FR3137413A1 - Reducer mixer for exhaust gas - Google Patents

Abstract

Reducer mixer for exhaust gas The invention relates to a mixer (1) for mixing exhaust gases with a reducing fluid, comprising a pipe (5-8) in which the exhaust gas circulates from an inlet (2) to an outlet. (3) and a sprayer (4) for spraying the reducing fluid into the exhaust gas circulation, where the pipe (5-8) comprises, from upstream to downstream, an upstream pipe (5) connecting the inlet (2) to a junction (9), at least two intermediate sub-pipes (6, 7) connecting the junction (9) to a junction (10) and a downstream pipe (8) connecting the junction (10) to the outlet (3), where, at the junction (10), the at least two sub-ducts (6, 7) are arranged in such a way that the circulation of exhaust gases forms a vortex in the downstream pipe (8 ) and where the sprayer (4) is arranged at the junction (10) so as to spray in the downstream pipe (8), preferably coaxially. Figure for abstract: 1

Description

Exhaust gas reducer mixer

The invention relates to a reducer mixer for exhaust gases. Such a mixer is typically arranged in an exhaust line of an internal combustion engine. Its function is to mix a reducing fluid, conventionally ammonia or an aqueous urea solution: AUS 32, DEF, with the exhaust gases, to prepare the treatment of the mixture by at least one selective catalytic reduction (SCR) in order to reduce nitrogen oxides.

To realize such a mixer, a mixing chamber is typically constructed in a sealed manner, excluding an inlet port, an outlet port and a spray port. The exhaust gases flow through the mixing chamber, from the inlet port to the outlet port. A sprayer is arranged in an outer wall of the mixing chamber, so as to spray the reducing fluid, through the spray port, into the exhaust gas flow.

Two problems are typically encountered when designing such a mixer.

First, it is necessary to optimize an intimate mixture of the reducing fluid with the exhaust gases. For this, it is generally planned to increase the length of the mixer as much as possible in order to increase the common circulation time of the two components: reducing fluid and exhaust gas. This is generally difficult in an environment, engine and/or exhaust, where space is very limited.

Secondly, it is advisable to avoid spraying the reducing fluid against an external, therefore cold, wall of the mixer, at the risk of forming a film of reducing fluid on said wall, a precursor to a deposit of reducing fluid, which is very harmful.

The invention proposes a novel mixer architecture, which addresses, in a particularly ingenious way, these two problems.

For this, the invention relates to a mixer, for mixing exhaust gas with a reducing fluid, comprising a pipe in which the exhaust gases circulate, from an inlet to an outlet and a sprayer for spraying the reducing fluid into the exhaust gas circulation, where the pipe comprises, from upstream to downstream, an upstream pipe connecting the inlet to a disconnection, at least two intermediate sub-pipes connecting the disconnection to a junction and a downstream pipe connecting the junction to the outlet, where, at the junction, the at least two sub-pipes are arranged so that the exhaust gas circulation forms a vortex in the downstream pipe and where the sprayer is arranged at the junction so as to spray into the downstream pipe, preferably coaxially.

Particular features or embodiments, usable alone or in combination, are:

- the arrangement of the sub-ducts is achieved by a geometric offset, the projections of the axes of the sub-ducts on a plane perpendicular to the axis of the downstream duct, preferably the axes of the sub-ducts themselves, not passing through the axis of the downstream duct,

- the arrangement of the sub-pipes is obtained by curvatures of the sub-pipes when they arrive at the junction,

- at least upon arrival at the junction, the projections of the sub-pipes on a plane perpendicular to the axis of the downstream pipe, preferably the sub-pipes themselves, are arranged axisymmetrically relative to the axis of the downstream pipe,

- the axis of the downstream pipe is substantially aligned with the axis of the upstream pipe,

- the downstream pipe comprises an open tube, of section smaller than the section of the downstream pipe, preferably perforated, arranged coaxially in the downstream pipe,

- the tube comprises at least one protuberance axially extending the tube towards the sprayer, said at least one protuberance not being placed opposite the arrival of a sub-duct,

- the sections of the sub-pipes are identical to each other, preferably substantially equal to the section of the upstream pipe divided by the number of sub-pipes,

- the section of the downstream pipe is substantially equal to or greater than the section of the upstream pipe.

According to a second aspect of the invention, an exhaust line comprising such a mixer.

The invention will be better understood by reading the following description, given solely by way of example, and with reference to the appended figures in which:

shows, in perspective view, a mixer according to the invention,

shows, in perspective view, the mixer of the from another point of view,

shows, schematically in view along the axis of the downstream pipe, a first mode of offset,

shows, schematically in view along the axis of the downstream pipe, another mode of offset,

shows, in perspective view, a detail of the mixer in fig 1, showing, in transparency, a tube.

In reference to the , the invention relates to a mixer 1. A mixer 1 is intended to mix a reducing agent with exhaust gases. Such a mixer is typically arranged in an exhaust line of an internal combustion engine. Its function is to mix a reducing fluid, conventionally ammonia or any other precursor of ammonia in gaseous or liquid form. A common solution is to use an aqueous urea solution: AUS 32, also called DEF or commercially known as AdBlue®.

Mixing this reducing fluid with the exhaust gases prepares them to be treated by a selective catalytic reduction (SCR) to reduce nitrogen oxides.

To realize such a mixer 1, a mixing chamber is typically constructed, sealed, excluding an inlet orifice 2, an outlet orifice 3 and a spray orifice. The exhaust gases flow through the mixing chamber, from the inlet orifice 2 to the outlet orifice 3. A sprayer 4 is arranged in an outer wall of the mixing chamber, so as to spray the reducing fluid, through the spray orifice, into the exhaust gas circulation.

The sprayer 4 may be a standard injector. It may or may not be heated, in order to preheat the reducing fluid when spraying it.

According to the invention, the mixer 1 comprises a pipe 5-8, forming the mixing chamber, connecting the inlet orifice 2 to the outlet orifice 3, in which the exhaust gases circulate. The direction of circulation, from upstream to downstream, corresponds to a direction going from the inlet 2 to the outlet 3. The mixer 1 further comprises a sprayer 4 intended to spray the reducing fluid into the pipe 5-8 and therefore into the circulation of exhaust gases.

According to one characteristic, the pipeline 5-8 comprises, successively, from upstream to downstream, an upstream pipe 5, at least two intermediate sub-pipes 6, 7, and a downstream pipe 8. The upstream pipe 5 connects the inlet 2 to a disconnection 9. Said at least two intermediate sub-pipes 6, 7 connect the disconnection 9 to a junction 10. The downstream pipe 8 connects the junction 10 to the outlet 3.

At the junction 10, the at least two sub-ducts 6, 7 are arranged so that the flow of exhaust gas forms a vortex in the downstream duct 8. Due to the arrangement, detailed further below, the at least two exhaust gas flows, formed at the disjunction 9, meet at the junction 10. The arrangement is such that this meeting creates a vortex in the downstream duct 8. Preferably, this vortex is centered on the axis A of the downstream duct 8.

The sprayer 4 is arranged at the junction 10 so as to spray into the downstream pipe 8. Thus the jet of reducing fluid is advantageously sprayed into the exhaust gas vortex. This vortex is very advantageous in that it creates a movement of the exhaust gases increasing the mixing of the exhaust gases with the sprayed reducing fluid, thus significantly improving the homogeneity of the exhaust gas/reducing fluid mixture. The spray axis of the sprayer 4 is preferably coaxial with the axis A of the downstream pipe 8, substantially merging with the axis of the vortex.

The arrangement is intended to create a swirling circulation of the exhaust gas flow. To obtain such a swirl, all methods are possible. Two examples are given here for information.

According to a first embodiment, more particularly illustrated in the , showing a detail of a mixer 1 along the axis A of the downstream pipe 8, the arrangement of the sub-pipes 6, 7 is achieved by a geometric offset F between the axes of the sub-pipes 6, 7 and the axis A of the downstream pipe 8. In other words, the projections of the axes of the sub-pipes 6, 7, on a plane perpendicular to the axis A of the downstream pipe 8, do not pass through the axis A of the downstream pipe 8 and do not intersect the axis A of the downstream pipe 8. This geometric offset F naturally creates a “lever arm” making it possible to apply a “torque” to the sub-flows of exhaust gas coming from each of the sub-pipes 6, 7. This is true in projection. Furthermore, according to a preferred embodiment, this is still true if the axes of the sub-ducts 6, 7 themselves do not pass through the axis A of the downstream duct 8.

According to another embodiment, more particularly illustrated in the , showing a detail of a mixer 1 along the axis A of the downstream pipe 8, the arrangement of the sub-pipes 6, 7 is achieved by bends of the sub-pipes 6, 7 at, or slightly before, their arrival at the junction 10. Here, "slightly before" is understood to mean a length of sub-pipe 6, 7 less than twice the diameter of the sub-pipe 6, 7, preferably less than once the diameter of the sub-pipe 6, 7 and even more preferably less than half the diameter of the sub-pipe 6, 7. Such a configuration makes it possible to create a non-homogeneous flow, which, when the exhaust gas sub-flows meet, will make it possible to create the desired swirl.

Obviously for those skilled in the art, it is possible to combine these two embodiments, by combining a curvature of the sub-ducts 6, 7 and a geometric offset F.

In order to optimize the energies arriving at the level of the junction 10, with a view to creating a homogeneous vortex and preferentially centered on the axis A of the downstream pipe 8, it is important that the configuration at the level of the junction 10 is axisymmetric.

Also, according to another characteristic, at least at, or a little before, the arrival at the junction 10, the projections of the sub-ducts 6, 7, on a plane perpendicular to the axis A of the downstream duct 8, are arranged axisymmetrically relative to the axis A of the downstream duct 8.

This is true in projection, and according to a preferred embodiment, this is still true if the sub-ducts 6, 7 themselves are arranged axisymmetrically relative to the axis A of the downstream duct 8.

Such an axisymmetry characteristic means that the sub-ducts 6, 7 have substantially identical shapes and sections. This also means that the sub-ducts 6, 7 are, in projection or per se, angularly equidistributed around the axis A of the downstream duct 8. Thus, two sub-ducts 6, 7 will be opposite each other, at 180°. Three sub-ducts 6, 7 will be equidistributed, angularly separated by 120° according to an angular periodicity around the axis A.

Everything described above applies at least in projection, parallel to the axis A of the downstream pipe 8, on a plane perpendicular to the axis A of the downstream pipe 8. However, the more "regular" the configuration, the greater the efficiency of the vortex.

Also, according to a preferred configuration, the axes of the sub-ducts 6, 7 are advantageously arranged on a cone with an axis coincident with the axis A of the downstream duct 8. In this case, the cone preferably has an opening facing upstream in order to facilitate circulation from the sub-ducts 6, 7 to the downstream duct 8.

According to another characteristic, the axes of the sub-ducts 6, 7 are coplanar, preferably in a plane perpendicular to the axis A of the downstream duct 8. This latter characteristic, with a plane perpendicular to the axis A, is included in the previous one, in that it corresponds to a cone with an opening angle of 180°.

According to another characteristic, the axis A of the downstream pipe 8 is substantially aligned with the axis of the upstream pipe 5.

According to another characteristic, more particularly illustrated in the , where the downstream pipe 8 is shown transparent, the downstream pipe 8 comprises a tube 11. This tube 11 is open at its two ends so as to allow the circulation of exhaust gas, reducing fluid and mixture of the two. The section of the tube 11 is less than the section of the downstream pipe 8, so that the wall of the tube 11 is distant from the wall of the downstream pipe 8. The ratio of the section of the tube 11 to the section of the downstream pipe 8 is advantageously between 40 and 80%. The tube 11 is arranged coaxially with the downstream pipe 8 and inside the latter. The tube 11 is preferably perforated.

The tube 11 performs a dual function. On the one hand, it prevents any possible projection of reducing fluid towards the colder internal wall of the downstream pipe 8, and thus limits the risks of deposits of reducing fluid on this internal wall. On the other hand, the tube 11 being advantageously arranged within the exhaust gas circulation, is heated by the latter. Also, any possible projection of reducing fluid on the tube 11 is advantageously vaporized, due to this heat, and a deposit on the tube 11 is not likely to occur.

It may be noted that at the level of the inlet of the tube 11, the parts of the tube 11 opposite the arrivals of the sub-ducts 6, 7 are removed, so as not to create an obstruction to the circulation of the sub-flows coming from the sub-ducts 6, 7.

The tube 11 comprises at least one protuberance 12 axially extending said tube 11 in the direction of the sprayer 4. Said at least one protuberance 12 is not placed opposite the outlet of the sub-ducts 6, 7.

It has been observed, unexpectedly, that this at least one protuberance 12 reduces the back pressure by 20%. These results were obtained by simulation. Like the tube 11 itself, said at least one protuberance 12 also makes it possible to collect part of the reducer spray on the tube 11 and not on the internal wall of the downstream pipe 8. This is all the more advantageous since the proximity between said at least one protuberance 12 and the sprayer 4 is significant.

According to another characteristic, the sections of the sub-ducts 6, 7 are identical to each other, preferably substantially equal to the section of the upstream duct 5 divided by the number of sub-ducts 6, 7.

According to another characteristic, the section of the downstream pipe 8 is substantially equal to or greater than the section of the upstream pipe 5. The incoming exhaust gas flow is found at the outlet. A (slightly) greater section of the downstream pipe 8 makes it possible to accommodate the negligible volume caused by the addition of reducing fluid and, above all, to avoid creating more pressure losses or back pressure.

The invention also relates to an exhaust line comprising such a mixer 1.

The invention has been illustrated and described in detail in the drawings and the preceding description. The latter should be considered as illustrative and given by way of example and not as limiting the invention to this description alone. Numerous variant embodiments are possible.

Claims (10)

Mixer (1) for mixing exhaust gases with a reducing fluid, comprising a pipe (5-8) in which the exhaust gases circulate, from an inlet (2) to an outlet (3) and a sprayer (4) for spraying the reducing fluid into the exhaust gas circulation, characterized in that the pipe (5-8) comprises , from upstream to downstream, an upstream pipe (5) connecting the inlet (2) to a disconnection (9), at least two intermediate sub-pipes (6, 7) connecting the disconnection (9) to a junction (10) and a downstream pipe (8) connecting the junction (10) to the outlet (3), and in that, at the junction (10), the at least two sub-pipes (6, 7) are arranged so that the exhaust gas circulation forms a vortex in the downstream pipe (8) and in that the sprayer (4) is arranged at the junction (10) so as to spray into the downstream pipe (8), preferably coaxially. Mixer (1) according to claim 1, where the arrangement of the sub-ducts (6, 7) is achieved by a geometric offset (F), the projections of the axes of the sub-ducts (6, 7), on a plane perpendicular to the axis (A) of the downstream duct (8), preferably the axes of the sub-ducts (6, 7) themselves, not passing through the axis (A) of the downstream duct (8). Mixer (1) according to any one of claims 1 or 2, wherein the arrangement of the sub-ducts (6, 7) is achieved by bends of the sub-ducts (6, 7) at, or slightly before, their arrival at the junction (10). Mixer (1) according to any one of claims 1 to 3, wherein, at least upon arrival at the junction (10), the projections of the sub-ducts (6, 7) on a plane perpendicular to the axis (A) of the downstream duct (8), preferably the sub-ducts (6, 7) themselves, are arranged axisymmetrically relative to the axis (A) of the downstream duct (8). Mixer (1) according to any one of claims 1 to 4, wherein the axis (A) of the downstream pipe (8) is substantially aligned with the axis of the upstream pipe (5). Mixer (1) according to any one of claims 1 to 5, where the downstream pipe (8) comprises an open tube (11), of section smaller than the section of the downstream pipe (8), preferably perforated, arranged coaxially in the downstream pipe (8). Mixer (1) according to the preceding claim, wherein the tube (11) comprises at least one protuberance (12) axially extending the tube (11) towards the sprayer (4), said at least one protuberance not being placed opposite the arrival of a sub-duct (6, 7). Mixer (1) according to any one of claims 1 to 7, where the sections of the sub-ducts (6, 7) are identical to each other, preferably substantially equal to the section of the upstream duct (5) divided by the number of sub-ducts (6, 7). Mixer (1) according to any one of claims 1 to 8, where the section of the downstream pipe (8) is substantially equal to or greater than the section of the upstream pipe (5). Exhaust line comprising a mixer (1) according to any one of the preceding claims.