FR3137413A1 - Reducer mixer for exhaust gas - Google Patents

Abstract

Reductant mixer for exhaust gas The invention relates to a mixer (1), for mixing exhaust gases with a reducing fluid, comprising a pipeline (5-8) in which the exhaust gas flows from an inlet (2) to an outlet (3) and a sprayer (4) for spraying the reducing fluid into the exhaust gas circulation, where the pipeline (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 gas forms a swirl 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

Reducer mixer for exhaust gas

The invention relates to a reducer mixer for exhaust gas. Such a mixer is typically placed 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 reducer (in English: “selective catalytic reduction” or SCR) in order to reduce nitrogen oxides.

To make such a mixer, a closed mixing chamber is typically constructed, sealed, excluding an inlet orifice, an outlet orifice and a spray orifice. Exhaust gases flow through the mixing chamber from the inlet port to the outlet port. A sprayer is disposed in an outer wall of the mixing chamber, so as to spray the reducing fluid, through the spray port, into the exhaust gas circulation.

Two problems are typically encountered when designing such a mixer.

Firstly, it is necessary to optimize an intimate mixing 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 so as 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 necessary 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 detrimental.

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

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 in the circulation of exhaust gases, where the pipe comprises, from upstream to downstream, an upstream pipe connecting the inlet to a junction, at least two intermediate sub-pipes connecting the junction to a junction and a downstream pipe connecting the junction to the outlet, where, at the junction, the at least two sub-ducts are arranged so that the circulation of exhaust gases forms a swirl in the downstream pipe and where the sprayer is arranged at the junction so as to spray into the downstream pipe, preferably coaxially.

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

- the arrangement of the sub-pipes is carried out by a geometric offset, the projections of the axes of the sub-pipes on a plane perpendicular to the axis of the downstream pipe, preferably the axes of the sub-pipes themselves, not passing by the axis of the downstream pipe,

- the arrangement of the sub-ducts is obtained by curvatures of the sub-ducts as they arrive at the junction,

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

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

- the downstream pipe comprises an unblocked 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 protrusion extending axially the tube towards the sprayer, said at least one protuberance not being placed opposite the arrival of a sub-pipe,

- 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 on reading the description which follows, 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 offset mode,

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 reducer with exhaust gases. Such a mixer is typically placed in an exhaust line of an internal combustion engine. Its function is to mix a reducing fluid, typically ammonia or any other ammonia precursor in gaseous or liquid form. A common solution is to use an aqueous urea solution: AUS 32, also called DEF or commercially called AdBlue®.

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

To produce such a mixer 1, a closed mixing chamber is typically constructed, sealed, excluding an inlet orifice 2, an outlet 3 and a spray orifice. The exhaust gases circulate through the mixing chamber, from the inlet port 2 to the outlet port 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.

Sprayer 4 can be a standard injector. It may or may not still be heated, in order to preheat the reducing fluid when it is sprayed.

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

According to one characteristic, the pipe 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 input 2 to a junction 9. Said at least two intermediate sub-pipes 6, 7 connect the junction 9 to a junction 10. The downstream pipe 8 connects the junction 10 to the outlet 3.

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

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 purposes only.

According to a first embodiment, more particularly illustrated in , 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 produced 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 gases coming from each of the sub-flows. -lines 6, 7. This is true in projection. Furthermore, according to a preferred embodiment, this is still true if the axes of the sub-pipes 6, 7 themselves do not pass through the axis A of the downstream pipe 8.

According to another embodiment, more particularly illustrated in , 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 produced by curvatures of the sub-pipes 6, 7 at, or a little before, their arrival at junction 10. What is meant here by “a little before” is 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 -pipe 6, 7 and even 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 meeting the sub-flows of exhaust gas , will 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 junction 10, with a view to creating a homogeneous vortex and preferably centered on axis A of the downstream pipe 8, it is important that the configuration at junction 10 is axisymmetric.

Also, according to another characteristic, at least at, or a little before, arrival at junction 10, the projections of the sub-pipes 6, 7, on a plane perpendicular to the axis A of the downstream pipe 8, are arranged axisymmetrically relative to the axis A of the downstream pipe 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 pipe 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 evenly distributed around the axis A of the downstream pipe 8. Thus two sub-ducts 6, 7 will be opposite each other, at 180°. Three sub-ducts 6, 7 will be equally distributed, angularly separated by 120° according to an angular periodicity around axis A.

Everything that has been described previously 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 is » and the greater the efficiency of the swirl.

Also, according to a preferred configuration, the axes of the sub-ducts 6, 7 are advantageously arranged on a cone of axis coincident with the axis A of the downstream pipe 8. In this case, the cone preferably has an opening facing upstream in order to facilitate circulation from the sub-pipes 6, 7, to the downstream pipe 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 pipe 8. This last 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 , where the downstream pipe 8 is shown transparent, the downstream pipe 8 comprises a tube 11. This tube 11 is unblocked 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. Tube 11 is preferably perforated.

Tube 11 performs a dual function. On the one hand, it prevents any 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 circulation of exhaust gases, is heated by the latter. Also, any 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 can be noted that at the inlet of tube 11, the parts of tube 11 facing the inlets of the sub-ducts 6, 7 are removed, so as not to create obstruction to the circulation of the sub-flows. from sub-ducts 6, 7.

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

It could be 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 reducing spray on the tube 11 and not on the internal wall of the downstream pipe 8. This is all the more advantageous that the proximity between said at least one protuberance 12 and the sprayer 4 is important.

According to another characteristic, the sections of the sub-pipes 6, 7 are identical to each other, preferably substantially equal to the section of the upstream pipe 5 divided by the number of sub-pipes 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) larger section of the downstream pipe 8 makes it possible to accommodate the negligible volume caused by the addition of reducing fluid and above all not to create further loss of load 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. This should be considered illustrative and given by way of example and not as limiting the invention to this description alone. Numerous embodiment variants 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 in 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 disjunction (9), at least two intermediate sub-pipes (6, 7) connecting the disjunction (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-ducts (6, 7) are arranged in such a way that the circulation of exhaust gases forms a swirl in the downstream pipe (8) and in that the sprayer (4) is arranged at the junction (10) so as to spray in the downstream pipe (8), preferably coaxially. Mixer (1) according to claim 1, wherein the arrangement of the sub-ducts (6, 7) is produced by a geometric offset (F), the projections of the axes of the sub-ducts (6, 7), on a perpendicular plane to the axis (A) of the downstream pipe (8), preferably the axes of the sub-pipes (6, 7) themselves, not passing through the axis (A) of the downstream pipe (8). Mixer (1) according to any one of claims 1 or 2, where the arrangement of the sub-ducts (6, 7) is produced by curvatures of the sub-ducts (6, 7) at, or a little before, their arrival at the junction (10). Mixer (1) according to any one of claims 1 to 3, where, at least on arrival at the junction (10), the projections of the sub-ducts (6, 7) on a plane perpendicular to the axis ( A) of the downstream pipe (8), preferably the sub-ducts (6, 7) themselves, are arranged axisymmetrically relative to the axis (A) of the downstream pipe (8). Mixer (1) according to any one of claims 1 to 4, where 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 unblocked 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, where the tube (11) comprises at least one protuberance (12) extending axially the tube (11) towards the sprayer (4), said at least one protuberance not being placed opposite of the arrival of a subduct (6, 7). Mixer (1) according to any one of claims 1 to 7, where the sections of the sub-pipes (6, 7) are identical to each other, preferably substantially equal to the section of the upstream pipe (5) divided by the number 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.