A burner including an acoustic damper

The present invention relates to a burner, a combustor for a turbomachine, particularly a gas turbine engine, and a method of manufacturing a burner. BACKGROUND OF THE INVENTION

Gas turbine engines, as one example of turbomachine, comprise as main components a compressor, a combustor, and an expansion turbine. For combustors, different designs exist, for example annular combustors or can-annular combustors. The combustor itself is comprised of a burner via which fuel is provided to a combustion space and a combustion chamber to encapsulate the combustion space. The combustion space is also provided with oxygen containing fluid for combustion, particularly air provided from the compressor.

Burners may be designed to operate under lean conditions so that the emissions, particularly NOx and CO is kept low. Un ^¬ der lean condition a mix of fuel and air is considered in which all or most of the fuel is burnt. For such a lean op ^¬ eration typically so-called main fuel is provided. For tran ^¬ sient operation, for example during start-up of the turbo ^¬ machine, additional fuel may be provided as so-called pilot fuel to stabilise the flame and to avoid combustion dynamics.

Dry Low Emission (abbreviation DLE) technology may be used to obtain reduction of the NOx emissions.

One problem related to DLE type of combustion is the occur- rence of combustion dynamic events. This phenomenon, caused by many different physical mechanisms, may result in struc ^¬ tural damages, negative impact on emissions and significantly affect commissioning. In general, one approach to suppress combustion dynamics is to use acoustic (passive) dampers directly installed in the combustion chamber of combustors (including both annular and can-annular combustors) . Such dampers include for example Helmholtz resonators or perforated liners to directly attack the acoustic pressure in the combustion chamber.

The above solution is not yet considered optimal. One of the mechanisms related to combustion dynamics is the presence of a fluctuating fuel profile entering the combustor and in particular the combustion zone. This modulation of the fuel is caused by the subsequent acoustic events and may result in a destructive, self-driven, feed-back loop. SUMMARY OF THE INVENTION

The present invention seeks to mitigate these drawbacks.

This objective is achieved by the independent claims. The de- pendent claims describe advantageous developments and modifi ^¬ cations of the invention.

In accordance with a first aspect of the invention, a burner of a turbomachine, particularly a gas turbine engine, is pro- vided, comprising at least one burner section having an annular wall surrounding a respective section of a burner interior, the annular wall including:

- an annular inner surface delimiting the burner interior,

- a plurality of dampening cavities for the dampening of thermo-acoustic vibrations in the burner interior, each damp ^¬ ening cavity being connected to the annular inner surface through at least a dampening hole.

By this configuration, by introducing acoustic dampers in one of the burner sections, the above described feed-back mecha ^¬ nism is reduced and the combustion dynamics related to this kind of phenomenon suppressed. According to the embodiments of the present invention, the at least one burner section having the annular wall is an intermediate burner section, particularly a mixing section, of the burner provided between:

- an upstream burner section for providing a first fuel and an oxygen containing fluid to an upstream end of a burner interior; and

- a downstream burner section for providing a second fuel to a downstream end of the burner interior or to a combustion chamber.

The upstream burner section may be arranged provide to mix main fuel and air, preferably via a swirler. The downstream burner section may be arranged to provide pilot fuel. The in- termediate burner section may be called a mixer or mixing section to allow better mixing of main fuel and air.

Positive effects by integrating passive acoustic dampers in an intermediate burner section are:

- No unwanted air consumption will be present as these acous ^¬ tic dampers which are placed up-streams the combustion zone. Thus, no negative impact on the emission levels due to unaf ^¬ fected (increased) flame temperature will be experienced;

- If dynamic issues occur on site related to the modulation phenomenon, corresponding burner adjustment can easily be realized for that particular site;

- If dynamic issues occur due to operation including different fuel specifications, proper acoustic adjustments of the burners may easily be realized to attack the problem.

In accordance with a second aspect of the invention, a com- bustor is provided, which comprises:

- a plurality of burners, at least one burner arranged as above specified; and

- at least one combustion chamber, particularly an annular or a can-annular combustion chamber, arranged downstream of the burner ( s ) . In accordance with a third aspect of the invention, a method of manufacturing the above defined burner is provided. Such manufacturing method includes the step of

- additive manufacturing of the annular wall as an integrally formed component, or

- additive manufacturing of the upstream burner section, the intermediate burner section and the downstream burner section as integrally formed component. According to possible embodiments of the present invention, the additive manufacturing step is performed by selective la ^¬ ser melting or selective laser sintering.

By use of the additive manufacturing technology burners can be designed with damping devices tuned for an optimal attenu ^¬ ation behavior, i.e. suppressing known specific frequency intervals .

According to other possible embodiments of the present inven- tion, each dampening cavity is connected to an annular outer surface of the annular wall through at least a purging hole.

According to further possible embodiments of the present in ^¬ vention, at least one of the dampening cavities has a quad- rangular or hexagonal or circular profile.

The plurality of dampening cavities may distributed in the annular wall along rows and/or column to form a pattern. Said rows and/or column may be parallel or inclined with respect to a longitudinal axis of the burner.

In the above and in the following description, the terms "longitudinal", "radial" and "circumferential" are referred, when not differently specified, to a longitudinal axis of the burner.

The terms "upstream" / "downstream" / "mid ( stream) " are used to indicate a direction along the longitudinal axis of the burner and are relative to a fuel flow direction. Even though some fluids will be swirled, in the end a main travel direc ^¬ tion can be given from an upstream end of the burner to the exit (downstream end) of the burner. The exit will release the fluid into the combustion chamber, which will therefore be again downstream of the burner.

The term "interior" and "exterior" is used in respect of a radial direction of the burner, perpendicular to the longitu- dinal axis of the burner. A radial inwards cavity - the burn ^¬ er interior - is surrounded radially outwards by the annular wall. Beyond that annular wall, i.e. further radially out ^¬ wards, a burner exterior is defined. The burner exterior is supposed to be a hollow space to guide compressed oxygen con- taining fluid provided from a compressor of the turbomachine .

It has to be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with ref- erence to apparatus type claims whereas other embodiments have been described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that in addition to any combination of features belonging to one type of subject mat- ter also any combination between features relating to different subject matters, in particular between features of the apparatus type claims and features of the method type claims is considered as to be disclosed with this application. The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi- ment but to which the invention is not limited. BRIEF DESCRIPTION OF THE DRAWINGS shows a lateral view of an exemplary embodiment of a burner according to the present invention; shows a longitudinal section of an intermediate burner section of a burner according to the present invention ;

FIG. 3 shows an axonometric view of the burner section

FIG. 2; shows the axonometric view of FIG. 3 without the annular outer surface to illustrate the inside of the annular wall of the intermediate burner sec ^¬ tion; schematically illustrates a first pattern for the dampers inside the annular wall of the intermediate burner section; schematically illustrates a second pattern for the dampers inside the annular wall of the intermediate burner section; schematically illustrates a third pattern for the dampers inside the annular wall of the intermediate burner section.

DETAILED DESCRIPTION OF THE INVENTION

The illustrations in the drawings are schematic. It is noted that in different figures, similar or identical elements or features are provided with the same reference signs. In order to avoid unnecessary repetitions elements or features which have already been described with respect to an embodiment are not described again further in the description. FIG. 1 represents one exemplary burner 1 according to the present invention. The burner 1 may be part of a gas turbine engine and may be used for an annular combustor. The burner 1 comprises an upstream burner section 2, an intermediate burner section 3 and a downstream burner section 4, longitudinally extended along a longitudinal axis Y of the burner 1. Up ^¬ stream of the upstream burner section 2 a burner shaft 5 provides fuel to the burner 1. The upstream burner section 2 may particularly be a swirler to mix fuel with air. Following in axial direction of the burner 1, the intermediate burner sec ^¬ tion 3 follows the upstream burner section 2 and provides a premixing zone to further mix the previously provided air and fuel. Further downstream, the downstream burner section 4 is present and specifically provides also a burner tip 42. The upstream burner section 2, the intermediate burner section 3 and the downstream burner section 4 altogether enclose a burner interior 6. Further ddownstream, the burner tip 42 is connected to the combustion chamber 70 of a combustor (not represented in further detail) .

The burner shaft 5, the upstream burner section 2 with the swirler, the downstream burner section 4 and the combustion chamber 70 are conventional and not a specific object of the present invention and therefore they are not described in further detail.

FIGS. 2 to 4 represent one embodiment of an intermediate burner section 3 according to the present invention.

In the embodiment, the intermediate burner section 3 substan ^¬ tially is configured as a cylindrical annular wall 10 around the longitudinal axis Y of the burner 1. The annular wall 10 may also be shaped differently, as long as being annular around a space. The annular wall 10 surrounds an intermediate section of the burner interior 6. The annular wall 10 includes an annular inner surface 16 delimiting intermediate section of the burner interior 6 and an annular outer surface 17 of the annular wall 10 in contact with an exterior environment 7.

The annular wall 10 comprises an annular cooling fluid pas ^¬ sage 11. To provide air into the annular cooling fluid pas- sage 11, a plurality of cooling fluid inlet holes 14 are pre ^¬ sent in the annular wall 10, provided on the annular outer surface 17 in a position longitudinally proximal to the up ^¬ stream burner section 2. The annular cooling fluid passage 11 extends the cooling fluid inlet holes 14 to a plurality of effusion holes 15 provided on the annular inner surface 16.

The annular wall 10 further comprises a plurality of dampen ^¬ ing cavities 30 for the dampening of thermo-acoustic vibra ^¬ tions in the burner interior 6. Each dampening cavity 30 is connected to the annular inner surface 16 through a respec ^¬ tive dampening hole 40. Each dampening hole 40 comprises a dampening opening 41 on the annular inner surface 16.

Each dampening cavity 30 represents a Helmholtz resonator having the following main geometrical parameters:

- the volume Vc of the dampening cavity 30,

- the cross sectional area A of the dampening hole 40,

- the length L of the dampening hole 40. Each dampening cavity 30 can be used to suppress a frequency f of the thermo-acoustic vibrations in the combustion gas inside the volume V, the frequency f being proportional to squared root of the ratio between area A and the product of volume Vc by the length L, i.e. in symbols:

f ~ sqrt (A/ (Vc*L) ) .

Each dampening cavity 30 is further connected to the annular outer surface 17 through a respective purging hole 50. Each purging hole 50 comprises a purging opening 51 on the annular outer surface 17. According to other possible embodiments of the invention (not shown in the attached figures) the purging holes 50 are not present . Each dampening cavity 30 is delimited by two base surfaces 31, 32, respectively radially internal and external, and one or more lateral surfaces 33, radially connecting the two base surfaces 31, 32. The two base surfaces 31, 32 may be, as shown in the embodiment of the attached figures FIGS. 2 to 4, oriented substantially parallel to the annular inner surface 16 and/or the annular outer surface 17.

According to respective embodiments of the present invention, the two base surfaces 31, 32 may have respective profiles. In particular the two base surfaces 31, 32 may a quadrangular profile (FIGS. 4 and 5), an hexagonal profile (FIG. 6) , a circular or elliptical profile (FIG. 7).

More in general, according to other embodiments (not shown) of the present invention, the tri-dimensional shape of each dampening cavity 30 may be arbitrary, e.g. globe, conical, rectangular, honeycomb, etc. The shapes of the dampening hole 40 and of the purging holes 50 may be round or oval, etc. According to respective embodiments of the present invention, the plurality of dampening cavities 30 may be distributed in the annular wall 10 along rows and/or column (represented by an axial orientation X) to form a plurality of respective patterns 101, 102, 103.

With reference to the embodiment in FIGS. 4 and 5, a plurali ^¬ ty of dampening cavities 30 having quadrangular, in particular diamond shape are distributed according to a first pat ^¬ tern 101 comprising a plurality of rows (or columns) X in- clined with respect to the longitudinal axis Y of the burner 1. With reference to the embodiment in FIG. 6, a plurality of dampening cavities 30 having hexagonal shape are distributed according to a second pattern 102 comprising a plurality of rows (or columns) X parallel to the longitudinal axis Y of the burner 1.

With reference to the embodiment in FIG. 7, a plurality of dampening cavities 30 having elliptical shape are distributed according to a third pattern 103 comprising a plurality of rows (or columns) X orthogonal with to the longitudinal axis Y of the burner 1.

The annular wall 10 and particularly dampening cavity 30, the dampening holes 40 and the purging holes 50 may be built by additive manufacturing techniques, for example by selective laser melting, selective laser sintering, electron-beam melting, selective heat sintering, or electron beam freeform fabrication. The additive manufacturing techniques

advantageously permits to manufacture a great variety of dampening cavity 30 shapes and patterns. Some possible examples of such variety are described above and shown in the attached figures.