Abstract:
A swirl generator, a method for preventing flashback in a burner with a swirl generator and a burner are provided. The swirl generator includes a central fuel distributor element, an outer wall enclosing the central fuel distributor element and bounding an axial flow channel for combustion air, swirl vanes extending in a radial direction to the outer wall and giving the flowing combustion air a tangential flow component, and a separating wall enclosing the central fuel distributor element and being positioned radially within the outer wall. The separating wall divides the flow channel into a radially inner channel segment and a radially outer channel segment. The radially inner channel segment allows the combustion air to pass without giving it a tangential flow component or while giving it a tangential flow component counter to the orientation of the tangential flow component in the radially outer channel segment.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of European Patent Office Application No. 09155904.7 EP filed Mar. 23, 2009, which is incorporated by reference herein in its entirety. 
     FIELD OF INVENTION 
     The present invention relates to a swirl generator having a central fuel distributor element and a burner having at least one swirl generator. The invention also relates to a method for preventing flashback in a burner, which comprises at least one swirl generator having a central fuel distributor element. 
     BACKGROUND OF INVENTION 
     Gas turbine burners having a central fuel distributor element and swirl generators enclosing the fuel distributor elements are described for example in DE 10 2007 004 394 A1, US 2004/0055306 A1 and U.S. Pat. No. 6,082,111. In the burners described in US 2004/055306 A1 and U.S. Pat. No. 6,082,111 the swirl generator extends in each instance from the central fuel distributor element to a wall enclosing the central fuel distributor element and bounding an axial flow channel for combustion air. The burners here each comprise a number of such arrangements. In such burners the profiles of the fuel injected into the flow channel are designed such that only very little fuel is fed to the zone around the central fuel distributor element, so that only a very lean mixture forms in this zone. This is with the intention of preventing flashback. A zone with reduced flow speed therefore results in the vortices, which form on the downstream side of the central distributor element. If too much fuel is now injected in proximity to the central distributor element, it may happen that this central region with low flow speed is supplied with too much fuel, which can result in flashback, which in the event of large loads is associated with very high temperatures downstream of the swirl generator. By diminishing the quality of the mixture, the very lean mixture in the region of the central fuel distributor element causes an increase in NO x  emissions, which however have to be tolerated to prevent flashback. 
     To prevent flashback it is proposed in DE 10 2007 004 394 A1 that the swirl vanes should be provided with cutouts in proximity to the central fuel distributor element, so that the swirl vanes in proximity to the central fuel distributor element are shorter in an axial direction than those some distance from the distributor element. The curvature of the swirl vanes in a circumferential direction therefore does not extend so far in proximity to the central distributor element as it does some distance from the central distributor element. This means that the air flowing through the flow channel is subject to less swirling in proximity to the distributor element and therefore flows faster in an axial direction than it does further away from the distributor element. A cylindrical wall can also be present in the region of the cutout on the inner edges of the swirl vanes facing the distributor element, separating the channel segment with less vortex formation from the channel segment with greater vortex formation. 
     SUMMARY OF INVENTION 
     An object of the present invention is to create an advantageous swirl generator and an advantageous burner compared with the cited prior art. A further object of the present invention is to provide an advantageous method for preventing flashback in a burner having at least one swirl generator. 
     The above objects are achieved by a swirl generator, a burner and a method for preventing flashback as claimed in the independent claims. The dependent claims contain advantageous embodiments of the invention. 
     An inventive swirl generator comprises a central fuel distributor element, an outer wall enclosing the central fuel distributor element and bounding an axial flow channel for combustion air, swirl vanes, which extend to the outer wall in a radial direction and give the flowing combustion air a tangential flow component, and a separating wall, which encloses the central fuel distributor element and is positioned radially within the outer wall. The separating wall divides the flow channel into a radially inner channel segment and a radially outer channel segment. The separating wall here can extend in the axial direction of the swirl generator at least over the axial length of the swirl vanes, but also particularly beyond their axial length. The radially inner channel segment allows the combustion air to pass without giving it a tangential flow component or while giving it a tangential flow component counter to the orientation of the tangential flow component in the radially outer channel segment. 
     The total avoidance of a tangential component in the inner channel region allows a flow enveloping the central fuel distributor element to be generated with a high axial flow speed around said element, which helps to prevent flashback in a reliable manner. However the generation of a counterswirl in the inner channel segment, in other words a swirl, the orientation of which is counter to the swirl in the outer channel segment, can also help to prevent flashback, since it has a positive influence on flow conditions in the vortex downstream of the central fuel distributor element. 
     The total avoidance of a tangential flow component in the inner channel segment can in particular be achieved by having no swirl vanes at all in this channel segment. To supply the swirl vanes in the radially outer channel segment with fuel, fuel lines can extend through the radially inner channel segment to the swirl vanes in the radially outer channel segment. To prevent flow interruptions at the fuel lines, these advantageously have a circular or teardrop-shaped cross section. 
     If there are swirl vanes in the radially inner channel segment, which give the combustion air flowing through the radially inner channel segment a tangential flow component, the orientation of which is counter to the tangential flow component in the radially outer channel segment, the fuel lines for the swirl vanes in the radially outer channel segment can extend through the swirl vanes in the radially inner channel segment, for example in the form of holes drilled through the swirl vanes. 
     In order to achieve a particularly uniform fuel profile in the inner channel segment, it is advantageous if there are fuel outlet openings in the fuel lines or swirl vanes in the inner channel segment. These can in particular be disposed so that they inject the fuel into the combustion air essentially perpendicular to the flow direction of the combustion air in the radially inner channel segment. Fuel outlet openings can similarly be present in the swirl vanes in the radially outer channel segment and these can in particular be disposed so that they inject the fuel into the combustion air essentially perpendicular to the flow direction of the combustion air in the radially outer channel segment. This also allows a uniform fuel profile to be achieved in the radially outer channel segment. However the injection direction does not necessarily have to be perpendicular to the flow direction of the combustion air. Rather the injection direction can in principle be selected freely. Alternatively or additionally to being supplied perpendicular to the flow direction of the combustion air, the fuel can therefore also be supplied for example perpendicular to the radial direction and/or counter to the flow direction of the combustion air flowing through the flow channel and/or parallel to the flow direction of the combustion air flowing through the flow channel. Other directions and combinations, which are not mentioned specifically, are also possible. This applies both to the fuel supply in the inner channel segment and to the fuel supply in the outer channel segment. 
     In order to increase the axial flow speed further in proximity to the central fuel distributor element, the separating wall can at least partially have a conical form, with the opening cross section of the radially inner channel segment decreasing in the flow direction of the combustion air. 
     In one development of the inventive swirl generator the separating wall projects out beyond the downstream end of the outer wall. This development can be realized both with a conically configured separating wall and with a separating wall that is not configured conically. 
     The relatively complicated geometric form of the inventive swirl generator compared with swirl generators according to the prior art can be realized advantageously, if the swirl generator is embodied as a cast part. If a casting model is first produced, the production costs for the inventive swirl generator as a cast part are not very different from the production costs for the swirl generator according to the prior art. 
     An inventive burner is equipped with at least one inventive swirl generator. This allows the advantages described with reference to the swirl generator to be achieved in a burner, which can in particular be a gas turbine burner. 
     According to the invention a method is also provided for preventing flashback in a burner, which comprises at least one swirl generator having a central fuel distributor element and an outer wall enclosing the central fuel distributor element and bounding an axial flow channel for combustion air. The combustion air flowing through the flow channel is given a tangential flow component in a radially outer channel region. In contrast in a radially inner region the combustion air flowing through the flow channel is not given a tangential flow component or is given a tangential flow component counter to the tangential flow component in the radially outer channel region. 
     The advantages that can be achieved with the inventive method in respect of preventing flashback have already been described in relation to the inventive swirl generator. Reference is made to this description to avoid repetition. 
     A particularly uniform fuel profile can be produced, if the fuel is supplied to the combustion air flowing through the flow channel. The fuel can be mixed in here in particular perpendicular to the flow direction of the combustion air flowing through the flow channel and/or perpendicular to the radial direction. Alternatively or in addition to the above-mentioned variants it can also be mixed in essentially counter to the flow direction of the combustion air flowing through the flow channel and/or parallel to the flow direction of the combustion air flowing through the flow channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, characteristics and advantages of the present invention will emerge from the description which follows of exemplary embodiments with reference to the accompanying figures, in which: 
         FIG. 1  shows a highly schematic diagram of a gas turbine. 
         FIG. 2  shows a perspective view of a gas turbine burner. 
         FIG. 3  shows a perspective view of a swirl generator of the burner from  FIG. 2 . 
         FIG. 4  shows a partial section of the swirl generator from  FIG. 3 . 
         FIG. 5  shows a section of the swirl generator from  FIG. 3  along its longitudinal axis. 
         FIG. 6  shows a partial section of an alternative embodiment of the swirl generator. 
         FIG. 7  shows a partial section of a further embodiment of the swirl generator. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The structure and function of a gas turbine are described below with reference to  FIG. 1 , which shows a highly schematic sectional view of a gas turbine. The gas turbine  1  comprises a compressor segment  3 , a combustion segment  4 , which in the present exemplary embodiment comprises a number of tubular combustion chambers  5  with burners  6  disposed thereon, but in principle can also comprise an annular combustion chamber, and a turbine segment  7 . A rotor  9  extends through all the segments and in the compressor segment  3  supports compressor blade rings  11  and in the turbine segment  7  supports turbine blade rings  13 . Rings of compressor vanes  15  and rings of turbine vanes  17  are disposed between adjacent compressor blade rings  11  and between adjacent turbine blade rings  13 , extending from a housing  19  of the gas turbine  1  radially outward in the direction of the rotor  9 . 
     During operation of the gas turbine  1  air is drawn in through an air inlet  21  into the compressor segment  3 . The air is compressed here by the rotating compressor blades  11  and routed to the burners  6  in the combustion segment  4 . In the burners  6  the air is mixed with a gaseous or liquid fuel and the mixture is combusted in the combustion chambers  5 . The hot combustion waste gases, which are at high pressure, are then fed to the turbine segment  7  as a working medium. On their way through the turbine segment the combustion waste gases transmit a pulse to the turbine blades  13 , causing them to decompress and cool. The decompressed and cooled combustion waste gases finally leave the turbine segment  7  through an exhaust  23 . The transmitted pulse produces a rotational movement of the rotor, which drives the compressor and a consumer, for example a generator to produce electrical current or an industrial machine. The rings of turbine vanes  17  serve here as nozzles for conducting the working medium, to optimize the pulse transmission to the turbine blades  13 . 
       FIG. 2  shows a perspective view of a burner  6  of the combustion segment  4 . As its main components the burner  6  comprises a fuel distributor  27 , eight fuel nozzles  29 , which extend out from the fuel distributor  27 , and eight swirl generators  31  disposed in the region of the tips of the fuel nozzles  29 . The fuel distributor  27  and the fuel nozzles  29  together form a burner housing, through which fuel lines extend to injection openings, which are disposed within the swirl generator  31  and are therefore not visible in  FIG. 2 . The burner can be connected to fuel supply lines by way of a number of sockets (not shown). A flange  35  secures the burner  6  to a tubular combustion chamber so the fuel nozzles  29  point toward the interior of the combustion chamber. 
     Although the burner  6  shown in  FIG. 2  has eight fuel nozzles  29 , it is also possible to equip it with another number of fuel nozzles  29 . The number of fuel nozzles here can be higher or lower than eight, for example six fuel nozzles or twelve fuel nozzles can be present, each having its own swirl generator. A pilot fuel nozzle is also generally disposed in the center of the burner. The pilot fuel nozzle is not shown in  FIG. 2  for purposes of clarity. 
     During the combustion process air from the compressor is conducted through the swirl generator  31 , where it is mixed with fuel. The air/fuel mixture is then combusted in the combustion zone of the combustion chamber  5  to form the working medium. 
       FIG. 3  shows a perspective view of a swirl generator of the burner  6 . The swirl generator  31  has a central fuel distributor element  37 , which is enclosed by an outer wall  39 , which forms an axial flow channel for compressor air. A separating wall  42 , which encloses the central fuel distributor element  37  and is positioned radially within the outer wall  39 , is also present in the flow channel  41  to divide the flow channel  41  into a radially inner channel segment  43  and a radially outer channel segment  45 . Swirl vanes  47  extend out from the separating wall  42  in a radial direction through the radially outer channel segment to the outer wall  39 . The swirl vanes  47  give the compressor air flowing through the radially outer channel segment  45  a tangential flow component, so the air forms a vortex after passing through the swirl generator  31 . 
     No swirl vanes are present in the radially inner channel segment  43 . Instead fuel lines  49  extend out from the central fuel distributor element  37  in a radial direction to the separating wall  42 . As is evident in particular in  FIG. 4 , which shows a partial section of the swirl generator  31 , the fuel lines  49  have a teardrop-shaped cross section, to prevent the flow being interrupted at the downstream edge of the lines  49 . However the lines  49  could in principle also have a round cross section instead of a teardrop shaped cross section. 
     The fuel lines  49  are disposed so that they are flush with the swirl vanes  47  in the radially outer channel segment, so that a fuel channel  51  can extend straight out from the central fuel distributor element  37  through the fuel lines  49  into the swirl vanes  47 . The fuel channels  51  can be seen in particular in  FIG. 5 , which shows a sectional view through the swirl generator  31  along its longitudinal axis. The fuel channels  51  are used to supply fuel to outlet openings  53  in the swirl vanes  47  and outlet openings  55  in the fuel lines  49 . The outlet openings  53 ,  55  here are disposed so that the fuel is injected into the radially outer channel segment  45  and the radially inner channel segment  43  essentially perpendicular to the flow direction of the compressor air. 
     The described swirl generator design means that the compressor air flowing through the radially inner channel segment  43  is not given any swirl. The flow speed of this compressor air in an axial direction is therefore greater than the speed of the compressor air flowing through the radially outer channel segment  45 , in which some of the axial flow is converted to a tangential flow component. The higher axial flow speed in the radially inner channel segment, i.e. in the region adjacent to the central fuel distributor element  37 , prevents the occurrence of zones with a low axial flow speed downstream of the central fuel distributor element  37 , which in turn prevents flashback. This allows more fuel to be injected in proximity to the central distributor element  37  compared with the prior art, thereby reducing NO x  emissions during combustion. 
     The separating wall  42  extends at least over the entire axial length of the swirl vanes  47  in the radially outer channel segment  45 , so that the introduction of a tangential flow component in the radially inner channel segment  43  can be reliably prevented. In the present exemplary embodiment the separating wall  42  also extends in an axial direction beyond the upstream and downstream edges of the swirl vanes  47 , to prevent the compressor air flowing through the radially inner channel segment  43  being influenced by the eddying air flowing in the radially outer channel segment  45 . 
     An alternative variant of the swirl generator  31  is shown in  FIG. 6 . Elements, which correspond to the swirl generator from the first exemplary embodiment, are identified in  FIG. 6  with the same reference characters as in the first exemplary embodiment and are not described again to avoid repetition. 
     The swirl generator  131  of the second exemplary embodiment differs from the swirl generator  31  of the first exemplary embodiment only by its separating wall  142 . In contrast to the first exemplary embodiment the separating wall  142  of the second exemplary embodiment has a conical segment  144 , which means that the cross section of the opening of the radially inner channel segment  43  decreases toward the outlet of the swirl generator  131 . The conical segment  144  causes the flow speed of the compressor air flowing through the radially inner channel segment  43  to be higher compared with the swirl generator  31  in the first exemplary embodiment. The central fuel distributor element  37  is thus enclosed by an air jacket, which has a particularly high axial flow speed and is thus able to prevent the formation of regions with low flow speed and the associated formation of flashback in a particularly reliable manner. 
     Although the separating wall  142  in the present exemplary embodiment only has a conical segment  144  on the downstream side, it can also be configured in a conical manner over its entire length. 
     A partial section of a third variant of the inventive swirl generator is shown in  FIG. 7 . As with the swirl generator of the second exemplary embodiment, with the swirl generator of the third exemplary embodiment all the elements that do not differ form the first exemplary embodiment are identified with the same reference characters as in the first exemplary embodiment and are not described again. 
     The swirl generator  231  of the third exemplary embodiment differs from the swirl generator of the first exemplary embodiment in that swirl vanes  149  are also present in the radially inner channel segment  43 . In contrast to the swirl vanes  47  in the radially outer channel segment  45  however, the intake and pressure sides of the vanes are reversed, so that the compressor vanes  159  give the compressor air in the radially inner channel segment a tangential component, which has a reverse orientation in respect of the axial flow direction compared with the tangential component given to the compressor air in the radially outer channel segment  45  by the swirl vanes  47  there. This measure also prevents flashback. Like the fuel lines  49  in the first two exemplary embodiments, the swirl vanes  149  in the radially inner channel segment  43  have fuel channels  51  and fuel outlet openings  155 , which are disposed so that they inject the fuel essentially perpendicular to the flow direction of the air flowing through the radially inner channel segment  43 . 
     Although the swirl generator  231  of the third exemplary embodiment in  FIG. 7  is shown with a cylindrical separating wall  42 , the swirl generator according to the third exemplary embodiment can also be equipped with an at least partially conical separating wall, as described in relation to the second exemplary embodiment. 
     In the exemplary embodiments shown in the figures the separating walls do not project beyond the downstream end of the respective outer wall. However the separating walls can also be extended on the downstream side—unlike in the figures—so that they project beyond the downstream end of the outer wall. This applies whether or not a separating wall is configured as conical. 
     The relatively complex geometric form of the swirl generators according to the exemplary embodiments described can be advantageously achieved, if the swirl generators are produced as cast parts.