U.S. patent application number 13/919806 was filed with the patent office on 2013-12-19 for reductant decomposition and mixing system.
The applicant listed for this patent is Cummins IP, Inc.. Invention is credited to Joseph M. Brault, Vinay K. Joshi, Dipesh M. Kadam, Abhijeet Nande, Jose Palacios, Timothy V. Splinter, Tamas Szailer.
Application Number | 20130333363 13/919806 |
Document ID | / |
Family ID | 49754656 |
Filed Date | 2013-12-19 |
United States Patent
Application |
20130333363 |
Kind Code |
A1 |
Joshi; Vinay K. ; et
al. |
December 19, 2013 |
REDUCTANT DECOMPOSITION AND MIXING SYSTEM
Abstract
A selective catalytic reduction (SCR) system includes a mixer
that has a plurality of blades spaced about a central tube. The
mixer can be configured to receive a reductant and exhaust gas
mixture. The SCR system also includes a closed-end tube that is
downstream of the mixer and configured to receive the reductant and
exhaust gas mixture from the mixer. The closed-end tube includes a
closed end opposing an open end. The closed end is substantially
perpendicular to an exhaust flow direction. The closed-end tube
further includes a sidewall that extends between the open and
closed end. The sidewall can include a plurality of perforations.
The flange plate of the SCR system includes an annular disk that
has a plurality of openings through which reductant and exhaust gas
mixture received from the closed-end tube is flowable. The flange
plate is coupled to and supports in place the closed-end tube.
Inventors: |
Joshi; Vinay K.; (Pune,
IN) ; Szailer; Tamas; (Seymour, IN) ; Kadam;
Dipesh M.; (Pune, IN) ; Palacios; Jose;
(Stoughton, WI) ; Splinter; Timothy V.;
(Stoughton, WI) ; Nande; Abhijeet; (Columbus,
IN) ; Brault; Joseph M.; (Columbus, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cummins IP, Inc. |
Columbus |
IN |
US |
|
|
Family ID: |
49754656 |
Appl. No.: |
13/919806 |
Filed: |
June 17, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61660532 |
Jun 15, 2012 |
|
|
|
Current U.S.
Class: |
60/301 ;
60/324 |
Current CPC
Class: |
F01N 3/035 20130101;
F01N 3/2066 20130101; Y02T 10/12 20130101; Y02T 10/24 20130101;
B01F 5/0614 20130101; B01F 3/04049 20130101; F01N 3/208 20130101;
B01F 5/0618 20130101; F01N 3/2892 20130101; B01F 5/0473 20130101;
B01F 5/0268 20130101; B01F 5/0689 20130101; B01F 5/0688
20130101 |
Class at
Publication: |
60/301 ;
60/324 |
International
Class: |
F01N 3/20 20060101
F01N003/20 |
Claims
1. A selective catalytic reduction (SCR) system, comprising: a
mixer comprising a plurality of blades spaced about a central tube,
the mixer configured to receive a reductant and exhaust gas
mixture; a closed-end tube downstream of the mixer configured to
receive the reductant and exhaust gas mixture from the mixer, the
closed-end tube comprising a closed end opposing an open end, the
closed end being substantially perpendicular to an exhaust flow
direction, wherein the closed-end tube comprises a sidewall
extending between the open and closed end, the sidewall comprising
a plurality of perforations; and a flange plate comprising an
annular disk, the annular disk comprising a plurality of openings
through which reductant and exhaust gas mixture received from the
closed-end tube is flowable, wherein the flange plate is coupled to
and supports in place the closed-end tube.
2. The SCR system of claim 1, wherein the SCR system is one of an
in-line system, end-to-end system, or end-to-side system.
3. The SCR system of claim 1, further comprising a linear exhaust
conduit extending between a reductant doser and an inlet of an SCR
catalyst, wherein the mixer, closed-end tube, and flange plate are
positioned within the linear exhaust conduit between the reductant
doser and the inlet of the SCR catalyst.
4. The SCR system of claim 3, wherein the linear exhaust conduit
comprises a decomposition tube and an SCR catalyst housing, and
wherein the mixer is positioned within the decomposition tube, at
least a portion of the closed-end tube is positioned within the SCR
catalyst housing, and the flange plate is positioned within the SCR
catalyst housing.
5. The SCR system of claim 3, wherein the mixer is a secondary
mixer, the SCR system further comprising a primary mixer positioned
within the linear exhaust conduit downstream of the reductant doser
and upstream of the secondary mixer.
6. The SCR system of claim 1, wherein the plurality of blades
induces a vortical swirling exhaust gas flow pattern and the
central tube induces a linear exhaust gas flow pattern within the
exhaust gas vortical swirling flow pattern.
7. The SCR system of claim 1, wherein the plurality of perforations
of the closed-end tube induces a radially outwardly directed
exhaust gas flow pattern.
8. The SCR system of claim 1, wherein the plurality of openings of
the flange plate induces a plurality of concentrated radially outer
regions of exhaust gas.
9. The SCR system of claim 1, wherein the SCR system is configured
such that an entirety of exhaust gas passing through the mixer is
received by the closed-end tube and passes through the plurality of
perforations of the closed-end tube, and wherein the SCR system is
configured such that an entirety of exhaust gas passing through the
plurality of perforations of the closed-end tube passes through the
plurality of openings of the flange plate.
10. An exhaust gas mixer, comprising: an inner tube defining a
central conduit; an outer tube positioned about the inner tube in
coaxial alignment with the inner tube; and a plurality of blades
coupled to and positioned between the inner and outer tubes.
11. The exhaust gas mixer of claim 10, wherein a radius of the
inner tube is equal to or more than half a radius of the outer
tube.
12. The exhaust gas mixer of claim 10, wherein each of the
plurality of blades comprises a radially inner edge fixed to the
inner ring and a radially outer edge fixed to the outer ring.
13. The exhaust gas mixer of claim 12, wherein the radially inner
edge and radially outer edge are non-parallel to a central axis of
the inner tube.
14. The exhaust gas mixer of claim 13, wherein the radially inner
edge defines a first angle relative to the central axis and the
radially outer edge defines a second angle relative to the central
axis, and wherein the first and second angles are different.
15. The exhaust gas mixer of claim 14, wherein the first angle is
between about 30.degree. and about 50.degree., and the second angle
is between about 50.degree. and about 70.degree..
16. The exhaust gas mixer of claim 10, wherein each of the
plurality of blades comprises a leading edge and a trailing edge,
and wherein each of the plurality of blades is curved in a
direction extending from the leading edge to the trailing edge.
17. An exhaust tube for receiving an exhaust gas stream flowing in
a flow direction, comprising: an open upstream end configured to
receive the exhaust gas stream flowing in the flow direction; a
closed downstream end defining an exhaust impact surface
substantially perpendicular to the flow direction, the exhaust
impact surface redirecting the exhaust gas stream in a redirected
direction substantially perpendicular to the flow direction; and a
sidewall extending between the open upstream and closed downstream
ends, the sidewall comprising a plurality of perforations through
which the redirected exhaust gas is flowable.
18. The exhaust tube of claim 17, wherein the exhaust impact
surface is convex.
19. The exhaust tube of claim 17, wherein an entirety of the
exhaust gas stream received by the exhaust tube flows through the
plurality of perforations.
20. The exhaust tube of claim 17, wherein the sidewall extends
parallel to the flow direction, and wherein the redirected exhaust
gas flows through the plurality of perforations in the redirected
direction.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 61/660,532, filed Jun. 15, 2012, which is
incorporated herein by reference.
FIELD
[0002] This invention relates to exhaust systems for internal
combustion engines, and more particularly to a system for
decomposing reductant and mixing together reductant and exhaust gas
in a selective catalytic reduction (SCR) catalyst of an exhaust
aftertreatment system.
BACKGROUND
[0003] Exhaust aftertreatment systems receive and treat exhaust gas
generated from an internal combustion engine. Typical exhaust
aftertreatment systems include any of various components configured
to reduce the level of harmful exhaust emissions present in the
exhaust gas. For example, some exhaust aftertreatment systems for
diesel powered internal combustion engines include various
components, such as a diesel oxidation catalyst (DOC), particulate
matter filter or diesel particulate filter (DPF), and a selective
catalytic reduction (SCR) catalyst. In some exhaust aftertreatment
systems, exhaust gas first passes through the diesel oxidation
catalyst, then passes through the diesel particulate filter, and
subsequently passes through the SCR catalyst.
[0004] Each of the DOC, DPF, and SCR catalyst components is
configured to perform a particular exhaust emissions treatment
operation on the exhaust gas passing through the components.
Generally, the DOC reduces the amount of carbon monoxide and
hydrocarbons present in the exhaust gas via oxidation techniques.
The DPF filters harmful diesel particulate matter and soot present
in the exhaust gas. Finally, the SCR catalyst reduces the amount of
nitrogen oxides (NO.sub.x) present in the exhaust gas.
[0005] The SCR catalyst is configured to reduce NO into less
harmful emissions, such as N2 and H.sub.2O, in the presence of
ammonia (NH.sub.3). Because ammonia is not a natural byproduct of
the combustion process, it must be artificially introduced into the
exhaust gas prior to the exhaust gas entering the SCR catalyst.
Typically, ammonia is not directly injected into the exhaust gas
due to safety considerations associated with the storage of liquid
ammonia. Accordingly, conventional systems are designed to inject a
urea-water solution into the exhaust gas, which is capable of
decomposing into ammonia in the presence of the exhaust gas. SCR
systems typically include a urea source and a urea injector or
doser coupled to the source and positioned upstream of the SCR
catalyst.
[0006] Generally, the decomposition of the urea-water solution into
gaseous ammonia occupies three stages. First, urea evaporates or
mixes with exhaust gas. Second, the temperature of the exhaust
causes a phase change in the urea and decomposition of the urea
into isocyanic acid (HNCO) and water. Third, the isocyanic acid
reacts with water in a hydrolysis process under specific pressure
and temperature concentrations to decompose into ammonia and carbon
dioxide (CO.sub.2). The ammonia is then introduced at the inlet
face of the SCR catalyst, flows through the catalyst, and is
consumed in the NO.sub.x reduction process. Any unconsumed ammonia
exiting the SCR system can be reduced to N.sub.2 and other less
harmful or less noxious components using an ammonia oxidation
catalyst.
[0007] To sufficiently decompose into ammonia, the injected urea
must be given adequate time to complete the three stages. The time
given to complete the three stages and decompose urea into ammonia
before entering the SCR catalyst is conventionally termed residence
time. Some prior art exhaust aftertreatment systems utilize a long
tube of a fixed linear decomposition length that extends between
the urea injector and SCR catalyst inlet face. The fixed linear
decomposition length of prior art systems must be quite long in
order to provide the necessary residence time. Long tubing for urea
decomposition often takes up valuable space that could be occupied
by other vehicle components and influences the design of the
exhaust aftertreatment system. However, shorter decomposition tubes
associated with some prior art end-in, end-out and end-in, side-out
SCR systems may not provide a sufficiently long residence time to
properly evaporate the injected urea.
[0008] Additionally, some prior art exhaust aftertreatment systems,
particularly those systems that utilize or require in-line or
end-to-end or end-to-side components, do not provide adequate
mixing of the urea/ammonia with the exhaust gas. Inadequate mixing
results in a low ammonia vapor uniformity index, which can lead to
crystallization/polymerization buildup inside the SCR catalyst or
other SCR system components, localized aggregation of ammonia,
inadequate distribution of the ammonia across the SCR catalyst
surface, lower NO conversion efficiency, and other
shortcomings.
[0009] Further, many exhaust aftertreatment systems with end-to-end
or end-to-side SCR systems fail to adequately distribute exhaust
gas across the inlet face of the SCR catalyst. An uneven
distribution of exhaust gas at the SCR catalyst inlet can result in
excessive ammonia slip and less than optimal NO.sub.x conversion
efficiency. For example, a low exhaust flow distribution index at
the SCR catalyst inlet results in a lower amount of SCR catalyst
surface area in contact with the exhaust gases. The lesser the
catalyst surface area in contact with the exhaust gases, the lower
the NO reduction efficiency of the SCR catalyst.
SUMMARY
[0010] The subject matter of the present application has been
developed in response to the present state of the art, and in
particular, in response to the problems and needs in the art that
have not yet been fully solved by currently available exhaust
aftertreatment systems employing an SCR system. Accordingly, the
subject matter of the present application has been developed to
provide a reductant decomposition and mixing system, and associated
apparatus, that overcomes at least some of the following or other
shortcomings of prior art reductant mixing and decomposition
techniques and devices.
[0011] According to one embodiment, a selective catalytic reduction
(SCR) system includes a mixer that has a plurality of blades spaced
about a central tube. The mixer can be configured to receive a
reductant and exhaust gas mixture. The SCR system also includes a
closed-end tube that is downstream of the mixer and configured to
receive the reductant and exhaust gas mixture from the mixer. The
closed-end tube includes a closed end opposing an open end. The
closed end is substantially perpendicular to an exhaust flow
direction. The closed-end tube further includes a sidewall that
extends between the open and closed end. The sidewall can include a
plurality of perforations. The flange plate of the SCR system
includes an annular disk that has a plurality of openings through
which reductant and exhaust gas mixture received from the
closed-end tube is flowable. The flange plate is coupled to and
supports in place the closed-end tube.
[0012] In certain implementations, the SCR system is one of an
in-line system, end-to-end system, or end-to-side system. The SCR
system may further include a linear exhaust conduit that extends
between a reductant doser and an inlet of an SCR catalyst. The
mixer, closed-end tube, and flange plate can be positioned within
the linear exhaust conduit between the reductant doser and the
inlet of the SCR catalyst. The linear exhaust conduit may include a
decomposition tube and an SCR catalyst housing. The mixer can be
positioned within the decomposition tube. At least a portion of the
closed-end tube can be positioned within the SCR catalyst housing,
and the flange plate can be positioned within the SCR catalyst
housing. The mixer can be a secondary mixer, and the SCR system can
further include a primary mixer positioned within the linear
exhaust conduit downstream of the reductant doser and upstream of
the secondary mixer.
[0013] According to some implementations, the plurality of blades
induces a vortical swirling exhaust gas flow pattern and the
central tube induces a linear exhaust gas flow pattern within the
exhaust gas vortical swirling flow pattern. The plurality of
perforations of the closed-end tube can induce a radially outwardly
directed exhaust gas flow pattern. The plurality of openings of the
flange plate can induce a plurality of concentrated radially outer
regions of exhaust gas. The SCR system can be configured such that
an entirety of exhaust gas passing through the mixer is received by
the closed-end tube and passes through the plurality of
perforations of the closed-end tube. Further, the SCR system can be
configured such that an entirety of exhaust gas passing through the
plurality of perforations of the closed-end tube passes through the
plurality of openings of the flange plate.
[0014] In another embodiment, an exhaust gas mixer includes an
inner tube that defines a central conduit. The exhaust gas mixer
may also include an outer tube that is positioned about the inner
tube in coaxial alignment with the inner tube. Additionally, the
exhaust gas mixer can include a plurality of blades that are
coupled to and positioned between the inner and outer tubes.
According to certain implementations, a radius of the inner tube is
equal to or more than half a radius of the outer tube.
[0015] According to some implementations of the exhaust gas mixer,
each of the plurality of blades may include a radially inner edge
fixed to the inner ring and a radially outer edge fixed to the
outer ring. The radially inner edge and radially outer edge may be
non-parallel to a central axis of the inner tube. The radially
inner edge may define a first angle relative to the central axis
and the radially outer edge may define a second angle relative to
the central axis. The first and second angles can be different. The
first angle can be between about 30.degree. and about 50.degree.,
and the second angle can be between about 50.degree. and about
70.degree.. Each of the plurality of blades may include a leading
edge and a trailing edge, where each of the plurality of blades is
curved in a direction extending from the leading edge to the
trailing edge.
[0016] In yet another embodiment, an exhaust tube for receiving an
exhaust gas stream flowing in a flow direction includes an open
upstream end that is configured to receive the exhaust gas stream
flowing in the flow direction, and a closed downstream end that
defines an exhaust impact surface that is substantially
perpendicular to the flow direction. The exhaust impact surface
redirects the exhaust gas stream in a redirected direction that is
substantially perpendicular to the flow direction. The exhaust tube
also includes a sidewall that extends between the open upstream and
closed downstream ends. The sidewall includes a plurality of
perforations through which the redirected exhaust gas is
flowable.
[0017] According to some implementations of the exhaust tube, the
exhaust impact surface is convex. In certain implementations, an
entirety of the exhaust gas stream received by the exhaust tube
flows through the plurality of perforations. According to yet
certain implementations, the sidewall extends parallel to the flow
direction, and the redirected exhaust gas flows through the
plurality of perforations in the redirected direction.
[0018] Reference throughout this specification to features,
advantages, or similar language does not imply that all of the
features and advantages that may be realized with the present
invention should be or are in any single embodiment or
implementation of the invention. Rather, language referring to the
features and advantages is understood to mean that a specific
feature, advantage, or characteristic described in connection with
an embodiment is included in at least one embodiment of the present
invention. Discussion of the features and advantages, and similar
language, throughout this specification may, but do not
necessarily, refer to the same embodiment or implementation.
[0019] The described features, structures, advantages, and/or
characteristics of the subject matter of the present disclosure may
be combined in any suitable manner in one or more embodiments
and/or implementations. In the following description, numerous
specific details are provided to impart a thorough understanding of
embodiments of the subject matter of the present disclosure. One
skilled in the relevant art will recognize that the subject matter
of the present disclosure may be practiced without one or more of
the specific features, details, components, materials, and/or
methods of a particular embodiment or implementation. In other
instances, additional features and advantages may be recognized in
certain embodiments and/or implementations that may not be present
in all embodiments or implementations. Further, in some instances,
well-known structures, materials, or operations are not shown or
described in detail to avoid obscuring aspects of the subject
matter of the present disclosure. The features and advantages of
the subject matter of the present disclosure will become more fully
apparent from the following description and appended claims, or may
be learned by the practice of the subject matter as set forth
hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] In order that the advantages of the subject matter may be
more readily understood, a more particular description of the
subject matter briefly described above will be rendered by
reference to specific embodiments that are illustrated in the
appended drawings. Understanding that these drawings depict only
typical embodiments of the subject matter and are not therefore to
be considered to be limiting of its scope, the subject matter will
be described and explained with additional specificity and detail
through the use of the drawings, in which:
[0021] FIG. 1 is a perspective view of an exhaust aftertreatment
system according to one embodiment shown with see-through housing
to more clearly show internal components of the system;
[0022] FIG. 2 is a perspective view of an exhaust mixer with a
central channel according to one embodiment;
[0023] FIG. 3 is a front view of the exhaust mixer of FIG. 2;
[0024] FIG. 4 is a partial cross-sectional side view of an exhaust
mixer with a central channel according to one embodiment;
[0025] FIG. 5 is a cross-sectional perspective view of a closed-end
perforated tube and perforated flange plate of an exhaust
aftertreatment system according to one embodiment; and
[0026] FIG. 6 is a cross-sectional side view of the closed end or
plug of a closed-end perforated tube according to one
embodiment.
DETAILED DESCRIPTION
[0027] Reference throughout this specification to "one embodiment,"
"an embodiment," or similar language means that a particular
feature, structure, or characteristic described in connection with
the embodiment is included in at least one embodiment of the
subject matter of the present disclosure. Appearances of the
phrases "in one embodiment," "in an embodiment," and similar
language throughout this specification may, but do not necessarily,
all refer to the same embodiment. Similarly, the use of the term
"implementation" means an implementation having a particular
feature, structure, or characteristic described in connection with
one or more embodiments of the subject matter of the present
disclosure, however, absent an express correlation to indicate
otherwise, an implementation may be associated with one or more
embodiments.
[0028] Described herein are various embodiments of a reductant
decomposition and mixing system for, among other things, enhancing
the decomposition of a reductant, such as urea, to ammonia in an
exhaust gas, improving the mixing of urea and ammonia with exhaust
gas, and improving the exhaust flow distribution uniformity into
the SCR catalyst. The reductant decomposition and mixing system
forms part of an exhaust aftertreatment system that has an SCR
catalyst and reductant injector. In one embodiment, the reductant
decomposition and mixing system is specifically configured for use
in an end-in, end-out (e.g., end-to-end), and/or end-in, side-out,
exhaust aftertreatment system configuration, as opposed to a
switch-back exhaust aftertreatment system configuration. The system
can include a secondary mixer with a central channel, a closed-end
perforated tube, and a perforated flange plate. The secondary mixer
creates a cyclone effect and the central channel obstructs the
formation of a low pressure zone between the swirling exhaust and
reductant mixture. The closed-end perforated tube prevents the
direct and forceful impact of exhaust and reductant mixture against
the SCR catalyst face, and the perforations of the tube direct the
mixture radially outwardly out of the tube. The flange plate
supports the secondary mixer and tube, and includes a perforation
pattern that increases the residence time, facilitates a radially
outwardly distribution of the mixture, and further mixes the
mixture prior to entering the SCR catalyst.
[0029] In one specific embodiment illustrated in FIG. 1, an exhaust
aftertreatment system 10 is coupled to an internal combustion
engine (not shown). The exhaust aftertreatment system 10 is capable
of receiving and treating exhaust gas generated by the engine as
indicated by directional arrow 12. After being treated by the
exhaust aftertreatment system 10, exhaust gas is expelled into the
atmosphere via a tailpipe (not shown) as indicated by directional
arrow 14. In certain implementations, the exhaust aftertreatment
system 10 is secured to a vehicle in which the engine is
housed.
[0030] The exhaust aftertreatment system 10 includes a plurality of
exhaust treatment devices. For example, the exhaust aftertreatment
system 10 has a housing that includes a diesel oxidation catalyst
(DOC) 20 and a diesel particulate filter (DPF) 30 downstream of the
DOC. The aftertreatment system 10 also includes a selective
catalytic reduction (SCR) system 40 that has an SCR catalyst 42.
The SCR catalyst 42 includes an open face 43 that is open to and
receives incoming exhaust flow. The SCR system 40 also includes a
reductant injector 46 coupled to a reductant supply source (not
shown). The reductant injector 46 injects reductant from the supply
source into a reductant decomposition tube 18 upstream of the SCR
catalyst 42. The DPF 30 is fluidly coupled with the SCR catalyst 42
via an exhaust tube 16. Although the exhaust aftertreatment system
10 of FIG. 1 shows one DOC 20, DPF 30, and SCR catalyst 42 in a
specific order relative to each other, in other embodiments, an
exhaust aftertreatment system can have fewer or more than the
number of exhaust treatment devices shown in FIG. 1 in a different
order relative to each other without departing from the essence of
the present disclosure.
[0031] The exhaust tube 16 includes a section 17 upstream of the
injector 46 and a section 18 downstream of the injector. The
downstream section 18 is defined as a reductant decomposition tube.
The system 10 further includes a reductant decomposition and mixing
system 45 positioned between the injector 46 and the open face 43
of the SCR catalyst 42. The system 45 can be positioned partially
within and proximate an outlet end of the decomposition tube 18,
and positioned at least partially within an SCR catalyst housing 41
that houses the SCR catalyst 42. The decomposition tube 18 and SCR
catalyst housing 41 can define a linear exhaust conduit (e.g., be
coaxially aligned) in certain implementations. Regardless of the
position of the reductant decomposition and mixing system 45,
exhaust gas and injected reductant pass through the system 45
before being presented at the SCR catalyst face 43 and entering the
SCR catalyst 42. In certain implementations, the reductant is urea.
The illustrated exhaust aftertreatment system 10 is an end-in,
end-out (e.g., end-to-end) system. In other words, the central axis
of each of the components of the system 10 (i.e., DOC 20, DPF 30,
decomposition tube 18, and SCR catalyst 42) are coaxially aligned.
However, in other embodiments, the reductant decomposition and
mixing system of the present disclosure is operable with an exhaust
aftertreatment system with an end-in, side-out type configuration.
Additionally, even in some embodiments, the reductant decomposition
and mixing system of the present disclosure can provide certain
advantages when used an exhaust aftertreatment system with a
switchback type configuration.
[0032] Despite the reductant decomposition and mixing system 45, in
certain implementations, the SCR system 40 includes a first or
initial mixer 106 positioned proximate an inlet end of the
decomposition tube 18. The mixer 106 is configured to facilitate an
initial mixing of injected reductant and exhaust gas flowing
through the exhaust tube 16. In some implementations, the reductant
injection and decomposition system 45 sufficiently mixes,
decomposition, and distributes the reductant spray (e.g., urea) and
exhaust gas that a dedicated first mixer 106 is not necessary.
However, without the reductant decomposition and mixing system 45,
the first mixer 106 does not sufficiently mix, promote
decomposition, and promote wide distribution to achieve adequate
NO.sub.x conversion efficiency within the SCR catalyst.
[0033] In the reductant decomposition tube 16, reductant injected
by the injector 46 mixes with exhaust gas flowing from the upstream
section 17 of the exhaust tube 16 and decomposes to gaseous ammonia
prior to being introduced, with the exhaust gas, into the SCR
catalyst 42. The gaseous ammonia reacts with NO in the presence of
the SCR catalyst 42 to reduce NO in the exhaust gas to less harmful
emissions. To facilitate reductant decomposition (e.g., urea to
ammonia), mixing of reductant and exhaust gas, and more uniform
distribution of reductant within the exhaust gas, the reductant
decomposition and mixing system 45 includes at least one of a mixer
50, a closed-end perforated tube 60, and a perforated flange plate
70.
[0034] Referring to FIG. 2, the mixer 50 includes a plurality of
blades 56 configured to induce a vortical swirling of the exhaust
and reductant mixture about a central axis 51 of the mixer. The
mixer 50 also includes concentric inner and outer rings or tubes
52, 53. The inner ring 52 defines a fluid conduit 54 through which
a portion of the reductant and exhaust mixture is flowable. The
outer ring 53 is substantially removed from FIGS. 1 and 2 for
convenience in showing the configuration of the blades 56. In some
implementations, the outer ring 53 is configured similar to the
inner ring 52, but with a larger inner diameter. In some
implementations, a radius of the fluid conduit 54 defined by the
inner ring 52 is about half, or more than half of, the radius of
the outer ring 53. The blades 56 are coupled to and positioned
between the inner and outer rings 52, 53. Each blade 56 includes a
leading edge 90, trailing edge 92, radially inner edge 94, and
radially outer edge 96. In the illustrated embodiment, the radially
inner edges 94 of the blades 56 are fixed to the inner ring 52 and
the radially outer edges 96 are fixed to the outer ring 53.
However, in some embodiments, the outer ring 53 is omitted and the
outer edges 96 of the blades 56 are fixed or coupled directly to an
inner surface of the decomposition tube 18. The leading edges 90
are positioned upstream of the trailing edges 92 when installed in
the SCR system 40 with the exhaust and reductant mixture flowing
into the decomposition and mixing system 45 as indicated by
directional arrow 13 (see FIG. 1). The blades 56 define curved
fluid conduits 58 between adjacent blades.
[0035] Generally, the blades 56 are curved in a direction from the
leading edge 90 to the trailing edge 92, and curved in a direction
from the inner edge 94 to the outer edge 96. The inner and outer
edges 94 are substantially non-parallel to the central axis 51 of
the mixer 50. For example, as shown in FIG. 4, the inner and outer
edges 94, 96 form respective angles (e.g., angles of incidence)
.theta., .beta. with the central axis 51 at the respective
junctions A, B with the leading edge 90. In some implementations,
the angle .theta. defined between the inner edge 94 and the central
axis 51 at the junction A is between about 30.degree. and about
50.degree.. In one specific implementation, the angle .theta. is
about 43.degree.. In some implementations, the angle .beta. defined
between the outer edge 96 and the central axis 51 at the junction B
is between about 50.degree. and about 70.degree.. In one specific
implementation, the angle .beta. is about 61.degree.. The angles of
incidence defined between the inner and outer edges 94, 96 and the
central axis 51 at the junction with the trailing edge 92 are
greater than the respective angles .theta., .beta. due to the
blades 56 curving about an axis extending perpendicular to the
central axis 51.
[0036] The swirling promotes mixing of the reductant and exhaust
gas, as well as increases the residence time, which leads to
increased reductant decomposition. More specifically, the vortical
or swirling pattern of exhaust effectuated by the mixer 50
effectively increases the distance the exhaust must travel relative
to linear flow through a conventional exhaust tube having the same
axial length. Because the exhaust is forced to travel a longer
distance, the residence time for decomposing the reductant into
ammonia is increased. The vortical pattern induced by the mixer 50
also improves the mixing of the reductant and/or ammonia with the
exhaust gas. However, vortical swirling patterns tend to create a
low pressure zone about which the swirling reductant and exhaust
gas mixture flows. Such low pressure zones produce a vacuum effect
that radially inwardly draws the circulating or swirling mixture.
In some applications, it may be desirable to force the exhaust gas
and reductant mixture radially outwardly to improve the special
distribution of the mixture across the SCR catalyst face 43.
Accordingly, in such implementations, the presence of a low
pressure zone to draw the mixture radially inwardly may be
undesirable. Additionally, centralized low pressure zones induced
by vertical swirling patterns can increase the backpressure within
the exhaust system, which may lead to undesirable effects, such as
reduced engine performance and fuel efficiency.
[0037] In addition to providing structural stability, the inner
ring 52 and associated conduit 54 are designed to prevent or at
least reduce the likelihood of the formation of centralized low
pressure zones within the exhaust and reductant mixture. As the
mixture flows through the mixer 50, a first portion of the mixture
(e.g., at the radially outward regions of the mixture stream)
engages the blades 56, which induces a swirling pattern in the
first portion. A second portion of the mixture (e.g., at the
radially inward regions of the mixture stream) flows through the
conduit 54. The second portion of the mixture remains relatively
unaffected by the blades 56 and thus maintains a substantially
linear, or non-swirling, flow pattern. Accordingly, the mixer 50
manipulates a portion of the exhaust and reductant mixture to swirl
around a relatively linearly directed flow at relatively high
pressure. The relatively high pressure of the central flow coming
out of the mixer 50 counters the tendency of the flow swirling
about the central flow to form a low pressure zone. Accordingly,
the swirling flow remains within a radially outward region of the
mixture stream exiting the mixer, which increases the decomposition
residence time and mixing while improving distribution uniformity
of the mixture at the face 43 of the SCR catalyst 42, and the
formation of low pressure zones is avoided, which helps reduce
backpressure within the exhaust system 10.
[0038] Enhanced decomposition, mixing, and distribution of the
reductant and exhaust mixture can be facilitated through use of the
closed-end perforated tube 60 positioned downstream of the mixer
50. As shown in FIGS. 1 and 5, the closed-end or plugged perforated
tube 60 includes a sidewall 62 extending between an open upstream
end 67 and a closed downstream end 66. The closed end 66 prevents
the portion of the exhaust and reductant mixture flowing in the
direction 13 from directly impacting the face 43 of the SCR
catalyst 42. The closed end 66 includes an impact surface that is
substantially perpendicular to the exhaust flow direction 13.
Referring to FIG. 6, the closed end 66 may be a plug or flange 66
that is seated and fixed to a downstream end of the tube 60.
Generally, the plug 66 redirects the mixture flow 13 radially
outward as shown in FIG. 6. In this manner, not only is the
relatively large concentration of the mixture about a central
region dispersed or redistributed radially outwardly to improve the
distribution uniformity of the mixture, but damage due to the
momentum of the large concentration of the mixture moving in the
axial direction 13 directly impacting the face 43 of the SCR
catalyst 42 is reduced. To facilitate radially outward redirection
of the mixture, the plug 66 may include a contoured end surface 68.
As shown, the end surface 68 may be substantially convex.
[0039] Radially outward redirection of the reductant and exhaust
mixture is further facilitated via a perforation pattern 64 formed
in the sidewall 62 of the tube. The perforation pattern 64 includes
a plurality of perforations or openings 65 arranged along the
sidewall 62 in a desirable pattern. The perforations 65 are
through-holes that allow the mixture entering the tube 60 to exit
the tube and enter a cavity of the housing 41 in which the SCR
catalyst 42 is housed. The perforation pattern 64 can include any
number of perforations 65 having any of various shapes and sizes,
and arranged in any of various patterns, as desired. For example,
in the illustrated implementation, the perforation pattern 64
includes axially aligned rows of perforations 65 each having the
same size.
[0040] In operation, reductant and exhaust mixture flows into the
tube 60 as indicated by directional arrow 13. As indicated with
directional arrows, the mixture is radially outwardly redirected by
virtue of the closed end 66 of the tube 66. The radially outwardly
directed flow passes through the perforations 65 of the perforation
pattern 64 into a radially outward region of the cavity defined by
the housing 41. The redirection of the mixture not only increases
the residence time of the mixture, but also further mixes the
mixture and distributes the mixture to the outer peripheries of the
housing away from a centralized region, which improves the
distribution uniformity of the mixture.
[0041] Again referring to FIGS. 1 and 5, the perforated flange
plate 70 couples a portion of the plugged or perforated tube 60 to
the sidewalls of the housing 41. Accordingly, the perforated flange
plate 70 acts to assist in holding the tube 60 in place during use.
The flange plate 70 is a generally annular disk with a plurality of
openings 72 spaced about the disk. A radially outer periphery of
the plate 70 is fixed to a wall of the housing 41, and a radially
inner periphery of the plate is fixed to the tube 60 (e.g., the
closed end 66 of the tube). The plurality of openings 72, which can
be any number of variously sized, shaped, and positioned openings,
are positioned between the inner and outer peripheries. Generally,
the openings 72 are larger than the openings 65 formed in the tube
60. The openings 72 act to at least partially concentrate
respective portions of the mixture exiting the tube 60 into
radially outer regions of the housing 41. In this manner, the
residence time, mixing, and distribution of the mixture is further
enhanced. After passing through the openings 72 in the flange plate
70, the mixture is allowed to expand and migrate radially inwardly
such that a high distribution uniformity of the mixture is achieved
at the face 43 of the SCR catalyst 42.
[0042] In the illustrated embodiments, the SCR system 40 includes
the mixer 50, tube 60, and flange plate 70. However, in some
embodiments, the SCR system includes fewer than all three of the
mixer 50, tube 60, and flange plate 70. For example, in one
embodiment, the SCR system may include only the mixer 50, without
the tube 60 and flange plate 70. In yet another embodiment, the SCR
system may include the tube 60, without the mixer 50, and with or
without the flange plate 70.
[0043] According to some embodiments, the mixer 50, which can be a
secondary mixer as described above, tube 60, and flange plate 70
are manufactured or formed separately from the decomposition tube
16 and SCR housing 41. The mixer 50 can be relatively easily
coupled to the inside of the decomposition tube 16 via a coupling
technique, such as welding. Similarly, the flange plate 70 can be
coupled to the interior of the SCR housing 41 upstream of the SCR
catalyst 42 via a coupling technique, such as welding. As shown in
FIG. 1, in certain embodiments, an entirety of the exhaust gas
flowing through the decomposition tube 16 (and, for example, the
DOC 20 and DPF 30) passes through the mixer 50, the tube 60, and
the flange plate 70.
[0044] In the above description, certain terms may be used such as
"up," "down," "upper," "lower," "horizontal," "vertical," "left,"
"right," and the like. These terms are used, where applicable, to
provide some clarity of description when dealing with relative
relationships. But, these terms are not intended to imply absolute
relationships, positions, and/or orientations. For example, with
respect to an object, an "upper" surface can become a "lower"
surface simply by turning the object over. Nevertheless, it is
still the same object. Further, the terms "including,"
"comprising," "having," and variations thereof mean "including but
not limited to" unless expressly specified otherwise. An enumerated
listing of items does not imply that any or all of the items are
mutually exclusive and/or mutually inclusive, unless expressly
specified otherwise. The terms "a," "an," and "the" also refer to
"one or more" unless expressly specified otherwise. Further, the
term "plurality" can be defined as "at least two."
[0045] Additionally, instances in this specification where one
element is "coupled" to another element can include direct and
indirect coupling. Direct coupling can be defined as one element
coupled to and in some contact with another element. Indirect
coupling can be defined as coupling between two elements not in
direct contact with each other, but having one or more additional
elements between the coupled elements. Further, as used herein,
securing one element to another element can include direct securing
and indirect securing. Additionally, as used herein, "adjacent"
does not necessarily denote contact. For example, one element can
be adjacent another element without being in contact with that
element.
[0046] The subject matter of the present disclosure may be embodied
in other specific forms without departing from its spirit or
essential characteristics. The described embodiments are to be
considered in all respects only as illustrative and not
restrictive. The scope of the invention is, therefore, indicated by
the appended claims rather than by the foregoing description. All
changes which come within the meaning and range of equivalency of
the claims are to be embraced within their scope.
* * * * *