The invention involves an apparatus for suppressing flame/pressure pulsations in a furnace having at least one burner for generating a flame and a combustion chamber into which the flame is directed, wherein the furnace has at least one gas outlet opening, from which flows gas that surrounds the flame in the form of a shroud or jacket and has a higher flow speed in the flame propagation direction than the outer regions of the flame. The invention is also related to a combustion chamber of a gas turbine that incorporates such an apparatus.
In industrial combustion systems such as gas turbines, combustion chambers, blast heaters, residue combustion systems or industrial ovens, but also for small furnaces such as gas boilers or heating furnaces in domestic use, unstable operating conditions occur under certain circumstances that are determined by the parameters of furnace operation, such as thermal output and air ratio, which are characterized by time-periodic changes of the flame that are accompanied by changes, in particular of the static pressure in the combustion chamber, as well as in pre-connected or post-connected system parts. These unstable conditions also occur in furnaces whose flames are sufficiently ignition-stabilized by known measures such as swirling flows, baffle structures, etc.
The occurrence of these combustion instabilities often causes a changed behavior compared to the steady-state operation of the system and also causes, besides an increased noise level, an increased mechanical and/or thermal stress of the combustion chamber and/or the combustion chamber lining. Such flame/pressure pulsations can, at unfavorable ratios, lead to damage of the system in which they occur, so that much expense is incurred in order to prevent such flame/pressure pulsations. Thus, for example, the combustion chamber geometry is changed by specially installed components, which, however, frequently leads only to a shift in the pulsation frequencies that occur, and thus does not contribute to a general solution of the problem. Otherwise, special measures are taken each time on an empirical basis for any occurring flame/pressure pulsations.
In European published patent application EP-A-0 754 908 (U.S. application Ser. No. 08/797,381), a device is proposed for this purpose, as mentioned above, in which the flame of a burner is surrounded as closely as possible by a flow of gas, such that the gas flow has a higher speed in the flame propagation direction than the outer and/or edge areas of the flame and/or of the fuel-containing burner main stream.
As used herein, insofar as mention is made of the "outer areas of the flame," this is understood to mean the reacting or reactable layers of a fuel and/or combustible gas/air flow. Upon these layers the gas shroud stream effects a transfer of the axial momentum.
As used herein, "flame propagation direction" shall mean the main propagation direction in the axial extension of a flame, and this is to be distinguished from the radial propagation direction of the flame.
The principle of the invention is thus based on the discovery that the pulsations are essentially caused or increased by ring vortices periodically forming in the edge area of the flame. These ring vortices, which arise from the rolling up of the edge areas of the fuel-containing burner stream, incorporate during their formation hot, already burned and no longer reactable flue gases that cause a quick heating up of the fuel/air mixture already contained in the ring vortex, and as a result cause a periodic pulse-type reaction of the fuel inside the ring vortex structures that excites pressure pulsations.
In order to now prevent this ring vortex formation, the flame, as described above, is surrounded by a gas shroud stream that exits at as small a radial distance as possible from the flame or from the burner main stream and that has a higher flow speed in the flame propagation direction than the outer or edge areas of the flame. Thus, an axial momentum exchange occurs between the shroud stream and the flame or the fuel gas/air stream which causes an acceleration of the free flame boundary layer or stream boundary layer of the fuel/air mixture, such that the periodic formation of reactable vortices in this area is effectively opposed.
To the extent that corresponding ring vortices then occur again at the boundary layer between the gas shroud stream and the surrounding medium (in the included case, generally flue gases), it is most favorable if the gas shroud stream does not contain any fuel, since then no fuel-containing vortex could then form from the (fuel-free) shroud stream, which could lead to a periodic pulse-type reaction of the fuel and thus to an excitation of flame/pressure pulsations as they occur for a non-shrouded flame or fuel/air stream.
In a more preferred manner, with the non-fuel containing gas of the shroud stream, this involves air, which is available everywhere in sufficient quantity. It is, however, also conceivable to use an inert gas here which, of course, would have a certain cost disadvantage as a result.