Patent Publication Number: US-8991160-B2

Title: Reductant aqueous solution mixing device and exhaust aftertreatment device provided with the same

Description:
TECHNICAL FIELD 
     The invention relates to a reductant aqueous solution mixing device and an exhaust aftertreatment device provided with the reductant aqueous solution mixing device. Specifically, the invention relates to a reductant aqueous solution mixing device used for supplying a reductant aqueous solution such as a urea aqueous solution to a selective catalytic reduction to purify exhaust gas and an exhaust aftertreatment device provided with the reductant aqueous solution mixing device. 
     BACKGROUND ART 
     An exhaust aftertreatment device that purifies nitrogen oxides (NOx) contained in exhaust gas of an engine with a selective catalytic reduction (abbreviated as “SCR” hereinafter) has been known. Urea aqueous solution injected by an injector is supplied to the SCR. The injector is attached to a mixing device provided upstream of the SCR. The urea aqueous solution is injected from the injector to exhaust gas flowing through the mixing device to mix the urea aqueous solution with the exhaust gas within the mixing device. As a result, the urea aqueous solution is thermally decomposed by the heat of the exhaust gas to produce ammonia. The ammonia is used as a reductant in the SCR. 
     If the injected urea aqueous solution is not sufficiently mixed with the exhaust gas in the exhaust aftertreatment device, a part of the urea aqueous solution may be adhered on an inner wall of the mixing device of which outside is cooled by an external air, possibly causing shortage of ammonia in the SCR. Further, the urea aqueous solution turned to droplets on the inner wall of the mixing device may be crystallized to be deposited on the inner wall to hinder the flow of the exhaust gas. In order to solve the above problems, the mixing device sometimes has a double-tube structure including an outer tube and an inner tube. Since both inner and outer surfaces of the inner tube of the mixing device are in contact with the exhaust gas to be heated thereby, the urea aqueous solution adhered on the inner wall is thermally decomposed by injecting the urea aqueous solution to the inside of the inner tube. Thus, the urea aqueous solution can be restrained from being adhered on the inner wall in a form of droplets to be crystallized or deposited. 
     In addition, Patent Literatures 1 and 2 disclose a mixing pipe provided downstream an injector in a mixing device so that a urea aqueous solution is sufficiently thermally decomposed. A plurality of openings are provided on an outer circumference of the mixing pipe. The exhaust gas flows into the mixing pipe through the openings to generate a turbulence or a swirl within the mixing pipe. The urea aqueous solution is injected from the injector into the flow of the exhaust gas to reduce the size of the urea aqueous solution particles, thereby facilitating the exhaust gas to be mixed with the exhaust gas and improving decomposition efficiency of the urea aqueous solution to ammonia. 
     CITATION LIST 
     Patent Literature(s) 
     Patent Literature 1: U.S. Patent Publication No 2010/0263359 
     Patent Literature 2: JP-A-2008-208726 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, an injection nozzle of the injector exposed to an inside of the mixing device is located in a concave recess provided in the mixing device, so that injection nozzle is surrounded by a wall face of the recess around an end thereof. With the presence of the recess, a part of the injected urea aqueous solution swirls to be returned toward the recess in which pressure becomes slightly negative to be resided therein. When the resided urea aqueous solution is crystallized to be deposited, the injection of the urea aqueous solution from the injection nozzle is disturbed. 
     It is speculated that, by also providing an opening for generating a swirl in the mixing pipe to the mixing pipe near the injection nozzle, the exhaust gas is flowed in the vicinity of the injection nozzle, thereby heating the vicinity to promote thermal decomposition of the urea aqueous solution, resulting in restraining residence of the urea aqueous solution. However, only with the arrangement that an opening is simply provided in the mixing pipe, the exhaust gas might not be able to be sufficiently flowed toward the vicinity of the injection nozzle, which is desired to be solved. 
     An object of the invention is to provide a reductant aqueous solution mixing device that is capable of restraining residence of a urea aqueous solution in a region around an injection nozzle of an injector, and an exhaust aftertreatment device provided with the reductant aqueous solution mixing device. 
     Means for Solving the Problems 
     According to a first aspect of the invention, a reductant aqueous solution mixing device interposed between a filter device that captures particulate matters in an exhaust gas and a selective catalytic reduction device disposed downstream of the filter device, the reductant aqueous solution mixing device adding a reductant aqueous solution in the exhaust gas, includes: an elbow pipe attached to an outlet pipe of the filter device, the elbow pipe changing a flow direction of the exhaust gas flowing from the filter device; a straight pipe connected to a downstream side of the elbow pipe, the straight pipe extending in a direction intersecting an axial line of the outlet pipe of the filter device; an injector attached to the elbow pipe, the injector injecting the reductant aqueous solution into an inside of the elbow pipe toward the straight pipe; and a mixing pipe disposed in the elbow pipe to serve as a cover for the reductant aqueous solution injected from the injector, in which a plurality of large openings are provided on a circumferential wall of the mixing pipe near the injector, and a plurality of small openings are provided on a circumferential wall of the mixing pipe near the straight pipe. 
     Herein, an opening area of each of the large opening is larger than an opening area of each of the small openings. However, a total opening area of the plurality of large openings and a total opening area of the plurality of small openings are not limited. 
     According to a second aspect of the invention, in the reductant aqueous solution mixing device, the plurality of large openings are provided in a circumferential direction of the mixing pipe. 
     According to a third aspect of the invention, in the reductant aqueous solution mixing device, when the mixing pipe is seen from an axial line of the straight pipe toward an inlet of the elbow pipe, the plurality of large openings are omitted for a predetermined width in the circumferential direction of the mixing pipe. 
     According to a fourth aspect of the invention, in the reductant aqueous solution mixing device, the plurality of large openings are provided only in a circumferential angular range of the circumferential wall. 
     According to a fifth aspect of the invention, in the reductant aqueous solution mixing device, the plurality of small openings are provided only in a circumferential angular range of the circumferential wall. 
     According to a sixth aspect of the invention, an exhaust aftertreatment device includes: a filter device that captures particulate matters in an exhaust gas; a reductant aqueous solution mixing device according to any one of the first to fifth aspects of the invention, the reductant aqueous solution mixing device being disposed downstream of the filter device in parallel to the filter device; and a selective catalytic reduction device disposed downstream of the reductant aqueous solution mixing device, the selective catalytic reduction device reducing and purifying a nitrogen oxide in the exhaust gas. 
     According to the first and sixth aspects of the invention, since the plurality of large openings each having a larger opening area than the small opening are provided to the mixing pipe near the injector, the exhaust gas can be more efficiently flowed into the mixing pipe through the large openings. With this arrangement, the exhaust gas flowing into the mixing pipe is directed toward the injection nozzle of the injector. Accordingly, even when the injection nozzle is surrounded with a recess, the region around the injection nozzle can be efficiently heated with the exhaust gas. Accordingly, the urea aqueous solution returning toward the injection nozzle in a form of a swirl can be reliably thermally decomposed, thereby restraining the urea aqueous solution from residing in the region around the injection nozzle. 
     According to the second aspect of the invention, since the plurality of large openings are provided in the circumferential direction, the exhaust gas can be more easily flowed toward the injection nozzle. 
     According to the third aspect of the invention, no large opening is provided in a region of the mixing pipe capable of being seen from the inlet of the elbow pipe. The region is provided by a circumferential wall. Accordingly, the exhaust gas entering through the inlet is kept from swiftly flowing into the mixing pipe and from being directly headed toward the injection nozzle. Thus, the reductant aqueous solution injected from the injection nozzle is not blown by the exhaust gas but the reductant aqueous solution can be injected in an appropriate direction. 
     According to the fourth aspect of the invention, since the large openings are provided only in a predetermined angular range, the exhaust gas flowing from the large openings can be swirled and the reductant aqueous solution can be mixed with the exhaust gas immediately after being injected, thereby further promoting such a mixture. 
     According to the fifth aspect of the invention, since the small openings are also provided only in a predetermined angular range, the exhaust gas flowing from the small openings can also be swirled in the mixing pipe depending on the angular range, thereby being mixed with the reductant aqueous solution more efficiently. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plan view showing an exhaust aftertreatment device according to a first exemplary embodiment of the invention. 
         FIG. 2  is a cross section showing a mixing device of the exhaust aftertreatment device. 
         FIG. 3  is a cross section showing a relevant part of the mixing device. 
         FIG. 4  is a cross section of a mixing pipe provided in the mixing device taken along IV-IV line in  FIG. 3 . 
         FIG. 5A  is another cross section of the mixing pipe taken along V-V line in  FIG. 3 . 
         FIG. 5B  is a further cross section of the mixing pipe seen in a direction different from the direction in  FIG. 5A . 
         FIG. 6  is a cross section showing a relevant part of a mixing device according to a second exemplary embodiment of the invention. 
         FIG. 7  is a cross section of a mixing pipe provided in the mixing device of the second exemplary embodiment taken along VII-VII line in  FIG. 6 . 
     
    
    
     DESCRIPTION OF EMBODIMENT(S) 
     First Exemplary Embodiment 
     A first exemplary embodiment of the invention will be described below with reference to the attached drawings. 
       FIG. 1  is a plan view showing an exhaust aftertreatment device  1  according to the first exemplary embodiment. It should be noted that, in the following description, the term “upstream” refers to an upstream side in a flow direction of the exhaust gas and the term “downstream” refers to a downstream side in the flow direction of the exhaust gas. 
     As shown in  FIG. 1 , the exhaust aftertreatment device  1  includes a diesel particulate filter (abbreviated as “DPF” hereinafter) device  2 , a mixing device  3 , a selective catalytic reduction (abbreviated as “SCR” hereinafter) device  4 . These devices  2  to  4  are provided in the exhaust pipe in which the exhaust gas from a diesel engine (not shown) flows. In a construction machine such as a hydraulic excavator, wheel loader and bulldozer, the exhaust aftertreatment device  1  is housed in an engine room together with the engine. 
     The DPF device  2  includes a cylindrical casing  21  and a cylindrical DPF  22  housed inside the casing  21 . The DPF  22  captures the particulate matters in the exhaust gas passing through the DPF  22 . An oxidation catalyst may be provided upstream of the DPF  22  in the casing  21 . The oxidation catalyst oxidatively activates a post-injection fuel or a dosing fuel (both the same as diesel-engine fuel) supplied at an upstream side thereof to raise the temperature of the exhaust gas entering the DPF  22  to a temperature at which the DPF  22  is regenerable. The high-temperature exhaust gas causes a self-burning (burnout) of the particulate matters captured by the DPF  22  to regenerate the DPF  22 . 
     The mixing device  3  adds reductant aqueous solution in a form of urea aqueous solution in the exhaust gas. The mixing device  3  includes: an upstream elbow pipe  31  connected to an outlet pipe  23  of the DPF device  2  and serving as an elbow pipe for changing the flow direction of the exhaust gas flowing out of the DPF device  2  by approximately ninety degrees; a straight pipe  32  connected to a downstream end of the upstream elbow pipe  31  and extending in a direction intersecting an axial line CL 2  ( FIG. 2 ) of the outlet pipe  23  of the DPF device  2 ; a downstream elbow pipe  33  connected to a downstream end of the straight pipe  32  for further changing the flow direction of the exhaust gas from the straight pipe  32  by approximately ninety degrees; and an injector  5  attached to the upstream elbow pipe  31  and injecting the urea aqueous solution inside the upstream elbow pipe  31  toward the straight pipe  32 . The SCR device  4  is connected to a downstream end of the downstream elbow pipe  33 . 
     The SCR device  4  includes a cylindrical casing  41  and a cylindrical SCR  42  housed inside the casing  41 . The SCR  42  reduces and purifies nitrogen oxides in the exhaust gas with ammonia (reductant) generated in the mixing device  3 . An ammonia reduction catalyst may be provided downstream of the SCR  42  in the casing  41 . The ammonia reduction catalyst oxidizes the ammonia unused in the SCR  42  to render the ammonia harmless, thereby further reducing emissions in the exhaust gas. 
     The urea aqueous solution injected from the injector  5  to the exhaust gas is thermally decomposed by the heat of the exhaust gas to become ammonia. The ammonia is supplied to the SCR device  4  as a reductant together with the exhaust gas. 
     The above-described DPF device  2 , the mixing device  3  and the SCR device  4  are juxtaposed so that the flow directions of the exhaust gas flowing in the devices become substantially parallel. In this arrangement, the flow directions of the exhaust gas flowing inside the DPF device  2  and the SCR device  4  are opposite to the flow direction of the exhaust gas flowing inside the mixing device  3 . Thus, these devices  2  to  4  are arranged substantially in an S-shape in a plan view. Accordingly, the size of the exhaust aftertreatment device  1  can be made compact as a whole, thereby allowing the exhaust aftertreatment device  1  to be securely disposed (e.g. mounted on an engine) in a limited housing space such as an engine room. 
       FIG. 2  is a cross section of the mixing device  3 . The mixing device  3  will be specifically described below with reference to  FIG. 2 . 
     In the mixing device  3  shown in  FIG. 2 , a part of the upstream elbow pipe  31  for changing the flow direction of the exhaust gas is defined as a direction-changing section  31 A. The upstream elbow pipe  31  includes: a circular inlet  31 B opened to and connected with the outlet pipe  23  of the DPF device  2 ; and a circular outlet  31  C opened to and connected with the straight pipe  32 . The direction-changing section  31 A is defined between the circular inlet  31 B and the outlet  31 C. An attachment portion  6  is provided to an outside of the direction-changing section  31 A. An injector  5  is attached to an outside of the attachment portion  6 . A mixing pipe  34  serving as a cover for the urea aqueous solution injected from the injector  5  is attached to an inside (i.e. interior side of the direction-changing section  31  A) of the attachment portion  6 . Details of the attachment portion  6  and the mixing pipe  34  will be described later. 
     The straight pipe  32  has a double-tube structure of an outer tube  35  and an inner tube  36  disposed inside the outer tube  35 . The inner tube  36  is welded or the like to a plurality of supporting recesses  35 A provided to the outer tube  35  and is welded or the like to an inner wall of the outer tube  35  through an annular support member  35 B at a downstream end thereof. Further, an upstream end of the inner tube  36  protrudes into the upstream elbow pipe  31 . The upstream end of the inner tube  36  is defined so that the urea aqueous solution injected by the injector  5  at an injection angle è 1  (see chain lines in  FIG. 2 ) of approximately 25 degrees securely enters an inside of the inner tube  36 . A plurality of openings  36 A are provided on an area near the downstream end of the inner tube  36 . 
     The exhaust gas flows into a gap between the outer tube  35  and the inner tube  36 . Since the supporting recesses  35 A are discontinuously provided in the circumferential direction, the entered exhaust gas flows to the support member  35 B through gaps between the supporting recesses  35 A. The annular support member  35 B blocks the flow of the exhaust gas, so that the exhaust gas flows into the inner tube  36  through the openings  36 A to be joined with the exhaust gas flowing inside the inner tube  36  to be further flowed toward the downstream. In other words, the inner tube  36  is efficiently heated by the exhaust gas flowing inside and outside the inner tube  36 . Thus, the urea aqueous solution injected to the inside of the inner tube  36  is securely thermally decomposed without being turned to droplets even when being adhered to an inner wall of the inner tube  36 . 
       FIG. 3  shows the upstream elbow pipe  31  of the mixing device  3  in an enlarged manner.  FIGS. 4 and 5A  show cross sections taken along IV-IV line and V-V line in  FIG. 3 , respectively.  FIG. 5B  shows the sectioned part seen from above in the figure. 
     As shown in  FIG. 3 , the attachment portion  6  for the injector  5  and the mixing pipe  34  includes a first plate  61  for closing an injector attachment opening  31 D provided to the direction-changing section  31 A and a second plate  62  attached to the first plate  61 . 
     A concave recess  63  enlarging toward an interior of the direction-changing section  31 A is provided at the center of the first plate  61 . An injection opening  64  is provided at a depth part of the recess  63 . An end of an injection nozzle  51  of the injector  5  protrudes through the injection opening  64 . An open degree è 2  ( FIG. 2 ) of a funnelform inclined wall  65  of the recess  63  is, though not specifically limited, defined at 90 degrees or more, preferably as large as 120 to 140 degrees so as for the exhaust gas to easily enter the depth side of the recess  63 , i.e. to the region around the injection nozzle  51 . 
     A flat annular fixing portion  66  orthogonal to an axial line CL 1  of the straight pipe  32  is provided at an outer periphery of the recess  63 . In this exemplary embodiment, an end of the mixing pipe  34  is welded to the fixing portion  66 . The mixing pipe  34  surrounds a downstream side of the injection nozzle  51 . The injection nozzle  51 , the mixing pipe  34  and the straight pipe  32  are sequentially disposed along the common axial line CL 1  from an upstream side. The exhaust gas is blown to the mixing pipe  34  housed in the direction-changing section  31 A from a lower side (in the figure) near the inlet  31 B of the upstream elbow pipe  31 . The direction of the flow of the exhaust gas from the lower side is changed to be along the axial line CL 1  at the direction-changing section  31 A. 
     On the other hand, a gap between the first plate  61  and the second plate  62  serves as a heat-insulation space. The heat-insulation space restrains a heat transmission from the first plate  61  exposed to the exhaust gas to the second plate  62 , thereby keeping the injector  5  attached to the second plate  62  from being directly affected by the heat. 
     As shown in  FIGS. 3 ,  4  and  5 A, the mixing pipe  34  has characteristic features of a plurality of circular holes  34 A (small openings) provided on a circumferential wall near the straight pipe  32  and three rectangular cutouts  34 B,  34 C and  34 D (large openings) provided to the circumferential wall near the injector  5 . The exhaust gas flows into the inside of the mixing pipe  34  through the circular holes  34 A and cutouts  34 B,  34 C and  34 D. The circular holes  34 A are substantially evenly provided from about a middle to an area near the straight pipe  32  in a longitudinal direction of the mixing pipe  34 . 
     The cutouts  34 B,  34 C and  34 D are provided to the mixing pipe  34  along the circumferential direction at a longitudinal end near the fixing portion  66 . The exhaust gas flowing into the mixing pipe  34  through the cutouts  34 B,  34 C and  34 D is directed toward the injection nozzle  51  due to the presence of the cutouts  34 B,  34 C and  34 D at the end of the mixing pipe  34 . A length L 2  of each of the cutouts  34 B,  34 C and  34 D is approximately 34% (L 2 /L 1 ≈0.34) of a length L 1  of the entirety of the mixing pipe  34 . 
     Thus, the exhaust gas passing through the cutouts  34 B,  34 C and  34 D smoothly flows toward the recess  63  while flowing closely over a surface of the fixing portion  66  to be directed toward the injection nozzle  51 . As a result, since the part around the recess  63  is heated by the exhaust gas to be temperature-raised, even when the urea aqueous solution injected from the injector  5  returns to the recess  63 , the urea aqueous solution is easily heated and decomposed, thereby restraining the urea aqueous solution from being resided in the recess  63  to be crystallized or deposited. 
     The mixing pipe  34  is provided by punching etc. a flat metal plate to form the circular holes  34 A and the rectangular cutout  34 B,  34 C and  34 D, curving the metal plate into a cylindrical form after the punching in a predetermined developed figure, and welding the butted portion of the curved plate. The diameter and length of the mixing pipe  34  are defined so that the urea aqueous solution injected from the injector  5  is not in contact with the mixing pipe  34  (see θ 1  shown in chain lines in  FIG. 2 ). 
     As shown in  FIG. 4 , when the mixing pipe  34  is circumferentially quadrisected at ninety degree intervals, in other words, when a first area A 1 , a second area A 2 , a third area A 3  and a fourth area A 4  are defined anticlockwise from the lowermost part at which the exhaust gas from the DPF device  2  is blown, the circular holes  34 A are provided only at the first area A 1  and the third area A 3  located point-symmetrically with the first area A 1 . The circular holes  34 A are provided over the entirety of the first and third areas A 1  and A 3 . 
     Since the circular holes  34 A are concentrated at the predetermined areas such as the first and third areas A 1  and A 3 , the exhaust gas flowing into the mixing pipe  34  through the circular holes  34 A generates a swirl, whereby the exhaust gas is efficiently mixed with the injected urea aqueous solution. The size and number of the circular holes  34 A are determined as desired considering the diameter and the length of the mixing pipe  34  and the mixing condition of the exhaust gas and the urea aqueous solution. 
     As shown in  FIG. 5A , the cutouts  34 B,  34 C and  34 D of the mixing pipe  34  are located axis-symmetrically about a center line Ov (symmetric line) perpendicularly drawn in the figure. The opening areas of the cutouts  34 B and  34 D are equal while the opening area of the cutout  34 C is larger than the opening areas of the cutouts  34 B and  34 D. 
     Specifically, the cutout  34 B occupies approximately two thirds of the first area A 1  and partially enters the second area A 2 . The cutout  34 C is provided so as to occupy approximately a half of the second area A 2  and a half of the third area A 3  around the center line Ov. The cutout  34 D is axis-symmetric with the cutout  34 B. The cutout  34 D occupies approximately two thirds of the fourth area A 4  and partially enters the third area A 3 . 
     The parts other than the cutouts  34 B,  34 C and  34 D at the end of the mixing pipe  34  near the fixing portion  66  define three supporting portions  34 E,  34 F and  34 G provided by the circumferential wall. The supporting portions  34 E,  34 F and  34 G are also axis-symmetric about the center line Ov (symmetric line). Edges of the supporting portions  34 E,  34 F and  34 G are welded to the fixing portion  66 . The circumferential length of the supporting portion  34 E is longer than the circumferential length of each of the supporting portions  34 F and  34 G. The circumferential length of the supporting portion  34 F is the same as that of the supporting portion  34 G 
     The supporting portion  34 E is present in both the first and fourth areas A 1  and A 4 , in other words, is interposed between the cutouts  34 B and  34 D to be axis-symmetric around the center line Ov as a symmetry line. An angle a formed by both circumferential edges of the supporting portion  34 E and a center O (the same as the axial line CL 1 ) of the mixing pipe  34  is roughly 60 to 70 degrees. The supporting portions  34 F and  34 G are present in the second and third areas A 2  and A 3  respectively. Further, when the mixing pipe  34  is seen in a direction toward the inlet  31 B ( FIGS. 2 and 3 ) of the upstream elbow pipe  31  from the axial line CL 1  (center O), the injection area (an area shown with the injection angle θ 1  in  FIG. 5B ) of the urea aqueous solution is within a projection width W of the supporting portion  34 E having no cutout in a direction orthogonal to the axial line CL 1  as shown in  FIG. 5B , thereby avoiding the exhaust gas flowing from the DPF device  2  from being directly blown to the urea aqueous solution. In other words, the supporting portion  34 E is an area of a predetermined width provided with no opening, within which the injection area is limited. 
     As described above, the supporting portion  34 E is located to cover the lowermost (in the figure) part of the mixing pipe  34 . Since a large part of the exhaust gas flowing from the DPF device  2  collides with the supporting portion  34 E, the exhaust gas does not swiftly enter the mixing pipe  34  without changing the flow direction thereof. Thus, the urea aqueous solution immediately after being injected is neither blown toward the second and third areas A 2  and A 3  nor greatly deflected by the exhaust gas. 
     The flow of the exhaust gas inside the upstream elbow pipe  31  will be described below with reference to  FIGS. 3 to 5 . In these figures, the flow of the exhaust gas is indicated in solid arrows. 
     As shown in  FIG. 3 , the exhaust gas flowing out of the DPF device  2  flows through the inlet  31 B of the upstream elbow pipe  31  toward the direction-changing section  31 A. The exhaust gas flowing along the inside of the direction-changing section  31 A flows directly along an inner wall of the direction-changing section  31 A toward the inner tube  36 . On the other hand, most of the rest of the exhaust gas flows toward the mixing pipe  34 . 
     In the vicinity of the mixing pipe  34 , the exhaust gas flows into the mixing pipe  34  through the circular holes  34 A as shown in  FIG. 4 . At this time, since the circular holes  34 A are provided only in the first and third areas A 1  and A 3 , the exhaust gas generates a swirl in the mixing pipe  34 . When the urea aqueous solution is injected into the swirl, the size of the urea aqueous solution particles is reduced to be efficiently mixed with the exhaust gas, thereby promoting a thermal decomposition. Subsequently, the exhaust gas flows toward the inner tube  36 . 
     In contrast, as shown in  FIGS. 5A and 5B , in the vicinity of the cutouts  34 B,  34 C and  34 D, even in the vicinity of the mixing pipe  34 , the exhaust gas flows into the mixing pipe  34  through the cutouts  34 B,  34 C and  34 D. After the exhaust gas flows along the fixing portion  66  especially at a side near the fixing portion  66 , the exhaust gas flows toward the injection nozzle  51  along the inclined wall  65  of the recess  63  that is enlarged at the large open degree  82  ( FIG. 2 ) to flow closely over the region around the injection nozzle  51  and, consequently, to be engulfed by the swirl. Accordingly, since the part around the recess  63  is heated by the exhaust gas to be kept at a high temperature, the urea aqueous solution swirled to be returned to the recess  63  is restrained from being resided in the region around the injection nozzle  51  and, consequently, crystallized to be deposited. 
     Additionally, as described above, at the part near the fixing portion  66 , most of the exhaust gas flowing from the DPF device  2  collides with the supporting portion  34 E provided between the cutouts  34 B and  34 D to be inhibited from flowing and does not directly flow into the mixing pipe  34 . Accordingly, the exhaust gas flows around toward the cutouts  34 B,  34 C and  34 D, whereby the exhaust gas flows into the mixing pipe  34  in three directions. Thus, the urea aqueous solution injected into the mixing pipe  34  is not blown to be deflected in one direction by the exhaust gas immediately after being injected, whereby the urea aqueous solution is injected in an appropriate direction to be efficiently mixed with the exhaust gas. 
     Second Exemplary Embodiment 
       FIG. 6  is a cross section showing the upstream elbow pipe  31  of the mixing device  3  according to a second exemplary embodiment of the invention.  FIG. 7  is a cross section of the mixing pipe  34  provided in the upstream elbow pipe  31  taken along VII-VII line in  FIG. 6 . It should be noted that the same reference numerals will be attached in the second exemplary embodiment to the same components or the components having similar functions in the second exemplary embodiment as those in the first exemplary embodiment to omit or simplify the explanation thereof. 
     As shown in  FIGS. 6 and 7 , the number, location and axial length of the cutouts of the mixing pipe  34  of the second exemplary embodiment significantly differ from those in the first exemplary embodiment. The other arrangements (e.g. the shape of the cutouts and areas for the circular holes to be provided) are the same as those in the first exemplary embodiment. 
     The mixing pipe  34  according to the second exemplary embodiment includes a pair of cutouts  34 H and  34 J (large openings) respectively provided only in the first and third areas A 1  and A 3 . Axial lengths of the cutouts  34 H and  34 J are shorter than those of the cutouts  34 B,  34 C and  34 D in the first exemplary embodiment but more number of circular holes  34 A are correspondingly provided. A length L 2  of each of the cutouts  34 H and  34 J is approximately 8% (L 2 /L 1 ≈0.08) of the length L 1  of the entirety of the mixing pipe  34 . 
     On the other hand, the cutout  34 H has a circumferential length extending substantially over the entirety of the first area A 1 . In other words, peripheries of the supporting portions  34 K and  34 L disposed between the cutout  34 H and the cutout  34 J only slightly enters the first area A 1  and the most part of the first area A 1  is occupied by the cutout  34 H. The cutout  34 J is shorter than the cutout  34 H in the circumferential length and is located closer to the second area A 2  in the third area A 3 . The supporting portions  34 K and  34 L are circumferentially long enough to completely cover the second and fourth areas A 2  and A 4 . 
     In this exemplary embodiment, the cutout  34 H is circumferentially widely opened and the adjacent supporting portion  34 L is not long enough to circumferentially cover the first area A 1  in the lower (in the figure) part of the mixing pipe  34 . 
     However, since the axial length of the cutout  34 H is short, the opening area is not as large as that in the first exemplary embodiment. Further, the cutouts  34 H and  34 J are point-symmetric in the same manner as the circular holes  34 A, and the exhaust gas flowing into the mixing pipe  34  through the cutouts  34 H and  34 J generates a swirl as in the exhaust gas flowing through the circular holes  34 A. In addition, the supporting portions  34 K and  34 L are circumferentially long enough to completely cover the second and fourth areas A 2  and A 4 . 
     Accordingly, the exhaust gas flowing into the mixing pipe  34  through the cutouts  34 H and  34 J contains swirling components immediately after being entered, so that the exhaust gas flows while swirling along the inclined wall  65  of the recess  63  toward the injection nozzle  51  to flow closely over the region around the injection nozzle  51 . Thus, since the part around the recess  63  is kept at a high temperature by the exhaust gas, even when the urea aqueous solution injected by the injection nozzle  51  returns to the recess  63  in a form of a swirl, the urea aqueous solution is kept from being resided. 
     Since the exhaust gas flowing through the cutouts  34 H and  34 J accompanies a larger swirling component, even when the side of the mixing pipe  34  near the DPF device  2  (lower side in  FIG. 7 ) is not sufficiently covered by the supporting portion  34 K, the urea aqueous solution from the injection nozzle  51  is kept from being excessively blown toward the second and third areas A 2  and A 3  by the exhaust gas but is injected in an appropriate direction. 
     Incidentally, it should be understood that the scope of the present invention is not limited to the above-described exemplary embodiments but includes modifications and improvements as long as the modifications and improvements are compatible with the invention. 
     For instance, though the cutouts  34 B,  34 C,  34 D,  34 H and  34 J (large openings) in the above exemplary embodiments are rectangular and the openings are provided by circular holes  34 A (small openings), the shape of the cutouts and the openings may be determined as desired, which is not limited to be rectangular and circular. In addition, the number and the like of the large openings may be determined as desired in implementing the invention. 
     The large openings are not limited to a cutout, but may be provided as a completely surrounded opening. 
     The supporting portion  34 E provided by a circumferential wall is provided at a part of the mixing pipe  34  near the inlet  31 B of the upstream elbow pipe  31  in the first exemplary embodiment and the cutouts  34 H and  34 J are provided to the first and third areas A 1  and A 3  in the second exemplary embodiment to restrain the urea aqueous solution injected into the mixing pipe  34  from being blown by the exhaust gas. However, since it is possible to heat the region around the injection nozzle  51  with the exhaust gas even when a cutout is provided at a side near the inlet  31 B, such an arrangement is compatible with the invention and is also within the scope of the invention. 
     Though the urea aqueous solution is used as the reductant aqueous solution, the other fluid may be used as the reductant aqueous solution in a modification of the invention.