Abstract:
An apparatus for distributing fluids in an exhaust system of an internal combustion engine. The apparatus includes an injection device, wherein a combination of a plurality of individual measures for achieving a uniform mixing of the fluids with the exhaust gas and a complete vaporizing of the fluids in the exhaust gas is provided. The individual measure comprises at least one of at least one swirl-generating device and at least one mixing apparatus. The individual measure also includes at least one catalyst. The individual measure further includes an injection nozzle of the injection device which is arranged at a predefined spacing from a wall of the exhaust tract.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims the benefit of priority to PCT Application no. PCT/EP2011/005438, filed Oct. 27, 2011, which claims the benefit of priority to German Patent Application No. 102010056314.5, filed Dec. 27, 2010, which are both incorporated herein by reference in their entirety. 
     FIELD OF THE INVENTION 
     The subject matter herein relates to an apparatus for distributing fluids. 
     Due to emission regulations for internal combustion engines becoming more strict, it has become necessary to reduce nitrogen oxides (NOx) in exhaust gas. A possibility consists of reducing the nitrogen oxides to nitrogen and water in a so-called selective catalytic reduction (SCR). This takes place in a so-called SCR catalyst while using a reductant injected into the exhaust gas. A water/urea mixture can be used for this purpose whose urea decomposes to ammonia in the exhaust gas which reacts with the nitrogen oxides. Liquid fuel in the form of different hydrocarbon compounds (HC) can also be injected into the exhaust gas. 
     It is generally desirable in such reduction processes that the reductant is mixed as uniformly as possible with the exhaust gas and that, while a vaporization or thermolysis of the liquid reductant is achieved, which is as complete as possible to obtain a high efficiency in the reduction and a residue-free operation. 
     SUMMARY 
     It is therefore a feature of the invention to improve the distribution of fluids in the exhaust gas and, where necessary, to ensure a complete vaporization of the fluids without residues in as simple a manner as possible. 
     This is satisfied by an apparatus for distributing fluids in an exhaust system of an internal combustion engine. The apparatus includes an injection device, wherein a combination of a plurality of individual measures for achieving a uniform mixing of the fluids with the exhaust gas and a complete vaporizing of the fluids in the exhaust gas is provided. The individual measure comprises at least one of at least one swirl-generating device and at least one mixing apparatus. The individual measure also includes at least one catalyst. The individual measure further includes an injection nozzle of the injection device which is arranged at a predefined spacing from a wall of the exhaust tract. It has been recognized in accordance with the apparatus that a good mixing and residue-vaporization of the fluids in the exhaust gas can only be achieved if a plurality of measures are combined with one another in a simple manner. In particular the injection characteristic or the spray characteristic of the injection device, the length of the distribution path or vaporization path, the temperature as well as the pipe shape and the pipe cross-section of the respective exhaust system section are decisive for the selection and configuration of the individual measures. A residue-free distribution and vaporization of the fluids in the exhaust gas flow can thus be achieved in a desired manner by taking account of the thermodynamic circumstances and the installation specifications as well as by a suitable matching of the different individual measures. 
     Further developments of the invention are set forth in the description and in the enclosed drawings. 
     In accordance with an embodiment of the invention, at least one swirl-generating device is provided which comprises a plurality of gas vanes arranged in the manner of a turbine. Such a swirl-generating device can be provided upstream or downstream of the injection device depending on the application. A good swirl generation in the exhaust gas flow can be achieved by the gas vanes. Furthermore, the gas vanes can act as a vaporizer surface and can thus increase the degree of vaporization. A particularly large vaporizer surface is provided due to the turbine-like arrangement of the gas vanes. The turbine-like embodiment additionally effects a good blocking of the exhaust gas cross-section, with the ratio between the exhaust back pressure and the mixing power being able to be optimized via the length and width as well as the angle of engagement of the vanes. 
     A mixing apparatus can furthermore be provided which comprises at least one swirl-generating device and one tubular vaporizing device. The mixing apparatus is arranged downstream of the injection device and serves to mix the injected fluids and optionally prehomogenized fluids uniformly with the exhaust gas. A particularly high degree of mixing can be achieved by a combination of a swirl-generating device—which can in turn in particular comprise a plurality of gas vanes arranged in the manner of a turbine—and a tubular vaporizing device, with the order and the spacing between the two components being able to be matched to the respective application. 
     In accordance with a further embodiment of the invention, a hydrolysis catalyst is provided which is in particular arranged upstream of a so-called SCR catalyst. Such a hydrolysis catalyst can assist an isocyanic acid conversion into ammonia and carbon dioxide and thereby relieve an SCR main catalyst, for example. 
     A flow guiding element can furthermore be provided which is arranged in the interior of the exhaust tract and is at least regionally spaced apart from a wall of the exhaust tract and which projects at least partly into a flexible decoupling section of the exhaust tract, wherein the flow guiding element in particular ends in a downstream region of the decoupling section. Flexible decoupling sections in the exhaust tract in particular serve to counter a transmission of vibrations or different thermal expansions of exhaust tract components. Such decoupling sections can, for example be designed as corrugated hose, wound hose or spiral wound metallic hose. Depending on the application, a combination of the named hose types can also be present, for example, a wound hose sheathed by a corrugated hose can be provided. Unwanted deposits of the injected fluids and/or of reaction products of the exhaust gas and of the fluids can collect in the recesses and wall irregularities present with such decoupling sections. Such depositions are avoided in that the exhaust gas is conducted through the smooth-walled, flow guiding elements and is thus at least sectionally prevented from flowing past the irregular wall of the decoupling section. The flow guiding element can further influence the flow of the exhaust gas in a desired manner by its special shape, for example, by dimples, depressions, indentations or the liner. 
     The injection device can in particular be arranged upstream of the flexible decoupling section of the exhaust tract. The advantage of the deposition avoidance by the flow guiding element is then particularly effective. If the application requires, the injection device can also be arranged between two flexible decoupling sections of the exhaust tract. 
     The flow guiding element can be configured as an inner pipe at least sectionally coaxial to the exhaust tract. Such an inner piper is also called an “inliner”. The inner pipe is designed fully walled. 
     The flow guiding element can have a prolongation section, for example, in the form of a piece of pipe halved in the longitudinal direction, at its downstream end, said prolongation section being interrupted peripherally and in particular being of half-shell shape. Such a prolongation section can be used for the direct catching of fluid residues in order subsequently to vaporizer them. 
     In accordance with an embodiment, the flow guiding element is sectionally tapered viewed in the direction of the exhaust gas flow. The tapering site is arranged downstream of a mixing apparatus. Alternatively or additionally, a tapering site can also be provided upstream of a mixing apparatus. A tapering effects an increase in the flow rate due to the Venturi effect, which facilitates the mixing of the fluid in the exhaust gas. 
     In accordance with a further aspect, the flow guiding element is integrated in the exhaust tract such that it can be flowed around by exhaust gas at the outside. The exhaust gas in such an embodiment is therefore not only conducted through the flow guiding element, but is also led past it outwardly. Some of the exhaust gas flow thus bypasses swirl-generating devices and/or mixing apparatus, which can be integrated in the flow guiding element, in a secondary flow. For this purpose, the flow guiding element can be fastened in the associated supporting exhaust pipe by means of a plurality of holding webs such that an air gap is present between the outer wall of the flow guiding element and the inner wall of the exhaust pipe. In this manner, the flow guiding element is heated by the hot exhaust gas flowing past outwardly, whereby a vaporizing of fluid in the interior of the flow guiding element is facilitated. 
     Furthermore, the swirl-generating device and/or the mixing apparatus can be integrated into a flow guiding element and/or the injection device can open into a flow guiding element. It can hereby be achieved that a fluid distribution apparatus takes up less construction space overall in that namely the flow guiding element is arranged overlapping with another functional component in the exhaust tract. 
     In accordance with a further embodiment, the injection nozzle is arranged at the wall of the exhaust tract. The fluids are therefore injected close to a pipe wall or line wall, i.e. at the outer margin of the flow. Such an arrangement of the injection nozzle can in particular be provided at apparatus in which the injection nozzle is arranged in a curved or angled section of the exhaust tract. If the application requires, the injection nozzle can also be arranged in a predefined, relatively small spacing from the wall. 
     In accordance with an alternative embodiment, the injection nozzle is arranged centrally with respect to a cross-section of the exhaust tract, i.e. the fluids are injected at the center of the flow. Such a central arrangement can also include an arrangement at the predefined small spacing from the center of the cross-sectional surface of the respective line section. 
     The injection nozzle can be configured to spray the fluids in the manner of a hollow cone or of a solid cone. Injection nozzles having spray characteristics of a hollow cone type are relatively inexpensive and available in a variety of variants. Injection nozzles having spray characteristics of a solid cone, in contrast, ensure a better degree of mixing of the sprayed fluid in the surrounding gas flow. 
     Two injection nozzles set against one another can also be provided for a spraying of the fluids in a flat-cone manner. Such a constellation is in particular advantageous with an oval line cross-section in the respective exhaust system section. 
     The injection device can be configured for spraying the fluids with a predefined average droplet size. The Sauter mean diameter (SMD) can be used as the parameter for the droplet size. The efficiency of the distribution apparatus can thus be optimized by a direct variation of the droplet size. For this purpose—provided the droplet size is known—the required components of the distribution apparatus as well as the corresponding parameters can be matched to the droplet size. Conversely, with predefined components and parameters, a specific ideal droplet size can be selected in order thus to ensure a deposition-free operation of the distribution apparatus. Depending on the application, the average droplet size of the injection device can therefore be an input parameter or an output parameter of the distribution apparatus to be configured. 
     In accordance with a further embodiment, the combination of a plurality of individual measures comprises at least one swirl-generating device and an injection nozzle of the injection device which is arranged centrally with respect to a cross-section of the exhaust tract and is configured for a spraying of the fluids in a solid cone manner with a droplet size of at most 25 μm Sauter mean diameter. It has namely been found that with a central injection having spray characteristics of a solid cone type and relatively small droplets, only one swirl-generating device—arranged upstream or downstream of the injection device—is sufficient to achieve a uniform distribution of the fluids vaporized in the gas phase. Further expensive components of the distribution apparatus can thus be dispensed with. 
     In accordance with a further embodiment of the invention, the combination of a plurality of individual measures comprises at least one swirl-generating device which is provided upstream of the injection device, at least one mixing apparatus which is provided downstream of the injection device and which comprises at least one tubular vaporizer device, and comprises an injection nozzle of the injection device which is arranged centrally with respect to a cross-section of the exhaust tract and is configured for a spraying of the fluids in a hollow cone manner with a droplet size of at most 25 μm Sauter mean diameter. Provided that therefore a mixing apparatus having at least one tubular vaporizer device and arranged downstream of the injection device is provided in addition to the swirl-generating device, a simple spraying injection nozzle of a hollow cone type can be made use of. 
     In accordance with a further embodiment of the invention, the combination of a plurality of individual measures comprises at least one swirl-generating device which is provided upstream of the injection device, at least one mixing apparatus which is provided downstream of the injection device and which comprises at least one swirl-generating device, and comprises an injection nozzle of the injection device which is arranged centrally with respect to a cross-section of the exhaust tract and is configured for a spraying of the fluids in a solid cone manner with a droplet size of at least 25 μm Sauter mean diameter. In the case of larger droplets, two swirl-generating devices are therefore required for a uniform distribution—one upstream and one downstream of the injection device. On use of an injection nozzle spraying in a full-cone manner, the arising of secondary products can be avoided. 
     In accordance with a further embodiment of the invention, the combination of a plurality of individual measures comprises at least one swirl-generating device which is provided upstream of the injection device, at least one mixing apparatus which is provided downstream of the injection device and which comprises at least one swirl-generating device and a tubular vaporizer device, and comprises an injection nozzle of the injection device which is arranged centrally with respect to a cross-section of the exhaust tract and is configured for a spraying of the fluids in a hollow cone manner with a droplet size of at least 25 μm Sauter mean diameter. In this constellation, two swirl-generating devices and one tubular vaporizer device are provided. In turn, only one simple injection nozzle spraying in a hollow-cone manner is to be provided. 
     In accordance with a further embodiment of the invention, the combination of a plurality of individual measures comprises at least one mixing device which is provided downstream of the injection device, and which comprises at least one swirl-generating device, and comprises an injection nozzle of the injection device which is arranged at the wall of the exhaust tract and is configured for a spraying of the fluids in a solid cone manner with a droplet size of at least 50 μm Sauter mean diameter. In the case of an injection of the fluids close to the wall, it is therefore expedient to design the injection nozzle for larger droplets. Furthermore, a swirl-generating device is to be provided downstream of the injection device. 
     In accordance with a further embodiment of the invention, the combination of a plurality of individual measures comprises at least one mixing device which is provided downstream of the injection device, and which comprises at least one swirl-generating device and a tubular vaporizer device, and comprises an injection nozzle of the injection device which is arranged at the wall of the exhaust tract and is configured for a spraying of the fluids in a hollow cone manner with a droplet size of at least 50 μm Sauter mean diameter. On a use of an injection nozzle spraying in a hollow cone manner and of an injection close to the wall, it is therefore expedient to use a mixing apparatus which is arranged downstream of the injection device and which comprises at least one swirl-generating device and at least one tubular vaporizer device. 
     In accordance with the invention, it was in particular taken into account in the determination of the embodiments that the injected droplets vaporize the faster, the smaller they are. However, smaller droplets are also more difficult to introduce into the exhaust gas flow, i.e. the penetration depth falls as the diameter of the droplets reduces or as the pulse of the droplets reduces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in the following by way of example with reference to the drawings. 
         FIG. 1  shows a partially cut-away perspective view of an exhaust tract section in which an embodiment of an apparatus for distributing fluids is integrated; 
         FIG. 2  shows a partially cut-away perspective view of another embodiment of an the apparatus in which is integrated in an angled exhaust tract; 
         FIG. 3  shows a partially cut-away perspective view of a further embodiment of an the apparatus; and 
         FIG. 4  is a schematic illustration of yet another embodiment of the apparatus; 
         FIG. 5  is a schematic illustration of yet another embodiment of the apparatus; 
         FIG. 6  is a schematic illustration of yet another embodiment of the apparatus; 
         FIG. 7  is a schematic illustration of yet another embodiment of the apparatus; and 
         FIG. 8  is a schematic illustration of yet another embodiment of the apparatus. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a pipe section  11  of an exhaust tract through which emissions to be purified of a diesel engine, not shown, flow along an exhaust gas flow direction A. An inner pipe  12  is arranged in the pipe section  11  and is designed as an exhaust mixer pipe as will be described more exactly in the following. The pipe section  11  with the inner pipe  12  is arranged before an SCR catalyst, likewise not shown, in the exhaust gas flow direction A. An apparatus  13  for distributing a fluid, e.g. a water-urea mixture, in the exhaust gas is integrated into the inner pipe  12 . The apparatus comprises an injection device  15  having an injection nozzle  17  opening into the inner pipe  12 . The fluid to be injected is sprayed into the flowing exhaust gas by means of the injection nozzle  17 . As can be seen from  FIG. 1 , the injection nozzle  17  is seated centrally, with respect to the injection nozzle  17 , in the pipe section  11 , that is on the central longitudinal axis L of the pipe section  11 . The injection nozzle  17  can be configured in dependence on the application to spur the fluid in a hollow cone manner or in a solid cone manner. Furthermore, the injection device  15 , including the injection nozzle  17 , is adapted such that a predefined average droplet size is generated on the spraying. 
     The apparatus  13  furthermore comprises a first swirl-generating device  19  arranged before the injection device  15  in the exhaust gas flow direction A as well as a mixing apparatus  20  arranged after the injection device  15  in the exhaust gas flow direction A. The mixing device  20  comprises a second swirl-generating device  21  and a tubular vaporizer device  23  arranged behind it in the exhaust gas flow direction A. The first swirl-generating device  19  and the second swirl-generating device  21  each comprise gas vanes  25  arranged and configured in the manner of turbines. The gas vanes  25  extend substantially radially outwardly from the central longitudinal axis L of the pipe section  11 , but are bent in on themselves so that their contact line with the pipe section  11  extends along a spiral line. A strong blockage of the cross-section of the pipe section  11  thereby results. On flowing through the swirl-generating devices  19 ,  21 , turbulences are generated in the exhaust gas flow which assist a mixing of the fluid with the flowing exhaust gas. To further improve the mixing, the gas vanes  25  can be shaped so that a turbulence-amplifying swirl direction reversal takes place between the first swirl-generating device  19  and the second swirl-generating device  21 . 
     The tubular vaporizer device  23  has a central pipe  27  as well as outer surfaces  29  which are formed in the manner of blooms and which further improve the mixing of the fluid and of the exhaust gas and moreover act as vaporizer surfaces and/or as impact plates in order thus to promote a vaporization. 
     The surfaces of the elements  19 ,  21  and  23  can be structured or perforated to further improve the atomization and distribution of the fluid. 
     Depending on the application, a hydrolysis catalyst, not shown, can furthermore be provided at any desired position of the pipe section  11  to relieve the SCR catalyst. 
     The exhaust gas flows through the pipe section  11  in accordance with the exhaust gas flow direction A during operation, with the swirl-generating devices  19 ,  21  and the tubular vaporizer device  23  being heated. The exhaust gas exiting the pipe section  11  subsequently enters into the SCR catalyst. To generate and/or facilitate a reduction of nitrogen oxides in the SCR catalyst, a water-urea mixture is e.g. injected into the exhaust gas flow by means of the injection device  15 . In the embodiment shown, only a single injection nozzle  17  is associated with the injection device  15 . In specific applications, the injection device  15  can, however, also comprise a plurality of injection nozzles  17 . A mixing takes place between the exhaust gas and the injected droplets of the water-urea mixture due to the turbulences generated in the exhaust gas jet by means of the first swirl-generating device  19 . The mixture thus produced then moves with the exhaust gas into the second swirl-generating device  21  in which the exhaust gas with the water-urea mixture is set into a rotation about the central longitudinal axis L of the pipe section  11 . A further improved mixing of the exhaust gas with the water/urea mixture thereby takes place. In addition, the gas vanes  25  of the swirl-generating device  21  also act as a vaporizer surface. 
     In the further extent, the partly vaporized water-urea mixture moves with the exhaust gas into the tubular vaporizer device  23  where a further mixing of the water-urea mixture with the exhaust gas and a further vaporization of the water-urea mixture takes place at the corresponding evaporator surfaces. 
     After the exit from the pipe section  11 , the exhaust gas optionally moves into a further vaporizer pipe. Any liquid portions of the water-urea mixture possibly still present can be vaporized here so that then only gaseous reductant moves into the SCR catalyst. The reduction of nitrogen oxides to nitrogen in water takes place in this. 
     The fluid distribution apparatus  13  is therefore divided into a plurality of functional units which are arranged in a plurality of stages with respect to the exhaust gas flow direction A. Which stages are to be provided in a specific application, the order in which the stages are to be arranged and which spacing is to be provided between the stages is determined by the respective spatial and thermodynamic circumstances and can be determined such that particularly a complete vaporization of the injected fluid results within the inner pipe  12 . The skilled person naturally endeavors in this process generally to provide as few functional groups as possible or to provide components which are as small and as inexpensive as possible. A configuration is therefore always desirable in which a deposition-free operation is possible while using as few means as possible which are as simple as possible. 
     It has resulted for the central or close-to-the-axis injection shown in  FIG. 1  and for an inner pipe diameter of 70 mm that only the first swirl-generating device  19  or the second swirl-generating device  21  selectively is sufficient to achieve a complete and residue-free vaporization provided that the injection nozzle  17  is configured to spray the fluid in a sold cone manner and the Sauter mean diameter of the droplets amounts to at most 25 μm. On a use of an injection nozzle spraying in the manner of a hollow cone, it is, in contrast, advantageous to provide both the first swirl-generating device  19  and the tubular vaporizer device  23 . Provided that an injection nozzle spraying in the manner of a sold cone is used and the Sauter mean diameter of the droplets amounts to at least 25 μm, it is to provide both the first swirl-generating devise  19  and the second swirl-generating device  21 . Provided, in contrast, that an injection nozzle spraying in the manner of a hollow cone is used and the Sauter mean diameter of the droplets amounts to at least 25 μm, both the first swirl-generating device  19  and the second swirl-generating device  21 , and additionally the tubular evaporator device  23 , are to be provided. All above-described constellations are very well suited to achieve a residue-free vaporization of the mixture. If it should, however, be necessary, further swirl-generating devices and/or vaporizer devices can also be provided. 
       FIG. 2  shows a similar apparatus  13  to the one shown in  FIG. 1  which is, however, integrated into an angled pipe section  11 ′ of the exhaust tract. The injection nozzle  17  is arranged in the region of the kink at the wall of the pipe section  11 ′. As in the embodiment in accordance with  FIG. 1 , a first swirl-generating device  19 , a second swirl-generating device  21  and a tubular vaporizer device  23  are provided following one another viewed in the exhaust gas flow direction A. It has been found that only the second swirl-generating device  21  is required to achieve a complete vaporization provided that an injection nozzle spraying in the manner of a solid cone is used and the Sauter mean diameter of the droplets amounts to at least 50 μm. Provided, in contrast, that an injection nozzle spraying in the manner of a hollow cone is used and the Sauter mean diameter of the droplets amounts to at least 50 μm, both the second swirl-generating device  21  and the tubular vaporizer device  23  are to be provided. Both described constellations are very well suited to achieve a residue-free vaporization of the fluid. With an enlarged inner pipe diameter, e.g. an inner pipe diameter of 140 mm, it may be expedient also to increase the droplet size, e.g. up to 100 μm. In another respect, the operation of the apparatus is as in the variant in accordance with  FIG. 1 . 
       FIG. 3  shows an apparatus  13 ″ in accordance with a further embodiment of the invention which is integrated into a exhaust tract  11 ″ substantially running in a straight line similar to the exhaust track  11  in accordance with  FIG. 1 . The injection device  15  is here arranged in an indentation  30  of the exhaust tract  11 ″ such that the injection nozzle  17  sprays the fluid obliquely to the front into the exhaust gas flow. The injection nozzle  17  in this respect opens into an inner pipe  12  into which a swirl-generating mixing apparatus  20 ″ is also integrated. As can be seen from  FIG. 3 , there is an air gap  33  between the inner pipe  12  and the wall of the exhaust tract  11 ″ so that some of the exhaust gas flow flows past the inner pipe  12  at the outside, which is illustrated by arrows in  FIG. 3 . The portion of the exhaust gas flow flowing through the inner pipe  12  and the portion of the exhaust gas flow flowing around the inner pipe  12  combine downstream of the inner pipe  12 . The inner pipe  12  has a tapering  35  at its downstream end which effects an increase in the flow speed due to the Venturi effect. The inner pipe  12  is heated by the flowing around and a vaporization of fluid at the surfaces of the inner pipe  12  and of the mixing apparatus  20 ″ is thus facilitated. The exhaust tract  11 ″ has a constriction point  37  directly downstream of the tapering  35  by which a faster mixing of the exhaust gases of the main flow and of the secondary flow can be achieved. 
     The exhaust tract  11 ″ has a decoupling section  31  which has a reduced stiffness with respect to the preceding and following sections of the exhaust gas flow  11 ″ in order thus to counter the transfer of vibrations and to avoid tension. The decoupling section  31  is designed as a spiral wound metallic hose which is sheathed by a flexible corrugated pipe of thin metal plate. The inner pipe  12  extends along the total length of the decoupling section  31  and only ends directly downstream of the decoupling section  31 . The fastening of the inner pipe  12  to the exhaust tract  11 ″ takes place only in the region of rigid components of the pipe section  11 ″ and not in the region of the flexible decoupling section  31 . In other words, the inner pipe  12  projects freely into the decoupling section  31  so that the movability of the exhaust tract  11 ″ made possible by the decoupling section  31  is maintained. At the same time, the inner pipe  12  shields the wall of the decoupling section  31  from the main flow of the exhaust gas and thus counters a fluid deposition on the irregular surface of the decoupling section  31 . An indentation  34  of the inner pipe  12  effects a direct guidance of the exhaust gas flow in the sense that the flow direction is adapted better to the injection direction. 
     It is understood that the mixing apparatus  20 ″ may comprise further components such as a plurality of different swirl-generating devices or a tubular vaporizer device. An arrangement having a flowed-around inner pipe  12  can also generally also be combined with the above-described measured to improve mixing. It can, however, also be used without such measures. 
       FIGS. 4 to 8  show further exemplary embodiments of the invention. In accordance with  FIG. 4 , the inner pipe  12  can form a part of a metering module  39  containing the injection device  15  as well as a mixing apparatus  20 ″ and can extend only partly into a decoupling section  31 .  FIG. 5  shows an embodiment in which a metering module  39  is arranged between two decoupling sections  31 . Since the mixing apparatus  20 ″ is so-to-say arranged in the decoupling section  31 , the required construction space is reduced in size with respect to a system having components arranged behind one another. In accordance with  FIG. 6 , the mixing apparatus  20 ″ associated with the inner pipe  12  can also be arranged upstream of the injection device  15  and can thus act as a pure swirl-generator. In the aspect in accordance with  FIG. 7 , a half-shell shaped prolongation  41  is provided at the downstream end of the inner pipe  12  and primarily protects the lower part of the decoupling section  31  from fluid droplets exiting the mixing apparatus  20 ″ in the position of use and vaporizes said droplets. As can be seen from  FIG. 8 , the mixing apparatus  20 ″ can also be provided at the end of a metering module  39 , with an extension pipe  43  adjoining the mixing apparatus  20 ″. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description.