WO2011086259A1

WO2011086259A1 - Device for mixing a main gas flow and a secondary gas flow

Info

Publication number: WO2011086259A1
Application number: PCT/FR2010/052738
Authority: WO
Inventors: Ludovic Thobois; Gilbert Blanchard; Grégory BLOKKEEL; Sylvie Honnet; Karine Pajot
Original assignee: Peugeot Citroën Automobiles SA
Priority date: 2010-01-18
Filing date: 2010-12-15
Publication date: 2011-07-21
Also published as: FR2955266B1; FR2955266A1

Abstract

The invention relates to a device for mixing a main gas flow (2) and a secondary gas flow (4), said device including a gas rotation means, characterized in that said device includes a means for accelerating a first fraction (15) of the main flow (2) and a means for moving the secondary flow (4). Said device is moreover characterized in that the gas rotation means is also a means for accelerating a second fraction (16) of the main flow (2). The acceleration means is fitted into said means in a substantially concentric manner such that, at the outlet of the acceleration means, the first fraction (15) drives the secondary flow (4) and is rotated by the second fraction (16) so as to obtain a uniform mixture of the main flow (2) and the secondary flow (4) and so as to break up the obtained mixture at the outlet of the device. The invention also relates to an exhaust line that includes the device of the invention.

Description

Device for mixing a main gas stream and an auxiliary gas stream Technical field of the invention

The present invention claims the priority of the French application 1050287 filed January 18, 2010 whose content (text, drawings and claims) is here incorporated by reference.

The present invention relates to the field of gas mixing means, in particular applied to the automobile.

Technological background

For many years, motor vehicle manufacturers have made great efforts to reduce the emission into the atmosphere of environmentally harmful chemical compounds produced by combustion engines during fuel combustion.

Among these compounds, there are in particular carbon dioxide C0 _{2 as} well as nitrogen oxides, mainly NO monoxide and nitrogen dioxide N0 ₂ , designated together under the abbreviation NO _x . It should be noted that the production of nitrogen oxides is greater for diesel engines than for gasoline engines because of their higher combustion temperature.

To limit the emission of nitrogen oxides in the atmosphere, a solution currently used is to place on the exhaust system of the vehicle an NO _x treatment system, called SCR system (Selective Catalytic Reduction), having as a function of chemically reducing nitrogen oxides to di-nitrogen molecules and water vapor by means of a reducing agent. In practice, the reducing agent is injected into the exhaust line upstream of a specific catalyst SCR in which the reduction reaction occurs.

There are two SCR channels, one with the aqueous solution of urea and the other with ammonia, NH ₃ , gaseous.

SCR with urea has been previously favored for reasons of vehicle implantation. The aqueous solution of urea requiring a reservoir, the liquid supply circuit is relatively simple. This path has the disadvantage of requiring a set of sequential physico-chemical long processes: the injection of the solution aqueous urea, the atomization of the urea spray droplets and droplets, evaporation and chemical decomposition of these droplets in NH ₃ and finally the mixture of NH ₃ with the exhaust gas. Moreover, given the space available in the exhaust line for the injection / mixing phenomena, the SCR solution with the aqueous urea solution greatly complicates the homogenization of the reducing agent with the oxidant.

The other route explored is SCR with ammonia, NH ₃ gas. In this case, the NH ₃ is injected as a gas directly into the exhaust gas where it must then mix.

It is known to leave an adequate distance between the reducing agent injector and the SCR catalyst to allow obtaining a homogeneous mixture as complete as possible between the exhaust gas and the reducing agent. In order to improve the homogeneity of this mixture and to shorten the distance required for homogenization, it is known to use a so-called static mixer, that is to say without moving parts, positioned inside. of the exhaust duct, between the point of injection of reducing agent in the exhaust gas and the entry of the catalyst SCR. JP2008049306 discloses in particular a device comprising a main duct in which circulates a main gas stream, which may be an exhaust gas stream, an auxiliary duct in which circulates an auxiliary gas stream, which may be a gas stream. reducing agent of nitrogen oxides. The auxiliary duct has an L shape extending from the wall of the main duct towards the center of the main duct to inject the auxiliary gas stream substantially along the central axis of the main duct. Downstream of the injection point in the direction of the main gaseous flow, the device comprises a means for dispersing the auxiliary gaseous flow from the center to the periphery of the main pipe, followed by means for rotating the main gas stream and auxiliary by inclined fins.

However, such a device does not make it possible to obtain a homogeneous mixture between the main gas flow and the auxiliary gas flow throughout the section of the main duct. Indeed, the auxiliary flow must first be distributed equitably over the entire surface of the dispersing means, which is not easy because of the low injection speed of the auxiliary gas flow. In addition a second disadvantage is that the device is not compact. Such an arrangement is not suitable in environments where space available is limited, as in the case of an exhaust system of a motor vehicle.

The object of the invention is to overcome the drawback of the prior art by proposing a new compact device which makes it possible to obtain a homogeneous mixture.

The invention therefore relates to a device for mixing a main gas flow and an auxiliary gas flow comprising means for rotating gas, characterized in that it comprises a means of accelerating a first fraction of gas flow main means for supplying the auxiliary gas stream at the outlet of the acceleration means, in that the means for rotating gas is also a means for accelerating a second fraction of the main gas stream, the means for accelerating being upstream of the rotating and accelerating means and nested in said rotation and acceleration means in a substantially concentric manner so that at the outlet of the acceleration means the first fraction of main gas flow entrains the auxiliary gas stream and is rotated by the second fraction of the accelerated main gas stream and rotated to obtain a homogeneous mixture between the main gas stream and the auxiliary gas flow and to disperse the mixture obtained at the outlet of the device.

An intense central recirculation zone is thus obtained which makes it possible to increase the residence time of the auxiliary gas flow such as ammonia and a main gas flow such as exhaust gases, which favors the homogenization of the flows. In addition, the invention may include one or more of the following features:

Preferably, the acceleration means and the means of rotation and acceleration are integral with the means for supplying the auxiliary gas flow. This simply ensures the maintenance of the acceleration means and the means of rotation and acceleration by their attachment to the supply means.

Alternatively, the acceleration means of the first fraction of the main gas stream comprises a convergent having an inlet surface of the first main gas stream fraction and an exit surface of the first main gas stream fraction, the surface input being greater than the output surface. Indeed a convergent is a simple and effective way to accelerate the gases. Preferably, the exit area is less than or equal to half of the entrance area, to at least double the speed of the first main gas stream fraction. Preferably, for reasons of simplicity of embodiment, the convergent is of frustoconical shape.

More preferably, the convergent is in the form of a nozzle, in order to limit the losses in the flow.

Preferably, the means for supplying the auxiliary flow comprise an auxiliary duct opening substantially in the center of the exit surface of the convergent. Thus the auxiliary flow is effectively driven by the first fraction of the main gas stream. Preferably, the means for rotating and accelerating the second fraction of main gas flow comprises two oblique half-truncated cones each comprising an inlet edge forming an inlet angle and an outlet edge forming an angle the exit angle being greater than the entry angle and said oblique half-truncated cones being arranged so that the entrance edge of one of the oblique half-truncated cones is next to the exit edge of the other half oblique cone trunk. This original design thus makes it possible to accelerate the second fraction of the main gas flow by convergence and to deflect it via the variation between the angles of entry and exit.

Preferably, the exit angle is between the entry angle plus five degrees and twice the entry angle, and more preferably, the exit angle is substantially equal to the entry angle plus 8 degrees.

In a variant, the means for rotating and accelerating the second fraction of main gas flow comprises two half-nozzles, in order to control the pressure drops of the device.

Preferably, the means for rotating and accelerating the second fraction of gas flow comprises a diffuser having the shape of a substantially circular ring disposed at the output of said means for rotating and accelerating and for channeling the second fraction of gas flow to the first main gas stream fraction. Furthermore, the invention also relates to a combustion gas exhaust line produced by an internal combustion engine, characterized in that it comprises a device of the invention.

Brief description of the drawings

Other features and advantages will appear on reading the following description of a particular embodiment, not limiting of the invention, with reference to the figures in which:

- Figure 1 is a schematic representation in section of the device of the invention.

FIG. 2 is a diagrammatic representation in perspective of a preferred embodiment of the gas mixing device according to the invention.

FIG. 3 is a diagrammatic representation in perspective of a preferred embodiment of the means for rotating and accelerating the gases.

FIG. 4 is a diagrammatic representation in perspective of a preferred embodiment of the gas acceleration means.

- Figure 5 schematically shows the dynamics of the flow in the mixing zone generated by the device of the invention.

detailed description

Figure 1 shows a portion of exhaust line of burnt gases produced by an internal combustion engine. The exhaust line comprises a main duct 1, of passage section S _in , in which circulates a stream 2 of flue gas said main gas stream and connected to a selective reduction catalyst 3 (or SCR) for the reduction of oxides of nitrogen NO _x present in the exhaust gases by selective catalytic reaction between _NOx and a reducing agent. By reducing agent is meant an agent that can react chemically with pollutants to transform them into less polluting products.

The main conduit 1 comprises, upstream of the catalyst 3 SCR, a device 6 for mixing gas between the main gas stream 2 and an auxiliary gas stream 4 of NH ₃ .

The device 6 comprises means for supplying the auxiliary gas flow. The reducing agent feed means such as ammonia, NH ₃ , in gaseous form in the main conduit 1 comprises an auxiliary conduit 5 opening substantially at the central axis XX of the main conduit 1. Figure 2 shows a more detailed perspective view of a preferred embodiment of the gas mixing device 6 of the invention. According to this preferred embodiment, the gas mixing device 6 comprises two main means.

The first principal means is an acceleration means 7 of a first fraction of the main gas stream 2.

Preferably, the acceleration means comprises a convergent 7 having an input surface S _c i and an output surface S _{c 2} , the input surface S _c i being greater than the output surface S _{c 2} . For reasons of simplicity of embodiment, preferably the convergent 7 is of frustoconical shape. The convergent 7 is shown, isolated from the rest of the device, in Figure 4. The convergent 7 further comprises a first notch 13 for passage to the mounting and placement of the auxiliary conduit 5.

The second principal means is a means for rotating and accelerating 8 a second fraction of the main gas stream 2. This means of rotation and acceleration 8, which may still be designated by the English term "swirler" includes in this preferred embodiment two oblique half-truncated cones 9, 9 '. The means of rotation and acceleration 8 is presented, isolated from the rest of the device, in FIG.

Each half-truncated oblique cone 9, 9 'comprises respectively an input edge 10, 10' and an output edge 11, 11 '. Each half-truncated cone is said to be oblique because the entry edge does not have the same angle of convergence as the exit edge. Indeed, it is expected that the input edge 10, 10 'of each of the oblique half-cones 9, 9' has an angle of convergence of input, that the output edge 1 1, 1 1 'of each oblique half-cones 9, 9 'have an output convergence angle β, the output convergence angle β preferably being greater than the input convergence angle α.

This variation of convergence angle favors the deviation of the second gas flow fraction and makes it possible to confer on it the expected rotational movement in addition to the acceleration conferred by the presence of the convergence. Preferably, the angle β of output convergence is between the input convergence angle a plus 5 ° and twice the angle of convergence of input. More preferably, the output convergence angle β is substantially equal to the input convergence angle α plus 8 °. Moreover, said oblique cone half-truncks 9, 9 'are arranged in such a way that the input edge 10, 10' of one of the oblique half-truncated cones is opposite the output edge 1 1, 1 1 'of the other half-truncated oblique cone.

The rotation and acceleration means 8 may comprise a diffuser 12. This diffuser 12 has the shape of a substantially circular ring disposed at the outlet of the rotation and acceleration means 8, in order to channel the second fraction of gas flow towards the central axis XX.

The means for rotating and accelerating 8 further comprises a second notch 14 for ensuring the passage to the mounting of the auxiliary conduit 5.

As shown in Figure 1, the first convergent 7 has a first inlet section S _c i, substantially orthogonal to the central axis XX between which the first fraction of the main stream 2 of exhaust gas and a first outlet section S _c2 , substantially orthogonal to the central axis XX through which the first fraction of the main stream 2 of exhaust gas.

As further shown in FIG. 1, the rotation and acceleration means 8 have an input plane P _e , substantially orthogonal to the central axis XX, through which the second fraction of the main gas flow 2 exhaust and a second outlet section S _or by which leaves the first and second fraction of the main stream and the auxiliary flow of NH ₃ gas.

As shown in FIG. 1 as well as FIG. 2, the convergent 7 is, with respect to the direction of flow of the main gas flow, upstream of the means of rotation and acceleration and preferably imbricated substantially concentrically in the means of rotation and acceleration 8 with respect to the central axis XX. By nested means that the first output section S _c i lies between the input plane P _e and the second output section S _or t-

In this embodiment, the mixing device 6 occupies the passage section S _in of the conduit, that is, the rotation and acceleration means 8 has a section input S _e substantially equivalent to the passage section. However, because of the presence of the convergent 7, the second fraction of exhaust gas rotated by the rotation and acceleration means 8 enters through an effective input section S _ee , peripheral to the convergent 7, substantially shaped like a crown. The second fraction of exhaust gas opens substantially orthogonally to the first output section S _c i of the convergent 7 by an effective output section, S _se , smaller than the effective input section S _ee, the section reduction resulting from the convergence of the rotation and acceleration means 8 and its position relative to the convergent 7 generating the acceleration of the gases of the second fraction of the main gas stream 2.

In order to inject the auxiliary flow 4 of gaseous ammonia appropriately into the mixing device 6 of the invention, preferably the auxiliary conduit opens at the central axis XX substantially in the center of the second output section S _c2 of the convergent 7.

The detail of the original operating principle of the invention is illustrated in FIG. 5. The main gas stream 2 of exhaust gas is therefore divided into two distinct streams. The first fraction 15 of exhaust gas, located in the central zone of the main duct 1 will be diverted and accelerated by the convergent 7. The second fraction 16 of exhaust gas located at the periphery of the main duct 1 will be accelerated and rotating around an axis parallel to the central axis XX by the rotating and accelerating means 8. This second fraction 16 will then join the first fraction 15 at the central axis XX that it will also rotate. The flow thus generated has a speed in the direction of the central axis XX greater than the flow upstream of the mixing device 6 and is rotated. This type of flow generates an intense central recirculation zone 17 in which the gaseous stream 4 of ammonia, driven by the first fraction of exhaust gas, is injected. This recirculation zone 17 makes it possible to increase the residence time of the ammonia and the exhaust gases, which makes it possible to better homogenize this mixture.

Subsequently, because of the rotational speed of the flow, it will rapidly extend from the center to the periphery (see reference 18) and thus disperse the mixture of exhaust gases and ammonia output of the device in the main duct 1 to water the SCR catalyst on the entire of its input surface. In order to ensure at least a doubling of the speed of the first exhaust gas fraction, preferably, the first convergent comprises an output surface S _c2 of the first convergent 7 less than or equal to half of its input surface S _c i.

Regarding the means of rotation and acceleration 8, preferably, the second output section S _0Lrt is substantially equivalent to the first output section S _c2 .

The diffuser 12 has a crown width D which depends on the convergence angles of input and output convergence, α, β. However, preferably the crown width D is greater than or equal to the diameter of the second output section S _0Lrt .

Preferably, it is intended to accelerate the second fraction of exhaust gas by a factor of at least two. To do this, the effective output area S _ee of the second fraction of the flux is less than or equal to the difference between the input section S _e of the rotation and acceleration means 8 and the input section S _c i of the convergent 7.

Advantageously, the convergent 7 and the rotating and accelerating means 8 are fixed to the auxiliary duct 5.

This mixing device 6 can be made from at least one sheet, by cutting, folding and assembly, for example by welding at the notches 13, 14, on the auxiliary duct 5. This advantageously makes it possible to provide a very efficient gas mixing device 6 for a particularly low manufacturing cost.

The embodiment described in this specification is not limiting. According to one variant, the forms of the acceleration means as well as the rotation and acceleration means 8 can have different shapes but with similar effects. What ultimately matters is the optimization of the means of acceleration and the means of rotation and acceleration in order to generate an aerodynamic motion around the central axis XX intense to improve the mixture while controlling the loss. of charge generated by the device 6. According to a variant advantageously to reduce the pressure drop the acceleration means can take a convergent form more complex than a simple convergent cone such as a nozzle shape with a progressive radius of curvature . Likewise, the means for rotating and accelerating, instead of to be composed of two half-truncated conical oblique cones, may have a shape approaching that of a nozzle and comprise two half-nozzles.

The invention has the advantage of improving the homogeneity of the mixture of the exhaust gases with the gaseous NH ₃ , which improves the overall efficiency of the SCR bread, and allows upstream to increase the combustion efficiency and reduce the consumption. In fact, rapid combustion reaches temperatures in the higher flue gases, which leads to a greater formation of nitrogen oxides. If the SCR device is more efficient by a better reducer / exhaust gas mixture, more nitrogen oxides can be formed in the combustion chamber which allows better phasing of the combustion for example and thus reduce consumption.

Claims

claims

1. Apparatus for mixing a main gas stream (2) and an auxiliary gas stream (4) comprising a means for rotating gas, characterized in that it comprises an acceleration means of a first fraction (15) main gas flow (2) means for supplying the auxiliary gas stream (4) at the outlet of the acceleration means, in that the means for rotating gas is also a means of accelerating a second fraction (16) of the main gas stream (2), the acceleration means being upstream of the rotation and acceleration means and nested in said rotation and acceleration means in a substantially concentric manner so that at the output of the acceleration means the first fraction (15) of the main gas stream (2) drives the auxiliary gas stream (4) and is rotated by the second fraction (16) of the accelerated main gas stream (2) and rotation to obtain a homogeneous mixture between the f main gaseous lux (2) and the auxiliary gas stream (4) and to disperse the mixture obtained at the outlet of the device.

2. Device according to claim 1, characterized in that the acceleration means and the rotating and accelerating means are integral with the means for supplying the auxiliary gas stream (4).

3. Device according to claim 1 or claim 2, characterized in that the acceleration means of the first fraction (15) of the main gas flow comprises a convergent (7) having an inlet surface (S _c i) of the first main gas stream fraction (15) and an exit surface (S _c2 ) of the first main gas stream fraction (15), the input surface (S _c i) being greater than the exit surface (S); _c2 ).

4. Device according to claim 3, characterized in that the output surface (S _c2 ) is less than or equal to half of the input area (S _c i).

5. Device according to claim 3 or claim 4, characterized in that the convergent (7) is of frustoconical shape.

6. Device according to claim 3 or claim 4, characterized in that the convergent (7) is in the form of a nozzle.

7. Device according to any one of claims 3 to 6, characterized in that the means for supplying the auxiliary flow comprise an auxiliary duct (5) opening substantially in the center of the outlet surface (S _c2 ) of the convergent (7 ).

8. Device according to any one of the preceding claims, characterized in that the means for rotating and accelerating the second fraction (16) of the main gas flow comprises two oblique half-truncated cones (9, 9 '). ) each comprising an input edge (10, 10 ') forming an input angle (a) and an output edge (1 1, 1 1') forming an exit angle (β), the exit angle (β) being greater than the entry angle (a) and said oblique tapered half-truncks (9, 9 ') being arranged in such a way that the edge (10, 10') of the inlet of the one of the oblique half-truncated cones is opposite the exit edge (11, 11 ') of the other oblique conical half-trunk.

9. Device according to claim 8, characterized in that the exit angle (β) is between the entry angle (a) plus five degrees and twice the entry angle (a) and preferably substantially equal to the entry angle (a) plus 8 degrees.

10. Device according to any one of claims 1 to 7, characterized in that the means for rotating and accelerating the second fraction (16) of the main gas stream comprises two half-nozzles.

1 1. Device according to any one of claims 8 to 10, characterized in that the means for rotating and accelerating the second fraction (16) of gaseous flow comprises a diffuser (12) having the shape of a substantially circular ring disposed at the outlet of said means for rotating and accelerating and for channeling the second fraction (16) of gas flow to the first fraction (15) of main gas flow.

12. exhaust line of combustion gases produced by an internal combustion engine, characterized in that it comprises a device according to any one of the preceding claims.