Abstract:
In a burner which essentially comprises a swirl generator (100) for a combustion-air flow (115) and means for injecting a fuel into the combustion-air flow (115), a mixing section (220) is arranged downstream of the abovementioned swirl generator. This mixing section (220) has inside a first part (200) of the section a number of transition passages (201) which run in the direction of flow and ensure the smooth passing of the flow (40) formed in the swirl generator (100) into a tube (20) arranged downstream. The outlet plane of this tube (20) to the combustion chamber (30) is formed with a breakaway edge which serves to stabilize and enlarge a backflow zone (50) forming downstream.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a burner according to the preamble. 
     2. Discussion of Background 
     A conical burner consisting of a plurality of shells, a so-called double-cone burner, for generating a closed swirl flow in the cone head has been disclosed by EP-B1-0 321 809, which swirl flow becomes unstable on account of the increasing swirl along the apex of the cone and changes into an annular swirl flow with backflow in the core. Fuels, such as, for example, gaseous fuels, are injected along the ducts, also called air-inlet slots, formed by the individual adjacent shells and are mixed homogeneously with the air before the combustion occurs by ignition at the stagnation point of the backflow zone or backflow bubble, which is utilized as a flame retention baffle. Liquid fuels are preferably injected via a central nozzle at the burner head and then vaporize in the conical hollow space. Under typical gas-turbine conditions, the ignition of these liquid fuels occurs early on near the fuel nozzle, whereby a sharp increase in the NOx values precisely on account of this lack of premixing cannot be avoided, which necessitates, for example, the injection of water. Furthermore, it was found that the attempt to burn hydrogenous gases in a similar way to natural gas led to problems of premature ignition at the gas bores with subsequent overheating of the burner. An attempt has been made to remedy this by a special injection method for such gaseous fuels being introduced at the burner outlet, the results of which, however, have not been completely satisfactory. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the invention, as defined in the claims, in a burner of the type mentioned at the beginning, is to propose measures by means of which perfect premixing of fuels of various types is achieved and operationally reliable and optimum flame positioning is obtained. 
     The proposed burner has a swirl generator on the head side and upstream of a mixing section, which swirl generator can preferably be designed to the effect that the basic aerodynamic principles of the so-called double-cone burner according to EP-A1-0 321 809 are utilized. However, the use of an axial or radial swirl generator is also possible in principle. The mixing section itself preferably consists of a tubular mixing element, called mixing tube below, which permits perfect premixing of fuels of various types. 
     The flow from the swirl generator is directed smoothly into the mixing tube: this is done by a transition geometry which consists of transition passages which are recessed in the initial phase of the mixing tube and which pass the flow into the adjoining effective cross section of flow of the mixing tube. This introduction of flow free of losses between swirl generator and mixing tube first of all prevents the direct formation of a backflow zone at the outlet of the swirl generator. 
     First of all the swirl intensity in the swirl generator is selected via its geometry in such a way that the vortex breakdown does not take place in the mixing tube but further downstream at the combustion-chamber inlet, the length of this mixing tube being dimensioned in such a way that an adequate mixing quality for all types of fuel is obtained. If, for example, the swirl generator used is constructed according to the features of the double-cone burner, the swirl intensity results from the arrangement of the corresponding cone angle, the air-inlet slots and the number thereof. 
     In the mixing tube, the axial-velocity profile has a pronounced maximum on the axis and thereby prevents flashbacks in this region. The axial velocity decreases toward the wall. In order to prevent flashbacks also in this region, various measures are taken: on the one hand, for example, the overall velocity level can be raised through the use of a mixing tube having a sufficiently small diameter. Another possibility consists in only increasing the velocity in the outer region of the mixing tube by a small portion of the combustion air flowing into the mixing tube via an annular gap or through prefilming bores downstream of the transition passages. 
     As far as the abovementioned transition passages for introducing the flow into the mixing tube from the swirl generator are concerned, it may be said that the path of these transition passages may be designed to be spirally convergent or widening, in accordance with the effective adjoining cross section of flow of the mixing tube. 
     A portion of the pressure loss possibly produced may be compensated for by attaching a diffuser to the end of the mixing tube. A venturi section may also be provided in this region or upstream. 
     The combustion chamber having a jump in cross section adjoins the end of the mixing tube. A central backflow zone forms here, the properties of which are those of a flame retention baffle. The generation of a stable backflow zone requires a sufficiently high swirl coefficient in the mixing tube. But if such a high swirl coefficient is undesirable at first, stable backflow zones may be generated by the feed of small, intensely swirled air quantities, 5-20% of the total air quantity, at the tube end. 
     In combination with the abovementioned jump in cross section, the end of the mixing tube is formed with a breakaway edge which gives the backflow zone high spatial stability. In general, the following advantages can be achieved by the abovementioned measures: 
     a) stable flame position; 
     b) lower pollutant emissions (CO, UHC, NOx); 
     c) minimization of the pulsations; 
     d) complete burn-out; 
     e) wide operational range covered; 
     f) good cross ignition between the various burners, in particular in the case of gradual provision of load, during which the burners are operated independently of one another; 
     g) the flame can be adapted to the corresponding geometry of the combustion chamber; 
     h) compact type of construction; 
     i) improved mixing of the flow media; 
     j) improved &#34;pattern factor&#34; of the temperature distribution in the combustion chamber (=compensated temperature profile of the combustion-chamber flow). 
     Advantageous and expedient further 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 burner with adjoining combustion chamber, 
     FIG. 2 shows a swirl generator in perspective representation, in appropriate cut-away section, 
     FIG. 3 shows a section through th two-shell swirl generator according to FIG. 2, 
     FIG. 4 shows a section through a four-shell swirl generator, 
     FIG. 5 shows a section through a swirl generator whose shells are profiled in a blade shape, 
     FIG. 6 shows a representation of the form of the transition geometry between swirl geometry and mixing tube, and 
     FIG. 7 shows a breakaway edge for the spatial stabilization of the backflow zone. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, all features not essential for directly understanding the invention have been omitted, and the direction of flow of the media is indicated by arrows, FIG. 1 shows the overall construction of a burner. Initially a swirl generator 100 is effective, the configuration of which is shown and described in more detail below in FIGS. 2 to 5. This swirl generator 100 is a conical structure to which a combustion-air flow 115 entering tangentially is repeatedly admitted tangentially. The flow forming herein, with the aid of a transition geometry provided downstream of the swirl generator 100, is passed over smoothly into a transition piece 200 in such a way that no separation regions can occur there. The configuration of this transition geometry is described in more detail under FIG. 6. This transition piece 200 is extended on the outflow side of the transition geometry by a tube 20, both parts forming the actual mixing tube 220, also called mixing section, of the burner. The mixing tube 220 may of course be made in one piece, i.e. by the transition piece 200 and tube 20 being fused to form a single cohesive structure, the characteristics of each part being retained. If transition piece 200 and tube 20 are constructed from two parts, these parts are connected by a sleeve ring 10, the same sleeve ring 10 serving as an anchoring surface for the swirl generator 100 on the head side. In addition, such a sleeve ring 10 has the advantage that various mixing tubes can be used. Located on the outflow side of the tube 20 is the actual combustion chamber 30, which is symbolized here merely by the flame tube. The mixing tube 220 fulfills the condition that a defined mixing section be provided downstream of the swirl generator 100, in which mixing section perfect premixing of fuels of various types is achieved. Furthermore, this mixing section, that is the mixing tube 220, enables the flow to be directed free of losses so that at first no backflow zone can form even in interaction with the transition geometry, whereby the mixing quality for all types of fuel can be influenced over the length of the mixing tube 220. However, this mixing tube 220 has another property, which consists in the fact that in the mixing tube 220 itself the axial velocity profile has a pronounced maximum on the axis so that a flashback of the flame from the combustion chamber is not possible. However, it is correct to say that this axial velocity decreases toward the wall in such a configuration. So that flashback is prevented also in this area, the mixing tube 220 is provided in the flow and peripheral directions with a number of regularly or irregularly distributed bores 21 having the most varied cross sections and directions, through which an air quantity flows into the interior of the mixing tube 220, and an increase in the velocity is induced along the wall for the purposes of a prefilmer. Another possibility of achieving the same effect is for the cross section of flow of the mixing tube 220 on the outflow side of the transition passages 201, which form the transition geometry already mentioned, to undergo a convergence, as a result of which the entire velocity level inside the mixing tube 220 is raised. In the figure, these bores 21 run at an acute angle relative to the burner axis 60. Furthermore, the outlet of the transition passages 201 corresponds to the narrowest cross section of flow of the mixing tube 220. The said transition passages 201 accordingly bridge the respective difference in cross section without at the same time adversely affecting the flow formed. If the measure selected for directing the tube flow 40 along the mixing tube 220 initiates an intolerable pressure loss, this may be remedied by a diffuser (not shown in the figure) being provided at the end of the mixing tube. A combustion chamber 30 adjoins the end of the mixing tube 220, there being a jump in cross section between the two cross sections of flow. Only here does a central backflow zone 50 form, which has the properties of a flame retention baffle. If a fluidic marginal zone forms inside this jump in cross section during operation, in which marginal zone vortex separations arise due to the vacuum prevailing there, this leads to intensified ring stabilization of the backflow zone 50. At the end face, the combustion chamber 30 has a number of openings 31 through which an air quantity flows directly into the jump in cross section and, inter alia, helps there to intensify the ring stabilization of the backflow zone 50. In addition, it must not be left unmentioned that the generation of a stable backflow zone 50 also requires a sufficiently high swirl coefficient in a tube. If such a high swirl coefficient is undesirable at first, stable backflow zones may be generated by the feed of small, intensely swirled air flows at the tube end, for example through tangential openings. It is assumed here that the air quantity required for this is approximately 5-20% of the total air quantity. As far as the configuration of the breakaway edge at the end of the mixing tube 220 is concerned, reference is made to the description under FIG. 7. 
     In order to better understand the construction of the swirl generator 100, it is of advantage if at least FIG. 3 is used at the same time as FIG. 2. Furthermore, so that this FIG. 2 is not made unnecessarily complex, the baffle plates 121a, 121b shown schematically according to FIG. 3 are only alluded to in FIG. 2. In the description of FIG. 2 below, the said figures are referred to when required. 
     The first part of the burner according to FIG. 1 forms the swirl generator 100 shown according to FIG. 2. The swirl generator 100 consists of two hollow conical sectional bodies 101, 102 which are nested one inside the other in a mutually offset manner. The number of conical sectional bodies may of course be greater than two, as shown in FIGS. 4 and 5; this depends in each case on the mode of operation of the entire burner, as will be explained in more detail further below. It is not out of the question in certain operating constellations to provide a swirl generator consisting of a single spiral. The mutual offset of the respective center axis or longitudinal symmetry axes 101b, 102b of the conical sectional bodies 101, 102 provides at the adjacent wall, in mirror-image arrangement, one tangential duct each, i.e. an air-inlet slot 119, 120 (FIG. 3) through which the combustion air 115 flows into the interior space of the swirl generator 100, i.e. into the conical hollow space 114 of the same. The conical shape, in the direction of flow, of the sectional bodies 101, 102 shown has a certain fixed angle. Of course, depending on the operational use, the sectional bodies 101, 102 may have increasing or decreasing conicity in the direction of flow, similar to a trumpet or tulip espectively. The two last-mentioned shapes are not shown raphically, since they can readily be visualized by a person skilled in the art. The two conical sectional bodies 101, 102 each have a cylindrical initial part 101a, 102a, which parts likewise run offset from one another in a manner analogous to the conical sectional bodies 101, 102 so that the tangential air-inlet slots 119, 120 are present over the entire length of the swirl generator 100. Accommodated in the region of the cylindrical initial part is a nozzle 103, preferably for a liquid fuel 112, the injection 104 of which coincides approximately with the narrowest cross section of the conical hollow space 114 formed by the conical sectional bodies 101, 102. The injection capacity of this nozzle 103 and its type depend on the predetermined parameters of the respective burner. It is of course possible for the swirl generator 100 to be designed to be purely conical, that is without cylindrical initial parts 101a, 102a. Furthermore, the conical sectional bodies 101, 102 each have a fuel line 108, 109, which lines are arranged along the tangential air-inlet slots 119, 120 and are provided with injection openings 117 through which preferably a gaseous fuel 113 is injected into the combustion air 115 flowing through there, as the arrows 116 are intended to symbolize. These fuel lines 108, 109 are preferably positioned at the latest at the end of the tangential inflow, before entering the conical hollow space 114, in order to obtain optimum air/fuel mixing. As mentioned, the fuel 112 fed through the nozzle 103 is a liquid fuel 112 in the normal case, a mixture formation with another medium being readily possible. This fuel 112 is injected at an acute angle into the conical hollow space 114. Thus a conical fuel spray 105 forms from the nozzle 103, which fuel spray 105 is enclosed by the rotating combustion air 115 flowing in tangentially. The concentration of the injected fuel 112 is continuously reduced in the axial direction by the inflowing combustion air 115 for mixing in the direction of vaporization. If a gaseous fuel 113 is injected via the opening nozzles 117, the fuel/air mixture is formed directly at the end of the air-inlet slots 119, 120. If the combustion air 115 is additionally preheated or enriched, for example, with recycled flue gas or exhaust gas, this provides lasting assistance for the vaporization of the liquid fuel 112 before this mixture flows into the downstream stage. The same considerations also apply if liquid fuels are to be supplied via the lines 108, 109. Narrow limits per se are to be adhered to in the configuration of the conical sectional bodies 101, 102 with regard to the cone angle and the width of the tangential air-inlet slots 119, 120 so that the desired flow field of the combustion air 115 can develop at the outlet of the swirl generator 100. In general it may be said that a reduction in the tangential air-inlet slots 119, 120 promotes the quicker formation of a backflow zone already in the region of the swirl generator. The axial velocity inside the swirl generator 100 can be changed by a corresponding feed (not shown) of an axial combustion-air flow. Corresponding swirl generation prevents the formation of flow separations inside the mixing tube arranged downstream of the swirl generator 100. Furthermore, the construction of the swirl generator 100 is especially suitable for changing the size of the tangential air-inlet slots 119, 120, whereby a relatively large operational range can be covered without changing the overall length of the swirl generator 100. The sectional bodies 101, 102 can of course also be displaced relative to one another in another plane, as a result of which even an overlap of the same can be provided. Furthermore, it is possible to nest the sectional bodies 101, 102 spirally one inside the other by a contra-rotating movement. It is thus possible to vary the shape, size and configuration of the tangential air-inlet slots 119, 120 as desired, whereby the swirl generator 100 can be used universally without changing its overall length. 
     The geometric configuration of the baffle plates 121a, 121b is now apparent from FIG. 3. They have a flow-initiating function, in which case, in accordance with their length, they extend the respective end of the conical sectional bodies 101, 102 in the oncoming-flow direction relative to the combustion air 115. The ducting of the combustion air 115 into the conical hollow space 114 can be optimized by opening or closing the baffle plates 121a, 121b about a pivot 123 placed in the region of the inlet of this duct into the conical hollow space 114, and this is especially necessary if the original gap size of the tangential air-inlet slots 119, 120 is to be changed dynamically. These dynamic measures may of course also be provided statically by baffle plates forming as and when required a fixed integral part with the conical sectional bodies 101, 102. The swirl generator 100 may likewise also be operated without baffle plates or other aids may be provided for this. 
     FIG. 4, in comparison with FIG. 3, shows that the swirl generator 100 is now composed of four sectional bodies 130, 131, 132, 133. The associated longitudinal symmetry axes for each sectional body are identified by the letter a. It may be said of this configuration that, on account of the smaller swirl intensity thus produced and in interaction with a correspondingly increased slot width, it is best suited to preventing the breakdown of the vortex flow on the downstream side of the swirl generator in the mixing tube, whereby the mixing tube can best fulfill the role intended for it. 
     FIG. 5 differs from FIG. 4 inasmuch as the sectional bodies 140, 141, 142, 143 here have a blade-profile shape which is provided for supplying a certain flow. Otherwise, the mode of operation of the swirl generator is kept the same. The admixing of the fuel 116 with the combustion-air flow 115 is effected from the interior of the blade profiles, i.e the fuel line 108 is now integrated in the individual blades. Here, too, the longitudinal symmetry axes for the individual sectional bodies are identified by the letter a. 
     FIG. 6 shows the transition piece 200 in a three-dimensional view. The transition geometry is constructed for a swirl generator 100 having four sectional bodies in accordance with FIG. 4 or 5. Accordingly, the transition geometry has four transition passages 201 as a natural extension of the sectional bodies acting upstream, as a result of which the cone quadrant of the said sectional bodies is extended until it intersects the wall of the tube 20 or the mixing tube 220 respectively. The same considerations also apply when the swirl generator is constructed from a principle other than that described under FIG. 2. The surface of the individual transition passages 201 which runs downward in the direction of flow has a form which runs spirally in the direction of flow and describes a crescent-shaped path, in accordance with the fact that in the present case the cross section of flow of the transition piece 200 widens conically in the direction of flow. The swirl angle of the transition passages 201 in the direction of flow is selected in such a way that a sufficiently large section subsequently still remains for the tube flow up to the jump in cross section at the combustion-chamber inlet in order to effect perfect premixing with the injected fuel. Furthermore, the axial velocity at the mixing-tube wall downstream of the swirl generator is also increased by the abovementioned measures. The transition geometry and the measures in the region of the mixing tube produce a distinct increase in the axial-velocity profile toward the center of the mixing tube, so that the risk of premature ignition is decisively counteracted. 
     FIG. 7 shows the breakaway edge already discussed, which is formed at the burner outlet. The cross section of flow of the tube 20 in this region is given a transition radius R, the size of which in principle depends on the flow inside the tube 20. This radius R is selected in such a way that the flow comes into contact with the wall and thus causes the swirl coefficient to increase considerably. Quantitatively, the size of the radius R can be defined in such a way that it is &gt;10% of the inside diameter d of the tube 20. Compared with a flow without a radius, the backflow bubble 50 is now hugely enlarged. This radius R runs up to the outlet plane of the tube 20, the angle β between the start and end of the curvature being &lt;90°. The breakaway edge A runs along one leg of the angle β into the interior of the tube 20 and thus forms a breakaway step S relative to the front point of the breakaway edge A, the depth of which is &gt;3 mm. Of course, the edge running parallel here to the outlet plane of the tube 20 can be brought back to the outlet-plane step again by means of a curved path. The angle β&#39;, which extends between the tangent of the breakaway edge A and the perpendicular to the outlet plane of the tube 20, is the same size as angle β. The advantages of this design have already been dealt with above in detail under the section &#34;SUMMARY OF THE INVENTION&#34;. 
     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.