Exhaust fluid mixing conduit

Abstract

An exhaust fluid mixing conduit 204 for mixing an exhaust gas with exhaust fluid has an outer wall 214 and a primary spiral mixing blade 320. The outer wall defines an inlet 202 that receives exhaust

Description

Field of the disclosure The present disclosure relates to an apparatus for treating exhaust gasses emitted during the operation of an internal combustion engine. In particular, the present disclosure relates to an exhaust fluid mixing conduit. Background Internal combustion engines output various substances when burning fuel, such as diesel, petrol and / or alternative fuels such as hydrogen. In some circumstances, it may be desirable to process one or more of the output substances. This processing may assist in meeting current and future emissions legislation. Most commonly those substances comprise hydrocarbons (HC), carbon monoxides (CO), mono-nitrogen oxides (NOx) and particulate matter, such as carbon (C), a constituent of soot. The emission of some of those substances may be reduced by careful control of the operating conditions of the engine, but usually it is necessary to provide apparatus downstream of the engine to treat at least some of those substances entrained in the exhaust fluid. For example, it is known to reduce or eliminate mono-nitrogen oxides (NOX) in diesel combustion emissions by conversion to diatomic nitrogen (N2) and water (H2O) by catalytic reaction with chemicals such as ammonia (NH3) entrained in the exhaust fluid. Generally, ammonia is not present in exhaust gases and must therefore be introduced upstream of a catalyst, typically by injecting an exhaust fluid, for example a reductant such as a urea solution, into the exhaust gas which decomposes into ammonia at sufficiently high temperatures. By this method, exhaust gases can be treated, meaning that a proportion of the substances which would otherwise be released to atmosphere are instead converted to nitrogen (N2) and water (H2O). The temperature of an engine is low following ignition of the engine. At low temperatures, exhaust fluid may not be efficiently converted into ammonia by the exhaust system. So, an engine may not be able to meet standards set by current and future emissions legislation when operating at low temperatures, e.g. during the period of time immediately following ignition of the engine. US-B-11,608,764 discloses a dosing and mixing arrangement including a mixing tube having a constant diameter along its length. At least a first portion of the mixing tube includes a plurality of apertures. The arrangement also includes a swirl structure for causing exhaust flow to swirl outside of the first portion of the mixing tube in one direction along a flow path that extends at least 270 degrees around a central axis of the mixing tube. The arrangement is configured such that the exhaust enters an interior of the mixing tube through the apertures as the exhaust swirls along the flow path. The exhaust entering the interior of the mixing tube through the apertures has a tangential component that causes the exhaust to swirl around the central axis within the interior of the mixing tube. The arrangement also includes a doser for dispensing a reactant into the interior of the mixing tube. Against this background there is provided an improved, or at least commercially relevant alternative, exhaust fluid mixing conduit. Summary of the disclosure According to a first aspect, an exhaust fluid mixing conduit for mixing an exhaust gas with exhaust fluid is provided. The exhaust fluid mixing conduit comprises an outer wall and a primary spiral mixing blade. The outer wall defines: an inlet configured to receive the exhaust gas in a first direction, and an outlet configured to output the exhaust gas in a second direction. The primary spiral mixing blade is provided within the exhaust fluid mixing conduit extending from the outer wall proximate the inlet. The outer wall and the spiral mixing blade define an exhaust gas flow path through the exhaust fluid mixing conduit which spirals from the inlet to the outlet about the second direction. In some embodiments, the exhaust fluid mixing conduit defines an exhaust gas flow path which spirals from the inlet to the outlet, wherein the spiral exhaust gas flow path spirals about the outlet (in the second direction). Due to the rotation, a vortex of exhaust gas may be created at the outlet, rotating about the second direction. This vortex flow may allow a greater quantity of the exhaust fluid to be mixed, and subsequently converted to ammonia by the exhaust fluid system. In addition, this vortex flow may allow a greater quantity of the exhaust fluid to be injected, converted to ammonia, and mixed by the exhaust fluid mixing conduit. In some embodiments, the exhaust gas flow path through the exhaust fluid mixing conduit spirals inward from the inlet to the outlet. The inward spiral shaped nature of the exhaust gas flow path improves the flow uniformity of the exhaust gas through the exhaust fluid mixing conduit, which in turn improves the uniformity of the subsequently converted ammonia downstream. Additionally, by mixing the exhaust fluid with the exhaust gas by means of a vortex, the exhaust fluid mixing conduit reduces the use of any substantial obstructions (e.g. baffles or other obstructions defining apertures) along the exhaust gas flow path which may give rise to a notable pressure drop along the exhaust fluid mixing conduit As such, the exhaust fluid mixing conduit may have a reduced pressure drop relative to exhaust fluid mixing devices which make use of flow obstructions for exhaust fluid mixing. Furthermore, by reducing the use of flow obstructions to improve exhaust fluid mixing, the exhaust fluid mixing conduit reduces and / or eliminates the risk of exhaust fluid deposits forming on surfaces of the exhaust fluid mixing conduit. Exhaust fluid deposits forming on surfaces of the exhaust fluid mixing conduit can lead to a build of urea within the exhaust fluid mixing device, which in turn can reduce the performance of the exhaust fluid mixing device over time. As such, the exhaust fluid mixing device of the first aspect may have an improved lifetime and / or reliability. Overall, the improved mixing capability of the exhaust fluid mixing device of the first aspect allows exhaust fluid to be mixed with exhaust gas and converted to ammonia at relatively low temperatures (e.g. when cold starting an engine and / or for a low load running engine). That is to say, the exhaust fluid mixing conduit may provide for the mixing of exhaust fluid with exhaust gas such that ammonia is produced downstream which reduces the need for application of external thermal energy and / or allows greater quantities of exhaust fluid to be mixed with exhaust gas. Such an exhaust fluid mixing conduit can thereby improve the overall emissions performance of an SCR system. Brief description of the drawings Embodiments of the disclosure will be set out with reference to the following non-limiting figures in which: Figure 1 shows an exhaust fluid injection system, not according to the claims; Figure 2 shows an exhaust fluid injection system comprising an exhaust fluid mixing conduit according to one or more embodiments; Figure 3 shows an exhaust fluid mixing conduit according to a first embodiment; Figure 4 shows an exhaust fluid mixing conduit according to a second embodiment; Figure 5 shows an exhaust fluid mixing conduit according to a third embodiment; Figure 6 shows an exhaust fluid mixing conduit according to a fourth embodiment; Figure 7 shows an exhaust fluid mixing conduit according to a fifth embodiment; Figure 8 shows an exhaust fluid mixing conduit according to a sixth embodiment; Figure 9 shows an exhaust fluid mixing conduit according to a seventh, eighth, ninth, tenth, and eleventh embodiment; Figure 10 shows a top view of an exhaust fluid mixing conduit according to a twelfth embodiment; Figure 11 shows a perspective view of the exhaust fluid mixing conduit of the twelfth embodiment; Figure 12 shows a perspective view of a spiral mixing blade and two sub-mixing blades of the exhaust fluid mixing conduit of the twelfth embodiment; Figure 13 shows a flow diagram of the exhaust fluid mixing conduit of twelfth embodiment; Figure 14 shows a flow diagram of a region proximate an exhaust fluid injector of the twelfth embodiment; Figure 15 shows an embodiment of an exhaust fluid injection system comprising an exhaust fluid mixing conduit, wherein an output of an exhaust gas is redirected before passing the exhaust gas through a selective catalytic reduction (SCR) system; Figure 16 shows an embodiment of an exhaust fluid injection system comprising an exhaust fluid mixing conduit, wherein an exhaust gas received in a first direction is output in a second direction, the second direction being aligned with the first direction; Figure 17 shows a cross sectional view of an exhaust fluid mixing conduit (left image) and a side-on view (right image); Figure 18 shows a perspective view of an exhaust fluid mixing conduit; Figure 19 shows the view of Figure 18, rotated by about 180 degrees about a first direction X; Figure 20 shows an exhaust fluid mixing conduit according to a thirteenth embodiment; Figure 21 shows an exhaust fluid mixing conduit according to a fourteenth embodiment; Figure 22 shows a perspective view of the exhaust fluid mixing conduit according to the fourteenth embodiment; Figure 23 shows a cross-sectional view of the exhaust fluid mixing conduit according to the fourteenth embodiment; Figure 24 shows a perspective view an arrangement of spiral mixing blades similar to the twelfth embodiment, wherein the spiral mixing blades extend over the outlet; and Figure 25 shows a cross-sectional view of the arrangement of spiral mixing blades of Figure 24. Detailed description Figure 1 shows an exhaust fluid injection system 100, not according to the claims. The exhaust fluid injection system 100 comprises an exhaust fluid injector 106 configured to inject exhaust fluid into the exhaust gas. The exhaust fluid injection system 100 further comprises an exhaust fluid mixing conduit 104 comprising one or more mixing elements 108. The exhaust fluid mixing conduit is provided downstream of the exhaust fluid injector 106. The exhaust fluid mixing conduit 104 may cause the exhaust fluid to mix with the exhaust gas. The exhaust fluid injection system 100 may further comprise a selective catalytic reduction system (SCR) 112 SCR 112 is provided downstream of the exhaust fluid mixing conduit 104. The SCR 112 may catalyse a chemical reaction between the exhaust fluid and the exhaust gas which may decrease the amount of Nitrogen Oxide in the exhaust gas. As such, the exhaust fluid injector 106 may inject an exhaust fluid which is a reductant, wherein the reductant reacts with the nitrogen oxide in the exhaust gas at the SCR 112. Figure 2 shows an exhaust fluid injection system 200 comprising an exhaust fluid mixing conduit 204 according to a first embodiment of the disclosure. Similar with the exhaust fluid injection system 100 of Figure 1, the exhaust fluid injection system 200 comprises an exhaust fluid injector 206 configured to inject exhaust fluid into the exhaust gas. Also similarly with the exhaust fluid injection system 100, the exhaust fluid injection system 200 may further comprise an exhaust fluid mixing conduit 204 and an SCR 212, wherein the exhaust fluid mixing conduit 204 may direct the exhaust gas to the SCR 212. It will be appreciated that the exhaust fluid mixing conduit of Figure 1 is configured to output the exhaust gas in substantially the same direction. By contrast, the exhaust fluid mixing conduit 204 of Figure 2 comprises an inlet 202 and an outlet 210. As shown in Figure 2, the exhaust fluid mixing conduit 204 is configured to output exhaust gas from the outlet 210 in a direction transverse to the direction in which the exhaust gas is received at the inlet 202. As such, the inlet 202 is configured to receive the exhaust gas in a first direction (indicated by the axis X), and the outlet 210 is configured to output the exhaust gas in a second direction (indicated by the axis Y). The first direction X may be transverse to the second direction Y. As will be described below, the exhaust fluid mixing conduit 204 may be configured to create a vortex downstream of the exhaust fluid mixing conduit 204 by means of a spiralshaped exhaust gas flow path. According to this disclosure, the inlet 202 to the exhaust fluid mixing conduit 204 is considered to be the part of the exhaust gas flow path at which the spiral-shaped exhaust gas flow path starts. That is to say, there may be an exhaust gas flow path upstream of the exhaust fluid mixing conduit 204 which follows a different trajectory. For example, in Figure 2 an exhaust gas conduit 211 is provided upstream of the exhaust fluid mixing conduit 204 which extends in a linear manner aligned with the X-direction of the exhaust gas system 200. In some embodiments (not shown in Figure 2), the exhaust gas conduit 211 may be provided upstream of the exhaust fluid mixing conduit 204 which extends in a curved manner. The exhaust gas conduit 211 provides exhaust gas to the inlet 202 of the exhaust fluid mixing conduit 204. As shown schematically in Figure 2, the exhaust gas mixing conduit 204 comprises one or more spiral mixing blades 208 provided within the exhaust fluid mixing conduit 204. The one or more spiral mixing blades 208 and an outer wall 214 of the exhaust gas mixing conduit 204 define an exhaust gas flow path through the exhaust fluid mixing conduit 204. The exhaust gas flow path spirals from the inlet 202 to the outlet 210 about the second direction Y. As such, by providing one or more spiral mixing blades 208, a vortex of exhaust gas may be created at the outlet 210. The vortex of exhaust gas which extends downstream of outlet 210 may provide for further mixing of the exhaust gas. As such, while some mixing of the exhaust gas and exhaust fluid may occur while the exhaust gas is travelling through the exhaust gas mixing conduit 204, mixing of the exhaust gas and exhaust fluid may continue downstream of outlet 210 due to the vortex created by the spiral exhaust gas flow path / spiral mixing blades 208. As such, in some embodiments, it will be appreciated that the spiral mixing blades promote mixing of the exhaust gas / exhaust fluid predominantly through the creation of a vortex. As shown schematically in Figure 2, exhaust fluid is injected in a direction aligned with the second direction such that the vortex of exhaust gas rotates to mix the exhaust gas with the exhaust fluid. Due to the vortex mixing, a greater quantity of exhaust fluid may be mixed with the exhaust gas relative to non-vortex-based mixing approaches. According to embodiments of the disclosure, the one or more spiral mixing blades 208 may be arranged within the exhaust fluid mixing conduit 204 in a variety of manners, as further described below. It will be appreciated that each arrangement of the one or more spiral mixing blades 208 in the exhaust fluid mixing conduit 204 may be used in the exhaust gas system 200 of Figure 2. Figure 3 shows a cross sectional view of the exhaust fluid mixing conduit 204 of the first embodiment. In Figure 3, the second direction Y (indicated in Figure 2, not shown in Figure 3) may be into the page. The first direction (indicated in Figure 2 by X) is indicated in Figure 3 by a large black arrow. In the first embodiment, the one or more spiral mixing blades comprise a primary spiral mixing blade 320 provided within the exhaust fluid mixing conduit 204 extending from the outer wall 214 proximate the inlet 202. In Figure 3, the inlet 202 to the exhaust fluid mixing conduit 204 is denoted by a dashed line. A substantially straight exhaust gas conduit 211 may be provided upstream of the inlet 202. In some embodiments, a curved exhaust gas conduit may be provided upstream of the inlet 202. The inlet 202 may have any suitable shape to allow exhaust gas to be received by the exhaust fluid mixing conduit 204, for example a circular opening, an elliptical opening, a square opening, a hexagonal opening, or any other regular or irregular polygon shape. The exhaust fluid mixing conduit 204 defines an exhaust gas flow path 318 through the exhaust fluid mixing conduit which spirals from the inlet 202 to the outlet 210 about the second direction. As shown in Figure 3, the outer wall 214 of the exhaust fluid mixing conduit 204 follows a spiral shaped trajectory in a plane normal to the second direction Y. In some embodiments (not shown in Figure 3) the exhaust fluid mixing conduit 204 may follow a spiral shaped trajectory in a plane at a 60 degree, a 70 degree, or an 80 degree angle to the second direction Y. As the spiral shaped trajectory of the outer wall 214 completes a revolution about the second direction, the primary spiral mixing blade 320 extends from the outer wall 214 in order to continue the spiral-shaped trajectory of the exhaust gas flow path. As shown in Figure 3, the primary spiral mixing blade 320 continues the spiral-shaped trajectory from the outer wall 214 towards the outlet 210. It will be appreciated that as the primary spiral mixing blade 320 extends along the spiral shaped trajectory, the primary spiral mixing blade 320 will overlap with outlet 210 formed in the exhaust fluid mixing conduit 204. In the embodiment of Figure 3, the primary spiral mixing blade 320 extends along the spiral shaped trajectory substantially all the way until it overlaps with the outlet 210. As will be appreciated from Figure 3, the outer wall 214 of the exhaust fluid mixing conduit 204 effectively defines a chamber 213. Exhaust gas flows through the chamber 213 along a spiral shaped exhaust gas flow path from the inlet 202 to the outlet 210. The primary spiral mixing blade 320 extends from the inlet 202 into the chamber 213 in order to define the spiral shaped exhaust gas flow path 318 in combination with the outer wall 214. Figure 17 shows a cross-sectional view of the exhaust fluid mixing conduit 204 (left image) and a side-on view (right image). The second direction Y is into the page for the cross-sectional view (left image) and the second direction Y is along the dot-dash line for the side view (right image). As can be seen in the right image, the shape of the outer wall 214 varies along the second direction Y. The outer wall 214 may comprise one or more sections of outer wall 214. For example, as shown in Figure 17, the outer wall 214 comprises a fluid injector section 231, a connector section 232 and a transition section 233. The fluid injector section 231 may provide a mounting surface for the exhaust gas fluid injector 206 (not shown in Figure 17) and / or may comprise the fluid injector inlet 216 configured to allow exhaust fluid to be injected into the exhaust gas flow path 318 (not shown in Figure 17). In some embodiments, the fluid injector section 231 may be a generally planar section. In the embodiment of Figure 17, the fluid injector section is may be generally conical section, or a generally dome-shaped section. Said conical section may be centred about the second direction, the fluid injector inlet or the outlet 210. As such, as shown in Figure 17, the fluid injector section 231 may taper outward along the second direction Y from the fluid injector inlet 216 to the connector section 232. In some embodiments (not shown in Figure 18), the fluid injector section 231 may taper inward or may not taper along the second direction Y to the connector section 232. The taper (whether inward or outward) of the fluid injector section 231 may be linear or non-linear. The connector section 232 may separate the fluid injector section 231 and the transition section 233. The connector section 232 may, at least in part, define the spiral shape of the outer wall 214 of the exhaust fluid mixing conduit 204. For example, as shown in Figure 17, the cross section shown (left image) is a cross section through the connector section 232. In the embodiment of Figure 17, the connector section 232 is a generally spiral prism shape which is aligned with the second direction (Y). Of course, in other embodiments, the connector section 232 may have a different shape / profile. As shown in Figure 17, the transition section 233 may taper inward along the second direction Y from the connector section 232 to the outlet 210. That is, a radius of the transition section 233 about a central axis of the transition section 233 decreases from the connector section 232 to the outlet 210. The exhaust gas conduit may continue downstream of the outlet 210, for example a substantially straight exhaust gas conduit is provided downstream in the embodiment of Figure 17. In other embodiments, the exhaust gas conduit downstream of the outlet 210 may be curved or any other shape (e.g. as shown in Figures 15 and 16). In some embodiments (not shown in Figure 17), the transition section 233 may taper outward along the second direction Y. The taper (whether inward or outward) of the transition section 233 may be linear or non-linear. Figures 18 and 19 shows perspective views of the exhaust fluid mixing conduit 204. The view in Figure 18 is rotated by around 180 degrees about the first direction X compared with the view in 19. The outer wall 214 may have different geometries. Shown in Figure 18, the fluid injector section 231 may have a dome shape. In some embodiments, the fluid injector section 231 may have a cylindrical, a conical inward or a conical outward shape. Shown in Figures 19 and 20, the transition section 233 may have an inward cone shape. In some embodiments, the transition section 233 may have a cylindrical, a dome, or a conical outward shape. In this context, inward / outward refer to an increase / decrease in a diameter of the outer wall 233 along the second direction Y. As shown in Figure 17, the spiral mixing blade 320 may extend in the second direction Y between the fluid injector section 231 and the transition section 233. The spiral mixing blade 320 may extend completely or partially between the fluid injector section 231 and the transition section 233. For example, the spiral mixing blade 320 may extend 80%, 90%, and / or 100% of a distance between the fluid injector section 231 and the transition section 233. The spiral mixing blade 320 may connect with and / or may be disconnected from the fluid injector section 231 and / or the transition section 233. In the embodiment of Figure 17, the spiral mixing blade 320 may be in direct contact with the fluid injector section 231 (i.e. the surface on which the exhaust fluid injector 206 is mounted). The spiral mixing blade may extend in the second direction (Y) such that it defines a spiral surface which has a component which is aligned with the second direction. The outlet 210 may be defined by an opening formed in the outer wall 214. Downstream of the outlet 210 exhaust gas may continue to mix with the exhaust fluid as it travels to the SCR 216. The outlet 210 may have any suitable shape to allow exhaust gas to exit the exhaust fluid mixing conduit 204, for example a circular opening, an elliptical opening, a square opening, a hexagonal opening or any other regular or irregular polygon shape. In some embodiments, the outlet 210 may be aligned with a bottom of the primary spiral mixing blade 320 As shown in Figure 3, the outer wall 214 and the primary spiral mixing blade 320 define an outer surface for the exhaust gas flow path 318. As such, the outer surface may be an internal surface of the exhaust fluid mixing conduit 204. For example, the outer surface may be the internal surface formed by the outer wall 214 and the primary spiral mixing blade 320. As shown in Figure 3, the outer surface may follow an inward spiral shape from the inlet 202 to the outlet 210. In some embodiments (e.g. not shown in Figure 3), the outer surface may follow an outward spiral shape from the inlet 202 to the outlet 210. The inward spiral shape or the outward spiral shape may be one of an Archimedean spiral, a hyperbolic spiral, a logarithmic spiral, and a Fibonacci spiral. Consequently, as the exhaust gas flows from the inlet 202 to the outlet 210, the exhaust gas may be rotated about the second direction Y, and a vortex may be created at the outlet 210 rotating about the second direction. As shown in e.g. Figure 3, the outer wall 214 may extend from the outer wall of the exhaust conduit 211 upstream of the inlet 202 to the exhaust fluid mixing conduit 204. That is to say, the outer wall 214 may provide a smooth transition from the exhaust conduit 211 to the exhaust fluid mixing conduit 204 which does not introduce any significant turbulence at the inlet 202. Referring again to Figure 2, the outer wall 214 may comprise a fluid injector inlet 216 configured to allow exhaust fluid to be injected into the exhaust gas flow path 318. The exhaust fluid injector 206 may be configured to inject exhaust fluid into the exhaust gas flow path 318. For example, a nozzle of the fluid injector 206 which ejects exhaust fluid may extend through the fluid injector inlet 216. In some embodiments, the outlet 210 may be configured to output the exhaust gas in the second direction Y and the fluid injector inlet 216 may be configured to receive injected exhaust fluid from the exhaust fluid injector 216 in the second direction Y. As such, the outlet 210 and the fluid injector inlet 216 may be aligned with the second direction Y and / or with each other. In some embodiments, the exhaust fluid injector 206 and / or the fluid injector inlet 216 may be angled relative to the second direction. That is to say, the exhaust fluid injector 206 and / or the fluid injector inlet 216 may be configured to inject exhaust fluid into the exhaust gas flow path 318 at an angle 0 (e.g. a non-zero angle) to the second direction Y. For example, the fluid injector inlet 216 may be configured to receive injected exhaust fluid aligned at an angle of no greater than 45, 30, 15, 10, or 5 degrees of the second direction Y. In some embodiments, the exhaust fluid injector 206 may be configured to inject exhaust fluid through a centre of the spiral of the exhaust gas flow path 318. For example, the spiral of the exhaust gas flow path 318 may be characterised by a flow of the exhaust gas in the second direction which rotates around an axis aligned with the second direction. The centre of the spiral may be the axis. The exhaust fluid injector 206 may be provided centrally with respect to the outlet 210. The outlet 210 may overlap with the centre of the inward spiral of the exhaust gas flow path 318. In some embodiments, a centre of the outlet 210 may be aligned with the central axis of the spiral. By injecting exhaust fluid through the centre of the inward spiral of the exhaust gas flow path 318, the exhaust fluid injector 206 may inject the exhaust fluid through the vortex. The exhaust fluid may be injected through the centre of the inward spiral from the fluid injector inlet 216 to the outlet 210. In some embodiments, the exhaust fluid injector 206 may be configured to inject exhaust fluid offset by from the centre of the spiral of the exhaust gas flow path 318. The offset may be distance of around 1 mm, around 5mm, around 10mm, around 15mm, or around 20mm, wherein the distance may be defined as a shortest distance from the centre of the spiral of the exhaust gas flow path to the fluid injector inlet 216. In some embodiments, the exhaust fluid injector 206 may be configured to inject exhaust fluid in extending from the exhaust fluid injector 206 in the second direction Y and expanding in the first direction X. In some embodiments, the exhaust fluid injector 206 may be configured to inject exhaust fluid generally in a cone extending from the exhaust fluid injector 206 in the second direction Y and expanding in the first direction X. In some embodiments, the cone may expand in a plane parallel with the first direction X. In some embodiments, the cone may expand in the first direction X or a plane parallel with the first direction X to a radius which may be less than a radius of the outlet 210. As such, in embodiments where the exhaust fluid injector 206 is offset from a central axis of the spiral, the spray cone may still substantially overlap with the central axis of the spiral exhaust gas flow. Furthermore, in some embodiments the radius of the expanded cone may be similar to the radius of the vortex. By injecting a cone of the exhaust fluid into a vortex of the exhaust gas, the mixing of the exhaust fluid with the exhaust gas may be improved. Of course, in other embodiments, the exhaust fluid injector 206 may inject exhaust fluid into the exhaust fluid mixing conduit 204 having a different profile. Further embodiments of the exhaust fluid mixing conduit 204 will now be described. It will be appreciated from the following discussion that the following exhaust fluid mixing conduits 204 may be provided in the exhaust mixing injection system 200. Figure 4 shows the exhaust fluid mixing conduit 204 according to a second embodiment. Compared with the first embodiment, the second embodiment of the exhaust fluid mixing conduit 204 may further comprise a secondary spiral mixing blade 420. The secondary spiral mixing blade 420 may be provided within the exhaust fluid mixing conduit 204 between the outer wall 214 and the primary spiral mixing blade 320. The secondary spiral mixing blade 420 may extend from the inlet 202 to the outlet 210 in order to define a plurality of exhaust gas flow paths 318a, 318b which spiral inward from the outlet about the second direction Y. For example, the primary spiral mixing blade 320 and the secondary spiral mixing blade 420 may each extend from the inlet 202 to the outlet 210 in order to define a first exhaust gas flow path 318a and a second exhaust gas flow path 318b. Figure 5 shows the exhaust fluid mixing conduit 204 according to a third embodiment. Compared with the first and second embodiments, the third embodiment of the exhaust fluid mixing conduit 204 may further comprise a tertiary spiral mixing blade 520. The tertiary spiral mixing blade 520 may be provided within the exhaust fluid mixing conduit 204 between the outer wall 214 and the secondary spiral mixing blade 420. The secondary spiral mixing blade 420 in the third embodiment may be provided within the exhaust fluid mixing conduit 204 between the tertiary spiral mixing blade 520 and the primary spiral mixing blade 320. The tertiary spiral mixing blade 520 may extend from the inlet 202 to the outlet 210 in order to define at least three exhaust gas flow paths 318a, 318b, 318c, which spiral inward from the inlet 202 to the outlet 210 about the second direction Y. For example, the primary spiral mixing blade 320, the tertiary spiral mixing blade 420, and the tertiary spiral mixing blade 520 may each extend from the inlet 202 to the outlet 210 in order to define a first exhaust gas flow path 318a, a second exhaust gas flow path 318b, and a third exhaust gas flow path 318c. Figure 6 shows the exhaust fluid mixing conduit 204 according to a fourth embodiment. Compared with the first embodiment, the exhaust fluid mixing conduit 204 according to the fourth embodiment may further comprise a sub-mixing blade 621a provided along the exhaust gas flow path 318. The sub-mixing blade 621a may divide the exhaust gas flow path 318 into two exhaust gas sub-flow paths 319a, 319b. The sub-mixing blade may extend along the exhaust gas flow path 318 to the outlet 210 of the exhaust fluid mixing conduit 204. For example, the sub-mixing blade 621a may extend along a length of the exhaust gas flow path 318 which is at least 5%, at least 10%, at least 15%, at least 20%, or at least 25% of the total length of the exhaust gas flow path 318. In some embodiments, one or more sub-mixing blade 621, or each sub-mixing blade 621a, 621b, 621c may extend along a length of the exhaust gas flow path by at least: 5 mm, 10 mm, 20, mm, or 30 mm. By providing one or more sub-mixing blades of such a length, the vortex created by the exhaust fluid mixing conduit 204 may more efficiently mix exhaust fluid with the exhaust gas. In some embodiments, the length of the exhaust gas flow path 318 may be defined by a length measured from the inlet 202 along a centre of the exhaust gas flow path to the outlet 210. In some embodiments, where the exhaust gas flow path 318 rotates in a plane generally transverse to the second direction (e.g. as shown in the cross section of Figure 3), the length of the exhaust gas flow path 318 may be defined by a length measured from the inlet 202 along a centre of the exhaust gas flow path to the outlet 210 in the plane transverse to the second direction. In some embodiments, the exhaust fluid mixing conduit 204 may comprise a plurality of submixing blades 621a, 621b. Each of the plurality of sub-mixing blades 621a, 621b may be provided similarly as for the sub-mixing blade 621a described above. Each of the plurality of sub-mixing blades 621a, 621b may divide the exhaust gas flow path 318. In some embodiments, the plurality of sub-mixing blades 621a, 621b may divide the exhaust gas flow path 318 into at least three exhaust gas sub-flow paths. For example, as shown in Figure 6, the exhaust fluid mixing conduit 204 may comprise a first sub-mixing blade 621a, and a second sub-mixing blade 621b. The first sub-mixing blade 621a may divide the exhaust gas flow path 318 into a first exhaust gas sub-flow path 319a and a second exhaust gas sub-flow path 319b. The second sub-mixing blade 621b may separate the exhaust gas flow path 318 into the second exhaust gas sub-flow path 319b and a third exhaust gas sub-flow path 319c. Figure 7 shows the exhaust fluid mixing conduit 204 according to a fifth embodiment. Similar to the second embodiment, the fifth embodiment of the exhaust fluid mixing conduit 204 comprises a primary spiral mixing blade 320 and a secondary spiral mixing blade 420 which define first and second exhaust gas flow paths 318a, 318b. The fifth embodiment further comprises a plurality of sub-mixing blades 621a, 621b, 621c, 621 d, 621e, 621f, which further sub-divide the first and second exhaust gas flow paths 318a, 318b. The first exhaust gas flow path 318a is divided by three sub-mixing blades 621a, 621b, 621c into three exhaust gas sub-flow paths. Similarly, the second exhaust gas flow path 318b is divided by three sub-mixing blades 621d, 621e, 621 f into three exhaust gas sub-flow paths. As such, describing the evolution of the number of paths from upstream of the inlet 202 to downstream of the outlet 210, there may a single upstream exhaust gas flow path upstream of the inlet 202, then two exhaust gas flow paths 318a, 318b, then six exhaust gas sub-flow paths. The six exhaust gas sub-flow paths may then flow into a single exhaust gas outflow path downstream of the outlet 210 (i.e. the exhaust gas sub-flow paths may recombine at the outlet 210). Figure 8 shows the exhaust fluid mixing conduit 204 according to a sixth embodiment. Compared with the third embodiment, the sixth embodiment of the exhaust fluid mixing conduit 204 further comprises the plurality of sub-mixing blades 621a and the plurality of exhaust gas sub-flow paths 319a. For simplicity of the diagram, each of the sub-mixing blades 621a and exhaust gas sub-flow paths 319a has not been labelled individually. In the example shown in Figure 8, each of the first exhaust gas flow path 318a, the second exhaust gas flow path 318b, and the third exhaust gas flow path 318c is divided by two sub-mixing blades 621a into three exhaust gas sub-flow paths 319a. As such, describing the evolution of the number of paths from upstream of the inlet 202 to downstream of the outlet 210, there may be a single upstream exhaust gas flow path upstream of the inlet 202, then three exhaust gas flow paths 318a, 318b, 318c, then nine exhaust gas sub-flow paths 319a, then a single exhaust gas outflow path downstream of the outlet (the flow paths may recombine at the outlet 210). Figure 9 shows the exhaust fluid mixing conduit 204a, 204b, 204c, 204d, 204e according to a seventh, eighth, ninth, tenth, and eleventh embodiment. The exhaust fluid mixing conduit 204a according to the seventh embodiment comprises a single inlet 202. The single inlet 202 of the seventh embodiment may comprise a primary spiral mixing blade 320a and five sub-mixing blades (not labelled). In some embodiments, the outer wall 214 may define a plurality of inlets 202a, 202b, 202c, 202d, 202e. Each inlet 202a, 202b, 202c, 202d, 202e may be configured to receive the exhaust gas in a direction transverse to the second direction Y. The exhaust fluid mixing conduit may comprise a plurality of spiral mixing blades 320a, 320b, 320c, 320d, 320e. Each primary spiral mixing blade 320a, 320b, 320c, 320d, 320e may be provided within the exhaust fluid mixing conduit 204a, 204b, 204c, 204d, 204e extending from the outer wall 214 proximate to the respective inlet 202a, 202b, 202c, 202d, 202e. In the eighth embodiment of the exhaust fluid mixing conduit 204b, the one or more inlets 202a, 202b may be radially separated. For example, the exhaust fluid mixing conduit 204b according to the eighth embodiment comprises a first inlet 202a and a second inlet 202b. Each of the first inlet 202a and the second inlet 202b may be at the same circumferential location, but radially separated. Therefore, the first inlet 202a maybe further than the second inlet 202b from the outlet 210 in a radial direction from the outlet 210. In the ninth to eleventh embodiments, the one or more separated inlets 202a, 202b, 202c, 202d of the exhaust fluid mixing conduit 204c, 204d, 204e may be circumferentially separated. For example, the exhaust fluid mixing conduit 204c according to the ninth embodiment comprises the first inlet 202a and the second inlet 202b. The first and second inlets 202a, 202b of the ninth embodiment may be at the same radial position, but circumferentially separated. In some embodiments, the first and second inlets 202a, 202b may be circumferentially separated by about 180°, although other circumferential separations may be used. Consequently, the exhaust fluid mixing conduit 204c according to the ninth embodiment may receive the exhaust gas in a first inflow direction through the first inlet 202a and in a second inflow direction through the second inlet 202b. The first and second inflow directions may each be transverse to the second direction Y. Compared with the ninth embodiment, the exhaust fluid mixing conduit 204d according to the tenth embodiment further comprises a third inlet 202c. The first, second, and third inlets 202a, 202b, 202c of the tenth embodiment may be at the same radial position, but circumferentially separated. In some embodiments, the first, second, and third inlets 202a, 202b, 202c may be circumferentially separated by about 120°. Consequently, the exhaust fluid mixing conduit 204d according to the tenth embodiment may receive the exhaust gas in a first inflow direction through the first inlet 202a, in a second inflow direction through the second inlet 202b, and in a third inflow direction through the third inlet 202c. The first, second, and third inflow directions may be transverse to the second direction Y. Compared with the tenth embodiment, the exhaust fluid mixing conduit 204e according to the eleventh embodiment further comprises a fourth inlet 202d. The first, second, third, and fourth inlets 202a, 202b, 202c of the eleventh embodiment may be at the same radial position, but circumferentially separated. In some embodiments, the first, second, and third inlets 202a, 202b, 202c, 202d may be circumferentially separated by 90°. Consequently, the exhaust fluid mixing conduit 204e according to the eleventh embodiment may receive the exhaust gas in a first inflow direction through the first inlet 202a, in a second inflow direction through the second inlet 202b, in a third inflow direction through the third inlet 202c, and in a fourth direction through the fourth inlet 202d. The first, second, third, and fourth inflow directions may be transverse to the second direction Y. Figure 10 shows a top view of an exhaust fluid mixing conduit 204 according to a twelfth embodiment. The exhaust fluid mixing conduit 204 of the twelfth embodiment comprises the outer wall 214, the primary spiral mixing blade 320, the first sub-mixing blade 621a, and the second sub-mixing blade 621b. Figure 11 shows a perspective view of the exhaust fluid mixing conduit 204 of the twelfth embodiment. The first direction X may be aligned with the large arrow labelled “Gas in’’(as shown in Figure 10). The second direction Y may be aligned with a direction into the page of Figure 10. As such, as can be seen in Figure 11, the exhaust fluid mixing conduit 204 may be provided as part of a U-shaped transition between two exhaust conduits. As such, the exhaust gas may flow upstream of the inlet 202 in a direction opposite to the second direction Y, and subsequently be redirected by the outer wall defining the exhaust gas conduit 211 upstream of the inlet 202 and the outer wall 214 of the exhaust fluid mixing conduit 204. Then, the exhaust gas may flow through the mixing blades 320, 621a, 621b, and be redirected through the outlet 210 in the second direction Y. As such, in this embodiment, the exhaust gas may be reversed in direction by passing through the exhaust fluid mixing conduit 204. For example, the shape of the outer wall 214 at the inlet 202 may be angled toward the outlet 210 such that, as the exhaust gas is received by the inlet 202, the exhaust gas is re-directed into the first direction X, such that the exhaust gas flows toward the outlet 210. Figure 12 shows a perspective view of the primary spiral mixing blade 320 and first and second sub-mixing blades 621a, 621b of the exhaust fluid mixing conduit of the twelfth embodiment. Each of the blades 320, 621a, 621b may be angled toward the outlet 210 such that, as the exhaust gas flows through the blade, the exhaust gas is re-directed in the second direction Y, such that the exhaust gas is output by the outlet 210 in the second direction Y. Figure 13 shows a flow diagram of the exhaust fluid mixing conduit 204 of the twelfth embodiment. The arrows represent the velocity of the exhaust gas at the location of the arrow. The flow diagram shows that the exhaust gas is guided by the outer wall 214 into the first direction X around the inlet 202. The flow diagram also shows that the exhaust gas is caused to rotate by the outer wall 214, the primary spiral mixing blade 320, and the first and second sub-mixing blades 621a, 621b. As such, the exhaust gas may rotate around the outlet 210. The exhaust gas may be prevented by the primary spiral mixing blade 320 from recirculating with the exhaust gas from the inlet 202. Figure 14 shows a cross-sectional flow diagram of the exhaust gas flow at the outlet 210 of the exhaust gas conduit 204 in a plane normal to the second direction. The arrows indicate the velocity of the exhaust gas in the plane normal to the second direction. As such, it will be appreciated that the exhaust gas passing though the outlet 210 is imparted with a rotational velocity such that a vortex is formed For example, the vortex may extend from fluid injector section 231 and extend through the outlet 210. As such, the vortex may be provided downstream of the outlet 210. The creation of the vortex improves the mixing of the exhaust gas with the exhaust gas fluid. In any of the above-described embodiments, the spiral mixing blade 208, 320a, 320b, 320c, 320d, 320e, 420, 520 may extend from the outer wall 204 along a spiral shaped trajectory until they overlap (in a plane normal to the second direction) with outlet 210 formed in the exhaust fluid mixing conduit 204. In some embodiments, one or more spiral mixing blades 208, 320a, 320b, 320c, 320d, 320e, 420, 520 may further extend along the spiral shaped trajectory such that they overlap the outlet 210 in the plane normal to the direction. By extending the spiral mixing blades 208 along the spiral shaped trajectory to overlap the outlet 210, the vortex of exhaust gas may be focused around the injected exhaust fluid. For example, the vortex may be focused around a cone of injected exhaust fluid. Figure 20 shows an exhaust fluid mixing conduit 204 according to a thirteenth embodiment. The exhaust fluid mixing conduit 204 according to the thirteenth embodiment is similar to the exhaust fluid mixing conduit 204 according to the first embodiment, but in the thirteenth embodiment the primary spiral mixing blade 320 overlaps the outlet 210 following the spiral shaped trajectory. In addition to the spiral mixing blades 208, 320a, 320b, 320c, 320d, 320e, 420, 520 extending over the outlet, in some embodiments, one or more of the sub-mixing blades 621 may also extend along a spiral shaped trajectory such that they overlap the outlet 210. For example, Figure 21 shows an exhaust fluid mixing conduit 204 according to a fourteenth embodiment. Figure 22 shows a perspective view of the exhaust fluid mixing conduit according to the fourteenth embodiment. The exhaust fluid mixing conduit 204 according to the fourteenth embodiment is similar to the exhaust fluid mixing conduit 204 according to the fourth embodiment, but in the fourteenth embodiment the primary spiral mixing blade 320 and the first and second sub-mixing blades 621a, 621b are each extended such that each overlap the outlet 210 in a plane normal to the second direction. In some embodiments, each of the spiral mixing blades 208, 320 may extend in the second direction a constant amount (i.e. along the spiral trajectory of the spiral mixing blade 208, the spiral mixing blade may have a constant length in the second direction). In some embodiments, the spiral mixing blades 208 may vary in length in the second direction Y along the spiral trajectory. By varying the length of the spiral mixing blades 208 in the second direction Y along the spiral trajecotry, it is possible to reduce impingement of the spiral mixing blades 208 on the injected exhaust fluid. As described previously, the injected exhaust fluid may extend in the second direction Y and expand in the first direction X (e.g. in a cone shape). As such, the variable length of the spiral mixing blades 208 along the spiral trajectory may accommodate the profile of the injected exhaust fluid. For example, where one or more spiral mixing blades 208, 320 overlaps the outlet 210 (or where one or more sub-mixing blades 621 overlaps the outlet 210) the spiral mixing blade 208 may reduce in length in the second direction along the spiral-shaped trajectory. As shown in Figure 23, by reducing the length of the spiral mixing blade 208 in the second direction as the spiral mixing blade approaches the centre of the spiral, the spiral mixing blade 208 may accommodate the profile of the injected exhaust fluid (conical shaped profile in Figure 23, but other profiled may be accommodated. As such, the length of each spiral mixing blade 208 and / or sub-mixing blade 621 may be varied along the spiral trajectory such that impingement of the injected exhaust fluid on the spiral mixing blade 208 / sub-mixing blade 621 is reduced or eliminated. In particular, by reducing (but not eliminating entirely) the impingement of the exhaust fluid on the spiral mixing blades 208 / sub-mixing blades 621, exhaust fluid droplets may be broken into droplets of smaller diameters, which in turn improves the conversion of exhaust fluid (e.g. urea water converted to ammonia). By controlling the impingement in this way, evaporation of the exhaust fluid from the mixing blades may be controlled such that deposits derived from the exhaust fluid are not formed on the spiral mixing blades 208 / sub-mixing blades 320. Figure 23 shows a cross-sectional view of the exhaust fluid mixing conduit 204 according to the fourteenth embodiment, taken along the dashed line marked A in Figure 21, and reproduced in Figure 23. As shown in Figure 23, the lengths of each of the primary spiral mixing blade 320, and the first and second sub-mixing blades 621a , 621b reduces along the spiral shaped trajectory. As will be appreciated from Figure 23, the length of the one or more spiral mixing blades 208 320 (e.g. primary spiral mixing blade 320 in Figure 23) may be a constant length along part of the spiral shaped trajectory, and may vary in length in other parts. For example, in Figure 23, where the primary spiral mixing blade does not overlap with the outlet 210 in the plane normal to the second direction, the length of the primary spiral mixing blade 320 in the second direction may be constant. At a point along the trajectory of the primary spiral mixing blade (e.g. at a point where the trajectory overlaps the outlet 210), the length of the primary spiral mixing blade 320 becomes variable. For example, in Figures 23, where the trajectory starts to overlap with the outlet 210, the length of the primary spiral mixing blade 320 starts to reduce along the spiral trajectory. Figure 24 shows a perspective view of an arrangement of spiral mixing blades similar to the twelfth embodiment, but wherein the spiral mixing blades extend over the outlet 210 (the outlet 210 is not shown in Figure 24). Similar to the spiral mixing blades shown in the twelfth embodiment, each of the spiral mixing blade 320 and the first and second submixing blades 621a, 621b may be angled toward the outlet 210 such that, as the exhaust gas flows through the blade, the exhaust gas is re-directed in the second direction Y, such that the exhaust gas is output by the outlet 210 in the second direction Y. Figure 25 shows a cross-sectional view of the arrangement of spiral mixing blades of Figure 24 taken along the dashed line marked B. The lengths of each of the primary spiral mixing blade 320, and the first and second sub-mixing blades 621a, 621b in the second direction Y varies along the length of the respective spiral shaped trajectory. As shown in Figure 25, the spiral mixing blade 320 and the first and second sub-mixing blades 621a, 621b may each partially impinge the cone-shaped injection profile of the exhaust fluid (indicated in dashed lines in Figure 25). In some embodiments, as described above with reference to Figure 2, the exhaust fluid mixing conduit 204 may be provided as part of an exhaust fluid injection system 200. As such, the exhaust fluid injection system 200 may comprise any of the embodiments of the exhaust fluid mixing conduits 204 described above. One or more other embodiments of the exhaust fluid injection system 200 will now be described. Figure 15 shows an embodiment of the exhaust fluid injection system 1500 comprising the exhaust fluid mixing conduit 204, wherein an output 210 of an exhaust gas is redirected before passing the exhaust gas through an SCR 212. Consequently, the exhaust gas may be received by the inlet 202 in the first direction X, output by the outlet 210 in the second direction Y, and then redirected through the SCR 212 in a direction opposite to the first direction X. Figure 16 shows an embodiment of the exhaust fluid injection system 1600 comprising the exhaust fluid mixing conduit 204, wherein the exhaust gas received in the first direction X, output by the outlet 210 in the second direction Y, and then redirected through the SCR 212 in a direction aligned with the first direction X. In any of the above embodiments, the exhaust fluid injector 206 may be mounted on the outer wall 214. For example, the exhaust fluid injector may be mounted on the outer wall via a mount provided on the internal surface of the outer wall 214. The skilled person would understand that arrangements other than those described in the are possible and would fall within the scope of the claims. In any of the embodiments, the fluid injection system may be for mixing an exhaust gas from a diesel engine with a diesel exhaust fluid. Industrial applicability According to embodiments of this disclosure, an exhaust mixing conduit is provided. The exhaust mixing conduit may be provided as part an exhaust fluid injection system for mixing an exhaust gas with exhaust fluid. In some embodiments, the exhaust fluid injection system may be provided as part of an internal combustion engine. The exhaust gas fluid injection system may be used with any type of internal combustion engine. In particular, the internal combustion engine may be a diesel engine. As such, the exhaust fluid injection system may be a diesel exhaust fluid (DEF) injection system. Such a DEF injection system may be configured to inject DEF (e.g. a fluid comprising urea water) into a DEF mixing conduit. The DEF may comprise a reductant or otherwise decompose into a reductant in the exhaust system 200 in such that a reduction reaction occurs at the SCR 212. Of course, in other embodiments, the exhaust fluid injection system may be configured to work with other, alternative fuels such as hydrogen and the like. As discussed above, the exhaust fluid mixing conduit 204 defines an exhaust gas flow path which spirals from the inlet to the outlet, wherein the spiral exhaust gas flow path spirals about the outlet (in the second direction). Due to the rotation, a vortex of exhaust gas may be created at the outlet, rotating about the second direction. This vortex flow may allow a greater quantity of the exhaust fluid to be mixed, and subsequently converted to ammonia (in the case of a DEF injection system) by the exhaust fluid system. In addition, this vortex flow may allow a greater quantity of the exhaust fluid to be injected, converted to e.g. ammonia, and mixed by the exhaust fluid mixing conduit. In some embodiments, the exhaust gas flow path through the exhaust fluid mixing conduit spirals inward from the inlet to the outlet. The inward spiral shaped nature of the exhaust gas flow path improves the flow uniformity of the exhaust gas through the exhaust fluid mixing conduit, which in turn may improve the uniformity of the subsequently converted ammonia downstream. Additionally, by mixing the exhaust fluid with the exhaust gas by means of a vortex, the exhaust fluid mixing conduit reduces the use of any substantial obstructions (e.g. baffles or other obstructions defining apertures) along the exhaust gas flow path which may give rise to a notable pressure drop along the exhaust fluid mixing conduit As such, the exhaust fluid mixing conduit may have a reduced pressure drop relative to exhaust fluid mixing devices which make use of flow obstructions for exhaust fluid mixing. Furthermore, by reducing the use of flow obstructions to improve exhaust fluid mixing, the exhaust fluid mixing conduit reduces and / or eliminates the risk of exhaust fluid deposits forming on surfaces of the exhaust fluid mixing conduit. Exhaust fluid deposits forming on surfaces of the exhaust fluid mixing conduit can lead to a build of urea within the exhaust fluid mixing device, which in turn can reduce the performance of the exhaust fluid mixing device over time. As such, the exhaust fluid mixing device of the first aspect may have an improved lifetime and / or reliability. Overall, the improved mixing capability of the exhaust fluid mixing device of the first aspect allows exhaust fluid to be mixed with exhaust gas and converted to ammonia at relatively low temperatures (e.g. when cold starting an engine and / or for a low load running engine). That is to say, the exhaust fluid mixing conduit may provide for the mixing of exhaust fluid with exhaust gas such that ammonia is produced downstream which reduces the need for application of external thermal energy and / or allows greater quantities of exhaust fluid to be mixed with exhaust gas. Such an exhaust fluid mixing conduit can thereby improve the overall emissions performance of an SCR system.

Claims

1. An exhaust fluid mixing conduit for mixing an exhaust gas with exhaust fluid, the exhaust fluid mixing conduit comprising:an outer wall defining:an inlet configured to receive the exhaust gas in a first direction; andan outlet configured to output the exhaust gas in a second direction;a primary spiral mixing blade provided within the exhaust fluid mixing conduit extending from the outer wall proximate the inlet;wherein the outer wall and the spiral mixing blade define an exhaust gas flow path through the exhaust fluid mixing conduit which spirals from the inlet to the outlet about the second direction.

2. The exhaust fluid mixing conduit of claim 1, wherein the second direction is transverse to the first direction.

3. The exhaust fluid mixing conduit of any preceding claim, wherein the outer wall and the spiral mixing blade define an exhaust gas flow path through the exhaust fluid mixing conduit which spirals inward from the inlet to the outlet about the second direction.

4. The exhaust fluid mixing conduit of claim 1, whereinthe outer wall and the primary spiral mixing blade define an outer surface for the exhaust gas flow path which follows an inward spiral shape from the inlet to the outlet.

5. The exhaust fluid mixing conduit of claim 4, whereinthe inward spiral shape is one of: an Archimedean spiral, a hyperbolic spiral, a logarithmic spiral, a Fibonacci spiral, a lituus spiral, and a Theodorus spiral.

6. The exhaust fluid mixing conduit of any preceding claim, further comprisinga secondary spiral mixing blade provided within the exhaust fluid mixing conduit between the outer wall and the primary spiral mixing blade, the secondary spiral mixing blade extending from the inlet to the outlet in order to define a plurality of exhaust gas flow paths which spiral inward from the inlet to the outlet about the second direction.

7. The exhaust fluid mixing conduit of claim 6, further comprisinga tertiary spiral mixing blade provided within the DEF conduit between the outer wall and the secondary spiral mixing blade, the tertiary spiral mixing blade extending from the inlet to the outlet in order to define at least three exhaust gas flow paths which spiral inward from the inlet to the outlet about the second direction.

8. The exhaust fluid mixing conduit of any preceding claim, further comprisinga sub-mixing blade provided along one exhaust gas flow path, wherein the sub mixing blade divides the exhaust gas flow path into two exhaust gas sub-flow paths.

9. The exhaust fluid mixing conduit of claim 8, whereinthe sub-mixing blade extends along the one exhaust gas flow path to the outlet of the conduit.

10. The exhaust fluid mixing conduit of claim 8 or claim 9, whereinthe sub-mixing blade extends along a length of the one exhaust gas flow path which is at least 5 % of a length of the exhaust gas flow path from the inlet to the outlet.

11. The exhaust fluid mixing conduit of any of claims 8 to 10, whereina plurality of sub-mixing blades are provided along one exhaust gas flow path, wherein the plurality of sub-mixing blades divide the one exhaust gas flow path into at least two exhaust gas sub-flow paths.

12. The exhaust fluid mixing conduit of any preceding claim, whereinthe outer wall defines a plurality of inlets, each inlet configured to receive the exhaust gas in a direction transverse to the second direction; andthe exhaust fluid mixing conduit comprises a plurality of primary spiral mixing blades, each primary spiral mixing blade provided within the exhaust fluid mixing conduit extending from the outer wall proximate to a respective inlet of the exhaust fluid mixing conduit.

13. The exhaust fluid mixing conduit of any preceding claim, wherein:the outer wall comprises a fluid injector inlet configured to allow exhaust fluid to be injected into the exhaust gas flow path.the outlet and the fluid injector inlet are aligned with the second direction.

15. An exhaust fluid injection system for mixing an exhaust gas with exhaust fluid, the exhaust fluid injection system comprisingan exhaust fluid mixing conduit according to any of claims 1 to 12; andan exhaust fluid injector configured to inject exhaust fluid through a centre of the inward spiral of the exhaust gas flow path to the outlet of the exhaust fluid mixing conduit.

16. The exhaust fluid injection system of claim 11, whereinthe exhaust fluid injector is configured to inject exhaust fluid through the exhaust fluid mixing conduit in the second direction.

17. The exhaust fluid injection system of claim 11 or claim 12, whereinthe exhaust fluid injector is configured to inject exhaust fluid in a cone extending from the exhaust fluid injector in the second direction and expanding in the first direction.

18. The exhaust fluid injection system of claim 17, whereinthe exhaust fluid injector is configured to inject exhaust fluid in a cone which expands in the first direction to a radius which is less than a radius of the outlet of the exhaust fluid mixing conduit.

19. The exhaust fluid injection system of any of claims 15 to 18, wherein the exhaust fluid injection system is for mixing an exhaust gas from a diesel engine with a diesel exhaust fluid.

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