EP1001214B1 - Burner - Google Patents

Description

Technical field

The present invention relates to the field of burners, in particular the burner for use in gas turbines, according to the generic term of the independent claim.

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 increasingly stringent. The corresponding optimization of the combustion can take place 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 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 special way.

EP-B1-0 321 809 describes a burner for liquid and gaseous fuels become known with premixing section, in which supplied from the outside Combustion air tangentially displaced between at least two inlet slots arranged, hollow semi-cones occurs and there in the direction of the combustion chamber flows, and in which either on the side facing away from the combustion chamber, tapered side of the half cones of liquid fuel centrally, or in the region of the entry slots is injected. The fuel becomes more certain masses captured by the combustion air and "enveloped", so that between the cones form a conical liquid fuel profile, which is in Expands towards the combustion chamber and burns there. Especially gaseous Fuel is obtained from fuel supply pipes that run along the air inlet slots run, injected transversely into the incoming air through rows of holes.

EP-A2-0 851 172 discloses a burner for operating a combustion chamber with a liquid and / or gaseous fuel. A burner for operating a combustion chamber with a liquid and / or gaseous fuel, the combustion air required for this through tangential air inlet ducts into an interior of the burner. By this flow guidance creates a swirl flow in the interior, which at the exit of the Brenner induced a backflow zone. In order to stabilize the flame front that forms there, at least one zone is provided for each partial body forming the burner, within which inlet openings for the injection of additional air into the swirl flow are provided. As a result of this injection, a part is formed on the inner wall of the partial body Film, which prevents the flame along the inner wall of the partial body into the interior of the burner can strike back.

US-A-5,375,995 discloses a double cone burner in which near the burner outlet at a distance of at least 30% from the burner diameter Nozzles for a medium-sized gaseous fuel containing highly reactive components Calorific value is attached to the circumference of the partial cone body. There is also a fuel line and a distribution channel placed in the area of the nozzles for the highly reactive Fuel presence. The fuel containing gaseous, highly reactive components is at high speed through the nozzles, which have a diameter of less than 1% of the nominal diameter of the burner have been injected into the zones of high air velocity and the depth of penetration and the depth of the fuel jets are coordinated so that the ignition only takes place behind the burner after mixing.

Problematic with such burners, and generally with burners where one Combustion airflow flowing into a combustion chamber in a similar manner is the Flow of the fuel-air mixture along the shuttering elements of the Burner, and the exit of the combustion air into the combustion chamber. So is both the sweeping of the combustion air in the burner on the walls of the Half cones mostly due to the flow and pressure conditions as well as the The geometry of the formwork is no longer laminar, but turbulent. The current in this boundary layer is the Reynolds number for a certain range of values coherently wavy, the wave crests running perpendicular to the direction of flow. This ripple in boundary layers is called Tolmien-Schlichting waves (TS waves), and is usually the first turbulent flow after the laminar flow, which occurs at lower Reynolds numbers. In addition, the combustion air is formed behind the front edge in the direction of flow the half-cones immediately a shear layer. This shear layer lies between the essentially stationary ones in the combustion chamber and hot combustion gases, and the emerging, flowing mixture of fuel and combustion air. It is in the nature of such shear layers, that, regardless of the Reynolds number, this will eventually roll up and turbulence result. This rolling up can be such that initially so-called Kelvin-Helmholtz waves appear on the shear layers form, whose wave crests are transverse to the direction of flow, and which then create vortices.

It turns out that these instabilities in boundary layers and their coupling the instabilities on shear layers in combination with the ongoing combustion process are primarily responsible for an important class of response rate fluctuations triggered, thermoacoustic oscillations. This largely coherent waves lead to a burner of the type mentioned above under typical operating conditions to vibrations with frequencies of about 100 Hz. Since this frequency with typical fundamental eigenmodes of many ring burners of gas turbines coincide, make the thermoacoustic Oscillations are a problem.

Presentation of the invention

The invention is therefore based on the object To provide burners which is the rocking interaction of coherent flow instabilities and acoustic field reduced.

This task is characterized by the features of the independent claim the formation of the first periodic, coherent is solved Flow instabilities in a boundary layer between the fuel-air mixture and the formwork elements disturbed and a coupling acoustic field in the combustion chamber is at such first flow instabilities reduced. The essence of the invention is that targeted prevention of coherent flow instabilities the resonant rocking of Prevents thermoacoustic oscillations when they are caused.

The first flow instabilities are Tolmien-Schlichting waves in the boundary layer between combustion air flow and formwork elements, which are capable of second flow instabilities, which change when the fuel-air mixture enters the Form combustion chamber on the shear layers that occur and which, for example Kelvin-Helmholtz waves can be clocked.

Brief explanation of the figures

Fig. 1

zeigt eine schematische Darstellung eines Brenners und das Aufrollen von Kelvin-Helmholtz-Wellen hinter der Austrittsöffnung; und

Fig. 2

zeigt eine schematische Darstellung eines Brenners mit Bohrungen, welche die Ausbildung von ersten, kohärenten Strömungsinstabilitäten verhindern.

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 burner and the rolling up of Kelvin-Helmholtz waves behind the outlet opening; and

Fig. 2

shows a schematic representation of a burner with holes which prevent the formation of first, coherent flow instabilities.

WAYS OF IMPLEMENTING THE INVENTION, INDUSTRIAL APPLICABILITY

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

Figure 1 shows schematically the flow conditions in a double-cone burner 19. The combustion air 10 passes laterally through the inlet slots 17 with light arranged displaced axes, hollow half-cones 13 and 16, flows to the front end of the burner 19, describing a slight curve, and enters the burner 19 after passing the front edges 18 of the half cones the combustion chamber 21 is located at the tapered end of the half-cones 13 and 16 a cylindrical part 15, in which a fuel nozzle 14 is arranged, which is the liquid fuel in this case centrally between the two half cones 13 and 16 injected. A gaseous fuel is preferred along the Entry slots 17 injected through a plurality of holes. The combustion air flow 10 envelops the injected fuel and a fuel cone is formed, which widens towards the front, and which after leaving the Combustion chamber 21 at the burner mouth 20 in one, schematically in the figure illustrated flame 12 burns. Rolling up is also indicated in FIG. 1 of the fuel-combustion air mixture behind the front edge 18 when entering the combustion chamber 21. Because for the thermoacoustic feedback Properties of the boundary layer between flowing mixture 11 and half cones 13 and 16 is crucial, the behavior of this boundary layer should first be more precise to be examined.

der Grenzschicht zwischen Halbkonussen 13 oder 16 und strömendem Brennstoff-Luft-Gemisch gegeben durch: δ1 = 1.7208 vx/U wobei x eine charakteristische Länge, v die kinematische Viskosität, und U die Strömungsgeschwindigkeit ausserhalb der Grenzschicht darstellt. Für turbulentes Strömungsverhalten der Grenzschicht ist die Verdrängungsdicke gegeben durch:

Obwohl die Strömung in einer solchen Grenzschicht bei planen Grenzschichten für einen grossen Bereich von Reynoldszahlen Re _x = Ux/v laminar ist, ist unter den Bedingungen, wie sie bei Doppelkegelbrennern 19 anzutreffen sind, d.h. konkave Wände und Störung durch den Brennstoffzumischungsprozess, ein turbulentes Strömungsverhalten zu erwarten. Die Verdrängungsdicke der Grenzschicht im Bereich der Vorderkante 18 der Verschalungselemente ist deshalb am besten durch die dritte Formel beschrieben.For laminar flow, the displacement thickness δ _{1 is} defined as

the boundary layer between half-cones 13 or 16 and the flowing fuel-air mixture given by: δ 1 = 1.7208 vx / U where x is a characteristic length, v is the kinematic viscosity, and U is the flow velocity outside the boundary layer. For turbulent flow behavior of the boundary layer, the displacement thickness is given by:

Although the flow in such a boundary layer is laminar for a large range of Reynolds numbers Re _x = Ux / v in the case of plane boundary layers, a turbulent flow behavior is to be found under the conditions found in double-cone burners 19, i.e. concave walls and disturbance due to the fuel admixing process expected. The displacement thickness of the boundary layer in the area of the front edge 18 of the formwork elements is therefore best described by the third formula.

bei den hier betrachteten Anwendungen normalerweise oberhalb von 1.8 liegt, kann ein wesentliches Wachstum von Tolmien-Schlichting-Wellen erwartet werden, wenn Reδ1 = Uδ1/v>104. For the coupling of the acoustic field to the turbulence in this boundary layer, it is necessary that the turbulence have a coherently wavy character, that in other words so-called Tolmien-Schlichting waves are formed. The ability of a turbulent boundary layer to build up Tolmien-Schlichting waves depends on the one hand on the form factor H _{12 of} the boundary layer and on the Reynolds number Re _δ formulated as a function of the displacement thickness ₁ = U δ ₁ / v from. Since the form factor H ₁₂ , defined as the quotient of the displacement thickness δ ₁ and the lack of impulse thickness δ ₂ , with

in the applications considered here is normally above 1.8, a significant growth of Tolmien-Schlichting waves can be expected if re δ 1 = U δ 1 / v > 10 4 ,

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 type EV18 from the applicant, the following conditions can be found in the Calculate edge region 18. The effective flow rate U is related to the nominal flow rate U _N according to U = U _N ξ , where ξ is the pressure drop coefficient of the burner. Type U _N [ m / s ] U [ m / s ] T [ K ] p [ bar ] Re _{δ 1} GT13E2 25.0 70.7 688 15.0 8.84 * 10 ³ GT8C 30.8 87.2 733 15.9 1.01 * 10 ⁴ GT11N2 38.0 107.5 693 13.7 1.14 * 10 ⁴ GT26 29.6 83.8 815 30.0 1.48 * 10 ⁴

The calculations show that the values of the Reynolds number for those in double-cone burners present conditions especially for the formation of Tolmien-Schlichting waves good range, and strong from the nominal flow rate and the temperature depend.

The coherent ripple in the boundary layer in the region of the leading edge 18, ie shortly before the mixture 11 separates and exits into the combustion chamber 21, is now able to regulate the turbulence in the trailing edge 18 between stationary air in the combustion chamber 21 and emerging mixture 11 to influence occurring shear layer. Since this shear layer is unstable regardless of the Reynolds number with respect to wavelengths greater than approximately 5 times the thickness of the shear layer, so-called Kelvin-Helmholtz waves (KH) 22 with wave combs 23 form there. These are coherent, and they can be clocked in particular in frequency and phase by the Tolmien-Schlichting waves of the boundary layer lying in front of them. The combustion in the eddies 24 of the Kelvin-Helmholtz waves 22 also pulsates at the same frequency and drives the acoustic field in the combustion chamber 21. Since the acoustic field is able to transmit the Tolmien-Schlichting waves (TS) in the to clock the first boundary layer, the following feedback circuit forms:
→ TS → KH → pulsating combustion → acoustic field → TS → KH → ....

Because such build-up processes reduce the efficiency of operations and in addition, the frequency also coincides with the natural frequencies of combustion chambers prevention is of enormous importance.

In principle, it is possible to prevent the feedback process by preventing one prevent any of the phenomena listed above. But it has been shown that the coherence of the Tolmien-Schlichting waves is being destroyed best suited in both theoretical and practical terms. The Tolmien-Schlichting waves take over the function of the to a certain extent vibrating lips when whistling with the mouth. If you prevent this, you hit one the resonant circle at a crucial and easy to influence Job.

The formation of the direction of flow of the fuel-air mixture 11 According to the invention, vertically extending TS waves can be attached of shark tooth-like, essentially the central axis of the torch Prevent 19 facing projections on the front edges 18. The coherent Cross waviness in the boundary layer is thus overlaid by a longitudinal disturbance and destroyed by this. Problems with such "shark teeth" can be that they can burn down due to the heat and radiation.

A ring of through bores 25 in the formwork elements acts in an analogous manner 13 and / or 16, as shown in Figure 2. The Bores 25 are in the region of the leading edge 18 and in their sequence arranged substantially parallel to the front edge. Is the pressure drop comparable to the pressure drop across the entire burner 19 via the bores 25, such bores are able to cross the boundary layer in the area of the leading edge 18 and thus disturb the TS waves in this region to destroy. The diameter of the bores 25 should at least match that The thickness of the boundary layer may be comparable. For an applicant's EV17 burner is therefore a diameter of a few millimeters, in particular 3 mm prefer. In addition, the distance between the holes 25 should be roughly in the range the wavelength of the highest occurring thermoacoustic frequency lie. To prevent the suction effects of neighboring holes 25 do not cancel each other out, you should also make sure that the distance between the bores 25 is not significantly smaller than the distance of holes 25 from the leading edge.

LIST OF REFERENCE NUMBERS

10

Combustion airflow upon entry

11

Mixture of combustion air and fuel

12

flame

13

first half cone

14

fuel nozzle

15

cylindrical part of the burner 19

16

second half cone

17

entry slot

18

Front edge of the half-cone 13, 16

19

Burner, double cone burner

20

burner mouth

21

combustion chamber

22

Kelvin-Helmholtz waves

23

Wave crests of the Kelvin-Helmholtz waves 22

24

Preliminary prelude to the Kelvin-Helmholtz waves 22

25

drilling

Claims (1)

1. Burner (19),

   which has two half-cones (13, 16), the axes of which are offset with respect to one another and thereby form inlet slots (17) for tangential inlet of a combustion air flow (10),

   a multiplicity of holes for injection of a gaseous fuel being arranged along the inlet slots (17),

   a cylindrical part (15), in which a central fuel nozzle (14) is arranged, being located at the narrowed end of the half-cones (13 16), and

   it being possible for the fuel/air mixture (11) formed in the burner (19) to be introduced into a combustion chamber (21) downstream of the burner (19),

   characterized in that a plurality of tooth-like projections are arranged at the front edges (18) of the half-cones (13, 16), facing the central axis of the burner (19), in order to perturb coherent flow instabilities in the boundary layer between the fuel/air mixture (11) and the half-cones (13, 16) and to reduce coupling of an acoustic field in the combustion chamber (21) downstream of the burner (19) to the coherent flow instabilities.

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