Mixer for exhaust aftertreatment systems

A swirl mixer for mixing a reducing agent with exhaust gas in a selective catalytic reduction (SCR) aftertreatment system is described. The swirl mixer may comprise a base permitting a flow of the reducing agent and the exhaust gas therethrough, and three arrays of fins projecting from the base in a direction of flow of the exhaust gas. The three arrays of fins may be arranged in a triangular configuration about a center of the mixer to induce a swirl motion to the reducing agent and the exhaust gas flowing through the mixer. The fins in each of the arrays may be oriented in a common direction that is rotated by about 60° from the common direction of the fins in an adjacent array.

TECHNICAL FIELD

The present disclosure generally relates to mixers for exhaust aftertreatment systems and, more specifically, to a swirl mixer for mixing a reducing agent with exhaust gas in a selective catalytic reduction (SCR) aftertreatment system.

BACKGROUND

Nitrogen oxide (NOx) gases, such as nitric oxide (NO) and nitrogen dioxide (NO2), are pollutants that may be produced when fuel is combusted at high temperatures in internal combustion engines. These gases may have adverse health effects, and may participate in the formation of smog and acid rain. In order to comply with increasingly demanding low NOxemission regulations, engine manufacturers may be compelled to use technologies that substantially decrease NOxemissions from engine exhaust. One such technology is selective catalytic reduction (SCR) aftertreatment systems which catalyze the reduction of NOxin exhaust gas to nitrogen and water prior to release of the exhaust gas from an exhaust outlet, such as a tailpipe. In a SCR aftertreatment system, a reducing agent is injected as a liquid into the exhaust gas stream of the exhaust pipe, and the mixture of the reducing agent and the exhaust gas is passed through a downstream SCR catalyst which uses the reducing agent to catalyze the reduction of NOxin the exhaust gas stream. The reducing agent may be ammonia, or it may be urea that is subsequently hydrolyzed to ammonia in the exhaust gas stream. In the context of diesel engines, a reducing agent consisting of urea and water is referred to as diesel exhaust fluid (DEF).

The reducing agent should be vaporized and well mixed with the exhaust gas prior to introduction to the SCR catalyst to ensure that the reduction of NOxat the SCR catalyst proceeds efficiently. Complete vaporization of the reducing agent not only assists even distribution of the reducing agent in the exhaust gas, but also avoids undesirable accumulation of reducing agent deposits in the exhaust pipe that could lead to decreased conversion efficiencies as well as increased back pressure in the exhaust pipe. To promote vaporization of the reducing agent and mixing of the reducing agent with the exhaust gas, a mixer may be provided in the exhaust pipe between the injector and the SCR catalyst. For example, U.S. Pat. No. 8,607,555 discloses a mixing element that includes a grid supporting rows of trapezoidal deflector elements that are oriented in different directions. The patent also discloses a mixing element that includes four fields of deflector elements that are turned 90° with respect to each other to generate rotational motion to the exhaust gases and reducing agent flowing through the mixer.

Although the above mixing elements are effective, there is still a need for improved mixer designs for exhaust aftertreatment systems which avoid droplets of reducing agent from being forced to the exhaust pipe walls. Additionally, there is also a need for mixer designs with improved structural robustness.

SUMMARY

In accordance with one aspect of the present disclosure, a swirl mixer for mixing a reducing agent with exhaust gas in a selective catalytic reduction (SCR) aftertreatment system is disclosed. The swirl mixer may comprise a base permitting a flow of the reducing agent and the exhaust gas therethrough, and three arrays of fins projecting from the base in a direction of flow of the exhaust gas. The three arrays may be arranged in a triangular configuration about a center of the mixer to induce a swirl motion in the reducing agent and the exhaust gas flowing through the mixer. The fins in each of the arrays may be oriented in a common direction that is rotated by about 60° from the common direction of the fins in an adjacent array.

In accordance with another aspect of the present disclosure, a swirl mixer for mixing a reducing agent with exhaust gas in an exhaust pipe of a diesel engine is disclosed. The swirl mixer may comprise a planar base permitting a flow of the reducing agent and the exhaust gas therethrough. The base may include radial legs each extending radially from a center of the base and being equally spaced from each other in a circumferential direction. The swirl mixer may further comprise a plurality of fins projecting from each of the radial legs in a direction of flow of the exhaust gas to induce a swirl motion in the reducing agent and the exhaust gas passing through the mixer. The fins projecting from each of the radial legs may be oriented in a common direction that is rotated by an angle with respect to the common direction of the fins projecting from an adjacent radial leg.

In accordance with another aspect of the present disclosure, a selective catalytic reduction (SCR) aftertreatment system for exhaust gas of a diesel engine is disclosed. The SCR aftertreatment system may comprise an exhaust pipe configured to carry the exhaust gas from the diesel engine to an exhaust outlet, and a reducing agent injector configured to inject a reducing agent into the exhaust pipe. The SCR aftertreatment system may further comprise a SCR catalyst downstream of the reducing agent injector configured to catalyze the reduction of NOxin the exhaust gas with the reducing agent. A dual mixer may be positioned in the exhaust pipe downstream of the reducing agent injector and upstream of the SCR catalyst. The dual mixer may include a first mixer configured to vaporize the reducing agent, and a swirl mixer downstream of the first mixer configured to induce a swirl motion in the reducing agent and the exhaust gas passing therethrough. The swirl mixer may include arrays of fins each projecting from the mixer in a downstream direction. Each of the arrays may include a plurality of parallel rows of fins that are oriented in a common direction that is rotated by about 60° from the common direction in an adjacent array.

These and other aspects and features of the present disclosure will be more readily understood when read in conjunction with the accompanying drawings.

DETAILED DESCRIPTION

Referring now to the drawings, and with specific reference toFIG. 1, an exhaust aftertreatment system10for an internal combustion engine12, such as a diesel engine14, is shown. The exhaust aftertreatment system10may include components that remove at least some of the pollutants in an exhaust gas16emitted by the engine12through an exhaust pipe18prior to release of the exhaust gas from an exhaust outlet20, such as a tailpipe. In particular, the aftertreatment system10may include a particulate filter22disposed in the exhaust pipe18that filters out particulates from the exhaust gas16. Downstream of the particulate filter22in the exhaust pipe18may be a selective catalytic reduction (SCR) aftertreatment system24for catalyzing the reduction of NOxin the exhaust gas16to nitrogen and water. Alternative arrangements of the aftertreatment system10may lack a particulate filter.

The SCR aftertreatment system24may include an injector26for injecting a reducing agent28from a supply source30into the exhaust gas16flowing in the exhaust pipe18. The reducing agent28may be a mixture of urea and water (also referred to as diesel exhaust fluid (DEF) if the engine12is a diesel engine), and the urea may be hydrolyzed to ammonia in the exhaust pipe18. Alternatively, the reducing agent28may be ammonia. The reducing agent28may initially be injected into the exhaust pipe18as a liquid, and later vaporized in the exhaust pipe18(see further details below). Downstream of the injector26may be a catalyst32that uses the reducing agent28to catalyze the reduction of NOxin the exhaust gas16to nitrogen and water prior to release of the exhaust gas through the outlet20.

The SCR aftertreatment system24may also include a mixing section34between the injector26and the SCR catalyst32where the reducing agent28is vaporized and mixed with the exhaust gas prior to introduction to the catalyst32. The mixing section34may contain a dual mixer36that consists of a first mixer37and a swirl mixer38downstream of the first mixer37. Flow of the exhaust gas16through the dual mixer36may promote vaporization of the reducing agent28and mixing of the reducing agent28with the exhaust gas16. Specifically, the first mixer37may vaporize liquid droplets of the reducing agent28, while the swirl mixer38may catch un-vaporized droplets of the reducing agent and induce a swirl motion to the vaporized reducing agent and the exhaust gas to promote thorough mixing. Due to the corrosive nature of the reducing agent28and vibrations in the exhaust pipe18, both of the first mixer37and the swirl mixer38may be formed from a material that is corrosion resistant and robust enough to withstand vibrations. For example, the first mixer37and the swirl mixer38may both be formed from stainless steel.

Turning now toFIG. 2, the first mixer37is shown in isolation. The first mixer37may include a planar grid40formed from a plurality of first support elements42arranged perpendicular to and intersecting a plurality of second support elements44to define holes46that allow the passage of the reducing agent28and the exhaust gas16through the mixer37. Protruding from the grid40in the direction of flow of the exhaust gas16(i.e., in the downstream direction in the exhaust pipe18) may be a plurality of fins48that promote vaporization of the reducing agent28. The fins48may have a trapezoidal shape or other alternative shapes such as, but not limited to, square, rectangular, triangular, spherical, oval shaped, or other polygonal and amorphous configurations. Moreover, the fins48may be oriented at a fixed angle with respect to the plane of the grid40that may vary between about 10° and about 80°. Additionally, the fins48may be formed integrally with and extend from the first support elements42to form a plurality of rows50of fins. The fins48in each of the rows50may alternate orientation direction, with one fin48pointed in one direction and an immediately adjacent fin48pointed in the opposite direction, as shown. AlthoughFIG. 2shows seven rows of fins and three to seven fins in each row, it will be understood that the number of rows and the number of fins in each row may vary depending on a number of design considerations in practice, such as the dimensions of the exhaust pipe18. The first mixer37may also include curved tabs52to allow attachment of the mixer37to the inner walls of the exhaust pipe18, such as by welding.

The swirl mixer38is shown in isolation inFIGS. 3-4. The swirl mixer38may include a base54that permits flow of the reducing agent28and the exhaust gas16therethrough. The swirl mixer38may also include a number of arrays56of swirl fins58projecting from the base54in a direction of flow of the exhaust gas16in the exhaust pipe18(i.e., in a downstream direction in the exhaust pipe18) . As used herein, an “array” is a group of swirl fins58arranged in parallel rows60, wherein all of the swirl fins58in the array are oriented in a common direction62with the tops59of the fins all pointed in the common direction62(seeFIG. 3). In addition, in each of the arrays56, the rows60may be equally spaced from each other, and the swirl fins58in each of the rows60may be equally spaced from each other to provide a regular, repeating pattern of swirl fins58. The arrays56may be identical to each other and may be arranged with respect to each other to provide a circling configuration about a center64of the swirl mixer38that may run either clockwise or counterclockwise to induce swirl motion in the reducing agent and the exhaust gas flowing through the mixer38. For example, the depicted swirl mixer38includes three arrays56in which the common direction62of each of the arrays56is rotated by about 60° from the common direction62of an immediately adjacent array56to create a triangular configuration about the center64, although other numbers of arrays having different rotation angles with respect to each other are possible. Accordingly, in the depicted embodiment, the swirl mixer38exhibits three-fold rotational symmetry. It is noted that the swirl mixer38is held stationary in the exhaust pipe18and does not rotate, and the swirl motion is induced by the circling configuration of the arrays56. In alternative configurations of the mixer38, the arrays38may not be identical to each other. In addition, althoughFIGS. 3-4show four rows60of swirl fins58in each of the arrays56, and three to four swirl fins58in each of the rows60, it will be understood that alternative designs of the swirl mixer38may have more or less rows and/or numbers of fins in each row.

Referring still toFIGS. 3-4, in the depicted embodiment having three arrays56, the base54of the swirl mixer38may include three radial legs66extending radially from the center64of the mixer38, and the three radial legs66may be equally spaced from each other by about 120° in a circumferential direction68(seeFIG. 3). Furthermore, a plurality of swirl fins58may be formed integrally with (or otherwise attached to) and may project from each of the radial legs66to form one of the rows60of fins in one of the arrays56. Namely, each of the radial legs66may support the last row60of fins in an array56before the orientation direction of the swirl fins58is rotated by 60° in an adjacent array56. Each of the radial legs66may also include a curved tab70that extends from the swirl mixer38to allow attachment of the mixer38to the inner walls of the exhaust pipe18, such as by welding. In other embodiments, more or less radial legs may be employed.

Turning now toFIG. 4, in the depicted embodiment having three radial legs66, the base54may further include three grids72between the radial legs66that support and interconnect the arrays56. The grids72may be constructed from a plurality of support elements74that each span two adjacent grids to provide interconnectivity and structural robustness to the mixer38. Specifically, each of the support elements74may include a first support element76in one of the grids72that is integrally formed with (or otherwise attached to) a second support element78in an adjacent grid72. In each of the grids72, a plurality of the first support elements76may be arranged perpendicular to and intersect a plurality of the second support elements78to define holes80that allow the passage of the reducing agent28and the exhaust gas16through the mixer38. Moreover, the first support elements76may be formed integrally with (or otherwise attached to) the swirl fins58to define one of the rows60in an array56. Furthermore, the first support elements76in each grid72may run parallel to the radial leg66that supports swirl fins58in the same array56, while the second support elements78may run perpendicular to and interconnect the first support elements76and the radial leg66in the array56. Moreover, in other embodiments employing a different number of radial legs66, a corresponding number of grids72may be formed between the radial legs66.

The base54of the swirl mixer38may be planar and extend along a plane81, and the swirl fins58may project from a downstream face83of the base at a fixed angle (α) with respect to the plane81of the base54, as shown inFIG. 5. The angle (α) may be about 45°, although other angles between about 5° and about 80° may also be used in some circumstances. Additionally, as shown inFIGS. 3-4, each of the swirl fins58of the swirl mixer38may have identical shapes and dimensions. Specifically, the swirl fins58may be trapezoidal (seeFIGS. 3-4) with a lengths (l) extending from a bottom82to the top59of each fin58being about 30 millimeters (seeFIG. 5). However, the swirl fins58may certainly have other shapes (e.g., square, rectangular, triangular, spherical, oval, other polygonal shapes, etc.) and dimensions in alternative designs of the mixer38.

As shown inFIG. 6, each of the support elements74may include slots86to permit connection to other support elements74when assembling the swirl mixer38. For example, the first support elements76may each have slots86presented on an upstream side88, while the second support elements78may each have slots86presented on a downstream side90. Accordingly, the grids72of the swirl mixer38may be assembled by connecting the slots86of the first support elements76with the slots86of the second support elements78. Likewise, as shown inFIG. 7, each of the radial legs66may have slots86presented on an upstream side92, such that the slots86of the radial legs66may be connected to the slots86of the second support elements78when assembling the swirl mixer38(see further details below).

INDUSTRIAL APPLICABILITY

In general, the teachings of the present disclosure may find applicability in many industries including, but not limited to, automotive, construction, agriculture, mining, power generation, and rail transport applications, among others. More specifically, the technology disclosed herein may find applicability in many types of engines and machines having SCR aftertreatment systems. It may also find applicability in other types of exhaust aftertreatment systems in which a reagent is mixed with exhaust gas.

Referring now toFIGS. 8-10, steps that may be involved in assembling the swirl mixer38are depicted. Namely,FIGS. 8-10depict steps involved in assembling the swirl mixer38with three arrays56, but it will be understood that the concepts disclosed herein may be similarly applied to swirl mixers having more or less numbers of arrays. Each of the three radial legs66may first be separately assembled with a plurality of the support elements74to form three units102. For example,FIG. 8shows one of the units102formed by inserting the slots86of the radial leg66into slots86of three of the second support elements78. Next, the three units102may be assembled together by interconnecting the slots86of the support elements74, as shown inFIGS. 9-10. In particular, this may be carried out by first assembling two of the units102together by inserting the slots86of the first support elements76of one of the units102into the slots86of the second support elements78of another unit102to provide one of the grids72interconnecting the two radial legs66(seeFIG. 9). The exposed first and second support elements76and78of the two assembled units102may then be assembled with the third unit102by interconnecting the slots86of the first support elements76and the second support elements78(seeFIG. 10).

Once assembled, the units102may be welded together at nodes106(or intersection points between the radial legs66and the first support elements76with the second support elements78) to provide the fully assembled swirl mixer38(seeFIG. 10). As shown inFIG. 10, the units102may be welded together on an upstream face109of the base54(also seeFIG. 5). It is noted here thatFIGS. 8-10depict one possible method to assemble the swirl mixer38, but numerous alternative ways to assemble the mixer38exist. For example, the radial legs66may first be welded together at the center64, and the grids72may be assembled between the radial legs66by interconnecting the support elements74and welding the support elements74together at the nodes106. Variations such as these also fall within the scope of the present disclosure.

The swirl mixer disclosed herein includes three arrays of fins arranged in a triangular configuration to induce swirl motion to a mixture of reducing agent and exhaust gas flowing through the mixer. The swirl mixer captures un-vaporized reducing agent droplets left behind from an upstream mixer, and promotes even distribution of vaporized reducing agent in the exhaust gas to improve NOxconversion at the downstream SCR catalyst. The rows of fins in each array have a smaller surface area than the solid blades used in some mixers of the prior art, thereby reducing the potential for build-up of reducing agent deposits on the surfaces of the mixer and enhancing the break-up of reducing agent droplets. Furthermore, the three arrays of fins impose a moderate swirl force onto the mixture of the reducing agent and the exhaust gas that is strong enough to provide adequate mixing, but weak enough to avoid undesirable forcing of reducing agent droplets to the walls of the exhaust pipe which could reduce the distribution of the reducing agent in the exhaust gas. Furthermore, an interconnected framework of grids with three-fold rotational symmetry provides a sturdier and more structurally robust structure than mixers of the prior art that are less interconnected. The technology disclosed herein may find wide industrial applicability in a wide range of areas such as, but not limited to, construction, mining, agriculture, automotive, and rail transport applications.