The present invention relates to a burner, particularly a burner of premixed type. A combustion process takes place in a burner, i.e. an exothermic oxidation reaction between a combustible substances and an oxidant (or comburent) , characterized by energy development in the form of light and heat, in addition to the production of burnt gases from reactant substances.

A burner is called of premixed type if, before the ignition of the combustion reaction, the reactants are finely mixed.

Known burners have two main problems. The first is related to the stability of the flame and the second is related to the production of pollutants.

Generally, the flame is stable in a burner when it moves towards the fresh mixture with a speed equal to that of flowing of the fresh mixture itself. In other words, the flame is stable when a condition of dynamic equilibrium is achieved between the flame speed and the speed of the fresh mixture. If the latter has a greater speed than the flame propagation speed, the flame is "pulled away" and tends to move away from the outflow zone until it is extinguished, due to the excessive thinning of the fresh mixture by the surrounding atmosphere. Such phenomenon is known as "blow off". On the contrary, if the flame speed is greater than the flow speed of the fresh mixture, and this often occurs in the burners of premixed type, it is possible that a flame return occurs in the burner duct until it extinguishes, due to the contact with the cold parts of the burner. Such phenomenon is known as "flashback".

In industrial applications, the flow speed of the fresh mixture is generally greater than the flame speed, so as to ensure high fresh mixture flow rates in small volume appliances and thus high combustion intensity and high heat release. It is therefore highly necessary to stabilize the flame so as to avoid the aforesaid blow off phenomenon.

In other words, it is necessary to "anchor" the flame in a suitable zone in order to ensure the ignition of the fresh mixture and hence the propagation of the flame itself. In turn, such zone must be stable, so that the flame is stationary.

Currently, the stabilization of the flame occurs by employing a non-aerodynamic body or a motion of swirl type, i.e. a spiral motion.

In both cases, the goal is to create a local slowing of the fresh mixture and a recirculation of the hot burnt gases, in such a manner that the latter continuously ignite the fresh mixture.

Such known devices, nevertheless, in some cases are unable to solve the problem, or they are unable to anchor the flame in a stable manner. In addition, the known burners which have such devices often have problems related to the emissions of pollutants.

Indeed, the use of combustion as source of energy leads to environmental problems and thus legal problems, associated both with the emission of pollutants such as nitrogen oxides, carbon monoxide, sulfur oxides, unburnt hydrocarbons and particulate, and with the formation of carbon dioxide (CO ₂ ) which, while being not a pollutant per se, is one of the gases responsible for the greenhouse effect and is therefore considered as such.

Carbon dioxide represents one of the primary products of hydrocarbon combustion and, up to now, it is not possible to avoid a production thereof.

Currently, in known burners, effort is made to limit the CO ₂ emission by increasing the efficiency of the energy generation processes and by using fuels which have a high hydrogen atom to carbon atom ratio.

Hydrocarbon combustion, apart from having the carbon dioxide as main product, produces a series of other substances, including those mentioned above, whose emission, even if lower in quantitative terms with respect to that of C02, constitutes a risk for the health of man and of the planet, even in small concentrations. For example, carbon monoxide is one of the most dangerous pollutants due to its toxicity.

The unburnt hydrocarbons, which are formed from non- reacted fuel or from pyrolysis products of larger molecules, are polluting and harmful substances, as well as particulate. In addition, carbon monoxide, unburnt hydrocarbons and particulate, beside having their own intrinsic toxicity, represent products of incomplete combustion, and hence indicate a loss of combustion efficiency and lack of use of the heat power provided by the fuel. Sulfur oxides, mainly composed of SO ₂ , are deemed pollutants since once released in the atmosphere, they are transformed into sulfurous acid (H ₂ SO ₃ ) and sulfuric acid (H ₂ SO ₄ ) , giving rise to acid rain, which apart from having a corrosive effect on a great number of materials, destroys vegetation and pollutes water and soil.

The combustion in air of all fuel types, including natural gas, gives rise to nitrogen oxides, constituted for over 90% by nitrogen monoxide (NO) , which is highly toxic. Currently, the scientific community is engaged in research and development of combustion processes which ensure a low level of polluting emissions. The environmental problems have in fact led to the setting of increasingly severe restrictions on such emissions, in particular on the nitrogen oxides which are the main pollutant from natural gas. Currently, in order to decrease their production, three parameters are mainly controlled:

-reduction of combustion temperature; -reduction of oxygen concentration in the maximum temperature zone;

-reduction of nitrogen compounds in the fuel.

Nevertheless, by using such techniques, fuels with unstable flame are obtained. In addition, these lead to improvement, but do not solve the problem of polluting emissions.

Therefore, one object of the present invention is that of providing a burner of premixed type which allows achieving a combustion with a low emission of pollutants and, at the same time, with extremely high flame stability.

A further object of the present invention is to provide a burner with low heat dispersions and high efficiency. A further object of the present invention is to provide a burner with high structural simplicity and thus having low production and maintenance costs.

These and other objects are achieved by means of a burner of premixed type comprising a flame confinement chamber of elongated shape, a spiral motion means, adapted to confer a tangential component of speed to a previously prepared fuel and comburent mixture flow, entering said flame confine chamber, a deviation means of the mixture flow, which is placed downstream of said spiral motion means and which is adapted to deviate the mixture flow motion downstream of the spiral motion means, in such a manner that a central portion of mixture flow is slowed and a peripheral portion thereof is accelerated, and in that it comprises a combustion triggering device positioned downstream of said mixture flow deviation means.

In the present description and in the subsequent claims, the terms "upstream" and "downstream" refer to the advancing direction of the mixture flow. Due to the combination of such features, it is possible to obtain a burner of premixed type having an extremely stable combustion flame and a low pollutant emission.

As a ' matter of fact, the combination of the aforesaid features confers a particular morphology to the combustion reaction. The latter develops inside the confinement chamber with a conoid shape, having the vertex immediately downstream of the mixture flow deviation means. Such characteristic shape allows obtaining a flame that, in the initial part, is at a distance from the walls of the confinement chamber and in the terminal part is in contact therewith.

This implies that, during operation, a portion of the confine chamber is subjected to extremely limited heat swings, and thus it does not require particular structural materials and does not dissipate heat energy. In addition, during operation, the terminal part of the flame occupies the entire cross section of the confinement chamber, preventing the unburnt gases from exiting and thus forcing them to complete the combustion reaction with considerable advantages from the pollutant emissions point of view.

In addition, the combination of a deviation means placed downstream of a spiral motion means with a flame confinement chamber of elongated shape, allows obtaining a kind of reverse Bunsen cone, in which the flame front is locally cooled by the fresh mixture which run over it, contributing to the reduction of NO _x formation, as will be better explained below. The combination of the aforementioned features therefore allows obtaining a burner with low heat dissipation and high heat efficiency.

The present invention, in addition, attains a burner with simple structure and maintenance and hence a burner with reduced costs.

In accordance with a preferred embodiment, the deviation means of the burner of the invention is axially movable inside the flame confinement chamber.

This allows obtaining the possibility of adjusting the exit level of the flame cone from the flame confinement tube, and therefore the morphology of the flame. In this way, it is possible to optimize the position and shape of the flame based on the operating conditions and use needs of the burner and therefore obtaining a versatile burner.

In addition, this allows obtaining a burner that does not exclusively function in the "on" and "off" states, but also in the "stand-by" state, avoiding the transitory lighting and extinguishing states. According to a preferred embodiment of the invention, the confinement chamber has an axial symmetric cross section, more preferably polygonal cross section and still more preferably substantially circular cross section. In particular, a circular or polygonal cross section with a high number of sides allows obtaining low pressure drops, a uniform mixture distribution and an optimal flame morphology.

According to an alternative embodiment of the invention, the burner comprises a flame confinement chamber with circular section walls and circular or frustoconical extension, in which a fuel and comburent flow is introduced to which a spiral rotary motion is conferred, a deviation means being placed aligned with such chamber in front of which a calm zone of the flow is formed where there are two electrical discharge lighting electrodes, which gives rise to a combustion reaction vertex which takes a conoid shape in order to be transformed into a flow with strong axial component towards the outlet of the confinement chamber, in the form of a dart formed by completely burnt gases proceeding into the external environment.

This allows obtaining a dart exiting from the combustion chamber having a maximum temperature compatible with the type of fuel and comburent employed, a high outflow speed of the dart and a high compactness of the dart exiting from the confinement chamber.

In order to better understand the invention and appreciate the advantages thereof, a description will be provided below of several non-limiting embodiments of the burner of the invention, making reference to the attached figures, in which:

- Figure 1 is a cross section view of a burner according to a first embodiment of the present invention; - Figure 2 is an enlarged view of a detail of figure 1;

- Figure 3 is a front view of the detail of figure 2 ; and

- Figure 4 is a partial cross section view of a burner according to a second embodiment of the invention.

With reference to figures 1 - 4, a burner is overall indicated with the reference number 15.

In particular, the reference 15 indicates a burner of premixed type which produces heat by means of the combustion of a mixture, generally comprising fuel gas and air. Preferably, such fuel gases are completely premixed, i.e. no further component is added to the mixture supplied to the burner.

Preferably, the burner 15 is supplied with a meager fuel mixture, or rather a mixture with an excess of air, for example equal to about 1.2.

The mixture can be preformed by means of a Venturi tube, through natural suction of external air by the mixture flow inside the supply duct. Therefore, the burner 15 may be provided with a Venturi tube or other mixing means not shown in the first embodiment .

According to the present invention, the burner 15 comprises a flame confinement chamber 17 with elongated shape.

In the present context, the wording "chamber with elongated shape" indicates a chamber whose extension in the direction of the mixture flow is greater than its extension in the direction perpendicular to the direction of the mixture flow.

If the flame confinement chamber 17 has a substantially cylindrical shape, the wording "chamber with elongated shape" indicates a chamber whose cylinder height is greater than its diameter. Preferably, the ratio between the size of the flame confinement chamber 17 along the flow direction of the mixture and the size of the same along the direction perpendicular to the flow direction is comprised between 3 and 30, more preferably between 4 and 15 and still more preferably it is equal to about 10.

Said flame confinement chamber 17 is adapted to confine the mixture flow, the combustion reaction and the flame that derives therefrom. The flame confinement chamber 17 at least partially comprises the combustion chamber of the mixture and, in some cases, coincides therewith.

Preferably, the flame confinement chamber 17 has an axial symmetric cross section, more preferably polygonal cross section and still more preferably substantially circular cross section.

According to embodiments of the invention, the flame confinement chamber 17 has substantially cylindrical or frustoconical shape. It comprises walls 7, a first inlet portion 107 of the mixture and a second outlet portion 207 of the flame.

The burner 15 according to the present invention also comprises spiral motion means 3 adapted to confer a speed component tangential to the previously prepared fuel and comburent mixture flow, entering into said flame confinement chamber 17.

The burner 15 also comprises a supply intake duct 1 and a feed case 2, which houses said spiral motion means 3.

The latter can be of any known type, but preferably comprises a swirl generator with guides. Such guides can be fixed or have adjustable slope.

In accordance with one embodiment, the spiral motion means 3 comprises a distributor having a flared circular head 4 equipped with a peripheral edge in which a rim is cut with eight tangential ducts 104, as it may be seen in figure 3. The head comprises a shank 103 provided with an axial through hole 203 leading on one side inside the head 4, and on the other side into a hole of the opposite wall of the case 2, as visible in figure 2. The head 4 communicates with an annular duct, formed between two elements 5 and 5' , which in turn communicates with a funnel-shaped element 6 for narrowing and conveying the gaseous flow to the flame confinement chamber 17.

The spiral motion means 3 reach a recirculation zone which leads to the stabilization of the flame and favors the fluid-dynamic mixing of the reactants, increasing the combustion efficiency.

According to the present invention, the burner 15 comprises mixture flow deviation means 9 which is placed downstream of said spiral motion means 3 and which is adapted to deviate the mixture flow motion downstream of the spiral motion means 3, in a such manner that a central flow mixture portion is slowed and a peripheral portion thereof is accelerated. In other words, the deviation means 9 create a local slowing down of the fresh mixture and a recirculation of the hot burnt gases, in such a manner that the latter can continuously ignite the fresh mixture. As a matter of fact, such deviation means 9 create, immediately downstream thereof, a stagnation zone in which the hot burnt gases heat the fresh mixture until ignition.

The deviation means 9 also act as a flame stabilizer.

The combination of the spiral motion means 3 with the deviation means 9 achieves a high recirculation of the burnt gases, thus ensuring the stabilization of the combustion process and thus of the flame, both in terms of position and morphology.

The combination of the spiral means 3 with the deviation means 9 and with the elongated confinement chamber 17 ensure that a stable flame is created with conoid morphology 12 at the center of the confinement chamber 17 and a peripheral fresh mixture flow is created that, proceeding at increased speed with respect to the mixture flow in the central portion of the chamber 17, does not take part in the combustion reaction at the first portion 107 of the confinement chamber 17, but cools the walls 7 of said chamber 17. Such fresh mixture therefore penetrates the flame from the outside, towards the inside, creating a kind of reverse Bunsen cone.

In such a manner, the conoidal flame 12 that enlarges increasingly as far as it proceeds towards the second portion 207 of the flame confinement chamber 17, comes to touch the walls 7 of such chamber 17, creating a closure of the same and thus preventing the outflow of the unburnt mixture from the chamber 17 itself.

Since two ends of the flame confinement chamber 17 are subjected to a difference of temperature, it is possible to provide that said first portion 107 is made of silicon dioxide and said second portion 207 is made of a refractory material.

Said first portion 107 can also be made of a different material, e.g. metal material. When it is made of silicon dioxide, due to the optical properties of such material, it is possible to visibly monitor the flame.

Analogously, said second portion 207 can be made of a different material, according to the uses of the burner 15. For example, when a contact zone of the flame front with the confinement chamber 17 is not required, and when such contact zone has a limited extension, it is possible to make such second portion 207 of metal alloy with high heat resistance.

The deviation means 9 preferably comprise a non- aerodynamic body, i.e. a bluff body, which is positioned coaxial with the flame confinement chamber 17.

According to the present invention, the burner 15 also comprises a combustion trigger device 10 which is positioned downstream of said deviation means 9.

In the two embodiments shown in the figures, the bluff body comprises a first end 19 with substantially spheroidal shape and a second end 108 with substantially- cylindrical shape whose height is axially developed with respect to the flame confinement chamber 17 and whose radius is less than the radius of the sphere of the first end 19.

In such case, the trigger device 10 comprises an electrode positioned on said second end 108.

Preferably, the trigger device 10 is positioned downstream of the deviation means 9 at a distance lower than the characteristic dimensions of such deviation means 9.

The deviation means 9 is preferably supported by a rod 8. Said rod 8 is preferably thin, i.e. it has a lower size in the direction perpendicular to the mixture flow than the characteristic dimensions of the deviation means 9.

In accordance with a particularly preferred embodiment, the deviation means 9 is axially movable inside the flame confinement chamber 17. In other words, it is possible to adjust the axial position of the deviation means 9 so as to move the vertex of the flame cone and thus to adjust the morphology of the flame and the outflow level of the flame from the confinement chamber 17. In other words, by axially translating the deviation means 9, preferably through the sliding of the rod 8, it is possible to place the entire combustion and consequently select the length of the confinement chamber portion 17 downstream of the deviation means 9 and thus the morphology and position of the flame based on the specific needs of the moment.

Preferably, the burner 15 is equipped with an automatic control system capable of operating on the rod 8, translating the deviation means 9 in response to the control of the selectable parameters, such as the mixture flow rate entering the burner, the temperature of the exiting flame, the temperature difference between two symmetric points of the confinement chamber or other parameters. The mobility of the deviation means 9 allows the closing and the opening of the terminal section of the confinement chamber 17 to be adjusted by means of the flame cone 12. In other words, it is possible to adjust the position of the flame cone 12 in such a manner that the cross section of the flame cone 12 at the end of the second portion 207 of the chamber 17 is less than or equal to the section of such end of the second portion 207. Even if it is preferable to close the confinement chamber 17 so as to avoid emissions of pollutants, adjusting its opening can be useful in some applications of the present invention, e.g. particularly ovens for glass or metal working in which an oxidizing or reducing environment is required.

In accordance with preferred embodiments, the deviation means 9 is positioned inside the confinement chamber 17, however it may also be placed for example in a connection zone comprised between the spiral motion means 3 and the confinement chamber 17.

In accordance with the second embodiment shown in figure 4, the burner 15 is provided with mixing means inside the burner 15 itself. In particular, the burner 15 is provided with a gas supply intake duct and an air supply intake duct, not shown in the figure, a gas distribution duct 20 and a mixing space 21. In such embodiment, the burner is also provided with a device for generating microturbulences 22 and a wire 23 positioned inside the rod 8 and connected to the trigger device 10.

In such second embodiment, the confinement chamber 17 is of cylindrical type and is connected to the case 2 which contains the spiral means 3 by means of a coupling flange 24.

It is possible to provide for coupling means of the confinement chamber 24 to the case 2 of different type, for example snap couplings of removable type, that allow achieving an interchangeability of the confinement chamber 17. In this way, it is possible to use time by time a different chamber 17 which optimizes the performances of the burner 15 on the basis of the specific needs at that moment.

A preferred embodiment of the functioning method of the burner 15 of the invention will now be described.

A gas-air mixture is conveyed at a spiral motion means 3 adapted to confer a speed component tangential to the flow of such mixture.

The mixture flow, exiting from such spiral motion means 3 with at least one flow speed axial component and one flow speed tangential component, enters into the flame confinement chamber 17. In said chamber 17, the mixture flow motion is deviated by the mixture flow deviation means 9, which slows a central portion of the mixture flow, accelerating a peripheral portion thereof so as to create a central zone of gas recirculation and stagnation and a peripheral zone of high speed gas. The thus modified flow is triggered by means of a trigger device 10 immediately downstream of the deviation means 9.

The combustion assumes the shape of a conoid 12, having a vertex 11 at the trigger device 10 and a terminal portion 14 that touches the walls 7 of the chamber 17 before exiting from the chamber 17 itself.

An alternative embodiment of the invention will now be described.

The confinement chamber 17 has a considerable length extension with respect to its inner diameter. Thus, for example, the ratio between the inner diameter and the length of the chamber 17 preferably varies from 1:4 to 1:15 up to even 1:4 to 1:30.

The rod 8 is inserted in a slidable manner inside the hole 203 of the tangential distributor shank of the spiral motion means 3. Such rod 8 bears, close to its end projecting inside the element 6, a globular element of the deviation means 9. Such rod 8 terminates downstream of the globular element with an end extension made of electrically insulating refractory material for the support of the electrode 10' , cooperating with the earth electrode 10' ' for the electrical discharge lighting of the gaseous mixture supplied into the confinement chamber 17. The operation of the burner 15, according to such alternative embodiment, is the following: a flow of an air and fuel gas mixture, having a given flow rate and consequent pressure, is introduced by means of the duct 1 into the case 2, and from this into the helix distributor 4 provided with a series of tangential jets 104, and from these into the annular duct 105, which will guide the rotation flow on a pre-established rotation diameter. At this point, the flow will proceed by engaging the element 6, which will induce such gas-air mixture flow to rotate with a given angular speed and a given spiral pitch, inside the chamber 17 delimited by the wall 7.

At the start of such chamber 17, a spheroid obstacle element is present with appropriate diameter and profile, borne by the rod 8 coaxial to the chamber 17. Such element bears, on its front, an electrically insulating refractory cylinder in whose axis an electrode 10' is placed for the electrical lighting, paired with an earth electrode 10''. Downstream of the spheroid element and coaxial with the system, there is the formation of a calm zone in the whirling motion of the flow that engages the chamber 17, which ensures the overall stability of the combustion. In this calm zone is placed the vertex 11 of the combustion reaction zone, whose development takes the shape of a conoid 12 containing the volume composed by the combustion reaction completion zone which is extended over the entire length of the confinement chamber 17 up to the contact zone 13 coinciding with the terminal section of the chamber 17 constituted by refractory material with a low heat conductivity level. Such contact zone 13 can also be activated with a catalyst in order to make its contact with the combustion zone more stable. Nevertheless, in specific cases, such contact can also be omitted.

Exiting from such zone 13, there is the production of the combustion dart or terminal portion 14 of the conoid 12. Such dart has nearly cylindrical shape with diameter slightly greater than the diameter of the chamber 17 and length which can vary from 25% to 75% of the total combustion length. Such dart has very little or none of the rotation phenomenon which is instead present in the supply gases.

The above described burner 15 was subjected to experimental tests aimed for the quantitative characterization of the combustion process, upon the variation of the apparatus operating conditions. The following Table 2 summarizes the nominal operating conditions used for several tests, given below as an example .

The employed fuel is a natural gas provided by the Italian distribution network.

Table 2

The speed of the mixture was evaluated "in cold conditions", i.e. in the absence of chemical reactions, inside the confinement chamber 17, taking on a constant

5 speed profile (average speed) . It is therefore to be considered as an approximate speed of the mixture upstream of the reaction zone. The same consideration is valid for the Reynolds number.

In such conditions, the (geometric) swirl number of

10 the current under examination is greater than 1, so that there is the generation downstream of the deviation means 9 of an intense recirculation zone characterized by negative values of the axial speed, which acts as a thermal energy source and unstable chemical species that

15 assist the self-maintenance of the flame and its propagation. Said recirculation zone also favors the mixing and premixing between the arriving reactant mixture and the burnt gases which are already formed due to the combustion reactions.

20 In such conditions, it was observed that the flame is stabilized downstream of the deviation means 9 in the lighting zone, remaining very suitably anchored therehere and showing high stability; the morphology of the flame is essentially conical, with length varying as a function of the supply flow rates and the position of the rod 8 inside the confinement chamber 17. The observed flame has:

- high stability; possibility of morphological variation as a function of the operating conditions and the position of the rod 8 inside the confinement chamber 17;

- an intense azure color, indication of a "clean" burning without the formation of CO, unburnt substances or particulate. The measurements carried out with regard to the polluting emissions produced by said flame have recorded, inside the functioning operating field, values of NO _x

(expressed as mg/Nm ³ of NO ₂ at 3% O ₂ ) comprised between 80 and 50, with negligible CO emission (i.e. below the detection limit of the measurement instrument) .

The confinement chamber 17, also downstream of the flame trigger, remains substantially cold. From measurements carried out by means of a wall thermocouple, the confinement chamber 17, in the first portion 107 made of quartz, has an external temperature which remains close to the environmental temperature up to about half the length of the chamber 17, after which point it reaches about 100 ⁰ C at the start of the second portion 207 and then has a strong gradient up to reaching about 430 ⁰ C close to the end of the chamber 17, where the flame tends to run over the walls of the tube itself.

Since the walls of the confinement tube are relatively cold, the heat dispersion by the apparatus is reduced, in practice this always generates 3-4% loss of useful yield.

The combustion intensity is particularly high (about 8*1O ^A4 kW/m ³ ) , thus ensuring the possibility to generate high heat power inside an apparatus that is compact and of simple assembly and structure. This leads to a great advantage in applications which require high local heat power, inside a simple and compact apparatus with high efficiency, e.g. in thermal cutting, in local thermal treatment of materials and in those applications which provide for the use of high speed burners.

In the scope of the preceding description and in the following claims, all the numerical quantities indicating amounts, parameters, percentages and so on should be intended as preceded in every case by the term "about", if not otherwise indicated. In addition, all the numerical quantity ranges include all the possible combinations of maximum and minimum numeric values, and all the possible intermediate ranges, in addition to those specifically indicated in the text.

The man skilled in the art, for the purpose of satisfying contingent and specific needs, can make further modifications and variants to the burner according to the present invention, all comprised in the protective scope of the present invention.