Title of Invention	"A SYSTEM FOR CONTROLLING FLAME PRESSURE IN A FIRING DEVICE"
Abstract	A system for controlling flame/pressure oscillations in a firing device having at least one burner for producing a flame (7); the flame (7) being directed to a combustion chamber (8)5 at least one gas-outlet opening (11,21), for gas (9,22) flow out, the flown out gas (9,22) enclosing the flame (7) in the form of i an envelope, the gas (9, 22) having a higher flow velocity in the flame-propagation direction (13) than the outer regions of the flame. A screen (19, 19, 20) surrounding the gas-outlet opening (11, 21) and running at a radial distance around the burner outlet is provided, and in that a flue-gas recirculation region (16) connected to the combustion chamber (8) is separated from said gas (9, 22) by means of said screen (15, 19, 20) thereby allowing the gas flow (9, 22) to expand as a free jet.

Title of Invention

"A SYSTEM FOR CONTROLLING FLAME PRESSURE IN A FIRING DEVICE"

Abstract

A system for controlling flame/pressure oscillations in a firing device having at least one burner for producing a flame (7); the flame (7) being directed to a combustion chamber (8)5 at least one gas-outlet opening (11,21), for gas (9,22) flow out, the flown out gas (9,22) enclosing the flame (7) in the form of i an envelope, the gas (9, 22) having a higher flow velocity in the flame-propagation direction (13) than the outer regions of the flame. A screen (19, 19, 20) surrounding the gas-outlet opening (11, 21) and running at a radial distance around the burner outlet is provided, and in that a flue-gas recirculation region (16) connected to the combustion chamber (8) is separated from said gas (9, 22) by means of said screen (15, 19, 20) thereby allowing the gas flow (9, 22) to expand as a free jet.

Full Text	The invention relates to a device for suppressing flame/pressure oscillations in a firing arrangement having at least one burner for producing a flame and a combustion chamber into which the flame is directed, the firing arrangement having at least one gas-outlet opening, from which gas flows out, this gas enclosing the flame in the form of an envelope and having a higher flow velocity in the flame-propagation direction than the outer regions of the flame, and to a gas turbine which is equipped with such a device. In industrial combustion ' plants, such as gas turbines, combustion chambers, hot-blast stoves, refuse incineration plants or industrial furnaces, or else in small firing arrangements, such as gas boilers or heating boilers in the domestic service sector, unstable operating states occur under certain conditions established by the firing parameters, such as thermal output and air coefficient, and these unstable operating states are characterized by periodic changes in the flame, which are accompanied by changes, in particular, in the static pressure in the combustion chamber and in plant parts connected upstream or downstream of the latter. These unstable states also occur in firing arrangements whose flames are adequately stabilized in terms of ignition by known measures, such as swirl flows, retention baffles, etc. The occurrence of these combustion instabilities often results in a changed behavior compared with the steady operation of the plant and, in addition to an increased noise nuisance, also causes intensified mechanical and/or thermal stressing of the combustion chamber or the combustion-chamber lining. Under unfavorable conditions, such flame/pressure oscillations may lead to the destruction of the plant -lain which they occur, so that a lot of effort is - 2 - made in order to avoid such flame/pressure oscillations. Thus, for example, the combustion-chamber geometry is changed by special built-in components, which, however, often only leads to a displacement of the oscillation frequencies which occur and therefore does not help to produce a general solution to the problem. Otherwise, special measures on an empirical basis are taken in each case when flame/pressure oscillations occur. Accordingly, EP-A-0 7 54 908 proposes a device as specified above in which the flame of a burner is enclosed as closely as possible with a flow of gas, the gas flow having a higher velocity in the flame-propagation direction than the outer or marginal regions of the flame or the burner main flow containing fuel. Insofar as the "outer regions of the flame" are referred to here as well as below, this refers to the reacting or reactive layers of a fuel or fuel-gas/air flow. The gas-envelope flow then transmits an axial impulse to these layers. "Flame-propagation direction" is intended here to designate the main propagation direction in the axial extent of a flame and in this respect is to be distinguished from the radial propagation direction of the flame. In this case, the principle of the invention is based on the knowledge that the oscillations are caused or amplified essentially by annular vortices forming periodically in the marginal region of the flame. These annular vortices, which are produced by curling-up of the marginal regions of the burner flow containing fuel, enclose with them, during their formation, hot flue gases which are already burnt out, are no longer reactive and cause rapid heating of the fuel/air mixture likewise - 3 - contained in the annular vortex and consequently an impulse-like, pressure-oscillation-exciting reaction of the fuel. In order to now prevent this formation of annular vortices, the flame, as described above, is surrounded with a gas-envelope flow, which discharges at as small a radial distance from, the flame or the burner main flow as possible and has a higher flow velocity in the flame-propagation direction than the outer or marginal regions of the flame. An axial impulse exchange thus occurs between envelope flow and flame or fuel-gas/air flow and results in an acceleration of the free flame or flow boundary layer of the fuel/air mixture, whereby the development of reactive vortices in this region is effectively countered. Insofar as corresponding annular vortices then occur again at the boundary layer between the gas-envelope flow and the surrounding medium (generally flue gases in the enclosed case), it is best if the gas-envelope flow contains no fuel, since no fuel-entrapping vortices can then form from the (fuel-free) envelope flow, which fuel-entrapping vortices may lead to a periodic reaction of fuel and thus to excitation of flame/pressure oscillations, as occur in the case of a flame or fuel/air flow without an envelope. The gas, which does not contain fuel, of the envelope flow is preferably air, which is available everywhere in sufficient quantity. However, it is also possible to use an inert gas here, although this would have a certain cost disadvantage. In particular in the case of inert gas,the object is to develop a device as described in such a way that a smaller gas flow or a smaller - 4 - gas quantity per unit of time is necessary in order to achieve the desired effect. However, even when air is used, it is desirable for less air to be provided for the envelope flow so that it is not necessary to provide too much compressor capacity or divert it in another way for this purpose. This object is achieved according to the invention in that a screen surrounding the gas-outlet opening and running at a radial distance around' the burner outlet is provided, by means; of which screen a flue-gas recirculation region connected to the combustion chamber is separated from said gas. It has been found that, on account of the spatial separation of the fuel/air mixture and the gases surrounding this fuel/air mixture in the form of an envelope from the outer recirculation flow of hot, burnt-out flue gases by means of the screen, the envelope flow is better protected against a lateral deflection and is therefore deflected in its direction to a less pronounced degree. As a result, lower impulse flow densities or gas velocities of the envelope flow are also sufficient in order to ensure that the envelope flow, with adequate impulse, that is to say with adequate excess velocity relative to the marginal regions of the flame, reaches those points located downstream of the burner outlet at which the periodic formation (referred to above) of reactive annular vortices, that is fuel-entrapping annular vortices, is to be avoided. The use of the screen thereby results in a considerable reduction in the requisite gas- or air-mass or air-impulse flow. Furthermore, by the flue-gas recirculation region being separated from the outlet location of the gas-envelope flow and thus from the gas-envelope flow, intermixing of hot flue gases from this region of the combustion chamber with the envelope flow. is - 5 - prevented. Such intermixing otherwise results in a pronounced reduction in the flow velocity of the envelope flow, the envelope fiow accordingly heating up at the same time. As a result, the design of the gas-envelope flow becomes less dependent on the rest of the construction of the firing arrangement surrounding it. In particular in the case of a plurality of burners possibly influencing one another, this is a further advantage of the invention. In this case, the screen itself may be designed to have a single shell, whereby it is also relatively simple to retrofit in already existing firing arrangements. The radial distance of the screen from the burner outlet is in this case to be selected in such a way that the envelope fiow is not decelerated too severely in an undesirable manner as a result of friction-induced adhesion on the wall. It must still be ensured that the envelope flow can reach the points at which it is intended to prevent the formation of annular vortices. In particular, with due regard to the fact that the gas-envelope flow expands starting from its outlet, the aim in this case is to achieve a situation in which the top edge of the screen is at a radial distance from the gas-outlet opening and the gas-envelope flow does not come into contact with the inside of the screen until just before this top edge. An undesirable inflow of hot flue gases along the inside of the screen, which would then be intermixed by the gas-envelope flow at its outlet location, can thus also be prevented. This aim may be achieved by the screen being of cylindrical design and being arranged concentrically to the burner outlet. However, it is also possible to give the screen itself a conical shape, the inclination of which is then adapted to the expansion angle of the envelope flow. In both cases, the screen extends - 5a - essentially parallel to the flame-propagation direction, compared with which, for example, the - 6 - extent of the conical screen in the radial direction is considerably smaller. In principle, the screen may also be designed to have a double shell and the gas forming the gas-envelope flow may flow through it. The gas envelope, for example completely or at least partly, cannot then discharge from the screen until the top edge of the latter. This enables the gas to still appropriately cool the screen, which, for example, is made of a high-temperature-resistant steel or else a ceramic material, and thus prevents the occurrence of thermal problems with regard to the screen. The invention is preferably used in gas turbines, in particular having a plurality of burners, preferably in annular combustion chambers, in which the effect according to the invention of the reduction in the mutual influencing becomes very noticeable. Further advantages and features of the invention follow from the description below of exemplary embodiments. In the accompanying drawing: Figure 1 shows a section through a firing arrangement equipped with a cylindrical screen; Figure 2 shows a plan view of a firing arrangement according to Figure 1 with a plurality of burners; Figure 3 shows a section through a firing arrangement equipped with a conical screen; Figure 4 shows a section through a firing arrangement with a screen which surrounds a shifted- forward burner outlet; Figure 5 shows a section through a firing arrangement with feed for the gas-envelope flow integrated in the screen. A firing arrangement equipped according to the invention is shown in section in Figure 1. This firing arrangement is a swirl burner to which a premixed fuel-gas/air mixture 1 is fed via a burner tube 2. This burner tube 2 ends - 7 - at a swirl front 3, which is rotationally symmetrical and has inclined guide blades 4 on its outer periphery. These guide blades have an inclination of about 30°, as a result of which the outflowing fuel-gas/air mixture is deflected and thus swirled. Furthermore, a plurality of holes 5 running through the swirl front 3 are made so as to be distributed radially over the periphery slightly further to the inside than the guide blades' 4, through which holes 5 a partial flow of the fuel-gas/air mixture can flow and thus helps to stabilize the flame by forming a pilot flame. The fue'1-gas/air mixture discharging from the burner is ignited on the outside 6 of the swirl front 3 and forms a flame 7, which enters a combustion chamber 8. In the example shown here, this combustion chamber 8 is the annular combustion chamber of a gas turbine, in which case the turbine sections arranged downstream of the combustion chamber on the right in Figure 1. are not shown. The flame 7 is formed by the outer regions of the reacting layers of the fuel-gas/air flow, which, with an intensive flame color, produce the flame contour which can be recognized by an observer. A gas envelope flows around this flame, which is formed by a fuel/air mixture bringing a reaction to completion. This envelope is produced by a gas flow 9 which is passed through the burner through an annular duct 10 parallel to the burner tube 2 and discharges from the burner at gas-outlet openings 11. A multiplicity of these gas-outlet openings 11 are distributed over the periphery of the burner. This multiplicity of openings is arranged close around the swirl front 3 of the burner, so that an envelope flow completely surrounding the flame forms from the plurality of gas flows 9 obtained in accordance with the number of gas-outlet openings 11. So that the flow velocity of the gas-envelope flows discharging from the gas-outlet openings 11 is accelerated to the desired value before they discharge from the annular - 8 - duct 10, quadrant nozzles 12, which produce a pronounced acceleration, in particular of the outer regions of the gas-envelope flow in the axial direction (that is, parallel to the axis 13 of the burner) , are fitted in the gas-outlet openings 11 in the exemplary embodiment shown here. On account of the quadrant nozzles 12, the flow velocity of the gas-envelope flow discharging from the gas-outlet openings 11 is accelerated to such an extent that the velocity in the direction of the axis 13 is considerably higher than that of the outer regions of the flame 7 of the burning fuel-gas/air mixture downstream of the swirl front '3. As a result, in the region between the fuel-gas/air mixture burning in a flame and the gas-envelope flow closely surrounding this flame, a boundary-layer acceleration of the partly burning, partly not yet ignited fuel-gas/air mixture is effected. Thus the formation of periodic, coherent annular-vortex structures in the marginal region of the flame is effectively prevented, which annular-vortex structures, due to a rapid reaction of the fuel contained in them by an in-phase supply of energy, may excite and amplify flame/pressure oscillations. The gas-envelope flow is normally discharged continuously from the gas-outlet openings 11. On the other hand, however, since the annular-vortex structures form periodically, it is also possible to operate tne air flow in a correspondingly periodic manner, that is in a discontinuous manner. Thus, although air-mass flow can be saved on the one hand, a considerably greater control input is necessary on the other hand. In particular, the control devices to be provided for this, such as valves, controllers, etc., involve high costs, and in addition such additional accessories result in an additional susceptibility to faults of the entire plant. - 9 - In the firing arrangement shown, a cylindrical screen 15 is welded to the end face 14 of the combustion chamber 8. This screen is at a radial distance from the burner and also surrounds the gas-outlet openings 11. Thus a flue-gas recirculation region 16 is separated from the gas flow 9 outside the screen 15. Thus flue gases, which flow essentially radially inward in this region on account of true rlow conditions and the flow path of which is indicated by arrow lines 17, are prevented from being sucked along into the gas flow 9 and from thus impairing the effect of the latter. Instead, the gas flow in the initial region can expand like free jets in an unaffected manner. This is also especially advantageous in a firing arrangement having a plurality of burners as shown in Figure 2 . Figure 2 is a section through an annular combustion chamber 8 of a gas turbine in which eight burners are distributed over the periphery. The swirl front 3 and the gas-outlet openings 11, which are arranged in an annular manner and produce a gas-envelope flow at the burner, can be seen in each case in the plan view of these burners. In order to screen this gas-envelope flow from the influence exerted by adjacent burners or flue-gas recirculations caused by the latter, each burner is surrounded by a corresponding screen 15. In the case of a cylindrical screen, as shown in Figure 1, the radial distance between the screen 15 and the gas-outlet opening 11 is selected in such a way that the gas-envelope flow 9, which expands like free jets starting from its discharge from the gasroutlet openings 11, does not come into contact with the inside of the screen 15 until in the vicinity of the top edge 18 of the screen 15. On the one hand, this makes it possible to prevent the gas flow from coming into contact with the screen 15 too prematurely and from being unintentionally decelerated by the friction with - 9a - the wall formed by the screen 15- On the other hand, it is thus ensured that a gap through which the flue gases can flow to the outlet regions of the gas-envelope flow, where they would be intermixed in an undesirable manner, does not - 10 - develop between gas-envelope flow and top edge 18 of the screen 15. The distance to be selected from these aspects, in the case of a Known aperture angle, dependent on gas density and gas temperature, of the gas-envelope flow, can be determined as a function of the extent of the screen parallel to the flame-propagation direction by simple trigonometrical relationships. For the sake of good order it may also be mentioned that, in the example described here, the flame-propagation direction coincides with the axis 13 of the burner. Instead of a cylindrical screen as shown in Figure 1, a conical screen 19 may also be used, the aperture angle of which is then to correspond approximately to the aperture angle of the gas-envelope flow 9 discharging from the gas-outlet openings 11. Furthermore, an embodiment in which the burner is shifted forward in the flame-propagation direction inside the cylindrical screen is shown in Figure 4 . The effect achieved here may essentially be attributed to the fact that the recirculation of the flue gas is effected with a flow component directed radially toward the burner in the flue-gas recirculation region 16, and thus the flue-gas recirculation, in the plane in which the gas-outlet openings 11 lie in the example shown in Figure 4, no longer has a noticeable radial flow component but is distinguished essentially by its axial flow. To this end, a further alternative is shown in Figure 5: provided in this case is a double-walled screen 20, through which the gas for the gas-envelope flow flows and which has corresponding gas-outlet openings 21. provided at the top edge, the gas-envelope flow 22 then discharging from the gas-outlet openings 21. - 11 - Here, too, in the region in which the gas-envelope flow 22 discharges from the gas-outlet openings 21, the recirculation of the flue gases has no substantial radial flow component, and the gas-envelope flow 22 can prevent the annular vortices without being impaired here by flue gases flowing in radially. In this case, it is assumed that the regions where annular vortices form periodically at the outer regions of the flame, as described above, lie downstream of the gas-outlet openings 21. In the example shown in Figure 5, the gas flowing through the double-walled screen at the same time also has a cooling function for the screen. In addition, the screens are in each case made of high-temperature-resistant steel or else of a corresponding ceramic material. In summary, it may thus be concluded that, by the use according to the invention of a screen, the influence of a flue-gas recirculation on the gas-envelope flow can be restricted, and therefore annular-vortex structures can be sufficiently prevented even with smaller gas volumetric flows or gas impulse flows. In particular in gas turbines which also have annular combustion chambers, operation with fewer problems can thus be achieved with the invention. -12- WE CLAIMS 1. A system for controlling flame/pressure oscillations in a firing device having at least one burner for producing a flame (7); the flame (7) being directed to a combustion chamber (8): at least one gas-outlet opening (11,21), for gas (9,22) flow out, the flown out gas (9,22) enclosing the flame (7) in the form of an envelope, the gas (9,22) having a higher flow velocity in the flame-propagation direction (13) than the outer regions of the flame, characterized in that a screen (15,19,20) surrounding the gas-outlet opening (11,21) and running at a radial distance around the burner outlet is provided, and in that a flue-gas recirculation region (16) connected to the combustion chamber (8) is separated from said gas (9,22) by means of said screen (15), 19,20) thereby allowing the gas flow (9,22) to expand as a free jet. 2. The system as claimed in claim 1, wherein the screen (15,19) has a single shell. 3. The system as claimed in claim 1, wherein the screen (20) has a double shell, the gas (22) flowing through the screen (20). 4. The system a© claimed in claim 1 wherein the screen (15,19,20) extends essentially parallel to the flame-propagation direction (13). 13. 5- The system as claimed in claim 1, wherein the screen (15,20) is cylindrical and concentric to the burner. 6- The system as claimed in claim 1, wherein the screen (19) has a conical shape. 7. The system as claimed in claim 1, wherein the top edge (18) of the screen (13) is at a radial distance from the gas—outlet opening (11), and wherein the gas-envelope flow (9) does not come into contact with the inside of the screen until the gas—envelope flow (9) reaches the end of the screen. 9. The system as claimed in claim 1, wherein the screen comprises ceramic material. 10. The system as claimed in claim 1, wherein the firing device has plurality of turbines. 11. The system as claimed in claim 1, wherein the combustion chamber (6) is an annular combustion chamber. A system for controlling flame/pressure oscillations in a firing device having at least one burner for producing a flame (7); the flame (7) being directed to a combustion chamber (8)5 at least one gas-outlet opening (11,21), for gas (9,22) flow out, the flown out gas (9,22) enclosing the flame (7) in the form of i an envelope, the gas (9, 22) having a higher flow velocity in the flame-propagation direction (13) than the outer regions of the flame. A screen (19, 19, 20) surrounding the gas-outlet opening (11, 21) and running at a radial distance around the burner outlet is provided, and in that a flue-gas recirculation region (16) connected to the combustion chamber (8) is separated from said gas (9, 22) by means of said screen (15, 19, 20) thereby allowing the gas flow (9, 22) to expand as a free jet.

Full Text

The invention relates to a device for suppressing flame/pressure oscillations in a firing arrangement having at least one burner for producing a flame and a combustion chamber into which the flame is directed, the firing arrangement having at least one gas-outlet opening, from which gas flows out, this gas enclosing the flame in the form of an envelope and having a higher flow velocity in the flame-propagation direction than the outer regions of the flame, and to a gas turbine which is equipped with such a device.
In industrial combustion ' plants, such as gas turbines, combustion chambers, hot-blast stoves, refuse incineration plants or industrial furnaces, or else in small firing arrangements, such as gas boilers or heating boilers in the domestic service sector, unstable operating states occur under certain conditions established by the firing parameters, such as thermal output and air coefficient, and these unstable operating states are characterized by periodic changes in the flame, which are accompanied by changes, in particular, in the static pressure in the combustion chamber and in plant parts connected upstream or downstream of the latter. These unstable states also occur in firing arrangements whose flames are adequately stabilized in terms of ignition by known measures, such as swirl flows, retention baffles, etc.
The occurrence of these combustion instabilities often results in a changed behavior compared with the steady operation of the plant and, in addition to an increased noise nuisance, also causes intensified mechanical and/or thermal stressing of the combustion chamber or the combustion-chamber lining. Under unfavorable conditions, such flame/pressure oscillations may lead to the destruction of the plant

-lain which they occur, so that a lot of effort is

- 2 -
made in order to avoid such flame/pressure oscillations. Thus, for example, the combustion-chamber geometry is changed by special built-in components, which, however, often only leads to a displacement of the oscillation frequencies which occur and therefore does not help to produce a general solution to the problem. Otherwise, special measures on an empirical basis are taken in each case when flame/pressure oscillations occur.
Accordingly, EP-A-0 7 54 908 proposes a device as specified above in which the flame of a burner is enclosed as closely as possible with a flow of gas, the gas flow having a higher velocity in the flame-propagation direction than the outer or marginal regions of the flame or the burner main flow containing fuel.
Insofar as the "outer regions of the flame" are referred to here as well as below, this refers to the reacting or reactive layers of a fuel or fuel-gas/air flow. The gas-envelope flow then transmits an axial impulse to these layers.
"Flame-propagation direction" is intended here to designate the main propagation direction in the axial extent of a flame and in this respect is to be distinguished from the radial propagation direction of the flame.
In this case, the principle of the invention is based on the knowledge that the oscillations are caused or amplified essentially by annular vortices forming periodically in the marginal region of the flame. These annular vortices, which are produced by curling-up of the marginal regions of the burner flow containing fuel, enclose with them, during their formation, hot flue gases which are already burnt out, are no longer reactive and cause rapid heating of the fuel/air mixture likewise

- 3 -
contained in the annular vortex and consequently an impulse-like, pressure-oscillation-exciting reaction of
the fuel.
In order to now prevent this formation of annular vortices, the flame, as described above, is surrounded with a gas-envelope flow, which discharges at as small a radial distance from, the flame or the burner main flow as possible and has a higher flow velocity in the flame-propagation direction than the outer or marginal regions of the flame. An axial impulse exchange thus occurs between envelope flow and flame or fuel-gas/air flow and results in an acceleration of the free flame or flow boundary layer of the fuel/air mixture, whereby the development of reactive vortices in this region is effectively countered.
Insofar as corresponding annular vortices then occur again at the boundary layer between the gas-envelope flow and the surrounding medium (generally flue gases in the enclosed case), it is best if the gas-envelope flow contains no fuel, since no fuel-entrapping vortices can then form from the (fuel-free) envelope flow, which fuel-entrapping vortices may lead to a periodic reaction of fuel and thus to excitation of flame/pressure oscillations, as occur in the case of a flame or fuel/air flow without an envelope.
The gas, which does not contain fuel, of the envelope flow is preferably air, which is available everywhere in sufficient quantity. However, it is also possible to use an inert gas here, although this would have a certain cost disadvantage.
In particular in the case of inert gas,the object is to develop a device as described in such a way that a smaller gas flow or a smaller

- 4 -
gas quantity per unit of time is necessary in order to achieve the desired effect. However, even when air is used, it is desirable for less air to be provided for the envelope flow so that it is not necessary to provide too much compressor capacity or divert it in another way for this purpose.
This object is achieved according to the invention in that a screen surrounding the gas-outlet opening and running at a radial distance around' the burner outlet is provided, by means; of which screen a flue-gas recirculation region connected to the combustion chamber is separated from said gas.
It has been found that, on account of the spatial separation of the fuel/air mixture and the gases surrounding this fuel/air mixture in the form of an envelope from the outer recirculation flow of hot, burnt-out flue gases by means of the screen, the envelope flow is better protected against a lateral deflection and is therefore deflected in its direction to a less pronounced degree. As a result, lower impulse flow densities or gas velocities of the envelope flow are also sufficient in order to ensure that the envelope flow, with adequate impulse, that is to say with adequate excess velocity relative to the marginal regions of the flame, reaches those points located downstream of the burner outlet at which the periodic formation (referred to above) of reactive annular vortices, that is fuel-entrapping annular vortices, is to be avoided.
The use of the screen thereby results in a considerable reduction in the requisite gas- or air-mass or air-impulse flow.
Furthermore, by the flue-gas recirculation region being separated from the outlet location of the gas-envelope flow and thus from the gas-envelope flow, intermixing of hot flue gases from this region of the combustion chamber with the envelope flow. is

- 5 -
prevented. Such intermixing otherwise results in a pronounced reduction in the flow velocity of the envelope flow, the envelope fiow accordingly heating up at the same time.
As a result, the design of the gas-envelope flow becomes less dependent on the rest of the construction of the firing arrangement surrounding it. In particular in the case of a plurality of burners possibly influencing one another, this is a further advantage of the invention. In this case, the screen itself may be designed to have a single shell, whereby it is also relatively simple to retrofit in already existing firing arrangements.
The radial distance of the screen from the burner outlet is in this case to be selected in such a way that the envelope fiow is not decelerated too severely in an undesirable manner as a result of friction-induced adhesion on the wall. It must still be ensured that the envelope flow can reach the points at which it is intended to prevent the formation of annular vortices.
In particular, with due regard to the fact that the gas-envelope flow expands starting from its outlet, the aim in this case is to achieve a situation in which the top edge of the screen is at a radial distance from the gas-outlet opening and the gas-envelope flow does not come into contact with the inside of the screen until just before this top edge. An undesirable inflow of hot flue gases along the inside of the screen, which would then be intermixed by the gas-envelope flow at its outlet location, can thus also be prevented.
This aim may be achieved by the screen being of cylindrical design and being arranged concentrically to the burner outlet. However, it is also possible to give the screen itself a conical shape, the inclination of which is then adapted to the expansion angle of the envelope flow. In both cases, the screen extends

- 5a -
essentially parallel to the flame-propagation direction, compared with which, for example, the

- 6 -
extent of the conical screen in the radial direction is considerably smaller.
In principle, the screen may also be designed to have a double shell and the gas forming the gas-envelope flow may flow through it. The gas envelope, for example completely or at least partly, cannot then discharge from the screen until the top edge of the latter. This enables the gas to still appropriately cool the screen, which, for example, is made of a high-temperature-resistant steel or else a ceramic material, and thus prevents the occurrence of thermal problems with regard to the screen.
The invention is preferably used in gas turbines, in particular having a plurality of burners, preferably in annular combustion chambers, in which the effect according to the invention of the reduction in the mutual influencing becomes very noticeable.
Further advantages and features of the invention
follow from the description below of exemplary

embodiments. In the accompanying drawing:
Figure 1 shows a section through a firing arrangement
equipped with a cylindrical screen; Figure 2 shows a plan view of a firing arrangement
according to Figure 1 with a plurality of
burners; Figure 3 shows a section through a firing arrangement
equipped with a conical screen; Figure 4 shows a section through a firing arrangement
with a screen which surrounds a shifted-
forward burner outlet; Figure 5 shows a section through a firing arrangement
with feed for the gas-envelope flow
integrated in the screen. A firing arrangement equipped according to the invention is shown in section in Figure 1. This firing arrangement is a swirl burner to which a premixed fuel-gas/air mixture 1 is fed via a burner tube 2. This burner tube 2 ends

- 7 -
at a swirl front 3, which is rotationally symmetrical and has inclined guide blades 4 on its outer periphery. These guide blades have an inclination of about 30°, as a result of which the outflowing fuel-gas/air mixture is deflected and thus swirled. Furthermore, a plurality of holes 5 running through the swirl front 3 are made so as to be distributed radially over the periphery slightly further to the inside than the guide blades' 4, through which holes 5 a partial flow of the fuel-gas/air mixture can flow and thus helps to stabilize the flame by forming a pilot flame. The fue'1-gas/air mixture discharging from the burner is ignited on the outside 6 of the swirl front 3 and forms a flame 7, which enters a combustion chamber 8. In the example shown here, this combustion chamber 8 is the annular combustion chamber of a gas turbine, in which case the turbine sections arranged downstream of the combustion chamber on the right in Figure 1. are not shown.
The flame 7 is formed by the outer regions of the reacting layers of the fuel-gas/air flow, which, with an intensive flame color, produce the flame contour which can be recognized by an observer. A gas envelope flows around this flame, which is formed by a fuel/air mixture bringing a reaction to completion. This envelope is produced by a gas flow 9 which is passed through the burner through an annular duct 10 parallel to the burner tube 2 and discharges from the burner at gas-outlet openings 11. A multiplicity of these gas-outlet openings 11 are distributed over the periphery of the burner. This multiplicity of openings is arranged close around the swirl front 3 of the burner, so that an envelope flow completely surrounding the flame forms from the plurality of gas flows 9 obtained in accordance with the number of gas-outlet openings 11.
So that the flow velocity of the gas-envelope flows discharging from the gas-outlet openings 11 is accelerated to the desired value before they discharge from the annular

- 8 -
duct 10, quadrant nozzles 12, which produce a pronounced acceleration, in particular of the outer regions of the gas-envelope flow in the axial direction (that is, parallel to the axis 13 of the burner) , are fitted in the gas-outlet openings 11 in the exemplary embodiment shown here.
On account of the quadrant nozzles 12, the flow velocity of the gas-envelope flow discharging from the gas-outlet openings 11 is accelerated to such an extent that the velocity in the direction of the axis 13 is considerably higher than that of the outer regions of the flame 7 of the burning fuel-gas/air mixture downstream of the swirl front '3. As a result, in the region between the fuel-gas/air mixture burning in a flame and the gas-envelope flow closely surrounding this flame, a boundary-layer acceleration of the partly burning, partly not yet ignited fuel-gas/air mixture is effected. Thus the formation of periodic, coherent annular-vortex structures in the marginal region of the flame is effectively prevented, which annular-vortex structures, due to a rapid reaction of the fuel contained in them by an in-phase supply of energy, may excite and amplify flame/pressure oscillations.
The gas-envelope flow is normally discharged continuously from the gas-outlet openings 11. On the other hand, however, since the annular-vortex structures form periodically, it is also possible to operate tne air flow in a correspondingly periodic manner, that is in a discontinuous manner. Thus, although air-mass flow can be saved on the one hand, a considerably greater control input is necessary on the other hand. In particular, the control devices to be provided for this, such as valves, controllers, etc., involve high costs, and in addition such additional accessories result in an additional susceptibility to faults of the entire plant.

- 9 -
In the firing arrangement shown, a cylindrical screen 15 is welded to the end face 14 of the combustion chamber 8. This screen is at a radial distance from the burner and also surrounds the gas-outlet openings 11. Thus a flue-gas recirculation region 16 is separated from the gas flow 9 outside the screen 15. Thus flue gases, which flow essentially radially inward in this region on account of true rlow conditions and the flow path of which is indicated by arrow lines 17, are prevented from being sucked along into the gas flow 9 and from thus impairing the effect of the latter. Instead, the gas flow in the initial region can expand like free jets in an unaffected manner.
This is also especially advantageous in a firing arrangement having a plurality of burners as shown in Figure 2 . Figure 2 is a section through an annular combustion chamber 8 of a gas turbine in which eight burners are distributed over the periphery. The swirl front 3 and the gas-outlet openings 11, which are arranged in an annular manner and produce a gas-envelope flow at the burner, can be seen in each case in the plan view of these burners. In order to screen this gas-envelope flow from the influence exerted by adjacent burners or flue-gas recirculations caused by the latter, each burner is surrounded by a corresponding screen 15.
In the case of a cylindrical screen, as shown in Figure 1, the radial distance between the screen 15 and the gas-outlet opening 11 is selected in such a way that the gas-envelope flow 9, which expands like free jets starting from its discharge from the gasroutlet openings 11, does not come into contact with the inside of the screen 15 until in the vicinity of the top edge 18 of the screen 15. On the one hand, this makes it possible to prevent the gas flow from coming into contact with the screen 15 too prematurely and from being unintentionally decelerated by the friction with

- 9a -
the wall formed by the screen 15- On the other hand, it is thus ensured that a gap through which the flue gases can flow to the outlet regions of the gas-envelope flow, where they would be intermixed in an undesirable manner, does not

- 10 -
develop between gas-envelope flow and top edge 18 of the screen 15.
The distance to be selected from these aspects, in the case of a Known aperture angle, dependent on gas density and gas temperature, of the gas-envelope flow, can be determined as a function of the extent of the screen parallel to the flame-propagation direction by simple trigonometrical relationships. For the sake of good order it may also be mentioned that, in the example described here, the flame-propagation direction coincides with the axis 13 of the burner.
Instead of a cylindrical screen as shown in Figure 1, a conical screen 19 may also be used, the aperture angle of which is then to correspond approximately to the aperture angle of the gas-envelope flow 9 discharging from the gas-outlet openings 11.
Furthermore, an embodiment in which the burner is shifted forward in the flame-propagation direction inside the cylindrical screen is shown in Figure 4 . The effect achieved here may essentially be attributed to the fact that the recirculation of the flue gas is effected with a flow component directed radially toward the burner in the flue-gas recirculation region 16, and thus the flue-gas recirculation, in the plane in which the gas-outlet openings 11 lie in the example shown in Figure 4, no longer has a noticeable radial flow component but is distinguished essentially by its axial flow.
To this end, a further alternative is shown in Figure 5: provided in this case is a double-walled screen 20, through which the gas for the gas-envelope flow flows and which has corresponding gas-outlet openings 21. provided at the top edge, the gas-envelope flow 22 then discharging from the gas-outlet openings 21.

- 11 -
Here, too, in the region in which the gas-envelope flow 22 discharges from the gas-outlet openings 21, the recirculation of the flue gases has no substantial radial flow component, and the gas-envelope flow 22 can prevent the annular vortices without being impaired here by flue gases flowing in radially. In this case, it is assumed that the regions where annular vortices form periodically at the outer regions of the flame, as described above, lie downstream of the gas-outlet openings 21.
In the example shown in Figure 5, the gas flowing through the double-walled screen at the same time also has a cooling function for the screen.
In addition, the screens are in each case made of high-temperature-resistant steel or else of a corresponding ceramic material.
In summary, it may thus be concluded that, by the use according to the invention of a screen, the influence of a flue-gas recirculation on the gas-envelope flow can be restricted, and therefore annular-vortex structures can be sufficiently prevented even with smaller gas volumetric flows or gas impulse flows. In particular in gas turbines which also have annular combustion chambers, operation with fewer problems can thus be achieved with the invention.

-12-
WE CLAIMS
1. A system for controlling flame/pressure oscillations in a firing device having at least one burner for producing a flame (7); the flame (7) being directed to a combustion chamber (8): at least one gas-outlet opening (11,21), for gas (9,22) flow out, the flown out gas (9,22) enclosing the flame (7) in the form of an envelope, the gas (9,22) having a higher flow velocity in the flame-propagation direction (13) than the outer regions of the flame, characterized in that a screen (15,19,20) surrounding the gas-outlet opening (11,21) and running at a radial distance around the burner outlet is provided, and in that a flue-gas recirculation region (16) connected to the combustion chamber (8) is separated from said gas (9,22) by means of said screen (15), 19,20) thereby allowing the gas flow (9,22) to expand as a free jet.
2. The system as claimed in claim 1, wherein the screen
(15,19) has a single shell.
3. The system as claimed in claim 1, wherein the screen (20) has a double shell, the gas (22) flowing through the screen (20).
4. The system a© claimed in claim 1 wherein the screen (15,19,20) extends essentially parallel to the flame-propagation direction (13).

13.
5- The system as claimed in claim 1, wherein the screen (15,20)
is cylindrical and concentric to the burner.
6- The system as claimed in claim 1, wherein the screen (19)
has a conical shape.
7. The system as claimed in claim 1, wherein the top edge (18) of the screen (13) is at a radial distance from the gas—outlet opening (11), and wherein the gas-envelope flow (9) does not come into contact with the inside of the screen until the gas—envelope flow (9) reaches the end of the screen.
9. The system as claimed in claim 1, wherein the screen
comprises ceramic material.
10. The system as claimed in claim 1, wherein the firing device
has plurality of turbines.
11. The system as claimed in claim 1, wherein the combustion
chamber (6) is an annular combustion chamber.
A system for controlling flame/pressure oscillations in a
firing device having at least one burner for producing a flame
(7); the flame (7) being directed to a combustion chamber (8)5 at
least one gas-outlet opening (11,21), for gas (9,22) flow out,
the flown out gas (9,22) enclosing the flame (7) in the form of
i an envelope, the gas (9, 22) having a higher flow velocity in
the flame-propagation direction (13) than the outer regions of
the flame. A screen (19, 19, 20) surrounding the gas-outlet
opening (11, 21) and running at a radial distance around the
burner outlet is provided, and in that a flue-gas recirculation
region (16) connected to the combustion chamber (8) is separated
from said gas (9, 22) by means of said screen (15, 19, 20)
thereby allowing the gas flow (9, 22) to expand as a free jet.

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Patent Number

201109

Indian Patent Application Number

IN/PCT/2000/00127/KOL

PG Journal Number

N/A

Publication Date

19-Jan-2007

Grant Date

19-Jan-2007

Date of Filing

11-Jul-2000

Name of Patentee

DVGW DEUTSCHER VEREIN DES GAS-UND WASSERFACHES-TECHNISCH-WISSENSCHAFTLICHER VEREINIGUNG

Applicant Address

JOSEF-WIRMER-STR. 1-3, D-53123 BONN,

Inventors:

#	Inventor's Name	Inventor's Address
1	BUCHNER HORST	3 RUE DU MARGRAVE DE BADE,F-67500 MARIENTHAL,
2	LEUCKEL WOLFGANG	AUF DER JUDENHUT 1 D-67098 BAD DURKHEIM,

PCT International Classification Number

F23C 7/00;9/00;7/02;

PCT International Application Number

PCT/EP99/00464

PCT International Filing date

1999-01-25

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	981011505	1998-01-23	EPO