CN112627951A

CN112627951A - SCR rotational flow mixing tube

Info

Publication number: CN112627951A
Application number: CN202011550297.0A
Authority: CN
Inventors: 王文晓; 左正兴
Original assignee: Beijing Institute of Technology BIT
Current assignee: Beijing Institute of Technology BIT
Priority date: 2020-12-24
Filing date: 2020-12-24
Publication date: 2021-04-09
Anticipated expiration: 2040-12-24
Also published as: CN112627951B

Abstract

The invention relates to an SCR (selective catalytic reduction) rotational flow mixing pipe, belonging to the technical field of engine aftertreatment. The invention aims to solve the problems of uneven distribution, slow urea decomposition rate, high crystallization risk caused by the fact that urea sprays into a mixing pipe and contacts with the wall, and the like. The spiral pipe structure can form turbulent flow in the mixing pipe at the rear end, accelerate the decomposition rate of urea in the mixing pipe and reduce the risk of urea crystallization. Meanwhile, a large amount of rotating airflow is formed around the inner wall of the mixing pipe by the rotating airflow generated by the spiral pipe structure, so that the urea solution sprayed to the inner wall of the mixing pipe is entrained and taken away, and the risk of urea crystallization is further reduced; the urea injection points are arranged in the middle, so that the distribution uniformity of urea in the mixing pipe can be improved, and the NO of the SCR system is finally improved_XThe conversion efficiency. The urea spraying point is arranged in the middle and matched with the mixing pipe with the pipe diameter increased at the rear end, so that the urea solution can be effectively prevented from contacting the wall, and the risk of urea crystallization is reduced.

Description

SCR rotational flow mixing tube

Technical Field

The invention relates to an SCR (selective catalytic reduction) rotational flow mixing pipe, belonging to the technical field of engine aftertreatment.

Background

The SCR technology can effectively solve the NO of the diesel engine_XThe technology is widely applied to the national IV and V stages of the vehicle diesel engine. The basic working principle of SCR is that a reducing agent is sprayed into an exhaust pipe of a diesel engine through a urea injection system, and the reducing agent reacts with NOx under the action of a catalyst to convert the NOx into N₂And the NOx emission control is carried out,in diesel NOx, more than 90% of the components are NO and NO₂. The reducing agent commonly used in SCR technology is NH₃Usually, NH is obtained after decomposition of urea by injecting it into the exhaust pipe₃。

Equation (1) shows that the injected urea decomposes endothermically to produce NH₃The formula (2) and the formula (3) represent NH₃Reacts with NOx in the exhaust gas on a catalyst and is converted into N₂And H₂O。

(NH₂)₂CO+H₂O→CO₂+2NH₃(1)

4NO+4NH₃+O₂→4N₂+6H₂O(2)

NO+NO₂+2NH₃→2N₂+3H₂O(3)

The urea injection amount needs to be changed in real time along with the working condition of the engine, so that the crystallization risk and the NOx conversion efficiency of the SCR system are determined by accurately controlling the urea injection amount. Because the working condition changes rapidly and the control system has hysteresis when controlling the urea injection quantity, the risk of urea crystallization in the urea mixing pipe is high.

The urea mixing pipe is an important component in the SCR system, can be used for preventing urea crystallization, accelerating the rapid mixing of urea solution and ensuring NH₃Is uniformly distributed.

In the urea mixing pipe, the position of the urea injection point is an important factor influencing the distribution uniformity and the crystallization risk after urea is injected into the mixing pipe. The mixing tube structure with urea injection points arranged on the wall of the mixing tube can cause urea to be difficult to uniformly distribute in the mixing tube after being injected into the mixing tube, the mixing effect is not ideal, and the SCR performance is influenced. Through the 90 mode of bending over through the pipeline and arranging urea injection point in mixing line central point position can guarantee that urea sprays back evenly distributed in the mixing tube, but this scheme can make the exhaust difficult to high-efficient direct mix with urea, influences urea decomposition rate to can produce great backpressure in the department of buckling, influence the diesel engine performance. Direct lean on into the mixing tube with urea injection part through physical structure, realize arranging placed in the middle of the point of spouting, can guarantee that urea spouts back evenly distributed in the mixing tube to can guarantee that exhaust evenly acts on the urea of spouting, but the physical structure who visits into can lead to the inhomogeneous disturbance that appears in the mixing tube, and this kind of disturbance can lead to urea to touch the wall when urea injection volume is great, causes the crystallization.

Therefore, it is necessary to design a novel SCR urea mixing tube to realize the central arrangement of the urea injection point, and reduce the risk of urea crystallization while ensuring the rapid decomposition of the urea solution and the uniform distribution of urea in the mixture.

Disclosure of Invention

The invention aims to solve the problems of uneven distribution, slow urea decomposition rate, high crystallization risk caused by the fact that urea sprays into a mixing pipe, and the like, and provides an SCR rotational flow mixing pipe. The spiral pipe structure can form turbulent flow in the mixing pipe at the rear end, accelerate the decomposition rate of urea in the mixing pipe and reduce the risk of urea crystallization. Meanwhile, a large amount of rotating airflow is formed around the inner wall of the mixing pipe by the rotating airflow generated by the spiral pipe structure, so that the urea solution sprayed to the inner wall of the mixing pipe is entrained and taken away, and the risk of urea crystallization is further reduced; the urea injection points are arranged in the middle, so that the distribution uniformity of urea in the mixing pipe can be improved, and the NO of the SCR system is finally improved_XThe conversion efficiency. The urea spraying point is arranged in the middle and matched with the mixing pipe with the pipe diameter increased at the rear end, so that the urea solution can be effectively prevented from contacting the wall, and the risk of urea crystallization is reduced.

In order to achieve the purpose, the invention adopts the following technical scheme:

the invention decomposes the pipeline between the air inlet pipe and the mixing pipe into branch pipelines with smaller diameters, and an installation space is generated at the circle center of the air inlet end surface of the mixing pipe in the mode and is used for installing the urea injection module. Meanwhile, gaps are reserved among the branch pipelines, urea supply and injection control of the urea injection module can be met, urea injection points are arranged in the middle, and the urea injection direction is parallel to the exhaust direction. The installation mode of the urea injection system provided by the invention can avoid additional disturbance caused by the fact that the solid structure enters the mixing pipe, and simultaneously ensures the distribution uniformity of urea after urea is injected into the mixing pipe.

The branch pipeline between the air inlet pipe and the mixing pipe is designed into a spiral pipe, airflow of the spiral pipe forms rotary airflow after entering the mixing pipe, the rotary airflow is disturbed to generate turbulence, urea decomposition is accelerated, urea spray is uniformly distributed in waste gas due to the turbulence, and the distribution uniformity of urea and urea decomposition products is improved. Meanwhile, due to the centrifugal effect generated by the rotation of the air flow in the spiral pipe, the air flow in the spiral pipe enters the mixing pipe and is distributed near the pipe wall in a centralized manner, a strong air flow layer is formed around the pipe wall, urea spray sprayed to the pipe wall is wrapped and carried away, and the urea solution is effectively prevented from contacting the wall for crystallization.

In order to better realize the function of the spiral pipe, the position of the urea injection point is adjusted along the axial direction of the mixing pipe by increasing the length of the urea spray gun. Defining the distance D from the injection point to the front end of the mixing tube, starting from the central point of the front end surface of the mixing tube, and when the injection point moves on the axis along the airflow direction, D is more than 0; otherwise, D is less than 0.

The truncated cone helix of the invention is as follows:

in the above formula, x, y, z are values of the spiral line in the three-dimensional space coordinate system, a is the radius of the spiral line, h is the pitch, and the smaller the value of h is, the higher the rotation degree of the spiral tube is. Theta is an angle, lambda is an adjusting coefficient of the text, the pattern of the circular truncated cone spiral line is adjusted by combining with the positive sign and the negative sign in front of the formula, when lambda is a positive sign, the radius of the circular truncated cone spiral line is larger and larger, when lambda is a negative sign, the radius of the circular truncated cone spiral line is smaller and smaller, lambda can be zero, and when lambda is 0, the circular truncated cone spiral line is a cylindrical spiral line. For z, the right helix takes a positive sign, and the left helix takes a negative sign.

When the front of the lambda is positive, the radius of the cylindrical spiral line is gradually increased, and the radius increasing speed is related to the value of the lambda. Along with the increase of the radius of the spiral line, the stronger the trend that the air current entering the mixing tube from the spiral tube moves to the mixing tube wall, so that the urea sprayed to the tube wall can be effectively taken away by a rotational flow layer formed around the mixing tube wall, the decomposition rate of the urea is accelerated by turbulent flow, and the urea crystallization risk in the mixing tube is reduced.

When the front of the lambda is a negative sign, the radius of the circular truncated cone spiral line gradually becomes smaller, and the speed of radius reduction is related to the value of the lambda. Along with the gradual reduction of the radius of the spiral line, the air flow entering the mixing pipe from the spiral pipe is gradually and intensively distributed at the axis position of the mixing pipe, so that the air flow directly acts on urea spray, and the urea decomposition rate is improved. However, the concentration of the rotational flow on the axis of the mixing tube may cause a portion of the urea to directly impact the mixing tube wall, so that the rotational flow is prevented from blowing the urea to the tube wall by adjusting the rotational direction of the helical tube at the front of the mixing tube and increasing the distance D between the urea injection point and the front end of the mixing tube. The urea solution is guaranteed to form a large amount of rotating turbulence near the inner wall surface of the mixing pipe before contacting the wall, urea sprayed to the inner wall of the mixing pipe can be taken away and decomposed, and crystallization risk is effectively reduced.

When the number of the spiral pipes is less than 3, the coverage degree of airflow generated on the inner wall surface of the mixing pipe is poor, part of the wall surface of the mixing pipe is not covered by the airflow, urea directly contacts the wall in an area without the airflow coverage, and the crystallization risk is extremely high. But after the pipeline number reaches 3, can realize comparatively outstanding air current cover effect, after the pipeline number reaches 4, rotatory air current has already can realize the even cover to the hybrid tube, and it does not have obvious promotion to the air current cover effect on hybrid tube surface to continue to increase the pipeline number.

The invention makes the sectional area of the air inlet of the spiral pipe larger than that of the air outlet of the spiral pipe, thereby realizing the increase of the air velocity in the spiral pipe.

Advantageous effects

1. The pipeline between the air inlet pipe and the mixing pipe is divided into branch pipelines with smaller diameters, and an installation space is generated at the circle center of the air inlet end face of the mixing pipe in the mode and used for installing the urea injection module; meanwhile, gaps are reserved among all branch pipelines, so that urea supply and injection control of a urea injection module can be met, the realization of central arrangement of urea injection points is ensured, and the urea injection direction is parallel to the exhaust direction; the installation mode of the urea injection system provided by the invention can avoid additional disturbance caused by the fact that the solid structure enters the mixing pipe, and simultaneously ensure the distribution uniformity of urea after urea is injected into the mixing pipe;

2. the urea mixing pipe can adjust the spiral line, the diameter of the inlet and the outlet of the spiral pipe and the outlet direction of the spiral pipe according to the use requirement in the application design process so as to realize airflow rotation effects of different degrees, achieve the best matching effect of the spiral flow and urea spraying, accelerate the urea decomposition rate, improve the distribution uniformity of urea and reduce the crystallization risk.

3. The diameter of the mixing pipe is increased, the diameter of the mixing pipe is 1.2 times of the diameter of the air inlet pipe, and the rotational flow formed by matching the wall of the mixing pipe reduces the possibility that urea directly contacts the wall, and further reduces the risk that the urea contacts the wall.

4. The sectional area of the air inlet of the spiral pipe is larger than that of the air outlet, so that the air flow speed in the spiral pipe is increased, the turbulence effect is accelerated, the urea decomposition rate is improved, the protection effect of the protection air flow is enhanced, and the crystallization risk is reduced.

Drawings

FIG. 1 is a general perspective view of an SCR system equipped with a coil;

FIG. 2 is a spiral pipe wireframe diagram marked with a spiral line;

FIG. 3 is a perspective view of an inlet tube-coil-mixing tube connection;

FIG. 4 is a side perspective view of the spiral pipe with D-0;

FIG. 5 is a side perspective view of the spiral pipe at D400 mm;

FIG. 6 is a side perspective view of the spiral pipe at D800 mm;

FIG. 7 is a cross-sectional view of the end of the inlet tube;

FIG. 8 is a wire frame diagram of a non-circular inlet-circular outlet volute;

FIG. 9 is a wire frame diagram of a spiral pipe with an inlet diameter of 600mm and an outlet diameter of 500 mm;

FIG. 10 is a wire frame diagram of a spiral pipe with an inlet diameter of 600mm and an outlet diameter of 400 mm;

FIG. 11 is a wire frame diagram of a spiral pipe with an inlet diameter of 600mm and an outlet diameter of 500 mm;

FIG. 12 is a right-hand rotary wire frame diagram of 8 spiral pipes with inlet and outlet diameters of 300 mm;

FIG. 13 is a left-hand rotary wire frame diagram of 8 spiral pipes with inlet and outlet diameters of 300 mm;

FIG. 14 is a cross-section of the inlet end of a double-turn helical tube.

The device comprises an air inlet pipe 1, a spiral pipe 2, a urea injector 3, a mixing pipe 4, a rear mixer 5, a reactor barrel 6, an SCR catalyst 7, an exhaust pipe 8, a spiral line 9 and a urea spray beam 10.

Detailed Description

Exemplary embodiments of the invention will be described in more detail below with the aid of figures and examples. While exemplary embodiments of the present disclosure are shown in the drawings, the patent rights are not intended to be limited by the embodiments set forth herein. Rather, these examples are provided for a more complete understanding of the patent and to fully convey the scope of the patent.

Example 1

As shown in fig. 1-4, an SCR swirl mixing tube is characterized in that: a spiral pipe 2 is added between an air inlet pipe 1 and a mixing pipe 4, a space is generated on the front section of the mixing pipe 4, and a urea injector 3 can be installed; the spiral pipe 2 rotates the exhaust gas to generate turbulent flow. Then the urea and the decomposition products thereof are rectified by a rear mixer 5, the distribution uniformity of the urea and the decomposition products thereof is improved again, finally the mixed gas is subjected to NOx reduction reaction on an SCR catalyst 7 in a reactor barrel body 6, and the treated exhaust gas is finally discharged through an exhaust pipe 8.

Through using spiral pipe 2, produce installation space at 4 front segments of mixing tube, can arrange urea sprayer 3 and control line etc. and this kind of structure can make urea evenly spout into mixing tube 4, carries out the optimum match with cylindrical urea spraying fog 10 and circular mixing tube 4, guarantees the urea distribution homogeneity in the mixing tube 4.

The air inlet pipe 1 is connected with a diesel engine exhaust pipe through a flange, and diesel engine exhaust enters the SCR system from the air inlet pipe 1.

The spiral wire 9 in fig. 2 leads to the production of a spiral wire 2 with a pitch h of 4000mm, a radius a of 424mm, a positive sign z, and a 0, in this case a cylindrical spiral. And z takes a positive sign and is a right helix. D-0 at the urea injection point.

The urea spray cone angle of the urea injector 3 is 60 °.

In this embodiment, 4 spiral pipes 2 are selected to ensure uniform protective airflow generated on the inner wall of the mixing pipe 4.

As can be seen from fig. 3 and 4, a part of the swirling air flow directly acts on the urea spray mist 10, and the turbulence generated by the swirling action accelerates the decomposition of the urea spray, thereby improving the urea distribution uniformity in the exhaust gas. Meanwhile, the protective airflow formed by the rotary airflow on the wall surface of the mixing pipe 4 can effectively prevent urea from hitting the wall, and the risk of urea crystallization is reduced.

The diameter of intake pipe 1 is 1500mm, and according to the relation of 1.2 times, the diameter of mixing tube 4 is 1800mm, and behind the increase pipe diameter, increased the time that urea moved to mixing tube 4 pipe wall, increased the urea decomposition time, combines the guard action of the 4 inner wall rotating air currents of mixing tube, has reduced the crystallization risk in the mixing tube 4 high-efficiently.

The whole set of urea mixing pipe can be matched with different SCR systems for installation and use. The aims of improving the urea decomposition rate and the urea distribution uniformity and reducing the crystallization risk are fulfilled.

Example 2

The apparatus was the same as in example 1 except that the urea injection point was 400mm apart from the front end surface of the mixing tube 4 as shown in FIG. 5.

At the moment, only a small part of rotating airflow can directly act on the urea spray mist beam 10, most of rotating airflow directly generates protective airflow on the wall surface of the mixing pipe 4, the urea spraying position moves backwards, the position where the urea spray can contact the wall also correspondingly moves backwards, the position where the urea spray contacts the wall after moving backwards has better protective airflow, the stronger rotating airflow can better wrap and carry away the urea sprayed to the wall of the mixing pipe, and the anti-crystallization capacity is stronger.

Example 3

The apparatus was the same as in example 1 except that the urea injection point was located at a distance D of 800mm from the front end surface of the mixing tube 4, as shown in fig. 6.

At the moment, all the rotating air flows directly generate protective air flows on the wall surface of the mixing pipe 4, no rotating air flow directly acts on the urea spray mist beam 10, the possible contact wall position of the urea spray moves backwards along with the urea spraying point, a large amount of protective air flows are formed at the contact wall position after the urea spray moves backwards, the strong rotating air flows can directly wrap and carry away the urea sprayed to the inner wall of the mixing pipe 4, and the anti-crystallization capacity is strong.

Example 4

The device is different from the device in embodiment 1 in that a straight line is used for guiding, and a circular truncated cone spiral line is not directly used, and the specific situation is shown in fig. 8.

Fig. 7 shows the shape of the interface of the spiral pipe 2 on the rear end surface of the air inlet pipe 1 in the embodiment 1, and the interface on the rear end surface of the air inlet pipe 1 is connected with the inlet of the spiral pipe 2. The front end face of the spiral tube 2 is circular at this time.

In fig. 8, the rear end surface of the intake pipe 1 is divided equally into 4 parts, each of the parts is taken as the inlet cross-sectional shape of one branch pipe, 4 branch pipes are used in total, and the final outlet shape of the branch pipes is circular. Space is provided for the central arrangement of the urea injector 3, and the structure shown in fig. 8 is mainly intended to achieve the central arrangement of the urea injection point, without involving the generation of a swirling flow.

Example 5

The device differs from the embodiment 1 in the parameters of the helix 9 leading the spiral 2 and in the shape of the outlet of the spiral 2, as shown in fig. 9.

The parameters of the helix 9 in fig. 9 are as follows: the pitch h is 2000mm, the starting radius a of the spiral line 9 is 424mm, and z is a positive sign, and is a right spiral line. λ is 0, which is a cylindrical spiral line, and the radius of the spiral line 9 is constant.

The diameter of the circular section of the inlet of the spiral pipe is 600mm, the diameter of the circular section of the outlet of the spiral pipe is 500mm, and the finally formed spiral pipeline is shown in figure 9.

The spiral pipe shown in figure 9 has smaller screw pitch, can form stronger rotational flow effect, generates stronger turbulent flow intensity and has stronger capability of accelerating the urea decomposition speed. The protective airflow formed near the inner wall of the mixing pipe has higher strength and stronger anti-crystallization capability.

Example 6

The device differs from the one of embodiment 1 in the parameters of the helix 9 leading the coil 2 and in the dimensions of the outlet of the coil 2, as shown in fig. 10.

The helix 9 parameters in fig. 10 are consistent with example 5: the pitch h is 2000mm, the initial radius a of the spiral line 9 is 300mm, λ is 0mm, and the spiral line 9 has a constant radius. And z takes a positive sign and is a right helix.

The diameter of the circular section of the inlet of the spiral duct 2 is still 600mm, which is different from that of the embodiment 5 in that the diameter of the circular section of the outlet is reduced to 400mm, and since the section of the outlet is reduced, the re-acceleration of the rotating airflow can be realized, and the spiral duct formed finally is as shown in fig. 10.

The spiral tube 2 of example 6 has a stronger swirling effect, a stronger intensity of generated turbulence, and a stronger ability to accelerate the urea decomposition speed than the spiral tube 2 of example 5. The protective airflow formed near the inner wall of the mixing tube 4 has higher strength and stronger anti-crystallization capability.

Example 7

The device differs from the one of embodiment 1 in the parameters of the helix 9 leading the spiral 2 and in the dimensions of the outlet of the spiral 2, as shown in figure 11.

The parameters of the helix 9 in fig. 11 are as follows: the pitch h is 2000mm, the initial radius a of the spiral line 9 is 424mm, and λ is 593mm, which is a circular truncated cone spiral line, and the radius of the spiral line 9 is gradually enlarged. And z takes a positive sign and is a right helix.

The diameter of the circular section at the inlet of the spiral pipe 2 is 600mm, the diameter of the circular section at the outlet of the spiral pipe 2 is reduced to 500mm, the rotating airflow can be accelerated again due to the reduced section at the outlet, and the finally formed spiral pipeline is shown in fig. 11.

Since the radius of the spiral line 9 is increased as the λ of 593mm in example 7, the generated gas flow also tends to move outward, and the rotating gas flow tends to move toward the inner wall of the mixing tube 4 in the mixing tube 4. Therefore, the rotating airflow in the embodiment 7 can form the protective airflow on the inner wall surface of the mixing tube 4 more quickly due to the effect of the circular truncated cone helical line under the condition of having the original advantages, and the anti-crystallization capability is strong.

Example 8

The device is the same as the embodiment 1, and is different in the number of the spiral pipes 2, the parameters of the spiral line 9 for guiding the spiral pipes 2 and the sizes of the inlet and the outlet of the spiral pipes 2, and the specific situation is shown in fig. 12.

The helix parameters in fig. 12 are as follows: the pitch h is 2000mm, the initial radius a of the spiral line is 424mm, the λ is 650mm, and the radius of the spiral line is gradually enlarged. And z takes a positive sign and is a right helix.

The diameters of the inlet and the outlet of the spiral pipe 2 in fig. 12 are both 300mm, and 8-path spiral pipe 2 is adopted, so that under the condition of having the original advantages, protective airflow can be formed on the inner wall surface of the mixing pipe 4 more uniformly, and the anti-crystallization capability is improved.

Example 9

The device is the same as the embodiment 8, except that the rotation direction of the spiral wire 9 is different, as shown in fig. 13.

The helix parameters in fig. 13 are as follows: the pitch h is 2000mm, the initial radius a of the spiral line is 424mm, the λ is 650mm, and the radius of the spiral line is gradually enlarged. z takes the negative sign and is the left helix.

The rotation direction of example 9 is different from that of example 8, but other parameters are consistent with example 8, so that the urea decomposition effect and the anti-crystallization ability are consistent with example 8.

Example 10

The device is the same as the embodiment 8, except that the number of the spiral pipes 2 is different, and the specific situation is shown in figure 14.

Fig. 14 is a cross section of the end of the intake pipe 1, which is connected to the front end of the spiral pipe 2, and it can be seen that 24-way spiral pipe 2 having two layers is used. Compared with the embodiment 8, the outer layer spiral pipe 2 can be used for forming protective airflow on the inner wall of the mixing pipe 4, and the inner layer spiral pipe 2 can be used for accelerating the urea decomposition. The urea in the mixing pipe 4 has high decomposition rate and strong anti-crystallization capability.

The above detailed description is intended to illustrate the objects, aspects and advantages of the present invention, and it should be understood that the above detailed description is only exemplary of the present invention and is not intended to limit the scope of the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims

1. An SCR cyclone mixing tube is characterized in that: the pipeline between the air inlet pipe and the mixing pipe is divided into branch pipelines with smaller diameters, so that a urea injection point can be arranged in the center of the mixing pipe, and the urea injection direction is parallel to the exhaust direction; the branch pipeline between the air inlet pipe and the mixing pipe is a spiral pipe, airflow in the spiral pipe enters the mixing pipe to form rotary airflow, the rotary airflow is disturbed to generate turbulence, urea decomposition is accelerated, the disturbance effect of the turbulence also promotes urea spray to be uniformly distributed in waste gas, and the distribution uniformity of urea and decomposition products of the urea is improved; meanwhile, due to the centrifugal effect generated by the rotation of the air flow in the spiral pipe, the air flow in the spiral pipe enters the mixing pipe and is distributed near the pipe wall in a centralized manner, a strong air flow layer is formed around the pipe wall, urea spray sprayed to the pipe wall is wrapped and carried away, and the urea solution is effectively prevented from contacting the wall for crystallization.

2. The SCR swirl mixing tube of claim 1, wherein: in order to better realize the function of the spiral pipe, the position of the urea injection point is adjusted along the axial direction of the mixing pipe by increasing the length of the urea spray gun. Defining the distance D from the injection point to the front end of the mixing tube, starting from the central point of the front end surface of the mixing tube, and when the injection point moves on the axis along the airflow direction, D is more than 0; otherwise, D is less than 0;

the truncated cone helix is as follows:

When the front of the lambda is a positive sign, the radius of the cylindrical spiral line is gradually increased, and the radius increasing speed is related to the value of the lambda; along with the increase of the radius of the spiral line, the stronger the trend that the air flow entering the mixing pipe from the spiral pipe moves to the wall of the mixing pipe, so that a rotational flow layer formed around the wall of the mixing pipe can effectively take away the urea sprayed to the wall of the mixing pipe, accelerate the decomposition rate of the urea through turbulence and reduce the crystallization risk of the urea in the mixing pipe;

when the front of the lambda is a negative sign, the radius of the circular truncated cone spiral line gradually becomes smaller, and the speed of radius reduction is related to the value of the lambda; along with the gradual reduction of the radius of the spiral line, the airflow entering the mixing pipe from the spiral pipe is gradually and intensively distributed at the axial position of the mixing pipe, so that the airflow directly acts on urea spray, and the urea decomposition rate is improved; however, the part of urea may be directly flushed to the wall of the mixing pipe by concentrating the rotational flow on the axis of the mixing pipe, so that the urea is prevented from being blown to the wall of the mixing pipe by the rotational flow at the moment by adjusting the rotating direction of the spiral pipe in front of the mixing pipe and increasing the distance D between the urea injection point and the front end of the mixing pipe; the urea solution is guaranteed to form a large amount of rotating turbulence near the inner wall surface of the mixing pipe before contacting the wall, urea sprayed to the inner wall of the mixing pipe can be taken away and decomposed, and crystallization risk is effectively reduced.

3. The SCR swirl mixing tube of claim 1, wherein: the spiral pipeline number can obtain better result when being more than 3, and the spiral pipeline number can be changed or can be set into a plurality of circles of pipelines.

4. The SCR swirl mixing tube of claim 1, wherein: increase the diameter of hybrid tube, the diameter of hybrid tube is 1.2 times of intake pipe diameter, and the whirl that the cooperation hybrid tube wall formed reduces the urea possibility of directly contacting the wall, further reduces urea and touches the wall risk.

5. The SCR swirl mixing tube of claim 1, wherein the size of the inlet and outlet of the spiral tube is adjustable to adjust the velocity of the air flow in the spiral tube.