EP2609996A1

EP2609996A1 - Mixing device

Info

Publication number: EP2609996A1
Application number: EP12199527.8A
Authority: EP
Inventors: Takashi Hashima
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2011-12-28
Filing date: 2012-12-27
Publication date: 2013-07-03
Also published as: JP2013136975A

Abstract

An oblique side (40) that is inclined so as to be closer to an exhaust gas upstream side nearer a low pressure region (51) is formed on a downstream end portion (37) of at least one blade (32), from among a plurality of blades (32) that form a mixing device.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a mixing device that is provided farther downstream in an exhaust pipe of an internal combustion engine than an adding valve that sprays a liquid additive into the exhaust pipe, and that mixes exhaust gas with the additive and leads the mixture toward an exhaust gas downstream side (i.e., downstream with respect to the direction in which exhaust gas flows).

2. Description of the Related Art

An exhaust gas control apparatus that purifies nitrided oxide (NOx) in exhaust gas by reducing the NOx to water and nitrogen is provided in an internal combustion engine. In such an exhaust gas control apparatus, a liquid additive that is a urea aqueous solution is sprayed from an adding valve into an exhaust pipe, and this additive is supplied to an exhaust gas control catalyst arranged on an exhaust gas downstream side (i.e., downstream with respect to the direction in which exhaust gas flows) of the adding valve. In this specification, "exhaust gas upstream side" refers to upstream with respect to the direction in which exhaust gas flows and may simply be referred to as "upstream", and "exhaust gas downstream side" refers to downstream with respect to the direction in which exhaust gas flows and may simply be referred to as "downstream". Also, a mixing device that is arranged between the adding valve and the exhaust gas control catalyst in the exhaust pipe is provided in the exhaust gas control apparatus (see Japanese Patent Application Publication No. 2008-274941 ( JP 2008-274941 A )).
As shown in FIGS. 7 and 8, the mixing device described in JP 2008-274941 A includes a plurality of blades 110 aligned in a circumferential direction on an inner wall of an exhaust pipe, and inclined at predetermined angles θ1 and θ2 in one direction with respect to a radial direction, within a virtual plane (a plane that is parallel to the surface of the paper on which the drawings are drawn) that is orthogonal to the flow direction of exhaust gas inside the exhaust pipe (i.e., a direction orthogonal to the surface of the paper). FIG. 8 is a view showing a frame format of a blade 110A and a blade 110B positioned on both sides sandwiching a center 100 of a virtual plane, when viewed from the center 100.
Also, a region that includes the center 100 of the virtual plane is a low pressure region 101 where the blades 110 that become flow resistance to the exhaust gas are not arranged. Also, some of the exhaust gas that flows inside of the exhaust pipe is led from upstream to downstream through this low pressure region 101, and the rest is led from upstream to downstream between the blades 110 that are adjacent to each other in the circumferential direction.
Exhaust gas that flows into the low pressure region 101 from upstream, flows downstream through the low pressure region 101 while maintaining substantially the same flowrate. On the other hand, the flowrate of exhaust gas that flows from upstream through a region to the radial outside of the low pressure region 101, i.e., a region where the plurality of blades 110 are positioned (hereinafter, also referred to as an "outer region 102"), slows because the blades 110 end up acting as flow resistance. Also, downstream of the mixing device, mixing of the exhaust gas and the urea aqueous solution is promoted by a flow created by the exhaust gas and the urea aqueous solution passing through the low pressure region 101, and a rotating flow created by the exhaust gas and the urea aqueous solution passing through the outer region 102.
Then, moisture in the urea aqueous solution mixed in with the exhaust gas vaporizes by absorbing heat from the exhaust gas. When this occurs, the urea in the urea aqueous solution and the moisture react with each other and hydrolyze, producing ammonia gas. This ammonia gas flows downstream and is taken into the exhaust gas control catalyst, together with the exhaust gas. As a result, in the exhaust gas control catalyst, the nitrided oxide in the exhaust gas is reduced to water and nitrogen by the ammonia gas.
As shown in FIG. 9, droplets 200 form by some of the urea aqueous solution that flows downstream with the exhaust gas adhering to a surface 111 of the blades 110 of the mixing device. These droplets 200 move from an upstream end portion 112 side of the blades 110 (i.e., the left end portion side in FIG. 9) toward a downstream end portion 113 side (i.e., the right end portion side in FIG. 9), and reach the downstream end portion 113. Even if the droplets 200 reach the downstream end portion 113, surface tension created between the droplets 200 and the blades 110 acts on the droplets 200, so the droplets 200 that have reached the downstream end portion 113 are not easily blown off downstream (i.e., to the right in FIG. 9).
Here, as described above, there are no blades that would become flow resistance to the exhaust gas in the low pressure region 101, so the transit velocity of the exhaust gas passing through the low pressure region 101 is faster than the transit velocity of the exhaust gas passing through the outer region 102. As a result, when the exhaust gas passes through the mixing device, the pressure at the low pressure region 101 will be lower than the pressure at the outer region 102. Therefore, at the outer region 102, a flow that drifts toward the low pressure region 101 is also created.
This flow in turn causes force that drifts toward the low pressure region 101 (hereinafter, this force will also be referred to as "pull") to act on the droplets 200 moving from the upstream end portion 112 side to the downstream end portion 113 side on the surface 111 of the blades 110. When this happens, the droplets 200 that have reached the downstream end portion 113 of the blades 110 move along the downstream end portion 113 toward the low pressure region 101 by this pull, as shown by the arrows in FIG. 9. As a result, a large amount of droplets collects on an end portion 113a of the downstream end portion 113 of the blades 110 that is on the side near the low pressure region 101, and consequently, the droplets 200 grow larger at this end portion 113a.
Even if the droplets 200 that have grown in this way are blown off of the blades 110 toward the downstream side against the surface tension created with the blades 110, the greater the mass of the droplets 200 is, the less easily the droplets 200 will vaporize. Therefore, there is a possibility that the moisture of the droplets 200 will be taken into the exhaust gas control apparatus without vaporizing. If the urea is taken into the exhaust gas control catalyst as liquid in this way, the amount of ammonia gas produced inside the exhaust pipe will be that much less. As a result, there is a possibility that the reduction efficiency of the nitrided oxide in the exhaust gas (i.e., the efficiency with which the nitrided gas in the exhaust gas is reduced) may decrease.

SUMMARY OF THE INVENTION

The invention thus provides a mixing device capable of inhibiting droplets adhered to a blade from growing.
Hereinafter, means and operation and effects thereof will be described. A first aspect of the invention relates to a mixing device that is provided farther downstream in an exhaust pipe of an internal combustion engine than an adding valve that sprays a liquid additive into the exhaust pipe, and that includes a plurality of blades that are arranged in a circumferential direction on an inner wall of the exhaust pipe and are inclined in one direction with respect to a radial direction, within a virtual plane that is orthogonal to a flow direction of exhaust gas inside of the exhaust pipe. In this mixing device, an oblique side that is inclined so as to be closer to an exhaust gas upstream side nearer a center of the virtual plane is formed on an end portion, on an exhaust gas downstream side, of at least one of the blades.
According to this structure, the flow direction of the exhaust gas changes due to the blades of the mixing device, and as a result, a rotating flow of exhaust gas is created downstream of the mixing device in the exhaust pipe. Here, some of the liquid additive that has been sprayed into the exhaust pipe from the adding valve adheres to the surface of the blades and forms droplets. In addition to force that moves the droplets along the surface of the blades from the exhaust gas upstream side toward the exhaust gas downstream side, force that pulls the droplets (hereinafter, also referred to as "pull") toward the center of the virtual plane that is orthogonal to the flow direction of the exhaust gas acts on the additive that has adhered to the surface of the blades and formed droplets in this way. As a result, when the additive that has adhered to the surface of the blades and formed droplets moves along the surface of the blades from the exhaust gas upstream side toward the exhaust gas downstream side, the additive moves toward the low pressure region that includes the center of the virtual plane in response to the pull. Therefore, in order to inhibit droplets that have adhered to the surface of the blades from collecting on an end portion on the low pressure region side of the end portion on the exhaust gas downstream side of the blades, droplets that have reached the end portion on the exhaust gas downstream side of the blades are preferably kept from moving along the end portion on the exhaust gas downstream side toward the low pressure region.
In order to inhibit droplets that have reached the end portion on the exhaust gas downstream side of the blades from coming near the low pressure region, a movement direction of the droplets that have moved along the surface of the blades to the end portion on the exhaust gas downstream side (hereinafter, this movement direction may also be referred to as the "droplet movement direction") is preferably orthogonal to an extending direction of a tangent line in which a position reached by the droplets on the end portion on the exhaust gas downstream side is the tangent point (hereinafter, this direction may also be referred to as the "tangential direction"). In this case, droplets that have reached the end portion on the exhaust gas downstream side will no longer move along the end portion on the exhaust gas downstream side toward the low pressure region. Therefore, droplets that have reached the end portion on the exhaust gas downstream side are inhibited from collecting on the end portion on the low pressure region side of the end portion on the exhaust gas downstream side of the blades, and consequently, the droplets on the blades are inhibited from growing. Also, when the droplet movement direction is orthogonal to the tangential direction, the force applied to the droplets in the droplet movement direction acts on the droplets as force that blows the droplets off of (i.e., so that they separate from) the exhaust gas downstream side of the blades toward the exhaust gas downstream side (hereinafter, this force may also be referred to as "separation force"). Therefore, the separation force for blowing the droplets off of the blades tends to become greater than the surface tension created between the droplets and the blades. That is, the droplets adhered to the blades are able to be blown off of the blades before they (i.e., the droplets) grow large.
However, the droplet movement direction ends up changing as a result of a change in the flowrate of the exhaust gas that flows through the exhaust pipe, and the like. Therefore, realistically it is extremely difficult to make the droplet movement direction orthogonal to the tangential line, regardless of the flowrate of the exhaust gas and the like.
Thus, in the invention, an oblique side that is inclined so as to be closer the exhaust gas upstream side nearer the center within a virtual plane is formed on an end portion on the exhaust gas downstream side of at least one of the blades. As a result, the angle formed between the droplet movement direction of the droplets that have moved along the surface of the blades to the oblique side of the end portion on the exhaust gas downstream side, and the tangential direction that is an extending direction of the tangent line in which the position reached by the droplets on the oblique side is the tangent point approaches a right angle. Accordingly, the separation force for blowing the droplets off of the end portion on the exhaust gas downstream side of the blades is able to be increased. As a result, the droplets adhered to the blades are able to be blown off of, and thus separated from, the blades before they (i.e., the droplets) grow large. Therefore, droplets adhered to the blades are able to be inhibited from growing. Also, the amount of liquid additive that is taken as a liquid into the exhaust gas control catalyst that is arranged to the exhaust gas downstream side of the mixing device is reduced.
Also, in the aspect described above, the oblique side may be inclined at a steeper inclination nearer the center of the virtual plane.
The pull that acts on the droplets that have reached the end portion on the exhaust gas downstream side of the blades is larger nearer the low pressure region. Therefore, in the structure described above, the oblique side is formed so as to be inclined toward the exhaust gas upstream side at an increasingly steeper inclination nearer the low pressure region. Accordingly, the difference between the separation force at a position close to the low pressure region and the separation force at a position away from the low pressure region on the oblique side is less than it is in a case in which the oblique side is inclined toward the exhaust gas upstream side at a constant inclination regardless of the distance from the low pressure region. As a result, at the end portion on the exhaust gas downstream side of the blades, droplets tend to be blown off toward the exhaust gas downstream side even from positions close to the low pressure region. Therefore, droplets adhered to the blades are able to be inhibited from growing.
Also, in the structure described above, a plurality of arc regions formed with different radii may be provided continuous on the oblique side, and the radii of the arc regions may be smaller nearer the center of the virtual plane.
According to this structure, the radii of the arc regions are smaller nearer the low pressure region. Therefore, the difference between the separation force at a position close to the low pressure region and the separation force at a position away from the low pressure region on the oblique side is less. As a result, on the oblique side, droplets tend to be blown off to the exhaust gas downstream side even from a position close to the low pressure region, so droplets are able to be more easily separated from the blades having the oblique side.
Also, in the structure described above, the oblique side may have a shape that is similar to a peripheral border of a simple ellipsoid.
Also, in the aspect described above, the oblique side may be inclined at a constant inclination.
According to the structure described above, droplets will not easily collect on the end portion on the low pressure region side of the end portion on the exhaust gas downstream side of the blades, so droplets adhered to the blades are able to be inhibited from growing.
In the structure described above, the oblique side may be formed on all of the plurality of blades. According to this structure, compared with when an oblique side is formed only on the end portion on the exhaust gas downstream side of a portion (i.e., one or some) of the blades, droplets are inhibited from growing on all of the blades, so large droplets are able to be inhibited from being blown off of the mixing device toward the exhaust gas downstream side.
In the structure described above, an end portion on an exhaust gas upstream side of at least one of the blades may be inclined so as to be closer to the exhaust gas downstream side nearer the center of the virtual plane.
It is thought that the flowrate of exhaust gas that flows through the exhaust pipe from the exhaust gas upstream side to the exhaust gas downstream side on the radially inner side in the exhaust pipe is faster, i.e., the flow volume of the exhaust gas on the radially inner side in the exhaust pipe is larger, than that on the radially outer side in the exhaust pipe. Therefore, with this structure, the end portion on the exhaust gas upstream side of at least one blade is formed inclined so as to be closer to the exhaust gas downstream side nearer the center of the virtual plane. Accordingly, the flow resistance at the inside in the exhaust pipe where the flow volume of the exhaust gas is large is less. As a result, the discharge efficiency of the exhaust gas with an exhaust pipe in which the mixing device is arranged is able to be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, advantages, and technical and industrial significance of this invention will be described in the following detailed description of example embodiments of the invention with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a view showing a frame format of an exhaust gas control apparatus provided with a mixing device according to one example embodiment of the invention;
FIG. 2A is a plan view of the mixing device viewed from a downstream side;
FIG. 2B is a perspective view of a portion of the mixing device;
FIG. 3 is an action diagram showing the manner in which exhaust gas and urea aqueous solution passes through the mixing device;
FIG. 4 is an action diagram showing the manner in which a droplet reaches a downstream end portion of a blade;
FIG. 5 is a view showing a frame format of a blade according to another example embodiment;
FIG. 6 is a view showing a frame format of a blade according to yet another example embodiment;
FIG. 7 is a front view showing a frame format of a related mixing device;
FIG. 8 is a view showing a frame format of blades of the related mixing device viewed from the center of a virtual plane; and
FIG. 9 is an action diagram showing the manner in which exhaust gas and urea aqueous solution pass through the related mixing device.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, an example embodiment of the invention will be described with reference to FIGS. 1 to 4. In this example embodiment, a vertical direction in FIG 1 matches the direction of gravitational force. As shown in FIG. 1, an exhaust gas control apparatus 20 of an internal combustion engine is an apparatus that reduces nitrided oxide (NO_x) in exhaust gas that flows inside an exhaust pipe 11, into water and nitrogen. The exhaust gas control apparatus 20 is provided with an adding valve 21 that sprays urea aqueous solution, that serves as a liquid reducing agent, toward an exhaust gas downstream side (i.e., downstream with respect to the direction in which exhaust gas flows; hereinafter also simply referred to as "downstream") (to the right in FIG. 1) in the exhaust pipe 11, and a supply pump 23 that operates to supply urea aqueous solution stored in a storage tank 22 to the adding valve 21.
Also, a mixing device 25 that mixes together exhaust gas that flows from an exhaust gas upstream side (i.e., upstream with respect to the direction in which exhaust gas flows; hereinafter also simply referred to as "upstream") to downstream (i.e., front left to right in FIG. 1) and the urea aqueous solution sprayed by the adding valve 21, is provided downstream of the adding valve 21 in the exhaust pipe 11. This mixing device 25 also serves to broaden the area across which the urea aqueous solution that has been sprayed by the adding valve 21 is dispersed.
Moisture in the urea aqueous solution that has been mixed with (i.e., added to) the exhaust gas in this way is vaporized as a result of absorbing heat from exhaust gas that flows with it in the exhaust pipe 11. When this happens, the urea in the urea aqueous solution and the water vapor produced by the evaporation of the moisture react and hydrolyze, producing ammonia gas. The ammonia gas produced in this way flows downstream with the exhaust gas.
An exhaust gas control catalyst 27 that takes in the exhaust gas and the ammonia gas is provided downstream of the mixing device 25 in the exhaust pipe 11. Then, a nitrogen compound in the exhaust gas taken into the exhaust gas control catalyst 27 is reduced to water and nitrogen by the ammonia gas taken into the exhaust gas control catalyst 27 together with the exhaust gas.
Next, the mixing device 25 of this example embodiment will be described with reference to FIGS. 2 and 3. FIG. 2A is a plan view of the mixing device 25 viewed from the downstream side. As shown in FIGS. 2A and 2B, a main body portion 30 that has a generally cylindrical shape about a central line 11a of the exhaust pipe 11 is provided in the mixing device 25. A flange 31 for attaching the mixing device 25 to the exhaust pipe 11 is provided on the upstream side of the main body portion 30. Also, a plurality of blades 32 protrude inward from the downstream side of the main body portion 30.
In this example embodiment, the main body portion 30 and the blades 32 are formed by plates oblong plates. That is, a plurality of inclined slits that intersect the short direction of the plates are formed at substantially equal intervals on one end side in the short direction of the plates. Then one end portion is overlapped with another end portion in the long direction of the plates, and strip-shaped portions created by the plurality of slits are folded inward. As a result, the cylindrical-shaped main body portion 30 and the plurality of blades 32 are formed.
These blades 32 are aligned in a circumferential direction of an inner wall 11b (see FIG. 1) of the exhaust pipe 11, and are inclined in one direction (i.e., the same direction) with respect to a radial direction, within a virtual plane (in FIG. 2A, a plane parallel to the surface of the paper on which FIG. 2A is drawn) that is orthogonal to the direction in which exhaust gas flows inside the exhaust pipe 11. Therefore, one end portion in the circumferential direction of the blades 32 is positioned farther upstream than the other end portion in the circumferential direction of the blades 32, and a surface 38 of the blades 32 is inclined with respect to the direction in which exhaust gas flows inside the exhaust pipe 11.
In this example embodiment, the upstream end portion of the blades 32 will be referred to as "upstream end portion 36", and the downstream end portion of the blades 32 will be referred to as "downstream end portion 37". Also, to facilitate understanding, the surface 38 of the blades 32 is marked with dots in FIG. 2B.
When exhaust gas passes through the region where the blades 32 are provided in the mixing device 25 (hereinafter, this region will be referred to as the "outer region 50"), the flow direction of the exhaust gas is changed by the surface 38 of the blades 32, and the exhaust gas flows from the upstream side to the downstream side along the surface 38. As a result, a rotating flow of the exhaust gas that rotates about the central line 11a is created downstream in the mixing device 25.
Also, a low pressure region 51 that includes a center 34 of the virtual plane and that is surrounded by tip ends 32a of the blades 32 is formed in the center of the mixing device 25. There are no blades 32 that would become flow resistance to the exhaust gas provided in this low pressure region 51. Therefore, the transit velocity of the exhaust gas that passes from upstream to downstream through this low pressure region 51 becomes faster than the transit velocity of the exhaust gas that passes from upstream to downstream through the outer region 50. Also, a flow from upstream to downstream is created downstream in the mixing device 25 by the exhaust gas that passes through the low pressure region 51. As a result, mixing of the exhaust gas and the urea aqueous solution that flows with the exhaust gas is promoted by the rotating flow created by the exhaust gas passing through the outer region 50, and the flow created by the exhaust gas passing through the low pressure region 51.
As shown in FIG. 3, an inner portion provided in a position on the side near the low pressure region 51, of the downstream end portion 37 of the blades 32 in this example embodiment is an oblique side 40 that is increasingly inclined toward the upstream side nearer the low pressure region 51. Also, the upstream end portion 36 of the blades 32 is inclined so as to be closer the downstream side nearer the low pressure region 51. That is, the width (i.e., the length in the circumferential direction) of the blades 32 becomes narrower nearer the low pressure region 51.
A plurality of (three in this example embodiment) arc regions 41A, 41B, and 41C with different radii are formed on the oblique side 40. These arc regions 41A, 41B, and 41C are provided continuous. In this example embodiment, the oblique side 40 that includes the arc regions 41A, 41B, and 41C is formed on the downstream end portion 37 of the blades 32 by cutting the portion indicated by the alternate long and two short dashes line in FIG. 3.
Of the arc regions 41A, 41B, and 41C, the curvature radius of the first arc region 41A positioned in a position farthest away from the low pressure region 51 will be denoted "R1", the curvature radius of the second arc region 41B positioned nearer the low pressure region 51 than the first arc region 41A will be denoted "R2", and the curvature radius of the third arc region 41C positioned in a position nearer the low pressure region 51 than the second arc region 41B and closest to the low pressure region 51 will be denoted "R3". At this time, the relationship among these curvature radii "R1", "R2", and "R3" is one in which "R1" > "R2" > "R3" is satisfied. That is, the oblique side 40 in this example embodiment is formed inclined toward the upstream side at an increasingly steeper inclination nearer the low pressure region 51.
Next, the operation when the exhaust gas and the urea aqueous solution pass through the mixing device 25 will be described with reference to FIGS. 3 and 4. Some of the exhaust gas that flows inside the exhaust pipe 11 passes through the low pressure region 51 positioned in the center of the mixing device 25, while the rest passes through the outer region 50. As described above, there are no blades 32 that would become flow resistance to the exhaust gas provided in the low pressure region 51. Therefore, the transit velocity of the exhaust gas that passes from upstream to downstream through the low pressure region 51 becomes faster than the transit velocity of the exhaust gas that passes from upstream to downstream through the outer region 50. As a result, the pressure inside the low pressure region 51 is lower than the pressure in the outer region 50. Therefore, force that drifts toward the low pressure region 51 (hereinafter, this force may also be referred to as "pull") acts in the outer region 50 by differential pressure between the pressure in the low pressure region 51 and the pressure in the outer region 50.
Also, when exhaust gas passes through the outer region 50, the exhaust gas passes through a gap between the blades 32 that are adjacent to each other in the circumferential direction. At this time, some of the urea aqueous solution that flows through the exhaust pipe 11 with the exhaust gas may adhere to the surface 38 of the blades 32. Droplets formed by the urea aqueous solution adhered to the surface 38 of the blades 32 in this way move from the upstream end portion 36 toward the downstream end portion 37 of the blades 32 with the flow of exhaust gas, as indicated by the arrows in FIG. 3.
As described above, pull acts in the outer region 50. Therefore, pull is applied to the droplets adhered to the blades 32. This pull becomes stronger nearer the low pressure region 51 even in the outer region 50. As a result, droplets heading toward the oblique side 40 of the downstream end portion 37 of the blades 32 gradually drift toward the low pressure region 51 side in the process of moving from the upstream end portion 36 toward the downstream end portion 37.
Also, droplets that have reached the downstream end portion 37 of the blades 32 move along the downstream end portion 37 toward the low pressure region 51 by this pull. However, the oblique side 40 that is inclined so as to be closer to the upstream side nearer the low pressure region 51 is formed on the downstream end portion 37 in this example embodiment. As a result, as shown in FIG. 4, an angle θ formed between a movement direction of a droplet 60 that moves along the surface 38 of the blade 32 to the oblique side 40 (i.e., the direction indicated by the dashed line in FIG. 4; hereinafter this direction may also be referred to as the "droplet movement direction"), and an extending direction of a tangent line with a position reached by the droplet 60 on the oblique side 40 as the tangent point (hereinafter, this direction may also referred to as the "tangential direction") approaches a right angle. As a result, force that blows the droplet 60 off of (i.e., away from) the oblique side 40 of the blade 32 (hereinafter, this force will be referred to as the "separation force A") increases. This separation force A is a component force that acts in a direction orthogonal to the tangential direction, of the force that moves the droplet 60 to the oblique side 40. That is, the separation force A becomes greater as the angle θ formed between the droplet movement direction and the tangential direction approaches a right angle. Accordingly, the force that moves the droplet 60 on the surface 38 of the blade 32 is effectively utilized to blow the droplet 60 off of the blade 32 (such that the droplet 60 separates from the blade 32), by the angle θ formed between the droplet movement direction and the tangential direction approaching a right angle. Therefore, the droplet 60 adhered to the blade 32 is more easily blown off of (i.e., separates from) the blade 32 before it (i.e., the droplet 60) grows larger on the blade 32.
Moreover, the three arc regions 41A, 41B, and 41C are formed such that the curvature radius is smaller in arc regions positioned nearer the low pressure region 51, on the oblique side 40 in this example embodiment. Therefore, on the oblique side 40, the difference in the angle θ formed between the droplet movement direction and the tangential direction at a position away from and a position close to the low pressure region 51 will not be large. That is, a difference between the separation force A at a position close to the low pressure region 51 where the pull B is large, and the separation force A at a position away from the low pressure region 51 where the pull B is small, will not be very large. Therefore, on the oblique side 40, the droplet 60 that has reached the oblique side 40 is easily blown off of the blade 32 against the surface tension before it grows on the blade 32, both at a position that is close to the low pressure region 51 and a position that is away from the low pressure region 51.
In this example embodiment, as described above, the droplet 60 before growing large is blown off of the blade 32 toward the downstream side. Therefore, the moisture included in the droplet 60 has been blown off of the blade 32 vaporizes more easily before being taken into the exhaust gas control catalyst 27, on account of the smaller mass of the droplet 60 that is blown off of the blade 32. As a result, the reaction amount of the water vapor and the urea in the droplet 60 is larger, so the amount of ammonia gas that is produced is larger. Therefore, the amount of ammonia gas taken into the exhaust gas control catalyst 27 is larger.
Also, a droplet that is not easily blown off downstream even after reaching the oblique side 40 of the blades 32 will move along the oblique side 40 toward the low pressure region 51 by the pull B. This pull B is a component force that acts in a direction parallel to the tangential direction, of the force that moves the droplet 60 to the oblique side 40, as shown FIG. 4. That is, the pull B becomes smaller as the angle θ formed between the droplet movement direction and the tangential direction approaches a right angle. Therefore, the droplet 60 that has reached the oblique side 40 slowly approaches the low pressure region 51 the closer the angle θ formed between the droplet movement direction and the tangential direction is to a right angle.
In the process of the droplet 60 slowly approaching the low pressure region 51 along the oblique side 40 in this way; the amount of heat absorbed by the droplet 60 from the blade 32 and the amount of heat absorbed by the droplet 60 from the exhaust gas that passes between the blades 32 that are adjacent to each other in the circumferential direction becomes larger the slower moving velocity of the droplet 60 is. As a result, the moisture in the droplet 60 that has reached the oblique side 40 is more easily vaporized in the process of moving slowly toward the low pressure region 51 side. Therefore, the droplet 60 that moves along the oblique side 40 toward the low pressure region 51 is more likely to disappear (i.e., evaporate) before reaching the end portion on the low pressure region 51 side of the downstream end portion 37. Also, even if the droplet 60 does reach the end portion on the low pressure region 51 side of the downstream end portion 37, the fluid volume of the droplet 60 that has reached the end portion is extremely small.
From this perspective as well, with the blades 32, the droplet 60 is less prone to grow, so the amount of ammonia gas that is produced inside the exhaust pipe 11 will be larger. As a result, the amount of ammonia gas taken into the exhaust gas control catalyst 27 is larger, so the reduction efficiency of nitrided oxide in the exhaust gas increases.
As described above, in this example embodiment, the effects described below are able to be obtained. (1) The oblique side 40 of the downstream end portion 37 of the blade 32 is inclined so as to be closer the upstream side nearer the low pressure region 51. Therefore, the angle θ formed by the droplet movement direction of the droplet 60 that moves along the surface 38 of the blade 32 to the oblique side 40, and the tangential direction that is the extending direction of a tangent line with a position reached by the droplet 60 on the oblique side 40 as the tangent point approaches a right angle. As a result, the separation force A that is force that blows the droplet 60 away from the oblique side 40 of the blade 32 becomes larger the closer the angle θ formed between the droplet movement direction and the tangential direction is to a right angle. As a result, the droplet 60 that has adhered to the blade 32 is more easily blown off of the blade 32 before it grows on the blade 32. Therefore, the droplets 60 tend not to collect on the end portion on the low pressure region 51 side of the downstream end portion 37, and thus the droplets 60 adhered to the blade 32 are able to be inhibited from growing.
(2) The oblique side 40 is formed inclined toward the upstream side at an increasingly steeper inclination nearer the low pressure region 51. More specifically, the curvature radii of the arc regions 41A, 41B, and 41C that form the oblique side 40 are smaller in arc regions in positions nearer the low pressure region 51. Thus, on the oblique side 40, the difference between the separation force A at a position close to the low pressure region 51, where the pull B is large, and the separation force A at a position away from the low pressure region 51, where the pull B is small, will be small. Therefore, on the oblique side 40, the droplet 60 that has reached a position near the oblique side 40 is able to easily be blown off downstream before it grows large. Accordingly, the droplet 60 that is adhered to the blade 32 is able to be inhibited from growing.
(3) Also, as described above, the pull B that is the force that brings the droplet 60 that has reached the oblique side 40 of the blade 32 along the oblique side 40 toward the low pressure region 51 becomes smaller as the angle θ formed between the droplet movement direction and the tangential direction approaches a right angle. As a result, a droplet 60 that is not blown off of the blade 32; from the droplets that have reached the oblique side 40 of the blade 32, will slowly approach the low pressure region 51. Therefore, the amount of heat absorbed by the droplet 60 when the droplet 60 slowly moves along the oblique side 40 toward the low pressure region 51 becomes larger, so the moisture in the droplet 60 vaporizes more easily. From this perspective as well, the droplets 60 tend not to collect on the end portion on the low pressure region 51 side of the downstream end portion 37, and thus the droplets 60 adhered to the blade 32 are able to be inhibited from growing.
(4) Because the droplets 60 are inhibited from growing on the blade 32, droplets 60 that have grown large are not as easily blown downstream off of the blade 32. Therefore, the urea aqueous solution is inhibited from being taken into the exhaust gas control catalyst 27 as it is. As a result, the amount of ammonia gas taken into the exhaust gas control catalyst 27 is larger by the amount of increase in the amount of ammonia gas produced by hydrolysis of urea in the urea aqueous solution. Thus, the reduction efficiency of nitrided oxide in the exhaust gas is able to be improved.
(5) In this example embodiment, the arc regions 41A, 41B, and 41C are formed on the oblique side 40 of the downstream end portion 37, by cutting a portion of rough material (i.e., the blades before the arc regions 41A, 41B, and 41C are formed) that forms the blades 32. Accordingly, the area of the surface 38 of the blades 32 in this example embodiment is narrower than the area of the surface of blades without the arc regions 41A, 41B, and 41C. As a result, the flow resistance with respect to the exhaust gas that flows through the exhaust pipe 11 is able to be reduced. Therefore, the discharge efficiency of the exhaust gas is able to be improved.
(6) Furthermore, in this example embodiment, the upstream end portion 36 of the blades 32 is inclined so as to be closer the downstream side nearer the low pressure region 51. Accordingly, the flow resistance on the center side of the exhaust pipe 11 where the flowrate of exhaust gas is large is able to be even smaller than it is when the up stream end portion 36 is not inclined downstream nearer the low pressure region 51. Therefore, the discharge efficiency of exhaust gas is able to be improved.
The example embodiment may also be modified to other example embodiments as described below. An arbitrary number of arc regions other than three (for example, one or four) may also be provided on the oblique side 40 of the downstream end portion 37 of the blades 32. If a plurality of arc regions are provided, the curvature radii of the arc regions are preferably smaller the closer an arc region is to the low pressure region 51.
As long as the oblique side 40 of the downstream end portion 37 of the blades 32 is shaped so that it inclines toward the upstream side at an increasingly steeper inclination nearer the low pressure region 51, the oblique side 40 may have another appropriate structure other than one that is provided with the plurality of arc regions 41A, 41B, and 41C having different curvature radii. For example, as shown in FIG. 5, the oblique side 40 of the blades 32 may have a shape similar to that of the peripheral border of an ellipsoid. Effects equivalent to those of the example embodiment described above are also able to be obtained with this kind of shape as well.
As long as the oblique side 40 of the downstream end portion 37 of the blade 32 is shaped so that it inclines so as to be closer to the upstream side nearer the low pressure region 51, the oblique side 40 may have another appropriate structure other than one that is provided with the plurality of arc regions 41A, 41B, and 41C having different curvature radii. For example, as shown in FIG. 6, the oblique side 40 may also be inclined toward the upstream end portion 36 side at a constant inclination. With this kind of structure as well, the angle θ formed between the droplet movement direction of the droplet that moves along the surface 38 of the blade 32 toward the downstream end portion 37, and the tangential direction that is an extending direction of a tangent line when a position reached by the droplet on the downstream end portion 37 is the tangent point, approaches a right angle. As a result, effects equivalent to those described in (1), (3), and (4) above are able to be obtained.
The low pressure region 51 where members that would impede the flow of exhaust gas, such as the blades 32, are not provided may also be positioned offset from the center of the exhaust pipe 11. In this case, the center 34 of the virtual plane described above is positioned in a different position than the central line 11a of the exhaust pipe 11.
The oblique side 40 may also be provided only on the downstream end portion 37 of only a portion (i.e., one or some) of the blades 32. For example, the blades 32 having the oblique side 40 may be alternatively arranged in the circumferential direction with blades that do not have the oblique side 40.
A structure in which only the upstream end portion 36 of a portion (i.e., one or some) of the blades 32 is inclined so as to be closer to the downstream side nearer the low pressure region 51 may also be employed. For example, blades 32 having the upstream end portion 36 that is inclined so as to be closer to the downstream side nearer the low pressure region 51 may be alternately arranged in the circumferential direction with blades having an upstream end portion that is not inclined so as to be closer to the downstream side nearer the low pressure region 51.
The blades 32 may also be members that are separate from the main body portion 30. In this case, the blades 32 are attached to the main body portion 30 by welding or the like. In this example embodiment, aside from urea aqueous solution, the liquid reducing agent may also be an aqueous ammonia solution, an aqueous hydrocarbon, or fuel for an internal combustion engine, such as diesel fuel or gasoline fuel.

Claims

A mixing device that is provided farther downstream in an exhaust pipe of an internal combustion engine than an adding valve that sprays a liquid additive into the exhaust pipe, and that includes a plurality of blades that are arranged in a circumferential direction on an inner wall of the exhaust pipe and are inclined in one direction with respect to a radial direction, within a virtual plane that is orthogonal to a flow direction of exhaust gas inside of the exhaust pipe, characterized in that:
an oblique side (40) that is inclined so as to be closer to an exhaust gas upstream side nearer a center (34) of the virtual plane is formed on an end portion (37), on an exhaust gas downstream side, of at least one of the blades (32).
The mixing device according to claim 1, wherein the oblique side (40) is inclined at a steeper inclination nearer the center (34) of the virtual plane.
The mixing device according to claim 2, wherein a plurality of arc regions (41A, 41B, 41C) formed with different radii are provided continuous on the oblique side (40), and the radii (R1, R2, R3) of the arc regions are smaller nearer the center (34) of the virtual plane.
The mixing device according to claim 2, wherein the oblique side (40) has a shape that is similar to a peripheral border of a simple ellipsoid.
The mixing device according to claim 1, wherein the oblique side (40) is inclined at a constant inclination.
The mixing device according to any one of claims 1 to 5, wherein the oblique side (40) is formed on all of the plurality of blades (32).
The mixing device according to any one of claims 1 to 6, wherein an end portion (36) on an exhaust gas upstream side of at least one of the blades (32) is inclined so as to be closer to the exhaust gas downstream side nearer the center (34) of the virtual plane.