Abstract:
A mixer assembly for mixing an injected reductant with an exhaust gas output from a combustion engine comprises a mixer housing including a wall defining an exhaust passageway having a longitudinal axis. A tubular swirling device housing extends along a first axis substantially transverse to the longitudinal axis. The tubular swirling device includes a plurality of openings through which exhaust gas enters. The exhaust gas within the tubular swirling device swirls about the first axis and exits at an outlet end of the tubular swirling device. A mixing plate is positioned immediately downstream of the tubular swirling device and includes apertures through which the exhaust gas exiting the outlet end of the tubular swirling device flows. The mixing plate swirls the exhaust about a second axis extending parallel to the longitudinal axis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 15/001,356 filed on Jan. 20, 2016, which claims the benefit of U.S. Provisional Application No. 62/106,398, filed on Jan. 22, 2015. The entire disclosures of the above applications are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to a mixer assembly for an exhaust aftertreatment system. 
       BACKGROUND 
       [0003]    This section provides background information related to the present disclosure and is not necessarily prior art. 
         [0004]    In an attempt to reduce the quantity of NO x  and particulate matter emitted to the atmosphere during internal combustion engine operation, a number of exhaust aftertreatment devices have been developed. A need for exhaust aftertreatment systems particularly arises when diesel combustion processes are implemented. Typical aftertreatment systems for diesel engine exhaust may include a hydrocarbon (HC) injector, a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF), and a selective catalytic reduction (SCR) system. 
         [0005]    SCR technology has been used in conjunction with reducing nitrogen oxides present in the exhaust of combustion engines. Some of these systems are constructed using urea-based technology including a container for storing a reductant (e.g., urea) and a delivery system (including an injector, for example) for transmitting the reductant from the container to the exhaust stream. A mixer is typically provided for mixing the injected reductant with the exhaust gas before the reductant reaches a catalyst with which the reductant reacts. While these systems may have performed well in the past, it may be desirable to provide an improved mixer to more efficiently and effectively mix the reductant with the exhaust stream. 
       SUMMARY 
       [0006]    This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
         [0007]    A mixer assembly for mixing an injected reductant with an exhaust gas output from a combustion engine comprises a mixer housing including a wall defining an exhaust passageway having a longitudinal axis. A tubular swirling device housing extends along a first axis substantially transverse to the longitudinal axis. The tubular swirling device includes a plurality of openings through which exhaust gas enters. The exhaust gas within the tubular swirling device swirls about the first axis and exits at an outlet end of the tubular swirling device. A mixing plate is positioned immediately downstream of the tubular swirling device and includes apertures through which the exhaust gas exiting the outlet end of the tubular swirling device flows. The mixing plate swirls the exhaust about a second axis extending parallel to the longitudinal axis. 
         [0008]    An exhaust aftertreatment system may include an exhaust gas passageway and a mixer assembly. The exhaust gas passageway may receive exhaust gas output from a combustion engine. The mixer assembly may be disposed along the exhaust gas passageway and may receive the exhaust gas. The mixer assembly may include a mixer housing, a mixing bowl and an injector housing. The mixing bowl may be disposed within the mixer housing and may include an outer diametrical surface that engages an inner diametrical surface of a wall of the mixer housing. The injector housing may extend through the wall and into an aperture in the mixing bowl. The aperture may define a flow path through which at least a majority of the exhaust gas entering the mixer assembly flows. The mixing bowl may include an upstream end portion having contours directing the exhaust gas toward the injector housing. 
         [0009]    In some configurations, the upstream end portion of the mixing bowl includes a pair of recesses that extend along lateral sides of the upstream end portion and diverge away from each other as the recesses extend toward a flange portion of the mixing bowl. 
         [0010]    In some configurations, the upstream end portion of the mixing bowl includes a generally U-shaped recess. 
         [0011]    In some configurations, all of the exhaust gas entering the mixer assembly flows through the aperture. 
         [0012]    In some configurations, the injector housing is an annular member having a plurality of openings through which exhaust gas enters before flowing through the aperture in the mixing bowl. The plurality of openings may be defined by a plurality of louvers. 
         [0013]    An injector may inject reagent (e.g., reductant or hydrocarbon fuel) into a mixing chamber defined by the injector housing and the mixing bowl. The exhaust gas and the reagent may mix with each other in the mixing chamber. 
         [0014]    In some configurations, the exhaust aftertreatment system includes an exhaust aftertreatment component (e.g., a selective catalytic reduction catalyst or an oxidation catalyst) disposed downstream of the mixing bowl and receiving the mixture of reductant and exhaust gas. 
         [0015]    In some configurations, the mixer assembly includes a mixing element (e.g., a mixing plate or other mixing structure) disposed between the mixing bowl and the selective catalytic reduction catalyst, the mixing element including a plurality of louvers and openings. 
         [0016]    In some configurations, the mixing bowl includes a flange portion and a collar portion. The flange portion may be disposed between the upstream end portion and the collar portion. The aperture may extend through the flange portion and is disposed between the collar portion and an inlet of the mixer housing. 
         [0017]    In some configurations, the collar portion is disposed further downstream than the upstream end portion. 
         [0018]    In another form, the present disclosure provides a mixer assembly receiving exhaust gas from an engine and mixing the exhaust gas with an injected fluid. The mixer assembly may include a mixer pipe, a mixing bowl and an injector housing. The mixing bowl may be disposed within the mixer pipe and may include an outer diametrical surface that engages an inner diametrical surface of a wall of the mixer pipe. The mixing bowl may have a diameter that is equal to an inner diameter of the mixer pipe. The injector housing may receive the injected fluid and may extend through the wall and into an aperture in the mixing bowl. The aperture may define a flow path through which exhaust gas flows. The mixing bowl may include contours directing the exhaust gas toward the injector housing. 
         [0019]    In some configurations, an upstream end portion of the mixing bowl includes a pair of recesses that extend along lateral sides of the upstream end portion and diverge away from each other as the recesses extend toward a flange portion of the mixing bowl. The aperture may be formed in the flange portion. 
         [0020]    In some configurations, the upstream end portion of the mixing bowl includes a generally U-shaped recess. 
         [0021]    In some configurations, all of the exhaust gas entering the mixer pipe flows through the aperture. 
         [0022]    In some configurations, the injector housing is an annular member having a plurality of openings through which exhaust gas enters before flowing through the aperture in the mixing bowl. 
         [0023]    In some configurations, the plurality of openings are defined by a plurality of louvers. 
         [0024]    In some configurations, the mixer assembly includes an injector injecting the injected fluid into a mixing chamber defined by the injector housing and the mixing. The exhaust gas and the injected fluid mix with each other in the mixing chamber. 
         [0025]    In some configurations, an outlet of the mixer pipe is connected to an inlet of a housing of a selective catalytic reduction catalyst. The selective catalytic reduction catalyst may receive the mixture of the injected fluid and exhaust gas. 
         [0026]    In some configurations, the mixer assembly includes a mixing plate disposed between the mixing bowl and the selective catalytic reduction catalyst. The mixing plate may include a plurality of louvers and openings. 
         [0027]    In some configurations, the mixing bowl includes a flange portion and a collar portion. The flange portion may be disposed between the upstream end portion and the collar portion. The aperture may extend through the flange portion and is disposed between the collar portion and an inlet of the mixer pipe. 
         [0028]    In some configurations, the collar portion is disposed further downstream than the upstream end portion. 
         [0029]    In another form, the present disclosure provides a mixer assembly receiving exhaust gas from an engine. The mixer assembly may include a mixer pipe, an injector housing and a mixing bowl. The mixer pipe may include an inlet, an outlet and a longitudinal axis extending through the inlet and the outlet. The mixing pipe may receive exhaust gas flowing in a first direction parallel to the longitudinal axis through the inlet. The exhaust gas may flow through the outlet in the first direction. The injector housing may receive a reagent and extend into the mixer pipe between the inlet and outlet. The mixing bowl may be disposed within the mixer pipe between the inlet and the outlet and may include an aperture receiving a portion of the injector housing. The mixing bowl may include a first contoured surface directing the exhaust gas downstream of the inlet in a second direction toward the injector housing. The second direction may be angled relative to the first direction or substantially orthogonal to the first direction. The mixing bowl may include a second contoured surface downstream of the first contoured surface directing exhaust gas through the aperture in a third direction opposite the second direction. 
         [0030]    In some configurations, the mixing bowl includes an outer diametrical surface that engages an inner diametrical surface of a wall of the mixer pipe. The mixing bowl may have a diameter that is equal to an inner diameter of the mixer pipe. 
         [0031]    In some configurations, the first contoured surface is defined by an upstream end portion of the mixing bowl having a pair of recesses that extend along lateral sides of the upstream end portion and diverge away from each other as the recesses extend toward a flange portion of the mixing bowl. The aperture is formed in the flange portion. 
         [0032]    In some configurations, the second contoured surface is defined by a collar portion of the mixing bowl, the aperture extends through the flange portion of the mixing bowl and is disposed between the collar portion and the upstream end portion. 
         [0033]    In some configurations, the first contoured surface is defined by an upstream end portion of the mixing bowl having a generally U-shaped recess. 
         [0034]    In some configurations, all of the exhaust gas entering the mixer pipe through the inlet flows through the aperture. 
         [0035]    In some configurations, the injector housing is an annular member having a plurality of openings through which exhaust gas enters before flowing through the aperture in the mixing bowl. 
         [0036]    In some configurations, the plurality of openings are defined by a plurality of louvers. 
         [0037]    In some configurations, the mixer assembly includes an injector injecting reductant into a mixing chamber defined by the injector housing and the mixing bowl. The exhaust gas and the reductant may mix with each other in the mixing chamber. 
         [0038]    In some configurations, the outlet of the mixer pipe is connected to an inlet of a housing of a selective catalytic reduction catalyst. The selective catalytic reduction catalyst may receive the mixture of the reductant and exhaust gas. 
         [0039]    In some configurations, the mixer assembly includes a mixing plate disposed between the mixing bowl and the selective catalytic reduction catalyst. The mixing plate may include a plurality of louvers and openings. 
         [0040]    Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0041]    The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
           [0042]      FIG. 1  is a schematic representation of an engine and exhaust aftertreatment system having a mixer assembly according to the principles of the present disclosure; 
           [0043]      FIG. 2  is a cross-sectional view of a portion of the aftertreatment system including the mixer assembly; 
           [0044]      FIG. 3  is an end view of the mixer assembly; 
           [0045]      FIG. 4  is a perspective view of a mixing bowl of the mixer assembly; 
           [0046]      FIG. 5  is a cross-sectional view of a portion of the aftertreatment system including another mixer assembly according to the principles of the present disclosure; 
           [0047]      FIG. 6  is an end view of the mixer assembly of  FIG. 5 ; 
           [0048]      FIG. 7  is a perspective view of a mixing bowl of the mixer assembly of  FIG. 5 ; 
           [0049]      FIG. 8  is a perspective view of another mixing bowl according to the principles of the present disclosure; 
           [0050]      FIG. 9  is an end view of the mixing bowl of  FIG. 8 ; 
           [0051]      FIG. 10  is an exploded perspective view of an alternate mixer assembly; and 
           [0052]      FIG. 11  is a fragmentary perspective view of the mixer depicted in  FIG. 10 . 
       
    
    
       [0053]    Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION 
       [0054]    Example embodiments will now be described more fully with reference to the accompanying drawings. 
         [0055]    Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail. 
         [0056]    The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
         [0057]    When an element or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0058]    Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments. 
         [0059]    Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
         [0060]    With reference to  FIG. 1 , an exhaust aftertreatment system  10  is provided that may treat exhaust gas output by a combustion engine  12 . The exhaust aftertreatment system  10  may include an exhaust gas passageway  14 , an oxidation catalyst (e.g., a diesel oxidation catalyst or DOC)  16 , a filter (e.g., a diesel particulate filter (DPF))  18 , a mixer assembly  20  and a selective catalytic reduction (SCR) catalyst  22 . The DOC  16 , filter  18 , mixer assembly  20  and SCR catalyst  22  are disposed within the exhaust gas passageway  14  such that some or all of the that exhaust gas output from engine  12  flows therethrough. 
         [0061]    A hydrocarbon (HC) injector  24  may be disposed at least partially within the exhaust gas passageway  14  upstream of the DOC  16 . The HC injector  24  may receive hydrocarbon fuel from a fuel source  26  and may inject into the fuel into the stream of exhaust gas upstream of the DOC  16 . A burner (not shown) may be disposed at least partially within the exhaust gas passageway  14  upstream of the DOC  16  at or adjacent the HC injector  24 . The burner may ignite the fuel injected by the HC injector  24  to regenerate the DOC  16  and/or the filter  18 . 
         [0062]    A reductant injector  28  may be disposed adjacent to or partially within the mixer assembly  20  upstream of the SCR catalyst  22 . The reductant injector  28  may receive a reductant (e.g., urea) from a reductant tank  30  and inject the reductant into an exhaust stream in the mixer assembly  20 . As will be described in more detail below, reductant may mix with exhaust gas within the mixer assembly  20  prior to flowing through the SCR catalyst  22 . 
         [0063]    Referring now to  FIGS. 2-4 , the mixer assembly  20  may include a mixer housing or pipe  32 , an injector housing  34 , a mixing bowl  36 , a first mixing plate  38  and a second mixing plate  40 . The mixer housing  32  can be generally cylindrical and may be directly or indirectly connected to a housing  42  of the SCR catalyst  22 . The mixer housing  32  may include an injector opening  44  through which the injector housing  34  and/or the reductant injector  28  may extend. 
         [0064]    The injector housing  34  may be an annular member including a flange  46 , a generally cylindrical portion  48 , and a generally frustoconical portion  50 . As shown in  FIG. 2 , a cap  52  may engage a first end  54  of the cylindrical portion  48 . While not specifically shown in  FIG. 2 , the reductant injector  28  may extend through an aperture  56  in the cap  52  and spray reductant into the cylindrical and frustoconical portions  48 ,  50 . The cylindrical portion  48  may include a plurality of apertures  58  extending therethrough. The frustoconical portion  50  may include a plurality of louvers  60  defining openings into the injector housing  34 . A distal end  62  of the frustoconical portion  50  may engage the mixing bowl  36  and may extend through an aperture  64  in the mixing bowl  36 . In some configurations, the distal end  62  may include a static mixer having louvers and apertures, for example, to provide additional turbulence. 
         [0065]    The mixing bowl  36  may be a generally bowl-shaped structure that may be stamped and/or otherwise formed from sheet metal, for example. It will be appreciated that the mixing bowl could be formed by any suitable process and from any suitable material. The mixing bowl  36  may include an upstream end portion  66 , a collar portion  68 , a step or flange portion  70  and a downstream rim  72  that cooperate to define a mixing chamber  74 . The flange portion  70  may be disposed between the upstream end portion  66  and the collar portion  68  and may include the aperture  64  through which the injector housing  34  extends. An outer diametrical surface of the rim  72  can be welded, fastener or pressed into engagement with the inner diametrical surface of the mixer housing  32 , for example. 
         [0066]    As shown in  FIG. 4 , an exterior surface  75  of the collar portion  68  (e.g., the surface of the collar portion  68  that generally faces an inlet  77  of the mixer housing  32 ) may be generally concave such that a central portion  71  of the exterior surface  75  is disposed further downstream than lateral portions  73  of an exterior surface  76 . As shown in  FIG. 2 , the exterior surface  76  of the upstream end portion  66  (e.g., the surface of the upstream end portion  66  that generally faces an inlet  77  of the mixer housing  32 ) may be generally convex such that an upper end  78  of the exterior surface  76  is disposed further upstream than a lower end  80  of the exterior surface  76 . 
         [0067]    As shown in  FIGS. 3 and 4 , a pair of recesses  82  may be formed in lateral side portions of the upstream end portion  66 . The recesses  82  extend along the perimeter of the upstream end portion  66  and generally diverge from each other as they extend from a lower portion of the upstream end portion  66  toward the flange portion  70 . In this manner, the recesses  82  are shaped to funnel or direct exhaust gas flowing into the mixer housing  32  toward the injector housing  34  and the collar portion  68  (as indicated by arrows A in  FIG. 3 ) so that exhaust gas will flow into the injector housing  34  (i.e., between the louvers  60  and/or through the apertures  58 ) and flow into the mixing chamber  74  through the aperture  64  in the flange portion  70 . In the particular embodiment depicted in  FIGS. 2-4 , all of the exhaust gas that enters the mixer housing  32  through the inlet  77  will be directed into the injector housing  34  and through the aperture  64  as indicated by the arrows A in  FIG. 2 . In this manner, the exhaust gas will be thoroughly mixed with the reductant from reductant injector  28  into the injector housing  34  and in the mixing chamber  74 . Mixing bowl  36  includes a curved interior or downstream surface  83  shaped in such a manner that exhaust passing through aperture  64  is redirected substantially ninety degrees to flow toward SCR  22 . 
         [0068]    The first mixing plate  38  may include a plurality of louvers  84  and apertures  86  through which the mixture of reductant and exhaust gas may flow. The second mixing plate  40  may include a plurality of apertures  88  through which the mixture of reductant and exhaust gas may flow. The first and second mixing plates  38 ,  40  may increase the turbulence of the flow upstream of the SCR catalyst  22  to facilitate further mixing of the exhaust gas and the reductant prior to entering the SR catalyst  22 . The mixing plates  38 ,  40  may also spread the mixture of reductant and exhaust gas over a larger cross-sectional area so that the mixture will flow into the SCR catalyst  22  over a larger cross-sectional area. It will be appreciated that the mixer assembly  20  could include any number of mixing plates and/or static mixers configured in any desired manner. In some embodiments, the contours of the mixing bowl and/or the shape and position of one or more mixing plates  38 ,  40  may direct the mixture of exhaust gas and reductant away from a lower portion (from the frame of reference of  FIG. 2 ) of the SCR catalyst  22  and toward a central region of the SCR catalyst  22 . 
         [0069]    While the mixer assembly  20  is described above as mixing exhaust gas with reductant from the reductant injector  28  and providing the mixture to the SCR catalyst  22 , it will be appreciated that the mixer assembly  20  could be used in conjunction with the HC injector  24  rather than the reductant injector  28 . That is, the mixer assembly  20  could be provided upstream of the DOC  16 , and the HC injector  24  could inject fuel into the injector housing  34  and mixing chamber  74  to be mixed with the exhaust gas prior to being ignited. 
         [0070]      FIGS. 5-7  depict another configuration of the mixing bowl  36  in which apertures  90  and louvers  92  are formed in the recesses  82  to allow a relatively small portion of the exhaust gas to flow therethrough rather than flowing into the injector housing  34  and the aperture  64 . Louvers  92  redirect the exhaust flowing through apertures  90  to flow along curved interior surface  83 . This flow pattern minimizes the number of large urea droplets contacting surface  83 . If liquid urea droplets do impinge upon surface  83 , the modified flow pattern quickly causes the liquid to evaporate. 
         [0071]    Additionally or alternatively, apertures and louvers could be formed in the exterior surface  76  between the recesses  82 . Additionally or alternatively, one or more bypass apertures or passages could be formed in the mixing bowl  36  at or near the aperture  64  and/or at or near the rim  72  so that relatively small amounts of exhaust gas can bypass the injector housing  34 . Additionally or alternatively, one or more bypass apertures or passages could be formed in the inner diametrical surface of the mixer housing  32  so that relatively small amounts of exhaust gas can bypass the injector housing  34  and the mixing chamber  74 . 
         [0072]    While the mixer assemblies  20  shown in  FIGS. 2 and 5  are oriented horizontally, it will be appreciated that these assemblies could be oriented vertically. As shown in  FIG. 2 , the injector housing  34  may have an axis of rotation (i.e., a longitudinal axis) that is disposed at a non-perpendicular angle relative to the longitudinal axis of the mixer housing  32 . In the vertical orientation, this configuration may be particularly advantageous in that liquid droplets can fall away from the injector  28  rather than forming deposits on the injector  28 . 
         [0073]      FIGS. 8 and 9  depict yet another configuration of a mixing bowl  136  that includes one continuous recess  182  in upstream end portion  166  instead of the pair of recesses  82  described above. Like the recesses  82  of the mixing bowl  36  described above, the recess  182  is shaped to funnel or direct exhaust gas outward and upward so that the exhaust gas will flow around collar portion  168  and into the injector housing  34  and through aperture  164  in flange portion  170  in a similar manner as described above and shown in  FIGS. 2 and 3 . 
         [0074]      FIGS. 10 and 11  depict an alternate mixer assembly  220  including a mixer housing or pipe  232 , an injector housing  234 , a mixing bowl  236 , a first mixing plate  238 , and a second mixing plate  240 . Mixer housing  232  is substantially similar to previously described mixing housing  32 . Similarly, injector housing  234  is substantially similar to the previously described injector housing  34 . In particular, injector housing  234  includes a flange  246  coupled to a swirling device  247 . Swirling device  247  includes a cylindrical portion  248  and a frustoconical portion  250 . A cap  252  is fixed to flange  246  and cylindrical portion  248 . Please refer to the earlier sections of the specification for greater detail. 
         [0075]    Mixing bowl  236  is substantially similar to previously described mixing bowl  236  and the alternate configurations depicted in  FIGS. 4-9 . Mixing bowl  236 , however, includes an aperture  290  associated with a louver  292  extending across pipe  232  a distance approximately half of the inner diameter of the pipe. Aperture  290  and louver  292  are positioned centrally within the circular cross-section of pipe  232 . Exhaust gas flows through aperture  290  and is re-directed by louver  292 . As described in detail with relation to the other embodiments, exhaust gas also flows through apertures extending through cylindrical portion  248 , frustoconical portion  250  to pass through aperture  264  of mixing bowl  236 . 
         [0076]    First mixing plate  238  includes a stepped shape when viewed from the side such that a first portion  243  is positioned axially closer to injector housing  234  than a second portion  245 . An intermediate portion  251  interconnects first portion  243  and second portion  245 . Intermediate portion  251  is spaced apart from a collar portion  268  of mixing bowl  236  to define a pinch point or orifice therebetween. The orifice generates a region of increased gas velocity to entrain the smaller droplets of reductant and urge these droplets toward second portion  245  positioned on the same side of pipe as the injector. First portion  243  includes a plurality of louvers  284  and apertures  286  through which the mixture of reductant and exhaust gas may flow. Second portion  245  includes a plurality of substantially cylindrically-shaped apertures  249  provided to allow reductant and exhaust gas to flow therethrough. Louvers  284  are substantially crescent shaped and arranged in an arc to create a swirling exhaust gas motion. As larger droplets of reductant attempt to increase their distance from the injector, the swirling exhaust gas pattern generated by louvers  284  urges the droplets toward the injector such that they evaporate near the center of the pipe  232 . 
         [0077]    Second mixing plate  240  may include a plurality of apertures  288  through which the mixture of reductant and exhaust gas may flow. The shape, size, and position of various apertures  288  are defined to further mix the exhaust gas and reductant prior to entering a downstream catalyst. Mixing plates  238 ,  240  may distribute the mixture of reductant and exhaust gas over a desired inlet area of the downstream catalyst to optimize efficiency of catalyst operation. 
         [0078]    The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.