Abstract:
In a premix burner having a swirl-stabilizing interior space (20) which is essentially formed by sectional shells (11, 12) nested one inside the other in a mutually offset manner as well as by a conically running inner body (13), one feed duct (11c, 12c) each extends upstream of the tangential air-inlet slots (11a, 12a) formed by the offset sectional shells, which feed duct (11c, 12c) is fitted at least with means (11d, 12d) for swirling an air flow (23) and with means for introducing a fuel (24). The introduction of the fuel is preferably arranged downstream of the means for swirling the air flow.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a premix burner. 
     2. Discussion of Background 
     Lean premixed combustion is a widespread method of achieving low pollutant emissions, in particular nitrogen-oxide emissions, during the combustion of fuels having a low content of nitrogen compounds. It has been disclosed by publications that a further reduction in the nitrogen-oxide emissions, in particular during combustion under high pressure, as is the case in gas turbines, is possible using experimental burners by improving the mixing quality of air and fuel. However, transferring such experimental burners to machine technology is not readily possible, since there are stringent requirements with regard to flame stabilization and flashback safety. Conventional swirl-stabilized premix burners suitable for machines do not mix the fuel into the combustion air until just before the flame zone. Tests of burners of the type have shown that homogeneous mixing of air and fuel at the flame zone still cannot be achieved. Shifting the fuel injection upstream to prolong the mixing time and thus improve the mixing quality is not permitted on account of the flashback risk associated therewith in a burner suitable for machines. 
     WO 93/17279 disclosed a burner which essentially comprises a cylindrical chamber which in turn has a plurality of tangentially arranged slots through which the combustion air flows into the interior of the chamber. In the region of these slots, at the transition to the interior space of the chamber, a number of fuel inject nozzles a gaseous fuel the combustion air flowing through there. Furthermore, the interior space of the chamber is provided with a conical body which tapers in the direction of flow, and additional fuel nozzles for a preferably liquid fuel are provided the tip region of this conical body. The combustion air is ignited downstream of the conical tip of this body. In order to keep the flame stable outside the premix section of the burner, the flow in the chamber itself must be subcritical, i.e. the swirl coefficient must be so small that no vortex breakdown occurs. The critical swirl coefficient can be achieved by three parameters at the correct location: by changing the width of the tangential slots, by adaptation of the angle of the conical body in the interior space of the chamber and by the addition of central assisting air, whether swirled or without swirl. However, due to the fuel injection in the region of the slots, the slots are greatly restricted in their design. In addition, optimum homogeneous mixing of air and fuel cannot be achieved directly. This applies in particular to those fuel injections which are located at the end of the burner and which are accordingly located in the immediate region of the plane of the flame front. What is more there is a potential risk of flashback due to this proximity. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the invention as defined in the claims, is to improve the mixing quality in a premix burner of the type mentioned at the beginning, and a further object is to eliminate the risk of flashback during the entire operation. 
     The essential advantage of the invention may be seen in the fact that the improvement can be used in all swirl-stabilized premix burners. The invention is particularly adventageous in premix burners which function with a critical swirl coefficient for forming a backflow zone, and which are constructed according to the principle of two or more offset sectional shells, the offset sectional shells forming air-inflow slots which run parallel to the burner axis. 
     The same also applies to those burners in which the outer shell comprises a uniform body in the form of a tube and the air flow into the interior takes place through a number of tangentially arranged ducts. 
     Advantageous and expedient developments of the achievement of the object according to the invention are defined in the further claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 shows a schematic representation of a premix burner having offset sectional shells and a conical inner body, 
     FIG. 2 shows a section through the premix burner along plane II--II from FIG. 1, the premix burner having extending ducts with vortex generators upstream of the air-inlet slots, 
     FIG. 3 shows a further representation according to FIG. 2, the extending ducts being equipped with a venturi mixer, and 
     FIG. 4 shows a further premix burner essentially according to FIG. 1 but having central assisting air. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, all elements not essential for directly understanding the invention have been omitted, and the direction of flow of the media is designated by arrows. It is of advantage if FIG. 2 or FIG. 3 is also used at the same time as FIG. 1 in order to understand the construction of the premix burner 1 better. Furthermore, so that FIG. 1 is not made unnecessarily complex, the feed ducts 11c and 12c according to FIG. 2 or FIG. 3 have not been shown in any more detail. In the description of FIG. 1 below, the other FIGS. 2 and 3 are referred to when required. 
     The premix burner according to FIG. 1 comprises two hollow sectional shells 11, 12 which are nested one inside the other in a mutually offset manner (cf. FIG. 2 for this). The shells 11, 12 enclose an interior space 20. The mutual offset of the respective center axis or longitudinal symmetry axis 11b, 12b (cf. FIG. 2) provides on both sides of the interior space 20, in mirror-image arrangement, tangential air-inlet slot 11a, 12a through which a combustion mixture 15 flows into the interior space 20. The configuration of these air-inlet slots 11a, 12a will be dealt with in more detail below. The shells 11, 12 run cylindrically in the direction of flow. However, the cross-section of flow formed by the interior space 20 may be designed to decrease or increase regularly or irregularly in the direction of flow, depending on use. A cross-section of flow of the interior space 20 may be shaped as a venturi tube in the direction of flow example. The possible constructions referred to are not shown in any more detail, since they can readily be envisaged by a person skilled in the art. Arranged in the interior space 20 is a conical inner body 13, which tapers in the direction of flow. The body 13 extends well into the interior space 20 and ends in the form of a point. The conical configuration of this inner body 13 is not restricted to the shape represented: an outer shape of this inner body 13 as diffuser or confuser is also possible. At least one bore 14 through which preferably a liquid fuel 16 is directed into the front region passes through the inner body 13. The injection of the liquid fuel 16 in the region of the tip of the inner body 13 forms the head stage of the premix burner 1. In this region, the inner body 13 can readily be supplemented by a swirl generator (not shown in the figure) which assists the mixing of the injected fuel 16. At the combustion space 22, the cross-section of flow area undergoes a jump in cross-section via a front wall 17, which connect the shell 11, 12 to the flame tube 21. The backflow zone or backflow bubble 18 which induces the flame stabilization also forms in this plane. The front wall 17 itself has a pluarlity of bores through which diluent air or cold air 19 is fed to the front region of the combustion space 22 according to requirements. The flame stabilization becomes important in assisting the compactness of the flame as a result of radial flattening, a factor which is also important with regard to the fuel injection through the head stage. The combustion mixture 15 comprises air and fuel (cf. FIG. 2). Of course, the combustion mixture 15 may also contain portions of a recycled exhaust gas or a quantity of steam. It is generally the case that a fluidic marginal zone forms inside the jump in crosssection in the region of the front wall 17. In the marginal zone, vortex separations occur due to the vacuum prevailing there, which vortex separations then in turn assist the flame stabilization. Depending on the degree of assistance, the diluent air or cold air 19 already mentioned is admixed. Swirling of the medium around the inner body 13 develops due to the combustion mixture 15 flowing tangentially into the interior space 20. Vortex breakdown forms in the region of the plane of the front wall 17 on account of the subcritical swirl flow there, homogeneous fuel concentration depending on the design of the tangential air-inlet slots or on the installation of vortex generators in the region of the air-inlet slots. Ignition is effected at the tip of the backflow zone 18: only at this location can a stable flame front develop. A flashback of the flame into the interior space 20 of the premix burner 1, as is always potentially the case in the premix sections which have been disclosed, where a remedy is attempted with complicated flame retention baffles, need not be feared here for the reasons mentioned. If the combustion air is additionally preheated or enriched with one of the media mentioned, this assists the evaporation of the liquid fuel 16 fed through the inner body 13. Narrow limits are to be adhered to in the design of the inner body 13 with regard to the conical configuration and the width of the tangential air-inlet slots 11a, 12a so that the desired flow field, i.e. the critical swirl coefficient, of the combustion mixture 15 can arise at the outlet of the interior space 20. In general it may be said that a reduction in the cross-section of flow of the tangential air-inlet slots 11a, 12a displaces the backflow zone 18 further upstream, as a result of which the mixture then ignites earlier, which in this case may give rise to the risk of a collision with the tip of the inner body 13 if the latter extends too far into the interior space 20. Nonetheless, it may be stated that the backflow zone 18, once fixed, is positionally stable per se, for the swirl coefficient increases in the direction of flow in the region of the conical shape of the inner body 13. The cross-section of flow of the tangential air-inlet slots 11a, 12a may of course be designed to be variable in the direction of flow, for example to decrease in the direction of flow, in order to make the backflow zone 18 more stable at the outlet of the interior space 20. The axial velocity of the combustion mixture 15 inside the interior space 20 of the premix burner 1 can be changed by feeding (not shown in any more detail) an axial combustion-air flow. Furthermore, the construction of the premix burner 1 is especially suitable for changing the size of the tangential air-inlet slots 11a, 12a, whereby a relatively large operational range can be covered without changing the overall length of the premix burner 1. 
     FIG. 2 shows the configuration of the sectional shells 11, 12 nested one inside the other. The sectional shells 11, 12 can of course also be displaced relative to one another over this plane, i.e. it is readily possible to effect an overlap of the same in the region of the tangential air-inlet slots 11a, 12a. Furthermore, it is also possible to nest the sectional shells 11, 12 spirally one inside the other by a contra-rotating movement. It is thus possible to vary the shape and size of the tangential air-inlet slots 11a, 12a in such a way that the swirl generation in the premix burner 1 can be adapted to the respective conditions. The tangential air-inlet slots 11a, 12a in each case form the discharge opening of a feed duct 11c, 12c in which, far removed from the interior space 20, the combustion mixture 15 is formed before it flows into this interior space 20. At an adequate distance upstream of the tangential air-inlet slots 11a, 12a, the feed ducts 11c, 12c have vortex generators l1d, 12d which integrally swirl the air 23 flowing in there. At a suitable distance downstream of these vortex generators 11d, 12d, a preferably gaseous fuel 24 is injected, whereby the intended air/fuel mixture can then form along the remaining section of the feed ducts 11c, 12c before this fuel/air mixture flows into the interior space 20 as combustion mixture 15 in an integrally identical consistency over the entire length of the tangential air-inlet slots 11a, 12a. The feed ducts 11c, 12c shown here have a largely cylindrical shape, the length and cross-section of flow of which are designed for optimum air/fuel premixing. The flow to be formed inside the feed ducts 11c, 12c must be set in such a way that no risk of flashback arises from the interior space 20 should instability of the flame front develop. Due to the arrangement shown of the vortex generators 11d, 12d relative to the fuel injection 24, there is no risk of flashback. The offsetting of the center axes 11b, 12b has already been dealt with in more detail under FIG. 1. 
     FIG. 3, unlike FIG. 2, shows feed ducts which have a venturi mixer 25a, 25b at a suitable distance from the tangential air-inlet slots 11a, 12a. The fuel injection 24 is carried out at the narrowest location. The greatest velocity also prevails there, whereby the best possible mixture formation is ensured, again while eliminating a risk of flashback. Otherwise, the construction of FIG. 3 corresponds to that of FIG. 2. 
     FIG. 4 largely corresponds to FIG. 1, the inner body 13 here being widened centrally with a duct for a flow of assisting air 26, which serves as a further measure for providing the critical swirl coefficient at the correct location. 
     As far as the number of shells is concerned, they are not restricted to two. A larger number can readily be used. If a spiral influx of the combustion mixture 15 into the interior space 20 is aimed at, this can readily be achieved via a single tangential air-inlet slot. 
     If the premix burner to be formed by the shells comprises a continuous tube, the tangential injections into the interior space can be achieved through duct-like leadthroughs through the wall thickness of this very same tube. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.