Abstract:
A burner having a plurality of pre-mixing chambers each having a fuel nozzle for two fuels, the fuel nozzle has a fuel lance extending in a flow direction, in which fuel lance first outlet openings for a first fuel are introduced. The fuel lance is surrounded by an outer pipe having at least one second outlet opening for a second fuel, the first outlet openings oriented radially and the second outlet opening oriented axially, a flow cross-section is formed between the fuel lance and the inside of the pre-mixing chamber and vortex generators are arranged on the fuel lance. At least one vortex generator is arranged upstream of the first outlet openings and downstream of the second outlet opening. The distance of the first outlet openings from an end of the pre-mixing chamber is at least three times as large as a cross-section of the pre-mixing chamber.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Stage of International Application No. PCT/EP2015/055881 filed Mar. 20, 2015, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 102014206446.5 filed Apr. 3, 2014. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a burner with a premixing chamber and with a fuel nozzle for two fuels. Furthermore, the invention relates to a gas turbine with such a burner. Furthermore, the invention relates to a fuel nozzle for two fuels. 
       BACKGROUND OF INVENTION 
       [0003]    In gas turbines a burner is typically provided with a premixing chamber, in which a fuel, in particular in gaseous form, is mixed with air, in order then to combust the resultant mixture. The efficiency of the gas turbine and also the formation of undesired emission products, in particular nitrogen oxides, are in this case essentially dependent on proper mixing of the fuel with the air. 
         [0004]    In particular in a gas turbine operated with natural gas, the natural gas is frequently injected radially, i.e. perpendicular to the direction of air flow (the “jet in crossflow” method). In this way, appropriate mixing of natural gas and air can be achieved. 
         [0005]    “Vortex generators” are additionally known for mixing, for example from DE 44 26 351 A1, in which a combustion chamber is disclosed which consists substantially of a first stage and a second stage arranged downstream in the direction of flow. In this case, the first stage comprises a mixer on the head side for forming a fuel/air mixture and vortex generators are present on the outflow side of the mixer. These serve in particular to swirl hot air, which is then guided into a premixing zone for mixing with fuel and then into a combustion zone of the second stage. 
         [0006]    It is additionally known to operate gas turbines with a plurality of fuels, for example with natural gas and hydrogen. Operation may proceed either simultaneously with a plurality of fuels or with just one of the fuels. This in particular increases the flexibility of the gas turbine, since fuels may be used according to availability. However, the different operating modes also result in additional requirements for the gas turbine and the burners thereof. 
         [0007]    When using or admixing hydrogen, however, there is an increased risk, over the use of just natural gas, of flareback and self-ignition. To reduce this risk, it is possible to add a less reactive gas to the hydrogen to form a hydrogen-containing fuel gas, which frequently however disadvantageously has a lower energy density than natural gas. It is thus in particular necessary to design injection so as to be suitable to accommodate different volumetric flow rates for the different fuels. 
         [0008]    To reduce further the risk of flareback and self-ignition, it is known to inject the hydrogen-containing fuel gas in a coaxial direction, i.e. in the direction of air flow. In this way, an air-side pressure drop is in particular also reduced, this being greater when hydrogen-containing fuel gas is used, due to the greater volumetric flow rate, than when natural gas is used. 
         [0009]    EP 2 604 919 A1 for example discloses a fuel nozzle for two fuels, with an inner pipe with radially oriented outlet orifices for a first fuel and with an outer pipe surrounding the inner pipe and having axially oriented outlet orifices for a second fuel. Using such a nozzle it is possible, for example, to inject a hydrogen-containing fuel gas in the direction of air flow via the axial outlet orifices. To improve mixing of the fuel gas with the air, a “lobe mixer” is additionally provided. The second fuel, for example natural gas, is then injected via the radial outlet orifices. A disadvantage here is that there are no further options for optimizing mixing in particular of the natural gas and the air. 
       SUMMARY OF INVENTION 
       [0010]    An object of the invention is to provide an improved burner, which in particular is suitable for operation with a plurality of fuels. It is additionally intended to improve mixture formation in the burner. It is further intended to provide a gas turbine with such a burner. In addition, it is intended to provide an improved fuel nozzle, which is suitable in particular for a plurality of fuels. 
         [0011]    The object is achieved according to the invention by a burner, a gas turbine having and a fuel nozzle having the features of the independent claims. Advantageous configurations, further developments and variants constitute the subject matter of the subclaims. 
         [0012]    To this end, provision is made for a burner to comprise a plurality of premixing chambers each with a fuel nozzle for two fuels, wherein the fuel nozzle has a fuel lance extending in a flow direction into which a number of first outlet orifices for a first fuel are introduced, and the fuel lance is surrounded by an outer pipe with at least one second outlet orifice for a second fuel, wherein the first outlet orifices are oriented radially and the second outlet orifice is oriented axially, wherein a flow cross section is formed between the fuel lance and the inside of the premixing chamber and wherein a number of vortex generators are arranged on the fuel lance which reduce a flow cross section oriented transversely to the direction of flow, wherein at least one vortex generator is arranged upstream of the first outlet orifices and downstream of the second outlet orifice and the premixing chamber has a cross section and an end and the distance between the first outlet orifices and the end of the premixing chamber is at least three times as great as the cross section of the premixing chamber. In particular, the premixing chamber comprises an air inflow channel, in which the fuel nozzle is arranged. Advantageously, air flows in the direction of flow, for mixing with the first and/or second fuel. 
         [0013]    The air, the first fuel and the second fuel are hereinafter denoted in general as gases. Use is in principle not restricted to gaseous media, however. Furthermore, use is not restricted to the gases stated below, namely natural gas, hydrogen and air. 
         [0014]    The fuel lance advantageously serves in the injection of natural gas, which is provided by means of the radial outlet orifices for mixing purposes. Radial is here understood to mean that the fuel lance extends in the direction of flow, i.e. axially, and, radially thereto, has a circumferential surface into which suitable, for example round, orifices are introduced. In this way, in particular a “jet in crossflow” mixture is produced, in which fuel inflow is substantially perpendicular to the air. 
         [0015]    The outlet orifices are advantageously at a common position in the axial direction and regularly distributed in the circumferential direction of the fuel lance. Alternatively, however, another suitable arrangement is selected. For example, outlet orifices are arranged one behind the other at multiple positions in the axial direction. 
         [0016]    The outer pipe, which surrounds the fuel lance, likewise extends in the direction of flow, i.e. in the axial direction. In this case, the outer pipe advantageously only partly surrounds the fuel lance in the axial direction, i.e. the fuel lance protrudes in the direction of flow. This makes it possible, in particular, to arrange the radial outlet orifices outside a region of the fuel lance covered by the outer pipe, so in particular improving mixing with air. Alternatively, the radial outlet orifices are however covered by the outer pipe or both covered and uncovered outlet orifices are present. 
         [0017]    The outer pipe advantageously serves for axial injection of the second fuel, for example hydrogen or a hydrogen-containing fuel gas. As a result of the axial injection, it is in particular possible to inject a fuel with a volumetric flow rate which is greater compared with natural gas. Advantageously, moreover, an air-side pressure drop, as may be present in the case of radial injection, may be reduced or avoided completely. 
         [0018]    To improve intermixing of two of the gases or all the gases, at least one vortex generator is provided, i.e. arranged in the burner. Mixing can in particular be achieved by means of the vortex generator in that a flow cross section of at least one of the gases is reduced at at least one position in the direction of flow. 
         [0019]    For example, the air flows in the direction of flow at a first position through a first area, which is oriented transversely, i.e. substantially perpendicular to the direction of flow. The first area here corresponds to the flow cross section at the first position. A vortex generator is for example arranged at a second position downstream of the first position, resisting the air with an additional blocking surface, whereby the second area through which the air flows at the second position is smaller than the first area. In other words, the flow cross section is smaller at the second position than at the first position. For example, the vortex generator is a surface angled relative to the direction of flow. In this case, the vortex generator appropriately has a contour, advantageously at least one edge, for generating turbulence. This serves in particular in mixing the then swirled gas with a second gas. Conveniently, mixing of the first and/or second fuel with air and/or intermixing of the fuels is hereby improved. Overall, therefore, as a result of the arrangement of the vortex generators, during operation efficient mixing of the gases is achieved in the case of a multi-substance fuel nozzle due to the turbulence generated, irrespective of the respective operating mode, i.e. which gas is currently being used as fuel. 
         [0020]    In the case of the burner according to the invention, at least one vortex generator is mounted on the fuel lance. When in particular the vortex generator is used to swirl the first fuel injected by means of the fuel lance, the vortex generator is advantageously adapted to the requirements of this fuel. In the event of changeover of the first fuel and thus possibly modified requirements with regard to mixing with air, it is thereby in particular possible, through changeover of the fuel lance, simultaneously to change the vortex generator. 
         [0021]    Furthermore, in the case of the burner according to the invention, at least one vortex generator is arranged upstream of the radial outlet orifices and downstream of the axial outlet orifice. In this way, swirling of the air and/or of the second fuel may in particular be achieved without the vortex generator directly influencing flow of the first fuel. Direct influencing is here understood to mean that the gas influenced by the vortex generator in question flows thereagainst. 
         [0022]    Finally, the premixing chamber of the burner according to the invention comprises a cross section and an end, wherein, with regard to good intermixing of fuel and air, the distance between the first outlet orifices and the end of the premixing chamber is at least three times as great as the cross section of the premixing chamber. In this way, it is ensured that the length of the path over which the fuel and air are able to mix is sufficiently great. 
         [0023]    In a configuration, the fuel lance and the outer pipe are arranged concentrically. In particular, the fuel lance and the outer pipe are substantially cylindrical in form and have a common longitudinal axis. The outer pipe is in this case advantageously configured as a pipe with an annular profile across the longitudinal axis. The longitudinal axis conveniently extends in the direction of flow. In particular, the second fuel and the air each flow in the direction of flow. In other words, the air and the second fuel flow in or are injected axially. In contrast, the first fuel is advantageously injected radially. 
         [0024]    In one suitable embodiment, at least one vortex generator is wedge-shaped in form. Wedge-shaped is here understood to mean that the vortex generator has a surface which extends obliquely relative to the axial direction and in particular obliquely relative to the direction of flow. The surface is for example of rectangular configuration. Alternatively, the surface is of triangular configuration, wherein one side of the triangle extends transversely to the direction of flow. The remaining two sides taper towards the point of the triangle either in or contrary to the direction of flow. Wedge-shaped is in particular also understood to mean tetrahedral. 
         [0025]    Instead of a wedge-shaped configuration, other geometric shapes are also suitable. The vortex generator is also possibly configured as a solid body or composed of various surface elements or indeed of multipart configuration. What is essential is that the flow cross section is adjustable in the direction of flow by means of the vortex generator, in particular in order to generate turbulence in the flow profile of the gas flowing against the vortex generator. Adjustable is here in particular understood to mean that the precise configuration and orientation of the vortex generator is established on production and installation thereof. 
         [0026]    In one advantageous configuration, at least one vortex generator is mounted on the outer pipe. The vortex generator is here mounted either on the outside on the outer pipe, in particular for swirling the air flowing appropriately therealong, or on the inside in the outer pipe, for swirling the second fuel advantageously flowing therealong. 
         [0027]    In a further advantageous configuration, the premixing chamber comprises an internal wall, on which at least one vortex generator is mounted. In this way it is possible in particular to achieve swirling that is substantially independent of the fuel nozzle. 
         [0028]    In a further advantageous configuration, at least one vortex generator is arranged downstream of the radial outlet orifices. In other words, at least one vortex generator is arranged downstream of each outlet orifice, whereby this vortex generator in particular influences, i.e. in particular swirls, each of the gases which have flowed in. 
         [0029]    Several or all of the above-stated configurations relating to the positioning of the vortex generator are combined in a further advantageous configuration. In this way, the correspondingly stated advantages are in particular also combined. For example, at least one vortex generator is arranged downstream of the axial outlet orifice and upstream of the radial outlet orifices and on the fuel lance. Alternatively or in addition, a plurality of vortex generators are for example arranged at various positions on the fuel lance in the axial direction. 
         [0030]    It is furthermore possible advantageously to arrange a plurality of vortex generators in groups, for example in series, one behind the other or offset in the axial direction; or in a plane, i.e. in particular both next to one another (for example in the circumferential direction) and one behind the other. It is also possible for a plurality of vortex generators advantageously to have different geometries and/or dimensions. 
         [0031]    A plurality of vortex generators are advantageously arranged at approximately the same position in the axial direction and in a circumferential direction with regard to the longitudinal axis. For example, a plurality of vortex generators are arranged in such a way around the circumference of the outer pipe that all the distances between adjacent vortex generators in the circumferential direction are of equal magnitude. 
         [0032]    In particular, it is advantageously possible to influence paired swirling of the various gases by a suitable combination of multiple, in particular different vortex generators. For example, a number of vortex generators are mounted on the outer pipe for swirling the air and for improved mixing with hydrogen injected axially downstream thereof. In order further to optimize the swirling of natural gas with air, vortex generators are for example additionally mounted downstream of the radial outlet orifices. 
         [0033]    In an advantageous further development, the outer pipe comprises an end region which is configured as a lobe mixer and comprises a number of lobes. These extend in particular in the direction of flow and as radially configured folds. This results, transversely of the longitudinal axis, in particular in a stellate cross section (or indeed a stellate profile). In the circumferential direction, an interspace is formed between each pair of lobes, by means of which in particular the flow cross section of the air is advantageously increased downstream. 
         [0034]    The lobes each comprise a vertex in the radial direction, which extends substantially in the axial direction. This means in particular that the radial distance between vertex and longitudinal axis is substantially constant in the direction of flow. In particular, the outer pipe comprises an outer jacket and the vertices of the lobes are substantially in line with the outer jacket. It is then possible for a slight inclination or slope to be provided in the axial direction. 
         [0035]    Mixing of the second fuel with air is advantageously achieved in that the air follows the direction of flow and stalls at the end of the end region. In particular, the stellate cross section of the end region comprises a contour line extended relative to the outer pipe (and correspondingly stellate). In this way, an edge larger than the circumference of the outer pipe is advantageously provided for stalling. 
         [0036]    In a suitable further development, the end region is turned or twisted in such a way that the lobes and thus also the vertices extend in a spiral around the longitudinal axis. This makes it possible additionally to swirl the air flowing past the lobes and thus to achieve improved mixing. 
         [0037]    In a further suitable further development, a number of the lobes are additionally configured as vortex generators. To this end, these lobes are in particular shaped in such a way that the vertices thereof are configured as surfaces extending obliquely in the axial direction. In other words, the distance from the vertex of a lobe to the longitudinal axis varies in the direction of flow. Advantageously, the distance is enlarged continuously in the direction of flow. In this way, an angled surface with an edge is in particular created, in such a way that swirling in the manner of a vortex generator may be achieved thereby. 
         [0038]    In an advantageous further development, at least one vortex generator is arranged in an interspace between two lobes. The stated interspace here corresponds to the interspace already mentioned above between two neighboring lobes in the circumferential direction of the outer pipe. This arrangement in particular makes it possible to produce vortex generators with comparatively large lateral surfaces, i.e. in comparison with vortex generators arranged for example on an annular pipe without lobe mixer. Swirling can advantageously be influenced in this way. 
         [0039]    A combination of the above-stated vortex generators with one of the above-stated designs for injecting the second fuel conveniently allows improved mixing of the gases concerned. Advantageously, mixing is improved both in the case of simultaneous injection of the first and second fuel (for example natural gas and hydrogen) and in the case of individual operation, i.e. injection of just one fuel (for example natural gas or hydrogen). 
         [0040]    A gas turbine advantageously comprises a burner with one or more of the above-stated features, resulting in the advantages accordingly stated above. Such a gas turbine is additionally particularly efficient and advantageously has lower pollutant emissions. 
         [0041]    A fuel nozzle for two fuels appropriately comprises a fuel lance extending in a direction of flow. A number of first outlet orifices for a first fuel are introduced thereinto. The fuel lance is then surrounded by an outer pipe with at least one second outlet orifice for a second fuel, wherein the first outlet orifices are oriented radially and the second outlet orifice axially, wherein a number of vortex generators are arranged on the fuel lance. At least one vortex generator is arranged upstream of the first outlet orifices and downstream of the second outlet orifice. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]    Exemplary embodiments of the invention are explained in greater detail below on the basis of drawings, in which: 
           [0043]      FIG. 1  is a side view of a burner with a fuel nozzle for two fuels and a plurality of vortex generators mounted on the fuel nozzle, 
           [0044]      FIG. 2  shows the burner according to  FIG. 1  with an alternative fuel nozzle and a premixing chamber, on the internal walls of which a plurality of vortex generators are mounted, 
           [0045]      FIGS. 3 to 17  show further exemplary embodiments of the fuel nozzle according to  FIG. 1 , wherein the fuel nozzle is shown in each of  FIGS. 3, 6, 9, 12 and 15  in side view, in each of  FIGS. 4, 7, 10, 13 and 16  in front view and in each of  FIGS. 5, 8, 11, 14 and 17  in perspective view, and 
           [0046]      FIGS. 18 to 23  each show an exemplary embodiment of a vortex generator in perspective view. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0047]    A schematic representation of a burner  2 , in particular for a gas turbine  4 , is shown by each of  FIGS. 1 and 2 . The burner  2  here comprises a premixing chamber  6 , downstream of which in the direction of flow S is arranged a combustion chamber  8 . In the exemplary embodiment shown here, two fuels and air are injected into the premixing chamber  6  when in operation. A fuel nozzle  10 , which extends in the direction of flow S, serves to inject the fuels. The air flows in the direction of flow S via an air inlet channel  12  surrounding the fuel nozzle  10 . 
         [0048]    The fuel nozzle  10  comprises a fuel lance  14  and an outer pipe  16  surrounding said lance, wherein the fuel lance  14  protrudes in the direction of flow S and relative to the outer pipe  16 . The fuel lance  14  and the outer pipe  16  are substantially cylindrical in the embodiment shown here, i.e. they have a circular or annular cross section transversely of the direction of flow S. In particular, the fuel lance  14  and the outer pipe  16  are arranged concentrically and accordingly have a common longitudinal axis L which extends in the direction of flow S. 
         [0049]    The fuel lance  14  comprises a number of radial outlet orifices  18 . These are embodied in circular manner in the exemplary embodiment shown here and arranged at a common position in the axial direction, i.e. in the direction of flow S. The outlet orifices  18  are here distributed in a circumferential direction U, in particular regularly. The radial outlet orifices  18  serve in particular to inject the first fuel, for example natural gas. 
         [0050]    The outer pipe  16  has a larger diameter than the fuel lance  14 , so forming in the axial direction an in particular annular, axial outlet orifice  20 . This is used to inject the second fuel into the premixing chamber  6 . In other words, the second fuel in particular also flows around the fuel lance  14 . 
         [0051]    In  FIG. 1  a number of vortex generators  22  are attached to the fuel lance  14 . These are arranged downstream of the axial outlet orifice  20  and upstream of the radial outlet orifices  18 . In the exemplary embodiment shown here, the vortex generators  22  are tetrahedral in form (cf. in particular also  FIG. 20 ). 
         [0052]      FIG. 1  additionally shows that the premixing chamber  6  has a cross section  50  and an end  52  and the distance between the first outlet orifices  18  and the end  52  of the premixing chamber  6  is at least three times as great as the cross section  50  of the premixing chamber  6 . 
         [0053]    In an alternative or supplementary configuration according to  FIG. 2 , the vortex generators  22  according to  FIG. 1  are fastened to the internal walls of the premixing chamber  6 . In this case, the vortex generators  22  are arranged at a position downstream of the radial outlet orifices. 
         [0054]    In the two exemplary embodiments shown in  FIGS. 1 and 2 , the vortex generators  22  each have a surface  24  angled relative to the direction of flow S, which surface is triangular here and tapers along the longitudinal axis L contrary to the direction of flow S. This arrangement is also designated as being directed forwards. In an alternative embodiment not shown here, the vortex generators  22  are in contrast directed backwards, i.e. turned through 180° in such a way that the surface  24  tapers along the longitudinal axis L in the direction of flow S. 
         [0055]    As a result of the surface  24 , here in particular in each case two edges  26  are formed, at which in particular swirling is generated during operation. In addition, a flow cross section Q, which is modified by the vortex generators  22  in the direction of flow S, is defined transversely of the direction of flow S in particular by the premixing chamber  6  and the elements arranged therein. In  FIG. 1  for example, the flow cross section Q is defined at a first position P 1  by the premixing chamber  6  and the fuel lance  14 . At this first position P 1 , the flow cross section Q is in particular greater than at a second position P 2 , at which the vortex generators  22  are arranged in the exemplary embodiment shown here. It is advantageously possible to adjust the flow cross section Q by appropriate configuration of the vortex generators  22  and thus in particular appropriately to influence the mixing of the gases. 
         [0056]      FIGS. 3 to 17  are schematic representations of further exemplary embodiments of a fuel nozzle  14 .  FIGS. 3, 6, 9, 12 and 15  each show the fuel nozzle  14  in side view, arrows indicating an inflow direction  28 ,  30 ,  32  for each of the gases. In this case, the first fuel flows in radially in the inflow direction  28  and the second fuel and the air flow in axially in the inflow directions  30 ,  32 . As a result of the axial inflow, in the premixing chamber  6  in particular the general direction of flow S is established, which the first fuel also substantially adopts at a sufficient distance from the radial outlet orifices  18 . 
         [0057]      FIGS. 4, 7, 10, 13 and 16  each show the corresponding fuel lance  14  in front view, and  FIGS. 5, 8, 11, 14 and 17  each show the corresponding fuel lance  14  in perspective view. 
         [0058]    The embodiment of the fuel nozzle  10  shown in  FIGS. 3 to 5  comprises a number of forwards-oriented tetrahedral vortex generators  22 , which are mounted on the fuel lance  14  downstream of the axial outlet orifice  20  and upstream of the radial outlet orifices  18 . Here, the vortex generators  22  each have a height H which is here selected such that the vortex generator  22  extends further in the radial direction than the outer pipe  16 . This is shown particularly clearly in  FIG. 4 . This makes it possible in particular for air to flow directly against the vortex generators  22  and be swirled. 
         [0059]      FIGS. 6 to 8  show the fuel nozzle  10  with vortex generators  22  mounted on the outer pipe  16 . These are here oriented forwards, the air which has flowed in around the outer pipe  16  flowing against them. The fuel lance  14 , in contrast, does not comprise any vortex generators  22 . 
         [0060]      FIGS. 9 to 11  show the fuel nozzle  10  with an end region  34  configured as a lobe mixer. To this end, a number of lobes  36 , six in this case, are formed in the end region  34 . These result in a stellate cross section, as is clear for example from  FIG. 10 .  FIG. 10  further shows that the lobes  36  substantially do not project beyond the outer pipe  16  in the radial direction. 
         [0061]    The lobes  36  each have a vertex  38  extending in the axial direction and are spaced apart in particular regularly in the circumferential direction U by interspaces  40 . At the end  42  of the outer pipe  16 , the lobes  36  form a contour  44  which here is stellate and by which in particular also a number of outlet channels  46  are created. The axial outlet orifice  20  therefore comprises six outlet channels  46  in the exemplary embodiment shown here. 
         [0062]    As  FIGS. 10 and 11  show, the vortex generators  22  mounted downstream on the fuel lance  14  may either be aligned in the direction of flow S with one of the outlet channels  46  or be offset relative thereto. Of the four vortex generators  22  present here, for example two vortex generators  22 A are arranged as notional extensions of outlet channels  46  and two vortex generators  22 B are arranged as notional extensions of interspaces  40 . As a result of an arrangement in particular configured in this way, it is possible to use the respective vortex generators  22  either principally to swirl the air flowing through an interspace  40  or principally to swirl the second fuel flowing through an outlet channel  46 . 
         [0063]      FIGS. 12 to 14  show an exemplary embodiment in which the outer pipe  16  of the fuel nozzle  10  has in the end region  34  a number of lobes  36 , here four, which are embodied at the same time as vortex generators  22 . The respective vertex  38  of a lobe  36  is configured as an angled surface  24  and has two edges  26  delimiting the substantially triangular surface  24 . These extend away from the longitudinal axis L in the downstream direction. The end region  34  has a number of outlet channels  46  for the second fuel corresponding to the number of vortex generators  22 . 
         [0064]    Furthermore, in the exemplary embodiment shown here the radial outlet orifices  18  are arranged substantially directly downstream of the outer pipe  16 . A respective radial outlet orifice  18  is here arranged either as a notional extension of an interspace  40  or as a notional extension of an outlet channel  46 . 
         [0065]    An alternative embodiment with both vortex generators  22  and lobes  36  in the end region  34  of the outer pipe  16  is shown in  FIGS. 15 to 17 . Here one vortex generator  22  is arranged in each interspace  40  between two adjacent lobes  36 . In the exemplary embodiment shown here, the vortex generators  22  are formed as far as the end  42  of the outer pipe  16 , i.e. the vortex generators  22  are in particular in line in the radial direction with the end  42  of the outer pipe  16 . Contrary to the vortex generators  22  shown in  FIGS. 12 to 14 , the vortex generators  22  shown in  FIGS. 15 to 17  do not have any outlet channels  46  at the end. 
         [0066]      FIGS. 18 to 23  each show an exemplary embodiment of a vortex generator  22 . The actual embodiment is not limited in this respect to the exemplary embodiments shown here. 
         [0067]      FIGS. 18 and 19  respectively show a triangular and rectangular surface  24  angled relative to a direction of flow S.  FIGS. 20 and 21  show similarly configured vortex generators  22 , but these are configured as solid bodies and have corresponding side faces  48 . In contrast, the vortex generators  22  shown in  FIGS. 22 and 23  each comprise two, in particular separately produced side faces  48 , which are angled relative to the direction of flow S. In  FIGS. 18 to 23  the vortex generators  22  are each oriented forwards with regard to the direction of flow S. Alternatively, however, the vortex generators  22  are oriented backwards, i.e. turned through 180° with regard to the direction of flow S (the arrow indicating direction of flow S in  FIGS. 18 to 23  then points in the opposite direction).