TECHNICAL FIELD

The present disclosure relates to the subject matter set forth in the claims. In particular, it relates to a burner. It further relates to a combustor and a gas turbine engine incorporating the burner.

BACKGROUND OF THE DISCLOSURE

From the art, combustion systems and burners are known for the combustion of a fuel with low nitric oxide generation. For this purpose, the fuel, in particular a gaseous fuel, is generally provided for combustion in an intensely premixed, lean fuel-oxidant mixture. The oxidant is most commonly air. For improved readability and ease of nomenclature, in the present disclosure the term “air” will be used to generically denote any oxidant. The skilled person will, by virtue of the aforesaid, readily understand the mention of air in the following as a disclosure of a generic oxidant. “Air”, to this extent, shall be broadly construed to represent a generic oxidant.

Lean premixed flames yield the issue of combustion stability, as they are generally operated at an equivalence ratio rather close to the lower extinction limit. Hence, in certain operation modes, the premix flames, or some of the premix flames, are replaced with, or supported by, so-called piloting flames. These are flames combusting less intensely premixed or even essentially non-premixed fuel-air mixtures, comprising zones of richer fuel-air mixture and thus providing for higher local combustion temperatures and yielding a combustion less sensitive to external influences. On the downside, however, nitric oxide formation increases disproportionally with the combustion temperature, and hence a balance needs to be found between nitric oxide formation and combustion stability. Diffusion flames, combusting a stream of fuel and air with zones having equivalence ratios close to 1 , i.e. near stoichiometric zones, yield excellent combustion stability, but with high nitric oxide formation. It is hence a goal in burner development and combustion engineering to design burners and operation concepts which yield minimum combustion instabilities in lean premix combustion and/or enable combustion over a large range of operation with as little piloting as possible.

Another aspect to be considered in the design of burners and combustion systems is an increasing demand for fuel flexibility. The combustion of so-called “blue” and “green” hydrogen, which is generated using renewable energy, may be found a suitable way to store and transport energy harvested from, for instance, solar and wind power plants. The operation of premix burners on fuels which yield a higher reactivity than natural gas, such as for instance, while not limited to, CO and hydrogen, or gas mixtures comprising high contents of C2+ species, i.e., hydrocarbon species having two or more carbon atoms, CO or hydrogen thereof, for a non-limiting instance 50% and more by volume, requires further considerations. Hydrogen, for instance, yields a short autoignition time, significantly higher flame velocity, and a wide flammability range. Thus, the operation of premix burners on, for instance, hydrogen or hydrogen-rich mixtures as fuel increases the risk of flame flashback and burner overheating, which need to be accounted for. The same is true for other fuels yielding, generally spoken, higher flammability than for instance natural gas. The combustion of hydrogen can yield locally higher flame temperatures when compared to the combustion of natural gas, which might result in an increased formation of nitric oxides. US 6,267,585 suggests the combustion of hydrogen by directly injecting hydrogen into air jets from perforated blades, by which the hydrogen is combusted in micro diffusion flames. The document states that in reducing the perforation matrix size of the perforated blades nitric oxide levels of as low as 10 ppmv (parts per million, volumetric) can be achieved. A burner type sometimes referred to as cluster burner or micro tube cluster burner is known in the art and suggested for the combustion of hydrogen. These generally comprise mixing tubes which are intended to be flown through by air and which extend through a fuel plenum. The mixing tubes are in fluid communication with the fluid plenum, whereby the fuel can be mixed into the combustion air stream through the mixing tubes.

US 2013/0232979 discloses a burner comprising mixing tubes which extend through a fuel plenum. Nozzles extend into the mixing tubes for discharging fuel from the fuel plenum into the mixing tubes. WO 2015/182154 and

US 2010/0218501 disclose further examples of burners of similar structure and function. US 2016/033133 suggests an arrangement of a multitude of individual cluster burner modules side by side, wherein each cluster burner module comprises an individual fuel plenum and is equipped with an individual supply line. US 2015/076251 describes a cluster burner in which the mixing tubes are combined with a fuel cartridge. Furthermore, a cooling air plenum is provided downstream of the fuel plenum. The cooling air plenum discharges the cooling air through the downstream front wall of the burner for effecting effusion and film cooling and is not fluidly connected to the mixing tubes. US 4,100,733 suggests a microtube cluster burner in which radially inner and radially outer micro tubes are arranged to be fed with fuel from distinct fuel plenums. In embodiments, the fuel plenums are stacked along the throughflow direction of the microtubes. A further cluster burner wherein fuel is discharged from fuel plenums into tubes which extend through the plenums is known from US 2013/074510. OUTLINE OF THE SUBJECT MATTER OF THE PRESENT DISCLOSURE

It is an object of the present disclosure to provide a burner as initially mentioned. In aspects, a burner shall be disclosed which avoids the drawbacks of the art outlined above. In more specific aspects, a burner shall be proposed which enables the combustion of hydrogen or hydrogen rich fuel or other highly reactive fuel gases or fuel gas mixtures in a wide load range and with minimized flashback risk and nitric oxide formation. Such gases and gas mixtures are, generally spoken, characterized by at least one of a significantly shorter autoignition time, significantly higher flame velocity, and a significantly broader range of the equivalence ratio in which they are flammable. In another aspect, a burner shall be proposed which allows robust lean premix operation over a wide load range with minimized flame blowoff risk. In still other aspects, the burner shall be suitable for operation on a wide range of fuels. In further aspects, the burner shall enable the use of, in addition to fuel, inert fluids for purposes of, for instance, while not limited to, reducing nitric oxide formation, mitigating potential flashback issues, and other purposes.

In still another aspect, the burner shall be suitable to replace existing burners in legacy combustors or combustion appliances, like for instance, while not limited to, gas turbine combustors. Such upgrading of legacy combustors may enable those combustors to be operated on fuels for which the legacy burners to be replaced were not suitable or inhibited limitations. Such upgrading may also be suitable to enhance fuel flexibility, emissions, operating range and other characteristics of a legacy combustor. For one, non-limiting instance, a legacy combustor may be upgraded for the combustion of hydrogen.

These objectives are achieved by the subject matter set forth in claim 1 and/or the specifics outlined in the dependent claims. Further effects and advantages of the disclosed subject matter, whether explicitly mentioned or not, will become apparent in view of the disclosure provided below.

Accordingly, disclosed is a burner comprising a first, upstream front wall and a second, downstream front wall. A general airflow direction is defined from the first, upstream front wall to the second, downstream front wall. Hence, the terms upstream and downstream used in the context of the herein described burner shall be understood as referring to the general airflow direction from the first, upstream, front wall to the second, downstream front wall, unless defined differently in the specific context. It is noted that generally a skilled person will be able to determine which of the front walls is intended to serve as the downstream front wall. The downstream front wall generally is, implicitly, intended to be provided bordering a combustion space and may thus define in terms of material use, cooling features, coatings and other features characteristic of a downstream front wall of a burner and by which the skilled person will readily distinguish the downstream front wall from the upstream front wall. At least one partition wall extends across the general airflow direction and between the first and second front walls, whereby the at least one partition wall divides a space between the upstream front wall and the downstream front wall into at least two separate fluid plenums stacked along the general airflow direction. The front walls and partition walls may in the following also be referred to as the “transverse walls”. The burner further comprises at least one peripheral wall extending between at least one of: the two front walls, at least two partition walls, and/or a front wall and a partition wall. The peripheral wall may in particular be leak-proof connected to the transverse walls to which it extends along the circumference of the respective transverse wall, thus forming an essentially closed plenum between the respective transverse walls. For the sake of clarity it is noted that within the framework of the present disclosure each space between two of the transverse walls inside the burner shall be understood as a plenum or fluid plenum, irrespective whether the space is further enclosed by a peripheral wall or not. A plenum or fluid plenum may be referred to as a “closed plenum” if further enclosed by a peripheral wall or an “open plenum” otherwise. In particular embodiments, the peripheral wall may extend from the upstream front wall to the downstream front wall and be leak-proof connected to the upstream and downstream front walls along their respective circumference and further in particular to all partition walls along their respective circumference. In another example, the peripheral wall may extend from the upstream front wall to the most downstream partition wall and be leak-proof connected to the upstream front wall and the most downstream partition wall along their respective circumference, and further in particular to all interposed partition walls along their respective circumference. It is understood that the most downstream plenum formed between the most downstream partition wall and the downstream front wall may in particular be intended to be used as a cooling air plenum and may hence be open at the periphery to receive air from the outside. Plenums intended to be used with fuel or other agents different from air, or more generally spoken, different from the oxidizing agent provided to the burner, may in contrast be closed by the peripheral wall and be provided with fluid supply connectors. It will be understood that at least the closed plenums comprise fluid supply connectors and are intended to be connected to supply lines fluidly connected to the plenums. A multitude of passages are provided through the upstream and downstream front walls and the at least one partition wall. These passages are provided by openings in the transverse walls, wherein openings in each transverse wall are aligned so as to form a passage through which another member may be formed or extend. A multitude of ducts are provided and extend through at least some of the passages and thereby through the fluid plenums, wherein the duct walls are leak-proof connected to the first front wall, the second front wall and the interposed partition walls. In particular, the ducts may extend from the upstream front wall to the downstream front wall. Such, the ducts provide fluid connection between an upstream side of the burner adjacent the first front wall and a downstream side of the burner adjacent the second front wall. Each duct has a first, upstream end adjacent the first, upstream front wall and a second, downstream end adjacent the second, downstream front wall. The first, upstream end of each duct opens out to the upstream side of the burner and the second downstream end of each duct opens out to the downstream side of the burner A longitudinal direction is defined between said ends of each duct. At their upstream ends, the ducts may be provided with a smooth inflow geometry, for instance a trumpet-shaped funnelling geometry or otherwise comprise a rounded transition geometry to the upstream front blade, so as to minimize losses of total pressure of the fluid intended to flow through the ducts. The ducts are intended to flow the oxidizing agent, most commonly combustion air, therethrough from the upstream side of the burner to the downstream side of the burner. The burner comprises discharge means for providing fluid communication between at least one of the fluid plenums and the interior of at least one of the ducts, the discharge means being configured for discharging a fluid from a respective fluid plenum into the duct. At least one of the discharge means is provided as at least one through hole through a duct wall and be hereinafter also be referred to as a “wall-opening type discharge means”, and at least one of the discharge means is a discharge nozzle which is suspended in a duct by a tube or by tubes and may hereinafter also be referred to as a “nozzle type discharge means”. The tube or tubes extend from a fluid plenum to the discharge nozzle and provide fluid communication between said fluid plenum and the discharge nozzle. It may be said that the nozzle is suspended inside the duct. Multiple through holes or a group of through holes through the wall of a duct may jointly form one discharge means. The discharge nozzle may for instance be tube-, drop-, tear-, cone- or pyramid-shaped, while not limited to said exemplarily given geometries, and be arranged with a longitudinal axis at least essentially parallel to or coaxial with a longitudinal axis of the duct. The nozzle comprises at least one discharge opening through which fluid originating from a fluid plenum and conveyed to the nozzle may be discharged into the respective duct. A discharge opening may be provided at an axial front face of the nozzle, in particular at a front face pointing towards the downstream end of the duct in which the nozzle is arranged, or on a side wall of the nozzle, in particular in a downstream region of the nozzle or adjacent the downstream front face of the nozzle. At least one of the ducts is provided with at least two discharge means, each discharge means fluidly connecting a fluid plenum out of the at least two fluid plenums to the interior of the duct. The at least two discharge means are provided to discharge a fluid from inside the respective fluid plenum at different positions along a longitudinal direction of the duct. Said discharge positions may in particular be referenced to and measured from the downstream end of the respective duct, and to a downstream edge or opening forming the discharge means. Said different longitudinal discharge positions for one and the same duct may differ from each other by for instance at least 2%, at least 5% or at least 10%, or even at least 20% of the length of the respective duct. As will be set forth below, it might be the case that two discharge means out of the at least two discharge means connect to one and the same plenum or to different plenums.

By virtue of the subject matter set forth above it is possible to discharge multiple fuel and non-fuel fluids into the combustion air flow through a duct of a micro-tube burner and discharge fluids into one and the same duct at different longitudinal positions of the duct, thus achieving specific and different mixing of the fluid and the combustion air at the downstream end of the duct. This allows superior versatility when operating the burner.

The outer geometry of the ducts corresponds to that of the passages. The inner cross-section of the ducts is in embodiments rounded and more in particular circular. In other embodiments, however, an inner cross-section of a duct may be oval, elliptical, or polygon-shaped, wherein in more particular examples the inner cross-section of a duct may have the shape of an equilateral polygon. It is understood that the ducts of the burner of the herein disclosed type are referred to a “microtubes” in the art, and hence imply to have comparatively small cross- sectional dimensions. A cross-sectional area of a single duct may be 2000 mm ² or less, 1500 mm ² or less, 1000 mm ² or less, 500 mm ² or less, 300 mm ² or less, 100 mm ² or less, or 64 mm ² or less. In other aspects, the hydraulic diameter, defined as four times the cross-sectional area divided by the length of the inner circumference of a duct, may be 50 mm or less, 40 mm or less, 35 mm or less, 25 mm or less, 20 mm or less, 10 mm or less, or 8 mm or less.

The skilled person will appreciate that the more upstream inside a duct, i.e. the further from the respective downstream end of the duct, a fluid is discharged into the duct the more intensely it is premixed with the combustion air, or other oxidizer, flowing through the duct, when it exits from the duct at the downstream end thereof. Accordingly, a fuel discharged at a relatively downstream position of a duct may be combusted in a relatively stronger diffusion combustion characteristic, while a fuel discharged at a relatively upstream position of a duct may be combusted in a relatively stronger premix combustion mode. In other words, the combustion of a fuel discharged at a relatively downstream position of a duct will generally spoken be more robust, i.e., less susceptible to flame extinction, when compared to the combustion of a fuel discharged at a relatively upstream position of a duct, while the latter generally spoken produces less nitrogen oxides.

The term “longitudinal direction”, as herein used, is not to be understood as vectored, but shall generally be understood as the orientation of a longitudinal extent in space, and may be equivalent to an axial direction, for instance, of a duct. However, a duct may be curved, whereby it has, strictly spoken, no axis, which might implicitly be understood as being straight, but has a curved longitudinal direction. An axis of a duct or passage may in particular extend parallel to the longitudinal direction of the duct or passage. A “longitudinal position” shall denote the position along said longitudinal direction measured from a specific reference position, such as a downstream end of a duct or passage.

It is noted that within the framework of the present disclosure the use of the indefinite article “a” or “an” does in no way stipulate a singularity nor does it exclude the presence of a multitude of the named member or feature. It is thus to be read in the sense of “at least one” or “one or a multitude of’. It is moreover noted that in the context of the present disclosure the terms “neighbouring” and “adjacent” are considered as synonyms.

In some embodiments, a cartridge may be provided in or may extend through at least one of the passages. At least one cartridge may in more particular embodiments be provided for providing a liquid agent like liquid fuel or water into a combustion space therethrough, and may more particularly comprise an atomizer, in particular at a distal end of the cartridge which points towards the combustion space. The cartridge may also be intended for providing steam, piloting fuel gas, or other gaseous agents therethrough. The cartridge may be a separate member and may be retractable from the burner. The cartridge may be provided directly through the passages, whereby an outer circumference of the cartridge seals with the transverse walls of the burner. In other embodiments, the cartridge may be inserted into a duct which in turn extends through a passage. It is appreciated that in said embodiment the duct through which the cartridge extends may be provided without discharge means for providing fluid communication with a fluid plenum. However, in other embodiments, such discharge means may be provided such that fluid from a fluid plenum may be discharged downstream a tip of the cartridge or in an annular space provided between the outer wall of the cartridge and the inner wall of the duct.

In particular, each of the passages may be provided with either a duct or a cartridge therein or therethrough.

The outer surfaces of the ducts or cartridges extending through the passages may seal gas-proof with the transverse walls through which they extend.

As will be appreciated and noted above, if a fluid is discharged into a duct at a relatively upstream position, it has a longer path to mix with, for instance, combustion air flowing through the duct before it is discharged into a combustion space at the downstream end of the duct when compared to a fluid which is discharged into the duct at a relatively downstream position. Thus, if for instance a gaseous fuel is discharged more upstream it may be considered as a premix fuel, while the fuel discharged more downstream is combusted in a relatively diffusion flame characteristic, hence providing additional robustness to the combustion of the premix fuel, in particular at low premix equivalence ratios. On the downside, the combustion of less intensely premixed piloting fuel will result in comparatively higher nitrogen oxide formation. However, one advantage of a micro-tube burner of the kind herein disclosed is the relatively small dimension of the individual flame emanating from an individual duct, and hence the relatively low residence time of the species at the elevated temperature level fostering nitrogen oxide formation. Discharge of fuel at a relatively upstream location and discharge of fuel at a relatively downstream location may take place simultaneously, and in one or in different ducts. The combustion of the less premixed fuel supports combustion of the more premixed fuel.

In non-limiting embodiments, at least one duct out of the multitude of ducts is in fluid communication with at least two fluid plenums, wherein at least one first discharge means is provided for providing fluid communication with a first one of said at least two fluid plenums and at least one second discharge means is provided for providing fluid communication with a second one of said at least two fluid plenums. This allows fluids to be selectively discharged into the duct from either of the thereto connected plenums. It will thus be found advantageous if supply lines to the plenums are equipped with individual control valves which allow controlling the supply of fluid to the plenums individually.

It may further be provided that at least one duct out of the multitude of ducts is configured with a first discharge means which is provided as a wall-opening type discharge means and further with a second discharge means which is nozzle type discharge means. This provides enhanced flexibility in discharging fluid into a duct. In particular, it will be appreciated that a fluid discharged into a duct through a nozzle-type discharge means may generally be discharged in or close to the centre of the duct, while fluid discharged through a wall-opening type discharge means is discharged into a boundary layer along the duct wall, and, dependent upon the momentum of the discharged fluid, may or may not penetrate the boundary layer. It will further be appreciated that if a fluid is discharged into a duct through a nozzle type discharge means the actual discharge position may be shifted along the general airflow direction, or the longitudinal direction of the duct, respectively, while in the case of a wall-opening type discharge means the discharge position is coupled to the position of the respective fluid plenum. In order to take advantage of this flexibility it might thus be provided that the first discharge means is in fluid communication with a first fluid plenum and the second discharge means is in fluid communication with a second fluid plenum, wherein the first fluid plenum is arranged downstream from the second fluid plenum along the general airflow direction. A discharge position of the second, nozzle type, discharge means providing fluid communication between the duct and the second, relatively upstream, fluid plenum may be positioned downstream the discharge position of the first discharge means. Thus, fluid from a relatively upstream fluid plenum may be discharged into the duct downstream from fluid originating from a relatively downstream plenum. In more specific embodiments, the discharge position of the second, nozzle type, discharge means may be at maximum 2 times the minimum hydraulic diameter of the duct downstream the discharge position of the first discharge means. In other specific and non-limiting embodiments, it may be provided that the discharge position of the second, nozzle type, discharge means is upstream of the discharge position of the first discharge means by at maximum 5 times the minimum hydraulic diameter of the duct and downstream the discharge position of the first discharge means by at maximum 2 times the minimum hydraulic diameter of the duct. The definition of the hydraulic diameter is outlined above. It will be appreciated that the duct may be provided with narrowing and widening cross-section passages along its longitudinal extent. Thus, the hydraulic diameter of the duct may vary along its longitudinal extent. Reference is made to a minimum hydraulic diameter of the duct. In other embodiments the discharge position of any discharge nozzle is positioned at a longitudinal position of the duct inside which the discharge nozzle is arranged which corresponds to at maximum 20 minimum hydraulic diameters of the respective duct when measured from a downstream end of the duct and along the longitudinal direction of the duct.

In non-limiting exemplary embodiments, at least two discharge means of one duct may fluidly connect to one and the same fluid plenum and are provided to discharge the fluid from the fluid plenum at two different positions along a longitudinal direction of the respective duct. The one duct may then discharge a fraction of relatively more intense premixed fuel together with a fraction of relatively less premixed fuel into a combustion space, which may serve to support combustion of the relatively more intense premixed fuel. It is noted that fluid from a particular plenum may likewise be discharged into ducts at more than two longitudinal positions inside the respective duct, which will yield more or less pronounced premix of diffusion flame characteristic of a resulting flame emanating from the respective duct. In non-limiting embodiments, a first duct may be provided with a first discharge means fluidly connecting the first duct to a first plenum and a second duct is provided with a second discharge means fluidly connecting the second duct to the first fluid plenum, wherein the first discharge means is arranged to discharge the fluid from the first fluid plenum into the first duct at a first longitudinal position of the first duct when measured from the downstream end of the first duct and the second discharge means is arranged to discharge the fluid from the first fluid plenum into the second duct at a second longitudinal position of the second duct when measured from the downstream end of the second duct. Further the second duct, in this non-limiting embodiment, is free from a discharge means fluidly connecting to the first fluid plenum. Hence, certain ducts may be operated as ducts with different premix characteristics of the fuel discharged from the first fluid plenum at the downstream end of the ducts. In addition, the first duct may be free from a discharge means fluidly connecting to the first fluid plenum and arranged to discharge the fluid from the first fluid plenum into the first duct at the second longitudinal position of the first duct when measured from the downstream end of the first duct. The more intensely premixed fuel and the less premixed fuel, in these embodiments, are provided from one and the same plenum, and the mass flow ratio of the more intensely premixed fuel and the less premixed fuel thus is fixed.

In still further embodiments, a first duct is provided with a first discharge means fluidly connecting the first duct to a first fluid plenum and arranged and configured for discharging into the first duct at a first position when measured from the downstream end of the first duct and a second duct is provided with a second discharge means fluidly connecting the second duct to a second fluid plenum and arranged and configured for discharging into the second duct at a second position when measured from the downstream end of the second duct. The first duct may be fluidly isolated from the second fluid plenum and the second duct may be fluidly isolated from the first fluid plenum. In yet other aspects, a first duct is provided with a first discharge means fluidly connecting the first duct to a first fluid plenum and arranged and configured for discharging into the first duct at a first position when measured from the downstream end of the first duct and the first duct is further provided with a second discharge means fluidly connecting the first duct to a second fluid plenum and arranged and configured for discharging into the first duct at a second position when measured from the downstream end of the first duct. The second position is in both cases different from the first position, wherein the positions may be expressed in absolute dimensions or in multiples of a minimum hydraulic diameter of the respective duct. The mass flow of fluid discharged from the plenums at the different first and second positions can be controlled independently from each other in providing independent control valves in the supply lines to the plenums. Thus, the mass flow ratio of the more intensely premixed fuel and the less premixed fuel can be controlled. While it is presumed to be self-evident, it is noted that generally a duct may be provided with discharge means for discharging fluid into the duct at more than two longitudinal positions of the duct.

The concept of axial staging of fuel discharge into a duct - i.e., the further upstream in a duct the fuel is discharged into the duct the more intensely mixed the fuel will be with combustion air upon exiting the duct, yielding a more premix type combustion and related low nitrogen oxides emissions, vs. fuel being discharged into the duct further downstream yielding a more diffusion type combustion and related higher nitrogen oxides generation, which on the other hand is less susceptible to interference - has been outlined in some detail above. In embodiments, the herein proposed burner comprises a duct which is dedicatedly configured as a piloting duct and is in fluid communication with a fluid plenum different from the most downstream fluid plenum through discharge means configured to discharge into the piloting duct at a longitudinal distance, when measured from the downstream end of the duct and along a longitudinal extent of the duct, corresponding to at maximum five times the minimum hydraulic diameter of the duct. Hence, the fuel may be virtually unmixed with the combustion air upon exiting from the duct and is combusted in a diffusion flame. In more specific exemplary embodiments, a multitude of piloting ducts are provided and each piloting duct is arranged in the centre of adjacent, i.e. immediately neighbouring, non-piloting ducts. Non piloting ducts, in this respect, are ducts which are in fluid communication with a fluid plenum different from the most downstream fluid plenum through discharge means configured to discharge into the non-piloting duct at a longitudinal distance, when measured from the downstream end of the respective duct and along a longitudinal axis of the respective duct, corresponding to more than five times the minimum hydraulic diameter of the respective duct, in particular six or more times, seven or more times, or ten or more times the . minimum hydraulic diameter of the respective duct. The non-piloting ducts may in particular be arranged on a concentric hexagon around the piloting duct. In more particular embodiments, Qn non-piloting ducts may be arranged adjacent the piloting duct, wherein n is a natural number.

Each three neighbouring passages may be arranged on the comers of an equilateral triangle. On an overall scale, this arrangement results in the passages being provided on an equidistant pattern, wherein each two neighbouring passages have the same distance from each other, and the mutual influence of the flows and flames emanating from them can easily be transposed from one pair of neighbouring passages to another pair of neighbouring passages. In more particular embodiments, this may result in an arrangement wherein a burner comprises a central passage which is encircled by a number of hexagonal rings in which further passages are arranged. Generally, on the n ^th hexagonal ring encircling a central passage, when counted from the central passage, 6n passages are provided. Apart from the passages of the outer ring, each passage is encircled by six neighbouring passages on a hexagonal ring. For instance, one duct provided as a dedicated piloting duct for a specific fuel may be encircled by six neighbouring ducts provided as dedicated premix ducts for said specific fuel. Likewise, a nozzle or cartridge may be arranged in or through a passage which is encircled by six other passages. In this embodiment, the nozzle or cartridge may comprise an atomizer and may be intended for supplying liquid fuel therethrough, while, on the liquid fuel operation, the six neighbouring ducts may be intended for providing the combustion air when the burner is operated on liquid fuel. Other arrangements not explicitly mentioned are readily conceived by a skilled person. The self-similar arrangement of passages, or ducts and/or cartridges, respectively, facilitates scalability of the burner. Moreover, providing the burner as an overall hexagon facilitates replacing existing burners of a legacy combustor with a number of burners of the type herein disclosed arranged beside each other.

In aspects, all ducts of the burner are parallel to each other. More in particular, the ducts may be parallel to a burner axis, wherein said burner axis is defined as a virtual axis perpendicular to the outer surface of the downstream front wall, or downstream face of the burner, which is intended to face a combustion space arranged downstream the burner. It may in other aspects be provided that at least a subset of the multitude of passages are arranged in at least one concentric hexagonal ring around a midpoint. Said concentric hexagonal ring may be provided as an equilateral hexagon, comprising in particular 6n passages, wherein n is a natural number. The passages arranged in the at least one concentric hexagonal ring are oriented such that fluid discharged at the downstream side of the burner from the passages in the concentric hexagonal ring has a velocity component which is tangential to a circle defined around the midpoint of the hexagonal ring. For instance, if a downstream face of the burner, which is the outer face of the downstream wall not facing a plenum, is a plain surface the passages, and accordingly any duct or cartridge provided therethrough, are accordingly inclined with respect to a normal to the downstream face. In particular, all passages in a given hexagonal ring may be inclined at an identical angle. As the skilled person will readily appreciate, said configuration is suited to generate a macroscopic vortex downstream the burner.

It is noted that the expression “subset” of passages or ducts, as used within the framework of this document, may as a special case also mean all passages or ducts of the burner, if, in a specific embodiment, the attributes conjugated with the subset apply to all passages or ducts.

In a discharge means, to the extent it comprises at least two discharge openings, the discharge openings may be provided such that the velocity vectors of all streams of fluid to be emanated from the discharge openings meet at or virtually originate from a common point in a view on a cross-section of a duct. For instance, if the duct inner cross-section is circular or elliptical or has the shape of an equilateral polygon, they may meet on or virtually originate from a centre of the duct cross-section. Likewise, a single discharge opening of a discharge means may be provided such that the velocity vector of a stream of fluid emanating from said opening is directed towards or virtually originates from a duct centre. It will be appreciated that geometric relations with respect to the duct in this context strictly relate to the open inner cross-section of the duct. In other embodiments, however, at least one discharge opening of a discharge means may be provided such the fluid is discharged with a tangential velocity component relative to the duct crosssection.

Further, a discharge opening of a discharge means may be provided so as to discharge fluid in a direction perpendicular to the longitudinal direction of the duct. It may also be inclined so as to discharge the fluid with a velocity component into the upstream or downstream direction of the duct.

In embodiments, vortex generators and/or blades may be arranged inside a duct. Vortexes in the flow of combustion air through a duct may serve to intensify mixing of fuel and other fluids discharged into the duct through discharge means of the duct with the flow of combustion air. A number of blades may be arranged along the circumference of the duct to generate a swirl of the combustion air inside the duct. In embodiments, the vortex generators and/or row of blades are arranged in an upstream section of a duct, for instance in the upstream 30% of the longitudinal extent of the duct, in the upstream 20% thereof, or in the upstream 10% thereof. The vortex generators and/or row of blades may be arranged upstream of any discharge means inside the duct. However, they may also be arranged differently with respect to the discharge means, for example between discharge means in the intended direction of combustion air flow or downstream thereof, according to the needs. It may moreover be the case that vortex generators and/or rows of blades may be provided at more than one longitudinal position of a duct and/or longitudinal position relative to discharge means.

At least three fluid plenums may be stacked along the general airflow direction. Plenums may be provided for the supply of different fluids, e.g., different fuels. For a specific fuel, two or more plenums may be provided so as to control the supply of said specific fuel to different ducts or the supply of said specific fuel at different longitudinal positions of the ducts for controlling, for instance, piloting. Further plenums may be added for supplying inert fluids like steam or nitrogen, which might be useful for flashback control, or for supplying, through internal atomizing nozzles inside on or more ducts, water or liquid fuel or another liquid agent into the flow through said one or more ducts. The herein proposed burner thus provides superior versatility for multi-fuel operation capability and the possibility to add other fluids, in that simply another plenum needs to be added to the stack of plenums.

The most downstream fluid plenum may be intended to be used as a coolant plenum. The most downstream fluid plenum may be provided with at least one of a fluid connection into at least some of the ducts, so as to discharge fluid from the most downstream fluid plenum into said at least some of the ducts, and/or with front wall through holes extending through the second, downstream front wall, different from the passages through the downstream front wall, so as to discharge fluid from the most downstream fluid plenum into an area downstream of the burner and to cool the downstream front plate by effusion cooling. The most downstream plenum may in particular be in fluid communication with at least one or with a fraction of the ducts or with all ducts through wall-opening type discharge means. The discharge means providing fluid communication between the most downstream plenum and a duct may be configured so as to discharge fluid from the most downstream plenum into the duct with a downstream velocity component tangential to the inner surface of the duct wall. The coolant then forms an inert boundary layer at the downstream end of the duct which helps to avoid flashback into the duct. It is noted that the most downstream fluid plenum may in embodiments be supplied with other coolant than air, such as for instance, while not limited to, steam.

Further, it may be provided, in non-limiting exemplary embodiments, that the second fluid plenum, when counted from the downstream side of the burner, is in fluid communication with the fluid surrounding the burner on its lateral sides. To this extent, in one aspect, no peripheral wall may be provided enclosing the second fluid plenum counted from the downstream side of the burner, such that said plenum is an open plenum. In another aspect, said plenum may be enclosed by a perforated peripheral wall extending between the transverse walls adjacent said plenum. Thus, when the burner is installed in a combustion appliance, said second fluid plenum counted from the downstream side of the burner is in fluid communication with a burner hood chamber inside which combustion and cooling air is provided. The most downstream fluid plenum then may be in fluid communication with said second fluid plenum when counted from the downstream side of the burner through openings in the partition wall delimiting the most downstream fluid plenum from the second fluid plenum when counted from the downstream side of the burner. Thus, for instance, impingement cooling of the downstream wall of the burner, which faces the combustion chamber, may be achieved. The most downstream fluid plenum, in this embodiment, may further be in fluid communication with the downstream side of the burner through the downstream front wall, in particular through openings provided therethrough, to provide further cooling of the downstream wall of the burner. The used cooling fluid is thus discharged into the combustion chamber. The most downstream fluid plenum may alternatively or in addition be configured to discharge the cooling air into the downstream part of at least one duct or at least some ducts.

In embodiments, at least a subset of the multitude of passages are arranged as outer passages on a concentric hexagonal ring around and adjacent at least one inner passage, wherein at least one inner duct is provided in the at least one inner passage and at least one outer duct is provided in the outer passages. In embodiments, the at least one inner passage might be one single central passage. In other embodiments, the at least one inner passage is arranged on an inner hexagon which is surrounded by an outer hexagon on which the outer passages are arranged. The at least one inner duct has a most downstream discharge position at which a discharge means in fluid communication with a fluid plenum different from the most downstream fluid plenum is configured to discharge into the at least one inner duct which is positioned at a first longitudinal distance from the downstream end of the at least one inner duct. The at least one outer duct has a most downstream discharge position at which a discharge means in fluid communication with a fluid plenum different from the most downstream fluid plenum is configured to discharge into the at least one outer duct which is positioned at a second longitudinal distance from the downstream end of the at least one outer duct. The second longitudinal distance is larger than the first longitudinal distance. Hence, a more intensely premixed fuel-air mixture exits from the at least one outer duct compared to the fuel-air mixture which exits from the at least one inner duct.

Each cross-section taken along the longitudinal extent of each duct, and more in particular perpendicular to the axis of the duct, out of the multitude of ducts may have one of a circular or polygonal shape.

If a fluid plenum is provided as a closed fluid plenum as defined above, supply connectors must be provided in fluid connection with the closed fluid plenum. In embodiments of the burner, the burner thus comprises supply connectors configured for fluid supply to at least some of the fluid plenums. In more specific embodiments, at least two of the fluid plenums are fluidly connected to a respective individual supply connector, wherein at least two supply connectors of different fluid plenums are arranged concentrically and coaxially.

The burner may be integrally formed by additive manufacturing. That means, the transverse walls, peripheral walls, duct walls, discharge means and, as the case may be, further elements like the above-mentioned vortex generators and blades, are provided as a seamless, monolithic one-piece member. The entire burner may be an additively manufactured seamless, monolithic one-piece member. In other embodiments, the burner may be assembled from layers, wherein each layer comprises at least two transverse walls and at least one plenum between the transverse walls. The burner may be assembled from seamless, monolithic one- piece members, each member comprising at least one plenum enclosed by two transverse walls and, optionally, by a lateral wall, stacked upon each other.

At least one of the through holes arranged in the wall of a duct out of the multitude of ducts for providing fluid communication with a fluid plenum may have an el I iptical ly shaped cross-section, wherein in particular the long axis of the ellipse includes an angle of at maximum 30 degrees with one of a longitudinal axis of the duct, the general airflow direction or a burner axis, and wherein further in particular the ratio of the length of the longer ellipse axis to the length of the short ellipse axis is 1 .25 or more. In the case that a duct is curved, the above-specified angle shall be measured against a local axis at the location of the through hole. A burner axis may be defined as an axis perpendicular to at least one of the upstream wall or a downstream wall. It may be found advantageous if the long axis of the ellipse is oriented at least approximately along a building direction of the burner during additive manufacturing of the burner. In applying such a geometry overhanging structures in the boundary of the through hole are reduced, which considerably facilitates applying an additive manufacturing process. The building direction might be chosen along the duct axis or the general air flow direction of the burner.

At least one of the through holes arranged in the wall of a duct out of the multitude of ducts for providing fluid communication with a fluid plenum may have a polygonal shaped cross-section having a polygonal shaped boundary, the polygonal shaped boundary comprising straight boundary segments, wherein said polygonal shaped boundary comprises an upstream boundary section and a downstream boundary section, wherein at least one of the upstream boundary section and the downstream boundary section is shaped such that an angle included between each straight segment of the respective boundary section and one of a longitudinal axis of the duct, the general airflow direction or a burner axis is smaller than or equal to 45 degrees. The statements as to the longitudinal axis of the duct and the burner axis made above apply. Also this shape of duct wall through holes reduces overhanging structures in the boundary of the through hole, which considerably facilitates applying an additive manufacturing process.

In aspects of the present disclosure, the burner may comprise at least some ducts differing form each other as to the minimum hydraulic diameter of the ducts.

In still further aspects, at least a subset of ducts out of the multitude of ducts are provided with a nozzle type discharge means inside the respective duct, whereby an outer boundary of a respective discharge nozzle provided inside a duct defines a closest residual flow cross-section between the discharge nozzle and the inner wall of the duct. At least two ducts out of the subset of ducts may be provided with different residual flow cross-sections. Generally spoken, a residual flow crosssection between a nozzle and an inner duct wall will represent a narrowest flow cross-section inside the duct, i.e. a metering section. Thus, the mass flow or volume flow through individual ducts of said subset of ducts may be adjusted by the choice of this residual flow cross-section. Said purposeful choice residual flow cross-sections may be achieved by a choice of the interacting geometries of the nozzle and the inner wall of the duct, i.e. by choice of size and/or shape.

It may in embodiments be provided that at least one duct out of the multitude of ducts comprises at least one tapering cross-section longitudinal portion, wherein within a tapering cross-section longitudinal portion the cross-sectional area of the at least one duct tapers downstream the general airflow direction, or in a direction from an upstream end of the duct towards the downstream end of the duct, respectively, from a first cross-sectional area to a second cross-sectional area smaller than the first cross-sectional area. A discharge nozzle is provided within said at least one duct with a downstream end of said discharge nozzle, relative to the intended flow direction of the duct and/or the general airflow direction, is positioned within a tapering cross-section longitudinal portion. Provided that the discharge nozzle is configured to discharge at the downstream end, or adjacent the downstream end, fluid from the discharge nozzle is discharged into an accelerated flow. Said tapering cross-section longitudinal portion, in which the downstream end of said discharge nozzle is provided, may be configured such that the hydraulic diameter of the duct at the position where the duct has the first cross-sectional area is 1 .12 times or more and 2.5 times or less the hydraulic diameter of the duct at the position where the duct has the second cross-sectional area.

For an even more specific instant, the downstream end of nozzles may be provided at different positions within the tapering cross-section longitudinal portions of different ducts, thus yielding different residual flow cross-sections and hence different mass or volume flows through the different ducts.

It may be provided, in non-limiting embodiments, that out of two fluid plenums arranged upstream of the most downstream fluid plenum the one which is arranged further upstream is fluidly connected through discharge means to a larger number of ducts than the one arranged more downstream. It may be intended to supply both of these fluid plenums with the same fuel.

It should be mentioned that in principle a plenum, as defined between two adjacent transverse walls, may be subdivided into sub-plenums in a direction across the burner axis, or over the cross-section of the burner, respectively. Such, the supply of fluids to the ducts may not only be controlled along the general airflow direction or burner axis, but also about the cross-section of the burner.

In other aspects, inserts may be provided inside at least one of the plenums, so as to mitigate eventual non-uniform ity of the volume flow to the various discharge means with which the plenum is in fluid communication.

Further disclosed is a combustor comprising a combustion space and further comprising at least any embodiments of a burner as set forth above, wherein the second, downstream, front wall of the burner faces the combustion space and the most downstream of the fluid plenums adjacent the second, downstream front wall is provided as a coolant plenum. The coolant plenum may be connectable, or connected, to a source of cooling air. In other aspects, the coolant plenum may be connectable, or connected, to a source of an alternative coolant like for instance, while not limited to, steam. Connectable, in this context, means that a connection line or feed line for connecting is present, but may be equipped with a stop and/or control device, so that fluid communication is not necessarily always present.

At least one fluid plenum may be fluidly connectable to a source of combustible gas. At least one fluid plenum may be connectable to a source of fuel containing at least 50% by volume of hydrogen. At least one fuel plenum may be connectable to a source of an inert fluid. In particular, if one plenum is connectable to a source of hydrogen-rich fuel or other highly reactive fuel, when compared to natural gas, it may be found useful if a plenum downstream therefrom is connectable to a source of an inert fluid in order to mitigate flashback risk.

At least plenums which are connectable to sources of combustible fluid may advantageously also be provided connectable to a purging fluid source, such as air or inert fluid, so as to avoid flashback into the plenum when not pressurized.

Further disclosed is a gas turbine engine comprising a combustor of the kind set forth above.

It is understood that the features and embodiments disclosed above may be combined with each other. It will further be appreciated that further embodiments are conceivable within the scope of the present disclosure and the claimed subject matter which are obvious and apparent to the skilled person by virtue of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present disclosure is now to be explained in more detail by means of selected exemplary embodiments shown in the accompanying drawings. The figures show

Fig. 1 shows an embodiment of the herein disclosed burner in a sectional view;

Fig. 2 illustrates a nozzle-type discharge means;

Fig. 3 shows a second embodiment of the herein disclosed burner in a sectional view;

Fig. 4 shows a third embodiment of the herein disclosed burner in a sectional view;

Fig. 5 shows an embodiment of the herein disclosed burner with a cartridge inserted through one passage in a sectional view;

Fig. 6 shows an embodiment of the herein disclosed burner, wherein ducts have different minimum cross-sectional areas, in a sectional view;

Fig. 7 shows an embodiment of the herein disclosed burner, wherein nozzle-type discharge means extend differently far into tapering sections of the ducts, in a sectional view;

Fig. 8 shows an embodiment of the herein disclosed burner, wherein two plenums are connected to coaxial and concentric supply connectors, in a sectional view;

Fig. 9 illustrates the possible streamwise orientation of discharge means;

Fig. 10 illustrates a duct of a burner in a cross-sectional view, wherein discharge means are arranged to discharge the fluid from a plenum with a tangential velocity component;

Fig. 11 outlines the arrangement of blades at the inlet of a duct to induce a vortex flow of combustion air inside the duct; Fig.12 illustrates the possible relative longitudinal discharge positions of a nozzle-type discharge means and a wall-opening type discharge means;

Fig. 13 outlines the hexagonal arrangement of passages and ducts and/or fuel nozzles of cartridges in embodiments of the herein disclosed burner;

Fig. 14 illustrates possible arrangements of piloting ducts, liquid fuel combustion nozzles or cartridges, steam or water injection nozzles or cartridges and the like with respect to a generally hexagonal passage arrangement;

Fig. 15 illustrates tilted passages for inducing a macroscopic vortex downstream the burner;

Fig. 16 shows an embodiment of the herein disclosed burner with coolant discharge into the ducts in a sectional view;

Fig. 17 illustrates an exemplary elliptical wall opening in a duct wall;

Fig. 18 illustrates an exemplary polygonal wall opening in a duct wall; and

Fig. 19 outlines geometric details of the polygonal wall opening of fig. 18.

It is understood that the drawings are highly schematic, and details not required for instruction purposes may have been omitted for the ease of understanding and depiction. It is further understood that the drawings show only selected, illustrative embodiments, and embodiments not shown may still be well within the scope of the herein disclosed and/or claimed subject matter.

EXEMPLARY MODES OF CARRYING OUT THE TEACHING OF THE PRESENT DISCLOSURE

Figure 1 shows an exemplary embodiment of a burner 1 . The burner may be particularly suitable for the combustion of hydrogen or hydrogen rich fuels and other, compared to natural gas, highly reactive fuels, while not being limited to this application. Burner 1 comprises upstream front wall 11 and downstream front wall 12. On an upstream side 2 of burner 1 a combustion air plenum may be located. On the downstream side 3 the combustion space is intended to be arranged. The combustion air plenum may for a non-limiting instance be provided to be supplied with compressed air from the compressor of a gas turbine engine, while the combustion space may be arranged to discharge into the expansion turbine of a gas turbine engine. Further, as noted above, “air” is to be understood as being representative of any oxidation agent, or fluid comprising an oxidation agent, which is suitable for the combustion of a fuel, and said oxidation agent or fluid comprising an oxidation agent is generically disclosed to a skilled person, although for the ease of description air is used as a common example representative of generic oxidation agents or suitable fluids containing oxidation agents. A general airflow direction is defined from the upstream wall 11 to downstream wall 12. Partition walls 21 , 22 and 23 are interposed between upstream front wall 11 and downstream front wall 12 and extend across the general airflow direction. Front walls 11 and 12 and partition walls 21 , 22 and 23 may generically be referred to as transverse walls, as they extend transverse to or across the general airflow direction of the burner. A space between upstream front wall 11 and downstream front wall 12 is divided into four plenums by the partition walls, namely most upstream plenum 31 between upstream front wall 11 and partition wall 21 , plenum 32 between partition walls 21 and 22, plenum 35 between partition walls 22 and 23 and most downstream plenum 33 between partition wall 23 and downstream front wall 12. Lateral wall 28 further encloses plenums 31 and 32, while lateral wall 29 encloses most downstream plenum 33. Plenum 35, in contrast, is not enclosed by a lateral wall and is thus in direct fluid communication with fluid provided on the sides of burner 1 , which in most common, while not limiting, cases is identical with the fluid on the upstream side 2 of burner 1 . Plenums 31 , 32 and 33 may thus be referred to as closed plenums, which require a supply line to be supplied with a fluid, while plenum 35 is referred to as an open plenum which may be supplied with fluid through open lateral sides. Plenum 35 might be in direct fluid communication with a combustion air plenum inside which the burner is installed. Most downstream plenum 33 is preferably used as a coolant plenum in order to cool downstream front wall 12 which is exposed to heat from a combustion space at the downstream side 3 of burner 1 . Plenum 33 is fed form a coolant supply through coolant feed connectors 331 , whereby generally an air flow bled from the combustion air may be used for cooling purposes. Also steam or any other suitable cooling fluid may be applied. It may be provided that coolant plenum 33 is not enclosed by a lateral wall and is thus configured as an open plenum and may be placed inside a larger air plenum of the combustor, such that cooling air may flow into cooling air plenum 33 form the entire circumference thereof. This may also be achieved in that lateral wall 29 of plenum 33 is perforated. Plenum 31 may be supplied with a fluid through supply connector 311 which joins into plenum 31 from the upstream side of the burner. Plenum 32 may be supplied through a fluid supply connector 321 which extends form the upstream side of the burner and through plenum 31 . As will be appreciated by virtue of figure 1 , transverse walls 11 , 21 , 22, 23 and 12 are provided with multiple sets of aligned openings, such that passages 40, of which only some are designated by reference numbers, are formed and extend through burner 1 from the upstream side 2 to the downstream side 3. Ducts 41 , 42 and 45 extend through passages 40, through transverse walls 11 , 21 , 22, 23 and 12 and through the plenums 31 , 32, 35 and 33. The duct walls are provided gas-leak proof with the transverse walls. It should be noted that in embodiments the ducts are seamlessly joined with the transverse walls, in that the duct walls and the transverse walls are manufactured in one monolithic piece. This may be achieved by additive manufacturing, while any suitable manufacturing method may be applied to manufacture burner 1 . Ducts 41 , 42 and 45 provide fluid communication from the upstream side 2 to the downstream side 3 of the burner along their longitudinal directions. In operation, combustion air flows through ducts 41 , 42 and 45 from the combustion air plenum of a combustion appliance, or the upstream side 2 of burner 1 , to the combustion space, or the downstream side 3 of burner 1 . Through holes 51 , only some of which are designated by reference numbers, are provided through the duct walls and provide fluid communication between plenum 31 and ducts 41 and 45. Through holes 51 are discharge means for discharging a fluid from plenum 31 into ducts 41 and 45. Through holes 52 are provided through duct walls of ducts 45 and provide fluid communication between plenum 32 and ducts 45. In addition, duct 42 if fluidly isolated from plenum 31 , and is fluidly connected to plenum 32 by a discharge means which is a nozzle 62 suspended inside duct 42. It is noted that the herein proposed burner is not limited to just one single duct of the type of duct 42, which is fluidly connected to only one plenum through a nozzle-type discharge means 62.

A nozzle type discharge means comprising a nozzle 62 suspended inside a duct 42 is shown in figure 2. Nozzle 62 suspended inside duct 42 by tubes 132, which provide a mechanical connection between the nozzle and the wall of the duct. Tubes 132 provide fluid communication between nozzle 62 and plenum 32. While two suspension tubes 132 are shown, or visible, respectively, in the exemplary embodiment, it is understood that more suspension tubes could be arranged, while in other embodiments only a single suspension tube may be provided. The suspension tube or suspension tubes may also be supplemented by suspension struts, which provide mechanical fixation to the duct wall, but otherwise do not serve to supply fluid to the nozzle. The nozzle, in the shown embodiment, is arranged coaxial with axis 451 of duct 42 and provided centrally within the duct. The nozzle is provided with an opening at a front face of the nozzle which is downstream with respect to the flow of combustion air 4 through duct 42. Fluid from plenum 32 may thus be injected into the flow of combustion air 4 at the downstream end of nozzle 62. A half angle a of injection may be 30 degrees or less. In embodiments, however, discharge from the nozzle may be effected through differently arranged discharge openings. In still further embodiments nozzle 62 may be equipped with a liquid agent atomizer.

It will be appreciated that the injection of a fluid into a duct through a nozzle 62 yields different characteristics than the discharge through openings in the duct wall. For instance, nozzle 62 discharges a fluid into a core flow through duct 42 rather than into a boundary layer at the duct wall, as is the case with discharge openings in the duct wall. In another aspect, the penetration of fluid discharged into a duct through a wall opening may strongly vary depending on the differential pressure between the respective duct and fluid plenum, and thus depend on the mass flow or volume flow of discharged fluid. The fluid discharged from wall openings 51 and 52 in figure 1 may penetrate differently deep into the axial flow through a duct dependent upon the mass flow or volume flow of discharged fluid. As will be appreciated, this is not the case if a fluid is discharged from plenum 32 through nozzle 62 in the embodiment of figure 2. However, if the fluid is discharged from nozzle 62 with a comparatively high momentum - i.e., high volume flow - and at a low cone angle, it might be the case that a streak of relatively unmixed fluid travels downstream the duct, while, if the fluid is discharged from wall openings 51 , 52 at a comparatively high momentum - i.e., high volume flow - it will penetrate deeper into the axial flow and can in some cases mix more intensely than at low volume flows. Further, as will be appreciated, the position of discharge from a wall opening is coupled to the position of the respective plenum, while a nozzle 62 may extend further downstream, or also upstream, inside the duct and allow the fluid from a plenum to be discharged into the duct remote from the position of the plenum from which the fluid originates. Further, a nozzle inside a duct results in a constriction of the flow cross-section through the duct, and may hence yield downstream eddies, which may foster mixture of the discharged fluid with the axial flow of, e.g., combustion air through the duct. Thus, providing discharge means from plenums to ducts in a burner of the type outlined above may result in excellent results when appropriately combining both types of discharge means in different ducts, at different distance from the downstream ends of the ducts. The absolute velocity or momemtum of the fluid upon being discharged from the discharge means may moreover have a significant impact on the premixing behaviour, as a person having skill in the art will readily appreciate.

It is noted that more than one fluid plenum may be provided in fluid communication with a duct through nozzle-type discharge means, of the type like discharge means 62 shown in figure 1 , and that more than one nozzle-type discharge means may be provided inside one duct. Also, more than one fluid plenum may be fluidly connected to one or more ducts through nozzle-type discharge means, and one or more ducts may be fluidly connected to one or more plenums through nozzle-type discharge means.

Referring again to figure 1 , it will further be appreciated that fluid discharged from fluid plenum 31 through wall openings 51 into ducts 41 and 45 travels a longer distance within the ducts before being discharged on the downstream side 3 of burner 1 that fluid discharged through discharge means 52 and 62. If for instance the same type of fuel, for a more specific instance gaseous fuel, is supplied to both plenums 31 and 32, the fluid provided through plenum 32 and discharged into a duct at a relatively downstream location may be less intensely premixed inside a duct than fuel discharged into a duct at a relatively upstream location. For one instance - dependent upon other parameters of the burner and the fuel - fuel discharged from plenum 32 may support, by a relatively diffusion type combustion in downstream space 3, combustion of more intensely premixed fuel from plenum 31. This can be important in particular at low, understochiometric, overall fuel/air ratios, which might result in the extinction of a completely premixed flame.

Dependent upon the particular requirements, all or only some of the ducts may be equipped with discharge means intended to supply fuel for combustion of an intensely premixed fuel/air mixture, i.e. positioned relatively upstream in the duct. Likewise, all or only some of the ducts may be equipped with discharge means intended to supply fuel for more diffusion type combustion, i.e. positioned relatively downstream in the duct. As seen in figure 1 , some ducts may be intended only for relatively far upstream, or premix type, fuel supply, while others may be intended only for relatively far downstream, or diffusion type, fuel supply, while yet other ducts may be equipped with discharge means for both. It is noted that the mass flow of fuel through plenums 31 and 32 may be controlled through control means provided upstream of the fluid supply connectors 311 and 321 . While the control means are not shown, they are familiar to a skilled person. The fuel mass flow to plenums 31 and 32 may thus be controlled individually, and thus the ratio between the mass flow or fuel discharged into the ducts relatively upstream and relatively downstream may be individually controlled. In other embodiments, however, discharge means may be provided in fluid communication with one and the same plenum and configured to discharge fluid from the plenum into one and the same duct or into different ducts, or a combination thereof, at different distances from the downstream end of a duct, which results in a fixed mass flow ratio of differently mixed fluids at the downstream end of the ducts.

Most downstream arranged coolant plenum 33 is fluidly isolated from the ducts and discharges the coolant though effusion cooling holes 121 in the downstream front wall and into downstream space 3, i.e. , into the combustion space, thus effecting cooling of downstream front wall 12.

In the embodiment of figure 1 , the ducts are shown to be slightly divergent at their downstream ends. Such flared outlet of the ducts may be useful to reduce exit velocity of the flow from a duct and hence reduce the risk of flame lift-off of the microjet flames and hence to ensure stabilization of the microjet flames. However, the shape of the ducts at the downstream end might be chosen as needed to meet requirements, for instance, as to the velocity of the gas flow emanating from a duct into downstream space 3. The upstream end of the duct may in instances be rounded or trumpet-shaped to reduce pressure losses.

The device shown in figure 3 differs from the embodiment of figure 1 mainly in comprising ducts 43 which are equipped with wall-opening type discharge means 51 fluidly connecting ducts 43 to upstream plenum 31 , while they are further equipped with nozzle-type discharge means 62 fluidly connecting ducts 43 with plenum 32 downstream from the position of wall-opening type discharge means 51. It should be noted that a duct may as well be equipped with more than one nozzletype discharge means, which may further be configured to discharge into the duct at different longitudinal positions of the duct. Two nozzle-type discharge means provided in one and the same duct may further be fluidly connected to one and the same fluid plenum or to different fluid plenums. It is further noted that in the framework of the present document a longitudinal or upstream-downstream position within a duct, or any other reference to a longitudinal position within a duct, is generally referenced to and measured from the downstream end of the respective duct, adjacent downstream plate 12. Said dimension defines the distance for mixing of fluids within a duct before discharging fluid into a combustion space and thus is a relevant parameter for a duct of the burner. Longitudinal dimensions inside a duct may further be expressed in multiples of the minimum hydraulic diameter of a duct along its longitudinal extent, wherein the hydraulic diameter is determined as four timed the local cross-sectional area divided by the circumference of a wall encircling said cross-sectional area.

The burner 1 shown as another exemplary embodiment in figure 4 comprises ducts 48 and 49 which are configured as dedicated piloting ducts and which are characterized in that the position of a downstream discharge location into ducts 48, 49 from a plenum different form most downstream plenum 33, wherein said most downstream plenum is configured as a coolant plenum, is provided 5 times or less the minimum hydraulic diameter of the respective duct upstream from the downstream end of the duct. This is so close to the downstream end of the respective duct that fuel discharged from any of wall-opening type discharge means 54 provided in ducts 48 or from nozzle-type discharge means 65 provided in piloting duct 49 will exit from the duct into combustion space or downstream space 3 virtually unmixed with the combustion air flowing through the respective duct and will thus yield in a markedly diffusion-type flame at the exit of the duct. As outlined above, such flames yield higher nitrogen oxide formation, but on the other hand superior robustness of combustion. Said marked pilot flames may support combustion of fuel discharged into ducts 41 , 48 and 49 relatively far upstream in the ducts through discharge means 51 , and thus intensely premixed when exiting the ducts, in particular at low fuel/air ratios. In the shown exemplary embodiment, each piloting discharge means, positioned less than five minimum hydraulic diameters upstream form the downstream end of the respective duct, is provided in a duct also comprising an upstream discharge means 51 . This is not necessarily the case. A piloting duct may also be provided as a standalone piloting duct having no other discharge means upstream from the piloting discharge means. However, distributing the most upstream discharge means in as many ducts as possible may yield advantages in that a given fuel mass flow through these upstream discharge means may be distributed into a maximum fraction of the combustion air, which results in good conditions to reduce nitrogen oxides emissions when operating the burner at high loads, i.e. with an comparatively high overall fuel/air ratio.

The skilled person will, in the light of the present description, readily appreciate that the herein disclosed burner is not limited to be provided with three plenums, but more generally may be usefully provided with any number of plenums of two and larger. For instance, the burner shown in figure 1 may be supplemented with two additional plenums which respectively are merely connected to ducts through relatively upstream discharge means and relatively downstream discharge means like ducts 41 and 42 in figures 1 and 3. It may likewise be supplemented with a combined piloting/premix duct as shown at 48 and 49 in figure 4. In being fluidly connected to different plenums the same ducts may thus be supplied with either natural gas and/or a different combustible, like, for instance, hydrogen. Thus, achieved is a highly versatile burner which is suitable for instance for the combustion of hydrogen or a hydrogen rich or other highly reactive fuel and for the combustion of natural gas in a variable premix/piloting operation, through the same ducts. The order in which the different plenums are arranged along a general airflow direction of the burner from the upstream side defined by upstream front wall 11 to the downstream side defined by downstream from wall 12, or the longitudinal position at which fluid from a specific plenum is discharged into the duct, will be determined by the skilled person in applying his general knowledge and taking into account the effect of the mixing distance of a fluid inside the duct from the discharge position of the fluid to the downstream end of the duct. The skilled person will readily appreciate that modules comprising fluid plenums intended for discharging a specific fluid into at least some of the ducts may, in a virtually modular manner, be stacked upon each other. This can be achieved in manufacturing burners with a different number of “slices” comprising a fluid plenum, sections of the ducts, appropriate discharge means for fluidly connecting the plenum to the interior of the ducts, and, optionally, a fluid supply connector for supplying fluid to the plenum. It might be considered to actually build the burner from stacked modules. The number of plenums is in principle not limited. Limiting factors might be seen in the increasing length of the ducts and thus increasing pressure losses, and other issues which might arise if fluids are discharged too far upstream of the downstream end of the duct. The fluid plenums, with their specific discharge means through which they are fluidly connected to at least one duct, can be stacked in any order a skilled person may find suitable to fulfil a certain purpose. However, it might be found advantageous if the most downstream plenum is a coolant plenum. It will be appreciated that due to the versatility of the herein disclosed burner a comprehensive description of possible embodiments is not practical. However, in understanding the principle behind the stacked micro duct burner herein disclosed, and the function and merits of certain specific embodiments of “modules”, the skilled person receives a comprehensive teaching.

Figure 5 outlines an embodiment of a burner 1 in which a cartridge 60 is provided through a passage 40. Cartridge 60 may in embodiments be a separate and retractable member. Cartridge 60 may be provided instead of or inside a duct. If cartridge 60 is provided instead of a duct, it is understood that cartridge 60 is preferably configured to achieve a gas-leak proof sealing of the plenums. In the embodiment shown, cartridge 60 extends through a duct, whereby sealing of the plenums is achieved by the duct wall which may be seamlessly manufactured with front walls 11 and 12 and partition walls 21 , 22 and 23 in the primary forming process. Longitudinally extending or spiralling flutes may optionally be provided on an outer surface of the cartridge and/or the inner side of the wall of the duct through which cartridge 60 extends. Combustion air may be provided through the flutes. The flutes may be spiralling, at least at their downstream ends adjacent downstream front wall 12 and when opening out to the downstream space 3, so as to generate a swirling flow of combustion air at or around the downstream end of cartridge 60. A fluid 6 may be provided to downstream space 3 through cartridge 60. Said fluid may be steam, or a gaseous fuel, or mixture of gaseous fuel and other gaseous agent, which may for instance be intended for diffusion-type combustion. The gaseous agent may be discharged with a swirl, which may co- or counter-rotate with a swirl of combustion air. In other embodiments, cartridge 60 may comprise a liquid atomizing nozzle, thus discharging fluid 6 as a spray cone 5. The liquid may be liquid fuel or water or a mixture thereof. It is understood that further agents, for instance atomizing air or steam, may be provided to and discharged through cartridge 60. A burner of the herein disclosed type may comprise more than one cartridge 60. Burner 1 of figure 5 further comprises ducts 43 which are equipped with relatively upstream nozzle type discharge means 61 in fluid communication with plenum 31 and relatively downstream wall-opening type discharge means 52 in fluid communication with plenum 32.

Figure 6 shows an exemplary embodiment in which some of the ducts have different diameters. For instance, ducts 41a and 41 b, which are equipped with relatively upstream wall-opening type discharge means 51 and in fluid communication with plenum 31 only, have different minimum hydraulic diameters. It will be appreciated that in a duct without an insert the longitudinal section having said minimum hydraulic diameter defines a metering section. These different diameters, as will be readily appreciated, have an impact on the mass flow of combustion air through the respective ducts. If discharge means 51 provided in ducts 41a and 41b have the same integral cross-section per duct, it will be appreciated that the flow through duct 41 b exhibits a higher fuel/air ratio from fuel discharged from plenum 31 than the flow through ducts 41a. If, however, the integral cross-section of discharge means 51 of ducts 41a is larger than the integral cross-section of discharge means 51 in ducts 41 b, the equivalence ratios may - or may not - be the same, and due to the different minimum hydraulic diameters the thermal load over the downstream surface of the burner adjacent downstream wall 12 may be varied. In ducts 42 and 43, however, a residual crosssection between an inner wall of the duct and an outer wall of nozzles 62 may define the metering section. The mass flow through ducts 42 and 43 may thus be varied in providing different hydraulic diameters of ducts 42 and 43 around nozzles 62, different outer diameters of nozzles 62, or both.

In the embodiment of figure 7, ducts 44a, 44b and 44c are fluidly connected to plenum 32 through wall-opening type discharge means 52, while plenum 31 is fluidly connected to ducts 44a, 44b and 44c through nozzle-type discharge means 61 a, 61 b and 61 c, respectively. In the region of the nozzle-type discharge means, each duct exhibits a tapering cross-section longitudinal portion in which the duct tapers downstream the general airflow direction, i.e., in a direction from first front wall 11 to second front wall 12, from a first cross-sectional area to a second cross- sectional area smaller than the first cross-sectional area. Nozzle-type discharge means 61 c extend further into the tapering cross-section longitudinal portion of ducts 44c than nozzle-type discharge means 61 b into ducts 44b, and nozzle-type discharge means 61 b extend further into the tapering cross-section longitudinal portion of ducts 44b than nozzle-type discharge means 61 a into duct 44a. Accordingly, the minimum flow cross-section, defined as a residual cross-section between the inner wall of a duct and the outer wall of a nozzle, is smaller in duct 44c than in duct 44b, and the minimum flow cross-section is smaller in duct 44b than in duct 44a.

The device shown in figure 8 outlines how two supply connectors 311 and 321 provided for feeding two plenums may be arranged concentrically and coaxially.

As illustrated in figure 9, wall-opening type discharge means may be arranged and configured to discharge the fluid form a plenum into a duct either • perpendicular to the longitudinal direction 451 , as for instance discharge means 56 of plenum 36;

• upstream the flow direction of combustion air 4, or inclined in the upstream direction of duct 46, respectively, as for instance discharge means 57 of plenum 37; or

• downstream the flow direction of combustion air 4, or inclined in the downstream direction of duct 46, respectively, as for instance discharge means 58 of plenum 38.

The direction along which the fluid is discharged into duct 46 from the respective discharge means 56, 57, 58 is indicated by the arrows originating from the discharge means. It is noted that the application of said teaching is not limited to wall-opening type discharge means.

Figure 10 shows a cross-section through an even more particular example of the embodiment shown in figure 9 along line X-X. The discharge means may be arranged and configured such that the fluid is discharged from plenum 36 into duct 46 with a tangential velocity component so as to form a vortex flow of discharged fluid as indicated by the circular arrows in figure 10. It is noted that such a configuration of the discharge means is not limited to the embodiment of figure 9 or any embodiment similar thereto. Such configuration suited to generate a vortex flow of fluid discharged from a plenum into a duct may be applied, for instance, while not limited to, discharge means discharging the fluid in an upstream or downstream direction of the combustion air flow inside a duct, as for instance depicted at 57 and 58 in figure 9, or a nozzle-type discharge means.

In further aspects, illustrated in figure 11 , at least one duct 46 may be provided with a row of vanes 452 disposed inside duct 46 and disposed around the circumference of duct 46 and inclined at an angle y with respect to the longitudinal direction 451 of duct 46, so as to induce a vortex flow as indicated at 401 onto the flow of combustion air 4. The vanes may in particular be provided at the upstream end of duct 46 such that the vortex flow of combustion air is present essentially throughout the longitudinal extent of duct 46. However, in other aspects, vanes 452 may be arranged further downstream, so as to discharge fluids from certain plenums into a purely axial flow of combustion air, while others may be discharged into a vortex flow of combustion air. The row of vanes 452 may be provided at the downstream end of duct 46. While it should be readily apparent to a person having skill in the art, it shall be mentioned that a vortex flow of combustion air may be combined with a tangential injection of at least one fluid from at least one plenum, as outlined in connection with figure 10. The vortexes of combustion air and of discharged fluid may be co- or counterrotating and may be affected with at least essentially equal tangential velocity components or different tangential velocity components and with essentially identical or different swirl numbers, as found suitable by a person having skill in the art when applying her or his common knowledge. The vanes 452 are in particular provided integrally and in one piece, i.e. , seamless, with the burner, or the inner wall of a duct. It will be appreciated that in particular manufacturing the burner with additive manufacturing methods enables providing such rather small and complex geometries in one integral workpiece.

It is apparent to a person having skill in the art that the relative velocities of the combustion air and the fluid discharged into the flow of combustion air inside a duct may have a major impact on the mixing of the fluids and may be applied to tune mixing and hence combustion behaviour.

Figure 12 illustrates a detail of a further possible embodiment of the herein disclosed burner. Figure 12 shows a section of a burner with plenums 36 and 37 and duct 141 extending therethrough. 4 denotes the flow of combustion air through duct 141. Plenum 36 is in fluid communication with duct 141 through nozzle-type discharge means 162, while plenum 37 is in fluid communication with duct 141 through wall-opening type discharge means 152. Nozzle-type discharge means 162 is configured to discharge fluid from plenum 36 at a position s1 measured from the downstream end of duct 141 , while wall-opening type discharge means 152 is configured to discharge fluid from plenum 37 at a position s2 measured from the downstream end of duct 141 . Although plenum 36 is positioned upstream from plenum 37, fluid from plenum 37 is discharged into duct 141 downstream from the discharge position s2 of fluid from downstream plenum 37. This illustrates the flexibility a nozzle-type discharge means offers with respect to the discharge location of a fluid from, a specific plenum into a duct.

Figures 13a through 13d show plan views on front faces of burners having different numbers of passages. As can easily be seen, in each burner each three neighbouring passages are provided on the comers of an equilateral triangle. All shown embodiments, irrespective of the number of passages, have in common that a centre passage 40a is concentrically encircled by concentric hexagonal rings of passages. The embodiment of figure 13a exhibits 7 passages, wherein a centre passage 40a is encircled by six passages 40b in a hexagonal arrangement. Each two neighbouring passages 40b have the same distance from each other than from central passage 40a. The embodiment of figure 13b exhibits 19 passages, wherein a central passage 40a is encircled by six passages 40b in a first hexagonal ring and 12 passages 40c of a second hexagonal ring. As is apparent when further considering figures 13c and 13d, the burner can easily be scaled by adding further concentric hexagonal rings of passages, wherein each additional hexagonal ring comprises 6 passages more than the adjacent inner hexagonal ring. Each passage, apart from the passages on the outermost hexagonal ring, may in turn be considered to form the centre of another hexagon of surrounding passages. In embodiments, centre passage 40a may be omitted. This self-similarity of the arrangement of passages, along with the equidistant arrangement of all neighbouring passages, facilitates scaling of the burner. A burner may, on the one hand, be adapted to different burner sizes in adding additional hexagonal rings of passages of the same size and distance from each other. A burner of a given size may, on the other hand, be provided with different cross-sectional dimensions of the passages, or ducts, respectively. It shall be noted that the ducts, or some of the ducts, may have a polygonal rather than a circular or, more generally spoken, rounded cross-sectional shape. In particular embodiments, at least some of the ducts may exhibit the cross-sectional shape of an equilateral hexagon.

Figures 14a through 14c illustrate possible arrangements of piloting ducts, liquid fuel nozzles, water or steam injection nozzles and so forth, in a burner of the described type. It is well understood and goes without saying that said ducts, nozzles and the like extend through passages of the burner. For one instance, as illustrated in figure 11a, a piloting duct 49, represented by the filled dot, may be arranged in a central passage, while all other passages may be provided with any other type of duct. The burner of figure 14b exhibits six piloting ducts arranged adjacent to each other on an equilateral hexagon. In the embodiment of figure 14c a piloting duct is provided in the centre of each hexagonal arrangement of neighbouring non-piloting ducts. The skilled person will appreciate that in the embodiments of figure 14 for instance cartridges for the supply of water, steam, liquid fuel, piloting gas and so forth may be arranged in the place of the piloting ducts 49.

Figure 15 illustrates how the passages 40b and 40c of the hexagonal rings, and, consequently, any means provided therethrough, may be inclined relative to a burner axis 101 , so as to discharge the flow emanating from said passages, or ducts, respectively with a macroscopic tangential component relative to the burner axis. Burner axis 101 is defined at least essentially perpendicular to the downstream front face of the burner. The longitudinal direction of central passage 40a, or duct respectively, is in particular aspects at least essentially parallel to burner axis 101 , while the longitudinal axis of passages 40b and 40c are inclined about an angle I3> relative to the burner axis, so as to generate a vortex flow indicated at 102 downstream the burner, or, more specifically, in the combustion space. In certain embodiments, no central passage may be present. References may in this case be made, for instance, to a midpoint of the hexagonal ring. It is understood that the inclination angle I3> of the passages may vary dependent on the distance of a respective passage from central passage 40a. For one instance, inclination angle I3> may increase with increasing distance from central passage 40a or from burner axis 101 , or, in other aspects, from a midpoint of the hexagonal ring. Of course, it is not excluded that, in embodiments, the inclination angle I3> may decrease or may remain the same with increasing distance from central passage 40a or from burner axis 101 , or, in other aspects, from a midpoint of the hexagonal ring. Due to the self-similarity of the arrangement of the passages, one burner may be subdivided into smaller areas, wherein each aera defines a local centre and an encircling hexagonal passage arrangement, wherein the passages encircling a specific centre may be inclined to provide a vortex around said local central passage. In other words, the passages in a concentric hexagonal ring are configured such that fluid discharged at the downstream side of the burner form the passages in the concentric hexagonal ring has a velocity component which is tangential to a circle defined around the midpoint of the hexagonal ring. An embodiment in which a macroscopic vortex is generated may be found useful for instance when replacing vortex burners in a legacy combustion appliance.

The example of figure 16 relates to a specific embodiment of the cooling of the burner. In the exemplary embodiments, plenums 31 and 32 are configured to discharge into ducts 47 through wall-opening type discharge means 51 and nozzle-type discharge means 62, respectively. Wall 23 is provided with through holes 122 fluidly connecting open plenum 35 and most downstream plenum or cooling plenum 33. As outlined above, open plenum 35 is in fluid communication with a space surrounding burner 1 , i.e. , for instance with a reservoir of air. Air from open plenum 35 flows through opening 122 into cooling plenum 33. Through jets of cooling air entering plenum 33 through holes 122 impingement cooling of adjacent areas of downstream wall 12 may be effected. Other areas of downstream wall 12 are effusion cooled by cooling air which is discharged into downstream space 3 through through holes 121 . A further fraction of the cooling fluid is discharged into the downstream part of the ducts through coolant discharge means 53. For the sake of completeness it is noted that coolant discharge means which provide fluid communication between the most downstream plenum and the interior of a duct may also be used in connection with embodiments in which coolant is supplied to plenum 33 through coolant feed connectors.

While the above shown exemplary embodiments all exhibit two fluid plenums in connection with the ducts, apart from the most downstream plenum which is configured as a cooling plenum to provide appropriate cooling for the thermally highly loaded downstream wall, the skilled person will appreciate that more plenums may be provided in fluid communication with at least one duct, and be intended for providing different types of fuel, fuel intended for different premixing prior to combustion, or steam or other inert fluids. In embodiments, a nozzle-type discharge means may be equipped with an atomizer, such that also liquids, like, for instance, liquid fuel or water, may be discharged into a duct through said nozzle-type discharge means. In embodiments, the burner may be modular, each module comprising at least a fluid plenum between two transverse walls, a duct segment or duct segments extending through the transverse walls and the plenum or the plenums, a discharge means to provide fluid from the plenum or the plenums to the duct segment or the duct segments, and, optionally, a fluid supply connector or fluid supply connectors in fluid communication with the fluid plenum or fluid plenums.

Figure 17 depicts a detail from an exemplary embodiment of a burner of the herein described type. A through hole 52a through the wall of duct 142 for providing fluid communication with plenum 32 is elliptically shaped, with a long ellipse axis 528 and a short ellipse axis 529. An angle included between the long ellipse axis 528 and the longitudinal direction of the duct 451 , the general airflow direction, or a burner axis, is in particular embodiments chosen to be 30 degrees or less, and in more particular embodiments, as shown in figure 17, an angle included between the long ellipse axis 528 and the longitudinal direction of the duct 451 is at least essentially zero. Moreover, the ratio between the length a of the long ellipse axis and the length b of the short ellipse axis may be chosen to be 1 .25 or more. This configuration yields advantages in particular if the burner or a burner module is manufactured by additive manufacturing, wherein the building direction may be chosen at least approximately parallel to the general airflow direction, the duct axis or burner axis. The elliptic shape of the opening may facilitate said manufacturing in that overhanging structures during manufacturing are largely reduced. In other embodiments, the through hole may for the same reason be lancet-shaped or shaped as a pointed arch with the apex pointing at least essentially in the building direction of the additively manufactured structure.

Essentially the same advantages may be achieved, for instance, with a polygonshaped wall opening 52b as shown in figures 18 and 19. Wall opening 52b as shown has a base and an apex pointing against the intended flow of combustion air 4. As outlined in more detail in figure 19, polygon-shaped wall opening 52b has a polygon-shaped boundary which comprises straight boundary segments 521 , 522, 523, 524 and 525. The boundary may also be considered as comprising an upstream boundary section 526 edging the opening in a direction against the flow of combustion air 4 and a downstream boundary section 527 edging the opening in the direction of the flow of combustion air. Upstream boundary section 526 comprises straight boundary segments 522 and 523, while downstream boundary section 527 comprises straight boundary segments 521 , 524 and 525. Straight boundary segments 522 and 523 of upstream boundary section 526 form an apex of opening 52b pointing upstream relative to the flow of combustion air. Straight boundary segments 522 and 523 include inner angles 5 and s with the longitudinal direction 451 of duct 142 which are 45 degrees or less. The exemplarily shown opening is particularly suitable in connection with a component which is additively manufactured with a building direction against the intended flow of combustion air 4. Generally, an opening being defined within a boundary having an apex pointing into the building direction of the component and the apex being enclosed between two straight boundary segments edging the opening in the building direction, wherein those two boundary segments include an angle of 45 degrees or less with a parallel to the building direction, may be found beneficial in connection with an additively manufactured component. Also a geometry with two apexes, like e.g. a diamond-shaped geometry, might be applied.

While the subject matter of the disclosure has been explained by means of exemplary embodiments, it is understood that these are in no way intended to limit the scope of the claimed invention. It will be appreciated that the claims cover embodiments not explicitly shown or disclosed herein, and embodiments deviating from those disclosed in the exemplary modes of carrying out the teaching of the present disclosure will still be covered by the claims.