METHOD FOR PREVENTING FLOW INSTABILITIES IN A BURNER

Abstract

In a method and an appliance for operating a burner (19), in which a flow of combustion air (10) transports fuel into a combustion chamber (21) where the fuel is burnt and, during transport, the flow of combustion air (10) is mixed with the fuel and guided by casing elements (13, 16), the resonant build-up interaction of coherent flow instabilities and acoustic field is reduced because the formation of first periodic, coherent flow instabilities in a boundary layer between the combustion air (10) and the casing elements (13, 16) is perturbed, and coupling of an acoustic field in the combustion chamber (21) to such first flow instabilities is reduced.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the field of the burner, in particular of the burner for use in gas turbines. It concerns an appliance and a method for operating a burner, in which a flow of combustion air transports fuel into a combustion chamber where the fuel is burnt and, during the transport, the flow of combustion air mixes with the fuel and is guided by casing elements.

[0003] 2. Discussion of Background

[0004] In modern burners, particularly in burners such as are employed in gas turbines, it is becoming increasingly important to keep the combustion both as efficient as possible and as free from pollutants as possible. Pollutant limits are specified by the authorities inter alia and the regulations concerning CO and NOx emission are becoming increasingly strict. The corresponding optimization of the combustion can take place in many ways, for example by the admixture of additives such as water to the fuel, by the employment of catalyzers or also by ensuring ideal fuel/air mixtures for the combustion. Optimum fuel/air ratios can be generated by premixing fuel and combustion air (so-called premixing burners) or by fuel and combustion air, mixed in a special manner, being jointly injected into the combustion space.

[0005] From EP-B1-0 321 809, a burner for liquid and gaseous fuels, without premixing section, has become known in which combustion air supplied from outside enters tangentially through at least two inlet slots between hollow half-cones with an offset arrangement and, in this region, the combustion air flows in the direction of the combustion chamber, and in which burner the liquid fuel is injected centrally either at the narrowed half-cone end facing away from the combustion chamber or is injected in the region of the inlet slots. The fuel is thus, to a certain extent, encompassed by the combustion air and “enveloped” so that a conical liquid fuel profile forms between the half-cones, this liquid fuel profile spreading out in the direction of the combustion chamber and burning there. Gaseous fuel, in particular, is injected through rows of holes transversely into the entering air from fuel supply tubes which extend along the air inlet slots.

[0006] The flow of the fuel/air mixture along the casing elements of the burner and the outlet of the combustion air into the combustion chamber are problematic in such burners, and generally in burners in which combustion air flows in a similar manner into a combustion chamber. Thus, the flow of the combustion air in the burner along the walls of the half-cones is usually no longer laminar but turbulent because of the flow and pressure relationships and the geometry of the casing. For a certain range of Reynolds number, the flow in this boundary layer is coherently undulatory, the undulation crests extending at right angles to the flow direction. This undulatory flow in boundary layers is usually designated as Tolmien-Schlichting waves (TS waves) and is usually the first turbulent flow form after the laminar flow which occurs at low Reynolds numbers. In addition, a shear layer forms immediately behind the front edge of the half-cones in the flow direction of the combustion air. This shear layer is located between the essentially stationary and hot combustion gases found in the combustion chamber and the emerging, flowing mixture of fuel and combustion air. It is in the nature of such shear layers that, independent of the Reynolds number, they roll up at some point with eddies as the result. This rolling-up can take place in such a way that so-called Kelvin-Helmholtz waves form initially on the shear layers, the crests of these waves extending transverse to the flow direction and the waves subsequently generating vortices.

[0007] It is found that these instabilities in boundary layers and their coupling to the instabilities on shear layers are, in combination with the combustion process taking place, mainly responsible for an important class of thermo-acoustic oscillations initiated by reaction rate fluctuations. In a burner of the type quoted above, these essentially coherent waves lead to vibrations with frequencies of approximately 100 Hz at typical operating conditions. Because these frequencies coincide with typical fundamental natural modes of many gas turbine annular burners, the thermo-acoustic oscillations present a problem.

SUMMARY OF THE INVENTION

[0008] Accordingly, one object of this invention is to provide a novel appliance or burner, and a method of operating such, which reduces the resonant build-up interaction of coherent flow instabilities and acoustic field.

[0009] This object is achieved in an appliance or a method of the type quoted at the beginning by perturbing the formation of first periodic, coherent flow instabilities in a boundary layer between the combustion air and the casing elements and by a coupling of an acoustic field in the combustion chamber to such first flow instabilities being reduced. The core of the invention is therefore located in the specific prevention of coherent flow instabilities preventing the resonant build-up of thermo-acoustic oscillations right at the source of their formation.

[0010] A first preferred embodiment of the invention is one wherein the first flow instabilities are Tolmien-Schlichting waves in the boundary layer between the flow of combustion air and casing elements, wherein furthermore these are preferentially capable of timing second flow instabilities which form at inlet of the fuel/air mixture into the combustion chamber on the shear layers then occurring and which can, for example, be Kelvin-Helmholtz waves. In addition, the perturbation of the first flow instabilities preferably takes place in the vicinity of the front edge of the casing elements facing toward the combustion space.

[0011] A further embodiment has the feature that the perturbation takes place by means of through-holes in the casing elements. The pressure drop through these holes leads to the fact that the transverse undulation in the boundary layer is interrupted.

[0012] Further embodiments of the method and the appliance are given in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description of embodiment examples when considered in connection with the accompanying drawings, wherein:

[0014] FIG. 1 shows a diagrammatic representation of a burner and shows Kelvin-Helmholtz waves rolling up behind the outlet opening; and

[0015] FIG. 2 shows a diagrammatic representation of a burner with holes which prevent the formation of first, coherent flow instabilities.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The effective principle of the approach described shall be initially rationalized and explained on the basis of some theoretic considerations. The technical embodiment examples are then described.

[0017] Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the views, FIG. 1 shows, diagrammatically, the flow relationships in a double-cone burner. The combustion air 10 enters laterally through the inlet slots 17 of the hollow half-cones 13 and 18, whose axes are arranged with a slight offset, flows to the front end of the burner while describing a slight curve and, after passing the front edges 18 of the half-cones, emerges from the burner 19 into the combustion chamber 21. At the narrowed end of the half-cones 13 and 16, there is a cylindrical part 15 in which is arranged a fuel nozzle 14 which injects the in this case liquid fuel centrally between the two half-cones 13 and 16. Gaseous fuels are preferably injected along the inlet slots 17 via a plurality of holes. The flow of combustion air 10 envelops the injected fuel and a fuel cone forms which spreads out in the forward direction and, after emergence into the combustion chamber 21 at the burner opening 20, burns in a flame 12 represented diagrammatically in the figure. The roll-up 22 of the fuel/combustion air mixture behind the front edge 18, on entry into the combustion chamber, is likewise indicated in FIG. 1. Because the properties of the boundary layer between the flowing air 11 and the half-cones 13 and 16 is decisive for the thermo-acoustic feedback, the behavior of this boundary layer shall be first investigated more precisely.

[0018] The displacement thickness δ1, defined as 1${\delta }_1={\int }_0^{\infty }\left(1-\frac{u\left(y\right)}{U}\right)dy,$

[0019] of the boundary layer between the casing 13 or 16 and the flowing fuel/air mixture is given, for laminar flow, by:

δ1=1.7208{square root}{square root over (xv/U)}

[0020] where x is a characteristic length, v is the kinematic viscosity and U is the flow velocity outside the boundary layer. For turbulent boundary layer flow behavior, the displacement thickness is given by: 2${\delta }_1=0.04625{x\left[\frac{v}{\mathrm{ux}}\right]}^{1/5}.$

[0021] Although the flow in such a boundary layer is laminar, in the case of plane boundary layers, for a large range of Reynolds numbers Rex=Ux/v, turbulent flow behavior is to be expected under conditions such as are met in double-cone burners, i.e. concave walls and perturbation due to the fuel admixture process. The displacement thickness of the boundary layer in the region of the front edge 18 of the casing elements is therefore best described by the third equation.

[0022] For coupling of the acoustic field to the turbulence in this boundary layer, it is necessary for the turbulence to exhibit a coherent undulatory character or, in other words, so-called Tolmien-Schlichting waves should form. The capability of a turbulent boundary layer to build up Tolmien-Schlichting waves depends, on the one hand, on the form factor H12 of the boundary layer and also on the Reynolds number Reδ1=Uδ1/v formulated as a function of the displacement thickness. Because the form factor H12, defined as a quotient of displacement thickness δ1 and momentum loss thickness δ2, where 3${\delta }_2={\int }_0^{\infty }\frac{u\left(y\right)}{U}\left(1-\frac{u\left(y\right)}{U}\right)dy,$

[0023] is normally located above 1.8 in the case of the applications considered here, a substantial growth of Tolmien-Schlichting waves can be expected, if

Re δ1 =Uδ 1 /v> 104.

[0024] Assuming a characteristic length of x=250 mm for a double-cone burner of the type EV17 and a characteristic length of x=(185/175) .250 mm for a burner of the applicant's type EV18, the following conditions in the edge region 18 can be calculated. The effective flow velocity U then behaves relative to the nominal flow velocity UN in accordance with U=UN{square root}ξ, where ξ is the pressure drop coefficient of the burner.

Type	Un[m/s]	U[m/s]	T[K]	p[bar]	Reδ1
GT13E2	25.0	70.7	688	15.0	8.84 * 103
GT8C	30.8	87.2	733	15.9	1.01 * 104
GT11N2	38.0	107.5	693	13.7	1.14 * 104
GT26	29.6	83.8	815	30.0	1.48 * 104

[0025] The calculations show that the Reynolds number values for the conditions present in double-cone burners are located precisely in the region favorable for the formation of Tolmien-Schlichting waves, and depend greatly on the nominal flow velocity and the temperature.

[0026] The coherent undulatory character of the boundary layer in the region of the front edge 18, i.e. shortly before the air flow 11 separates and emerges into the combustion chamber 21, is now capable of influencing the turbulence in the shear layer occurring behind the front edge 18 between stationary air in the combustion chamber and emerging air 11. Because, whatever the Reynolds number, this shear layer is unstable with respect to wavelengths greater than approximately five times the thickness of the shear layer, so-called Kelvin-Helmholtz waves (KH) 22 form there. These are coherent and can be timed, particularly in frequency and phase, by the Tolmien-Schlichting waves of the boundary layer located upstream. The combustion in the eddies 24 of the Kelvin-Helmholtz waves likewise pulsates with the same frequency and, in the process, drives the acoustic field in the combustion chamber 21. Because the acoustic field is capable of timing the Tolmien-Schlichting waves (TS) in the first boundary layer, the following feedback circuit forms:

→TS→KH→pulsating combustion→acoustic field→TS→KH→ . . .

[0027] Because such resonant build-up processes reduce the efficiency of the operation and, furthermore, the frequency can additionally coincide with natural frequencies of combustion chambers, their prevention is of enormous importance.

[0028] In principle, it is possible to forestall the feedback process by preventing an arbitrary one of the phenomena listed above but it is found that destroying the coherence of the Tolmien-Schlichting waves is most suitable from both theoretical and practical points of view. To a certain extent, the Tolmien-Schlichting waves here undertake the function of the vibrating lips when whistling with the mouth. Forestalling this affects the resonant circuit at a decisive and easily influenced location.

[0029] The formation of TS waves extending at right angles to the flow direction of the fuel/air mixture 11 can be prevented by, for example, attaching shark-tooth-type projections which essentially face toward the central axis of the burner 19. In this way, a longitudinal perturbation is superimposed on the coherent transverse undulation in the boundary layer and destroys the latter. A problematic feature of such “shark's teeth”, however, is that they can be burnt off because of the heat and the radiation.

[0030] A ring of through-holes 25 in the casing elements 13 and/or 16, such as are represented in FIG. 2, acts in an analogous manner. The holes 25 are then arranged in the region of the front edge 18 and their rows are essentially parallel to the front edge. If the pressure drop over the holes 25 is comparable with the pressure drop over the whole of the burner 19, such holes are then capable of strongly perturbing the boundary layer in the region of the front edge 18 and, therefore, of destroying the TS waves in this region. The diameter of the holes 25 should then be at least comparable with the thickness of the boundary layer. For one of the applicant's EV17 burners, therefore, a diameter of a few millimeters, in particular of 3 mm, is to be preferred. Furthermore, the distance between the holes 25 should be located roughly in the region of the wavelength of the highest thermo-acoustic frequency occurring. In order to prevent the suction effects of adjacent holes 25 mutually canceling one another, attention should additionally be paid to ensuring that the distance between the holes 25 is not substantially smaller than the distance of the holes 25 from the front edge.

Claims

1. A method of operating a burner (19), in which a flow of combustion air (10) transports fuel into a combustion chamber (21) where the fuel is burnt and, during the transport, the flow of combustion air (10) is mixed with the fuel and is guided within the burner by casing elements (13, 16), wherein the formation of first periodic, coherent flow instabilities in a boundary layer between the combustion air (10) and the casing elements (13, 16) is perturbed in such a way that a coupling of an acoustic field in the combustion chamber (21) to such first flow instabilities is reduced.

2. The method as claimed in claim 1, wherein the first flow instabilities in the boundary layer between the combustion air (10) and the casing elements (13, 16) are Tolmien-Schlichting waves.

3. The method as claimed in one of claims 1 or 2, wherein the first flow instabilities time coherent second flow instabilities (22) of a shear layer between essentially stationary air in the combustion chamber and combustion air (11) emerging from the burner (19).

4. The method as claimed in claim 3, wherein the second flow instabilities are Kelvin-Helmholtz waves (22).

5. The method as claimed in one of claims 3 or 4, wherein the combustion of the mixture (11) of combustion air and fuel takes place in the shear layer in a combustion zone (24), wherein this combustion essentially takes place while pulsating with the frequency of the second flow instability (22), and wherein the frequency of the acoustic field in the combustion chamber (21) is essentially determined by this pulsating combustion.

6. The method as claimed in one of claims 1 to 5, wherein the perturbation of the coherence of the first flow instabilities is undertaken in the region of a front edge (18), facing toward the combustion chamber (21), of the casing elements (13, 16).

7. The method as claimed in claim 6, wherein the perturbation of the coherence of the first flow instabilities is undertaken by a plurality of through-holes (25) through the casing elements (13, 16) in the region of the front edge (18).

8. The method as claimed in claim 6, wherein the perturbation of the coherence of the first flow instabilities is effected by a plurality of tooth-shaped projections, which are applied to the casing elements (13, 16) in the region of the front edge (18) and which are essentially directed in the direction of the central axis of the burner (19).

9. A burner (19) for carrying out a method as claimed in one of claims 1 to 8, wherein means (25) are provided which reduce the coherent periodicity of the first flow instabilities.

10. The burner (19) as claimed in claim 9, wherein the burner (19) is a double-cone burner whose casing elements are formed by two half-cones (13, 16) which are slightly offset relative to one another, and wherein the means (25) are configured as a plurality of holes (25), which holes (25) are essentially arranged in a row parallel with the front edge (18) of the half-cones (13, 18).

11. The burner (19) as claimed in claim 10, wherein the holes have a diameter in the region of the thickness of the boundary layer between the combustion air (10) and the casing elements (13, 16), which boundary layer causes the first flow instabilities.

12. The burner as claimed in claim 11, wherein the holes have a diameter of 2 to 5 mm, in particular of 3 mm.

13. The burner as claimed in one of claims 9 to 12, wherein the distance apart of the holes (25) is located in the region of the highest occurring thermo-acoustic wavelength, and wherein the distance apart is essentially larger or equal to the distance of the holes (25) from the front edge (18).