the leading edges (24) of the half cones (18, 21) in such a way that they inject disturbance air (22), perpendicularly to the main flow direction of a fuel/air mixture (15) discharging into the combustion chamber (28), into the shear layer (10) forming downstream of the leading edges (24) and essentially parallel to this shear layer.
3. Premix burner according to Claim 2, characterized in that a multiplicity of disturbance nozzles (16) are arranged, and in that the disturbance nozzles (16) are distributed uniformly over the circumferences of the half cones (18, 21).
4. Premix burner according to Claim 3, characterized in that

   the uniform spacing of the disturbance nozzles (16) is selected in such a way that it is equal to or less than a critical value, and in that

   this critical value is obtained form the flow velocity of the combustion air and the frequency, occurring at the burner without disturbance nozzles (16), of the coherent flow instabilities of the fuel/air mixture (15),

   in such a way that the critical value is obtained as quotient, multiplied by 0.312, of the flow velocity of the combustion air (15) and the frequency, occurring at the burner without disturbance nozzles (16), of the coherent flow instabilities of the combustion-air flow (15).
5. Premix burner according to Claim 4, characterized in that, at a frequency, occurring at the premix burner (26) without disturbance nozzles (16), of the coherent flow instabilities of the combustion-air flow (15) within the range of 100 to 125 Hz and a flow velocity of the combustion air (15) within the range of 20 to 30 m/s, the disturbance nozzles (16) on the half cones (18, 21) have a spacing within the range of 3 to 5 cm, in particular within the range of 4.5 to 5 cm.

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