The present invention relates to a flameholder (1) for holding a flame (84) comprising a flow of combusting fluid. The flameholder (1) comprises an inlet (32) and an outlet (34) which defines a flow path between them. A magnetic-field generator (10, 20) is arranged to generate a magnetic field (40) across the flow path such that in use the fluid flows in the flow path through the magnetic field (40). As the fluid flows through the magnetic field an electric current is induced in the fluid. This results in a force (86) being generated on the fluid which opposes the flow direction (82). This force acts to hold the flame in place. If the flow path is in the form of a closed loop, in a plane perpendicular to the flow direction (82), then the current (50) induced in the fluid can flow in a closed loop entirely within the fluid.

The invention relates to a flameholder, and particularly, although not exclusively, to a controllable magnetic flameholder for a gas turbine engine.

A gas turbine engine comprises a compressor, a combustion chamber and a turbine. The compressor draws in air and pressurises it. This pressurised air is then fed to the combustion chamber where it is combusted with fuel. This causes the temperature and volume of the air to increase. The high-pressure, high-temperature air then expands through the turbine, thereby generating energy.

In the combustion chamber a flame is generated by the combustion of fuel. Because of the fast flow of air through the combustion chamber it is necessary to shield the flame in order to prevent it from being extinguished. It is known to use a device known as a flameholder (or flame can) for this purpose. As shown inFIG. 1, one type of flameholder comprises a perforated metal can3which shields the flame5from the flow of air6through the combustion chamber. The perforations4in the can allow air into the can3so as to maintain combustion. The perforations4in the can are designed so as to allow just enough air into the can for stoichiometric combustion.

The above described flameholder is simple and is satisfactory for some circumstances. However, it is inflexible because the flameholding is not controllable.

It is therefore desirable to provide a flameholder which allows the flameholding to be controlled.

According to a first aspect of the present invention there is provided a flameholder for holding a flame comprising a flow of combusting fluid, comprising: an inlet and an outlet defining a flow path between them; and a magnetic-field generator arranged to generate a magnetic field across the flow path, the magnetic-field generator comprises a first pole piece having a cavity within which a second pole piece is located in such a way that an opening is formed between the first and second pole pieces which in use provides the flow path for the fluid and across which the magnetic field is generated; wherein in use the fluid flows in the flow path through the magnetic field, which then induces a flow of electric current in the fluid, thereby generating a force on the fluid which opposes the flow direction, wherein the induced current is in the form of a closed loop in a plane perpendicular to the flow direction such that in use the induced current can flow in a closed loop entirely within the fluid.

The force generated on the fluid which opposes the flow direction acts to hold the flame in place, thereby preventing the flame from being blown out. Because the current flows entirely within the fluid it is not necessary to provide electrodes which would be susceptible to erosion and would need to be replaced periodically.

In one embodiment the first pole piece is generally annular and the second pole piece is generally cylindrical, the first and second pole pieces being concentric, so that the flow path is annular in a plane perpendicular to the flow direction.

In a preferred arrangement the magnetic-field generator comprises first and second electromagnets, each including a pole piece and a winding. The electromagnets may be superconducting electromagnets and comprise superconducting windings. The use of electromagnets allows the strength of the flame-holding force to be controlled. The magnetic-field generator may generate an alternating magnetic field for various electromagnetic effects. This may help to suppress combustion instabilities such as rumble. The alternating magnetic field may have an alternating component and a steady component and the steady component is always in the same direction, this helps to prevent the flame from being blown out. The alternating magnetic field may be a combination of two or more frequencies.

In one embodiment the magnetic-field generator is arranged to generate a magnetic field that is stronger in the region of the outlet than the inlet. This promotes a current, and hence a flame-holding force, to be formed at the outlet as opposed to the inlet. The magnetic field generator may be arranged to generate a magnetic field that is stronger in the region of the inlet than the outlet.

The first pole piece may have a recess on an inner surface and a winding is located in the recess. The second pole piece may have a recess on an outer surface and a winding is located in the recess.

The magnetic field generator may be positioned closer to the outlet than the inlet. The winding(s) may be positioned closer to the outlet than the inlet. The magnetic field generator may be positioned closer to the inlet than the outlet. The winding(s) may be positioned closer to the inlet than the outlet.

At least one fuel burner may be located within the inlet between the first pole piece and the second pole piece. A plurality of fuel burners may be located within the inlet between the first pole piece and the second pole piece.

The first pole piece and/or the second pole piece may have at least one cooling duct for the passage of a cooling fluid to cool the first pole piece and/or the second pole piece.

The at least one fuel burner may be positioned upstream of the magnetic field generator. The at least one fuel burner may be positioned upstream of the winding(s).

The inlet of the flameholder may have an end cap, the end cap having a plurality of apertures and each aperture has a respective one of the plurality of fuel burners, the outlet of the flameholder has a plurality of spokes extending between the first pole piece and the second pole piece. Each spoke comprises a ferromagnetic core and an insulating refractory coating.

The invention is also concerned with a gas turbine engine including a flameholder according to any statement herein.

According to a second aspect of the invention there is provided a method of holding a flame, comprising: causing a flame comprising a flow of combusting fluid to flow along a flow path from an inlet to an outlet; and generating a magnetic field across the flow of the fluid in such a way that current is induced in the fluid, thereby generating a force on the fluid which opposes the flow direction, the induced current flows in the form of a closed loop in a plane perpendicular to the flow direction and the induced current flows in a closed loop entirely within the fluid. The closed loop may be generally annular.

In a particularly preferred arrangement the magnetic field is an alternating magnetic field. The alternating magnetic field may have an alternating component and a steady component and the steady component is always in the same direction. The alternating magnetic field may be a combination of two or more frequencies.

The magnetic field may be stronger in the region of the outlet than the inlet.

The invention may comprise any combination of the features and/or limitations referred to herein, except combinations of such features as are mutually exclusive.

FIGS. 2 and 3show a first embodiment of a magnetic flameholder1. The flameholder1comprises a magnetic-field generator in the form of an outer annular pole piece10and an inner cylindrical pole piece20. The outer and inner pole pieces10,20are concentric with one another and an annular opening (or gap)30between them defines a flow path having an inlet32and an outlet34.

Referring now toFIG. 3, the outer pole piece10comprises an annular recess12on an inner surface14and the inner pole piece20comprises an annular recess22on an outer surface24. An outer winding16is located in the recess12of the outer pole piece10and an inner winding26is located in the recess22of the inner pole piece20. The windings16,26are connected to a controller (not shown) which supplies a flow of current to the windings16,26. This produces a radial magnetic field40between the pole pieces10,20. The direction of the magnetic field40depends on the direction of the flow of current in the windings16,26.

The pole pieces10,20may be made of any suitable material such as a low-loss magnetic material. Examples include laminated electric steels and high-resistivity magnetic materials such as ferrite. Spacers, struts or supports (not shown) may be provided in the gap between the inner pole piece20and the outer pole piece10in order to maintain the relative positions of the inner and outer pole pieces. The spacers, struts or supports may carry electrical connections to the windings16,26. As shown inFIG. 4, in use the flameholder1is positioned at the opening of a fuel supply conduit80that supplies combustible fluid flowing in a first direction82. The fluid flows through the annular opening30of the flameholder1and when ignited generates a flame84that is positioned in the region of the annular opening30.

The flame84is considered to be a region where combustion of the combustible fluid occurs. There is flow of particles through this region that include fuel particles and products of combustion. The flame84can therefore be considered to be a flow of combusting fluid. The term ‘flame flow’ will be used to mean a flow of combusting fluid.

Due to the heat generated by combustion of the fluid, the fluid undergoes thermal ionization. This means that the flame is capable of conducting current.

With reference toFIGS. 2 and 3the annular flame flow in the annular opening30interacts with the radial magnetic field40generated between the windings16,26of the outer and inner pole pieces10,20as follows.

The flame84flows perpendicularly through the magnetic field40in the annular gap30. Since the flame84is an ionized gas, a current50is induced in the flame84in a direction perpendicular to both the flow direction82and the magnetic field40. As shown inFIG. 2, this results in an annular current50flowing in the annular flame84. In this embodiment two annular currents50,52are induced in the flame84, one at the inlet32and one at the outlet34of the flameholder1. Since the direction of the magnetic field is opposite at the inlet32and the outlet34, the annular currents50,52flow in opposite directions.

The annular current flows50,52interact with the magnetic field to produce a Lorentz force86on the flame84. This force is mutually perpendicular to the current flow50and the magnetic field40and is in the opposite direction to the flame flow. This Lorentz force86holds the flame84in the desired position and is known as the flame-holding force.

The flame-holding force86can be altered by changing the strength of the magnetic field40in the annular opening30. This is can be done by changing the current supplied to the windings16,26. The magnetic flameholder1therefore allows the position of the flame to be readily controlled.

The magnetic flameholder1is able control combustion instabilities that are known as ‘rumble’. This can be done by supplying AC current to the windings16,26. The waveform of the AC current is chosen to produce a varying magnetic field40that reduces or suppresses rumble. It may be desirable to superimpose an AC current on a DC current so that the magnetic field is always in the same direction.

It is also possible to suppress rumble by supplying two or more AC currents of different frequencies to the windings16,26. For example, if two frequencies f1and f2are used, non-linear effects in the annular currents50,52will generate additional frequencies including the sum frequency (f1+f2) and the difference frequency (f1−f2) of the original two frequencies. Heterodyne operation could therefore be used to improve the performance of the magnetic flameholder.

For example, if the magnetic flameholder1operates efficiently over a band of frequencies including f1and f2, but rumble control is required at a different frequency fr, the frequencies f1and f2can be chosen so that the frequency fr required for rumble control is the sum or the difference of the frequencies f1, f2. For example, if rumble control at fr=20 Hz is required but the magnetic flameholder operates more efficiently in the kilo hertz band, f1can be made 2020 Hz and f2can be made 2000 Hz so that the difference between f1and f2is the frequency required for rumble control (i.e. fr=f1−f2=20 Hz).

Heterodyne operation could be achieved by using transformers, filters or other suitable devices to supply two or more AC voltages to the windings16,26.

In order to improve the electrical conductivity of the flame84the combustible fluid may be seeded with easily ionisable materials such as alkali or alkaline-earth metals or their compounds.

The performance of the magnetic flameholder1can also be improved by further ionisation of the flame84within the flameholder1. This may be done by irradiating the flame84with electromagnetic radiation such as microwaves, ultraviolet, X-rays or gamma rays, for example, or with corpuscular radiation such as alpha rays, beta rays, or beams of ions, for example. The flame may also be seeded with chemicals such as alkali metals or their compounds or with radioactive substances. Modulating the means of ionisation may also improve the combustion and may also improve the control of combustion instabilities such as rumble by tuning the modulation to relevant frequency components in the rumble.

Rumble may be a particular problem when burning fuel of having a low or a variable calorific value, such as municipal refuse or coal having a high ash content.

Heterodyning could be applied to the means of ionization in a similar way as described above for heterodyne control of the current in the windings. Also, heterodyne operation could also be achieved or assisted by varying the current in the windings at one frequency and the means of ionisation at a different frequency.

FIG. 5shows a second embodiment of a magnetic flameholder101in which only one annular current150is induced in the flame, in the region of the inlet132. This is done by positioning the windings116,126on the outer and inner pole pieces110,120closer to the inlet132than the outlet. This means that the magnetic field140is stronger at the inlet132and therefore an annular current150is induced in the flame in this region. The magnetic field140towards the outlet134is too weak to induce an annular current in this region or the annular current induced is small.

FIGS. 6 and 7show a third embodiment of a magnetic flameholder201in which only one annular current250is induced in the flame84, in the region of the outlet234. This is done by positioning the windings216,226on the outer and inner pole pieces210,220closer to the outlet234than the inlet232. This means that the magnetic field240is stronger at the outlet234and therefore an annular current250is induced in the flame in this region. The magnetic field240towards the inlet232is too weak to induce an annular current in this region or the annular current induced is small.

The above described arrangement allows fuel burners260to be located in the annular opening230at the inlet232. This provides a more compact arrangement. A gap is provided between the pole pieces210,220and the burners260to allow air to be drawn into the flameholder201. In order to improve the magnetic circuit of the flameholder201, the burners260can be made of a low-loss magnetic material such as laminated electric steels, or high-resistivity magnetic materials such as ferrite. This reduces losses due to eddy currents and magnetic hysteresis.

FIG. 8shows a fourth embodiment of a magnetic flameholder301. The inner and outer pole pieces310,320are provided with cooling ducts318,328. These cooling ducts318,328are supplied with a cooling fluid, such as air, which acts to cool the pole pieces310,320. As the air cools the pole pieces310,320its temperature increases. This therefore pre-heats the air which is then used in the combustion process, thus saving energy. The electrical conductors in the windings may be made hollow for the circulation of cooling fluid.

FIGS. 9,10and11show a fifth embodiment of a magnetic flameholder401. The inlet432end of the opening430has an end cap470that has holes472in it for burners460. The end cap470is integrally formed with the outer and inner pole pieces410,420. The outlet434end of the opening430has an arrangement of spokes476that bridge the gap between the outer and inner pole pieces410,420. Each spoke476comprises a ferromagnetic core477and an insulating refractory coating478. The end cap470and the spokes476allow magnetic flux to pass more easily between the outer and inner pole piece410,420.

As shown inFIG. 11, the outer pole piece410has two annular recesses412,413and the inner pole piece420has two annular recesses422,423. First and second outer windings416,417are located in the recesses412,413of the outer pole piece410and first and second inner windings426,427are located in the recesses422,423of the inner pole piece420. Current is supplied to the first outer winding416and the first inner winding426in the same direction, and an opposite flow of current is supplied to the second outer winding417and the second inner winding427. This produces a concentrated magnetic field440in a region between the first and second recesses. In use, an annular current450is therefore induced in the flame in this region, thus generating a Lorentz force in the opposite direction to the flow of the flame. This flame-holding force acts to hold the flame in place.

The high-temperatures that the magnetic flameholder1is exposed to during use may adversely affect its performance. There are a number of ways of mitigating this. These include: coating the surfaces with refractory materials such as thermal barrier coatings, using high-temperature insulation in the electrical windings such as glass fibre, applying similar techniques to those used in fire-resistance cables to protect the electrical windings, using high-temperature conductors such as tungsten for the windings.

Although in the foregoing embodiments it has been described that the magnetic field is generated by electromagnets, it is possible to make the pole pieces10,20either partially or entirely out of permanent magnets. Also, it is not essential that the flow path defined by the opening30is annular. In other embodiments the flow path in a direction perpendicular to the flow direction may be a closed loop of any shape such that current can flow entirely in the flame.

However, with reference toFIG. 12, in yet a further embodiment the current550induced in the flame does not flow in a closed loop within the fluid. Instead, electrodes592,594are provided that allow the flow of current550through the flame584. However, the basic principle is the same. The flame584flows through a magnetic field540generated a magnetic-field generator, shown schematically, comprising two permanent magnets510,520, although other techniques of generating a magnetic field may be used. The magnetic field induces a current550in the flame584which flows through the flame between the two electrodes592,594. The current550induced in the flame584interacts with the magnetic field540to generate a Lorentz force that opposes the flow of the flame584. This flame-holding force acts to hold the flame in place.

FIG. 13shows a turbofan gas turbine engine500comprising in flow series an intake502, a fan section504, a compressor section506, a combustion section508, a turbine section510and an exhaust512. The fan section502comprises a fan514. The compressor section504comprises an intermediate pressure compressor516and a high pressure compressor518. The turbine section510comprises a high pressure turbine520, an intermediate pressure turbine522and a low pressure turbine524. The low pressure turbine524is arranged to drive the fan514via a first shaft526. The intermediate pressure turbine522is arranged to drive the intermediate pressure compressor516via a second shaft528and the high pressure turbine520is arranged to drive the high pressure compressor518via a third shaft530. The combustion section508comprises an annular combustion chamber532and a plurality of fuel burners534are arranged to supply fuel into the annular combustion chamber532. A fuel supply, fuel tank,536is arranged to supply fuel to the fuel burners534via a fuel pipe538. The annular combustion chamber532comprises a flameholder according to the present invention as discussed with reference toFIGS. 2 to 12.

Although it has been described that the flameholders are for use with a gas-turbine engine, they may be used with other combustion systems. Examples include, but are not limited to oil burners and pulverised fuel burners used in installations such as power station boilers, space heating boilers and refuse incinerators.