Effects of SCR injection system parameters on UWS atomizationand mixing characteristics at low exhaust temperature

HEKUN JIA* , ZEYUAN ZHOU, BIFENG YIN, ZHIYUAN LIU and SHUAI WEN

School of Automobile and Traffic Engineering, Jiangsu University, Zhenjiang 212013, China

e-mail: jiahk@ujs.edu.cn; stuzhou@foxmail.com; ybf@ujs.edu.cn; 2221704007@stmail.ujs.edu.cn;

806764328@qq.com

MS received 25 December 2020; revised 13 July 2021; accepted 13 September 2021

Abstract. Several models involving spray atomization, wall impingement, evaporation and urea pyrolysis

were built through a commercial code to study the influence of parameters of Urea Selective Catalytic Reduction

(Urea –SCR) injection system on critical performances at low exhaust temperatures. The spray development,

NH3 concentration distribution and wall film thickness have been studied. Results suggest that the wall

impingement of the Urea Water Solution (UWS) is more at the temperature of 180�C than at 430�C. Lowertemperatures tend to hinder evaporation and pyrolysis. NH3 concentration is uneven at low temperatures and the

fuel film generated on the wall is also incomplete. Low temperatures could help injection system with a tilting

nozzle raise its injection pressure and accelerate the evaporation and pyrolysis of the droplets. The distribution

of spray impingement, however, is narrower and the fuel film thickness on the wall is increased. Smaller

injection angle made it easier to expel gas stream and the droplets would then follow the direction of the gas

stream. The droplets distribution is therefore more uniform and the spray wall-impingement reduced.

Keywords. Diesel engine; SCR; atomization; mixing characteristics; uniformity.

1. Introduction

Diesel engines are widely used in transportation and con-

struction machines because of high fuel efficiency and long

durability. However, emissions of the diesel engine such as

NOX and smoke should be considered as well because they

are detrimental to the environment.

It is difficult to reduce NOX and smoke simultaneously

simply through in-engine purification. A promising choice

except the in-engine purification to help reduce emissions is

Selective Catalytic Reduction (SCR), which has been

proved as an effective way of solution (Koebel et al [1],Han et al [2] and Johnson [3]).

Researchers (Dong et al [4]) studied the performance of

SCR systems with different injection parameters under

medium and high loads. The results suggested that low-

quality atomization could reduce the life of SCR system

and NOx conversion efficiency. Shi et al [5] found that the

wall impingement could be avoided when injection pres-

sure was appropriate. Lower injection pressures reduced the

mixing distance, and the axial concentration of UWS dro-

plet would be more uniform.

Jeong et al [6] studied parameter effects on the unifor-

mity of ammonia concentration distribution and the maxi-

mum ammonia conversion efficiency in varied load

conditions. The parameters involved injection angle,

injection position, and number of holes on the injector. The

results showed that the ammonia conversion efficiency was

the best when the injection was vertical to exhaust gas

stream. The difference between the uniformity of the 6-hole

nozzle and 8-hole nozzle is small and the local evaporation

ratio on the monolith face from both nozzles reached

almost 95%.

Hua et al [7] used a CFD code to simulate the spray

atomization and evaporation of UWS. The simulation was

conducted without air-assisted urea injection system. The

atomization characteristics was optimized to improve NOx

conversion efficiency. Kawauchi et al [8] studied the

evaporation and pyrolysis of UWS. The results suggested

that smaller size droplets can effectively promote the

evaporation and pyrolysis process of UWS as well as the

conversion efficiency from UWS to NH3. Wang et al [9]studied the evaporation of UWS at temperatures ranging

from 373K to 873K and found that evaporation has a link

with ambient temperatures. The behaviors were quite

complicated at high temperatures, which involved bubble

formation, distortion, and partial rupture after a linear D2-

law period. These behaviors were weakened as tempera-

tures decreased. Grout et al [10] found that increasing the

gas flow rate and temperatures would slow the formation of

the liquid film. Postrioti et al [11] studied the characteristicsof UWS spray at different temperatures and the results*For correspondence

Sådhanå (2021) 46:238 � Indian Academy of Sciences

https://doi.org/10.1007/s12046-021-01745-7Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

showed that gas flow had an important effect on

atomization.

Previous studies by Sadashiva et al [12], Lee [13] and Oh[14] suggested that parameters such as the injection pres-

sure, injection angle and position of the SCR injection

system had an important effect on urea atomization and

uniformity of ammonia concentration distribution. Addi-

tionally, works of Sadashiva et al [12] suggested UWS

atomization would affect the ammonia generation rate and

the subsequent reactions. The atomization was strongly

affected by the exhaust gas temperatures. However, these

studies mainly focused on the working condition when the

load was medium or high, and the exhaust temperatures

were at a high level (673 K–773 K). The low-speed low-

load conditions are common in WLTC (World Harmonized

Transient Cycle), in which exhaust temperatures are lower

and performance of the SCR system is different. The per-

formance of the injection system at lower temperatures

remains for further researches. Meanwhile, Tutuianu [15]

suggested a method to let catalytic work well at lower

temperatures (373*473 K).

In this paper, a numerical simulation on the UWS spray

atomization mixing process of Urea-SCR system at low

temperature was performed.

The influence of exhaust gas temperature, injection

pressure, and injection angle on the droplet distribution

pattern and the uniformity of ammonia concentration dis-

tribution was analyzed.

A commercial code (AVL FIRE) was used to numeri-

cally analyze the UWS spray atomization.

2. SCR System numercial simulation

2.1 Geometric model and mesh generation

In this paper, a diesel engine (Model 4G33T) was used to

provide exhaust parameters from real conditions, preparing

for numerical simulation. The main parameters of the diesel

engine are shown in table 1. A straight pipe geometry

(figure 1) was built to serve as a working part of the non-

air-assisted Urea-SCR system. The diameter of the exhaust

pipe is 67 mm, the distance between the injector and the

catalyst is 407 mm. The nozzle used in the Urea-

SCR injection system has 6 holes, which were fabricated

uniformly on the nozzle tip. The diameter of the hole is 0.2

mm. The UWS is injected with an angle of a with respect to

the center line of the SCR monolith. The size of the cat-

alytic reactor is 198.5 mm 9 274 mm. The density of the

pore of the catalyst is 62 pores/cm2. The thickness of wall

and coating of the SCR monolith are both 0.01 mm.

A module of the commercial code (AVL) was used to

mesh the geometry. Boundaries of the model involving

inlet, outlet, injection location and inlet section of the cat-

alyst are given in figure 2. The distribution of the ammonia

at the SCR entrance has a direct influence on the subse-

quent chemical reaction and the NOx conversion efficiency.

Therefore, the Uniform Index (UI) c (Weltens et al [16]) isused to describe the ammonia concentration distribution,

which is given as follows:

c ¼ 1� 1

2n

Xni¼1

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiCi � C� �2q

Cð1Þ

Where n is the number of grids on the measured section,

Ci is the parameter value on the grid i, C is the average

parameters across the entire section. c increases along with

the uniformity of ammonia.

The mesh grid size sensitivity analysis (figure 3) was

performed by calculating on the SCR monolith entrance the

uniformity of ammonia concentration distribution. In this

way the effects of grid size on the accuracy of the simu-

lation results can be reduced to the level that would not

affect the precision of simulation. The exhaust temperature

is 180�C, the injection pressure is 0.75 MPa and the spray

angle 35�. One can see that the uniformity coefficient is

stable of close to 0.422 after the number of grids passes one

million. To save computation time, the number of grids of

1.25 million was selected.

2.2 Mathematical model and verification

According to Jeong et al [6] and Fischer et al [17], theinternal gas flow of the SCR system is turbulent. The

conservation of mass, momentum and energy should be

considered. Therefore, the k � n� f system (Durbin [18]),

as is given as follows, was adopted to balance the com-

putation time and accuracy.

k equation (Durbin [18]):

qDk

Dt¼ q Pk � eð Þ þ o

oxjlþ lt

rk

� �okoxj

ð2Þ

e equation(Durbin P.A[18]):

qDk

Dt¼ q

C�e1Pk � Ce2e

Tþ ooxj

lþ ltrk

� �oeoxj

ð3Þ

n equation (Durbin [18]):

Table 1. Parameters of the diesel engine.

Properties Parameters

Engine Type Two-valve

Turbocharged intercooled

Stroke(mm) 100

Cylinder bore(mm) 95

Rated power(N�m) 230

Speed(r/min) 2200

Exhaust pipe diameter(mm) 67

238 Page 2 of 13 Sådhanå (2021) 46:238

qDnDt

¼ qf � qnkPk þ o

oxjlþ lt

rn

� �oeoxj

� �ð4Þ

f equation(Durbin P.A[18]):

f � L2o2

oxjoxj¼ C1 þ C2

Pk

n

� � 2=3� n

Tð5Þ

Where T is the turbulent time scale and L is the length

scale:

T ¼ max mink

e;

affiffiffi6

pCl Sj jn

!;CT

v

e

1=2 !ð6Þ

L ¼ CLmax mink3=2

e;

k1=2ffiffiffi6

pCl Sj jn

!;Cg

v3=4

e1=4

!ð7Þ

The parameters used are shown in table 2.

The DPM discrete phase model (Yue et al [19]) was usedas to analyze the droplet trace as well as heat and mass

transfer. The Huh-Gosman model (Huethorst [20], Huh

[21]) was adopted to compute the secondary breakup. SCR

Evaporation Model including Urea Thermolysis (Birkhold

[22]) module was picked up to compute the process of

UWS droplets decomposed into ammonia after evaporation

and pyrolysis. The material property of catalyst in SCR

monolith was analyzed using a porous media model

(Millington [23]).

The pyrolysis reaction follows UWS evaporates. Eva-

poration of urea solution is followed by the thermolysis of

pure urea. According to Arrhenius’s theory (Birkhold [24],

Birkhold. [25, 26]), the thermolysis rate is given as follows:

dmurea

dt¼ �p � A � Dd � exp � Ea

RT

� �ð8Þ

Figure 1. SCR system.

Figure 2. Mesh grid of the pipe.

Figure 3. Mesh grid size sensitivity analysis.

Sådhanå (2021) 46:238 Page 3 of 13 238

Where A is 0.4 kg/ms and Ea is 69000 J/mol.

The impingement of the droplets and the formation of

liquid film were analyzed using the wall film model

(O’Rourke [27]). The evaporation was also considered in

this model. The diameter and velocity of the droplet after

the impingement as well as the mass of liquid film on the

wall were analyzed using the Kuhnke model (Kalb [28]).

This model takes into account simultaneously the K factor

and wall temperature Tw. The interaction between the spray

and the wall was also analyzed. The equation for the cal-

culation of the diameter of droplets is given as follows:

D32 ¼Pk

i¼1 nid3iPk

i¼1 nid2i

ð9Þ

Where ni is the number of the particle with a diameter di.The working process of the catalyst was analyzed using a

porous media model, which cuts the calculation domain

into individual pore channel. The energy and mass equation

were calculated in such individual channel. Transport

equation for the gas flow is given as follows (Kaviani [29],

Verein [30]):

eg �oqg � wk;g

ot¼ �eg �

oqg � wk;g � mgoz

þ eg

� ooz

qg � Deff � owk;g

oz

� �þMk;g �

Xli

vi;k

� ri cLk ; Ts� �

ð10Þ

eg � oot qg �XKk

wk;g � hk !

¼

� eg � ooz qg �XKk

wk;g � hk � mg !

þ eg � ooz kg � oTgoz

� �

þXKk

eg � ooz qg � Deff � owk;g

oz� hk

� �

þ GSA � kh � Ts � Tg� ��XI

i

�hi � ri cLk ; Ts� �

ð11Þwhere wk;g is the mass fraction of the component k, Deff

is the effective diffusion coefficient, ri is the molar reaction

rate on the catalyst surface.

Gas phase energy equation is given as follows (Kaviani

[29], Verein [30]):

Where Tg is the gas temperature, hk is the total enthalpy

of the component k, kg is the thermal conductivity of the

gas, kh is the heat transfer coefficient between the gas and

the solid wall, GSA is the channel surface area per unit

volume, Dhi is the reaction heat, Ts is the solid wall of the

catalyst temperature.

In order to certify the accuracy of the spray model (AVL

[31]), a UWS spray test system was built. It involved a

strong light source, a digital ICCD camera, a constant

volume bomb, an UWS injection system, a signal control

device and a set of information acquisition system. The

geometry of holes for the urea spray injection used in the

test are fabricated according to the three-dimensional

geometry used in the simulation to ensure conformability.

The spray pattern was photographed using backlight

method. The spray cone angle and the spray penetration can

be calculated in the after-treatment of the spray pattern

images. Experimental data would be compared and ana-

lyzed together with the data from the numerical computa-

tion. Parameters of the test are shown in table 3.

The simulation result of the spray angle and the spray

penetration was compared with the experimental data in

figure 4. One can see the result of the simulation which

shows good agreement with experimental data. The biggest

difference between the experimental data and simulation

result in figure 4(a) is only 4.5%, in figure 4(b) it is 1.2%.

The spray pattern from the model is compared with

experimental image in figure 5. The images of the experi-

mental data and simulation result show good agreement.

Therefore, the computation model was robust enough to

predict urea injection, and the aim of the study was to study

the advantages of spray atomization.

2.3 Boundary conditions and calculation method

A diesel engine with low load and low speed was used to

provide boundary conditions. The exhaust temperature was

Table 2. Parameters of k � n� f model.

Ce1 Ce2 C1 C2 rk re rn Cl CT Cg

1.4 � 1þ 0:012ð Þ=n 1.9 0.4 0.65 1 1.3 1.2 6.0 0.36 85

Table 3. Experimental parameters.

Boundary conditions Parameters

Injection pressure (MPa) 0.75

Backpressure (MPa) 0.1

Bulk temperature (K) 300

Injection duration (ms) 5

238 Page 4 of 13 Sådhanå (2021) 46:238

180�C and the exhaust flow rate was 250 kg/h. The char-

acteristic length was 10% of the inlet diameter, the turbu-

lent energy was 5% of the square of inlet average speed,

and the outlet pressure was 0.1 MPa. The UWS nozzle has

6 holes, which are uniformly set on a circle of 1.2 mm in

diameter on the injector. The diameter of the nozzle hole is

0.2 mm. Urea solution of 32.5% was used. The simulation

period is 0.35 s. The injection started at 0.1s and the

duration is 5 ms.

3. Result and discussion

3.1 Spray development and ammoniaconcentration distribution at varied exhausttemperatures

Figure 6 shows the spray pattern at high and low temper-

ature, respectively. It can be seen that at the phase of the

end of the injection (EOI), the impingement within the pipe

occurred. The injection pressure is 0.75 MPa and the

injection angle is 35�. The spray atomization quality is bad.

The droplet is large. The specific surface area is small,

which is not conducive to the evaporation of the droplets.

Meanwhile, the momentum of a single droplet is large and

it is difficult for an individual droplet to follow the exhaust

gas flow. The direction of the drop movement changed little

and impingement emerged.

The droplet patterns have changed to 4 ms after EOI at

both calculation temperatures. The atomization quality is

not good and the UWS droplet is large. Droplets continue

moving in the spray direction, many of them would hit the

wall. The low temperature contributes to increasing the

deposition and splashing. Then liquid film forms on the

wall and urea tends to deposit. High temperatures help

droplets evaporate rapidly. The rebound of the droplets is

also boosted (Li et al [32]). Higher wall temperatures boost

the heat transfer between the droplet and the wall, and the

evaporation rate increases swiftly. The upward momentum

produced by evaporation increase the power of rebound.

Smaller droplets tend to rebound more severely owing to

equivalent amount of momentum. The increase in the

ambient flow rate driven by energy from heat transfer leads

to the droplet easier to break up. The decomposition of urea

is also accelerated. Therefore, the number of droplets is

reduced and the formation of liquid film on the wall is also

hindered.

The influence of temperature on spray pattern is obvious

8 ms and 12 ms after the EOI. The temperature of the gas

flow and the wall surface are high so that heat transfer is

boosted. Strong heat transfer accelerates evaporation and

thermolysis. The number of droplets then decreases and

more ammonia is generated. Weak evaporation and

upstream atomization let the droplets be concentrated on a

lower layer of the exhaust pipe at low temperature. The

attachment of UWS particles on the wall makes the droplets

easy to deposit in liquid film and the conversion from urea

to ammonia is also blocked. The consequent ammonia gas

is low in concentration and in maldistribution. The con-

centration of NH3 on the front of the catalyst at high and

low temperatures are shown in figure 7. It can be seen that

the UI of ammonia distribution is more concentrated at high

temperature than at low temperature. The lack of a mixer

made the ammonia distribution a little nonuniform at high

temperature. The high concentration region is broader at

high temperatures, indicating that more ammonia is gen-

erated. That the ammonia concentration distribution at low

(a) Spray penetration

(b) Spray angle

Figure 4. Data from the simulation and the experiment.

(a) 2ms ASOI (b) 5ms ASOI

Figure 5. Images of spray from the experiment and the

simulation.

Sådhanå (2021) 46:238 Page 5 of 13 238

temperatures is nonuniform owes to poor atomization and

the impingement. The low catalytic conversion rate is

caused by weak evaporation and insufficient ammonia.

According to the above analysis, evaporation and ther-

molysis could be strongly affected by temperature. Low

temperature reduces atomization and suppresses evapora-

tion and thermolysis. That would lead to broad range of

impingement, urea crystallization, maldistribution of

ammonia and a reduction in production of ammonia. That

means the importance of solving those difficulties. Low

temperature could worsen atomization of UWS because the

evaporation rate decreases at low temperatures, therefore

some droplets remain in big size and the momentum of the

droplet is larger as well. Large droplets tend to deposit on

the well. On the other hand, the spray penetration would

increase at low temperatures. That could help the

(a) Exhaust temperature of 430°C

(b) Exhaust temperature of 180°C

Figure 6. Spray development in exhaust pipe at varied exhaust temperatures.

238 Page 6 of 13 Sådhanå (2021) 46:238

deposition of droplets with large momentum on the wall as

wall. Finally, temperatures have a direct impact on

atomization of UWS, and the process of atomization is

certainly dependent on the ambient temperature.

3.2 Effect of injection pressure on UWSatomization mixing characteristics

The influence of injection pressure on atomization has been

studied. The injection pressures are 0.75 MPa, 1.2 MPa, 1.5

MPa and 2 MPa. Figure 8 shows the development of the

spray pattern in those conditions. One can see the different

spray patterns. As the injection pressure increases, the spray

droplets decrease significantly in size. Smaller droplets tend

to move toward the direction of exhaust gas, owing to the

smaller momentum and leading to a reduction of

impingement.

One can see from figure 8(b) that the area of wall

impingement of the UWS droplets increases along with the

injection pressure. The direction of movement of the dro-

plets changes quickly and the droplets are distributed in the

lower and center portion of the exhaust pipe, leading to a

better mixture of droplets with the exhaust gas flow.

Meanwhile, the atomization is better and the specific sur-

face area increases when the injection pressure increases

and the droplet size decreases. The contact area among the

droplet and the gas flow is increased to promote the evap-

oration and pyrolysis of the droplet.

The subsequent developments of the UWS droplets in the

exhaust pipe are shown in figures 8(c) and (d). It can be

seen that with the injection pressure increased the number

of droplets on the wall is reduced sharply. That could

hinder the formation of liquid film and the deposition.

Meanwhile, the spray quality is better and the size of the

droplet is decreased. That leads to the increasing injection

pressure, because the number of droplets in the tube is low

and droplets are capable of moving further. That pushes the

acceleration of evaporation and pyrolysis. Small droplets

moving further with the help of the gas flow. The time for

mixing is then reduced.

Figure 9 shows the ammonia concentration distribution

and UI in the front of catalyst in varied injection pressure

conditions. When the injection pressure is low, the

ammonia would concentrate on the lower part of the SCR

monolith. When the injection pressure increases, ammonia

gradually moves toward the center of the monolith. The

area of heavy ammonia concentration decreases, and the

ammonia distribution becomes more uniform. That is

because the high injection pressure helps in breaking up the

droplets and increase its velocity. Therefore, higher injec-

tion pressure facilitates promoting the propagation of the

droplets, leading to the droplets moving toward the center

of the SCR monolith. The size of the droplet is smaller, the

velocity is increased to help the interaction between the

urea and the exhaust gas. Therefore, the decomposition of

the urea becomes better. The UI of ammonia concentration

increases from 0.423 to 0.511, or 20.8%, with the injection

pressure increasing from 0.75 MPa to 2 MPa. The

improvement would help the following chemical reaction.

The injector is installed in places several times of

exhaust diameter up stream, and the injection pressure are

0.75 MPa, 1.2 M Pa, 1.5 MPa and 2 MPa. One can see from

figure 10 that the SMD is small in high injection pressure.

Injection pressure increases from 0.75 MPa to 1.2 MPa, the

SMD decreases from 138 lm to 60 lm, and the injector is

placed one diameter upstream. When the injection pressure

is 2 MPa, the SMD is the smallest of 38 lm.

Smaller droplet would help the acceleration of evapora-

tion, thermolysis and further reactions. The SMD decreases

when the place of the injector is farther from upstream,

because longer distance between the injector and the SCR

monolith upstream helps to increase time of the interaction

between the droplet and exhaust gas, and the evaporation is

promoted. The difference among the computation cases

becomes smaller as the injector moves away from the

upstream. The difference does not increase even if the

injection pressure is increased. When droplets become

bigger, the inertia force becomes more important, and

impingement occurred by the inertia force, in which the

calculation of the SMD does not involve. The smaller

droplets moving along with the gas flow could be counted.

The low SMD number does not lead to a good atomization

when the injection pressure is low, and impingement could

be seen in the area where the SMD decreases rapidly.

According to figure 10, the SMD is small when injection

pressure is high, the atomization is good at this point.

Smaller droplets help in reducing maldistribution of

ammonia.

Figure 11 shows the thickness of the liquid film at the

bottom of the exhaust pipe, the injection pressures are 0.75

MPa, 1.2 MPa, 1.5 MPa and 2.0 MPa. Liquid film dis-

tributes broadly in the pipe when the injection pressure is

0.75 MPa. The catalyst is intended to be blocked by liquid

Figure 7. Distribution and UI c of NH3 concentration in the

front of catalytic converter at varied temperatures.

Sådhanå (2021) 46:238 Page 7 of 13 238

(a) EOI

(b) 4ms after EOI

(c) 8ms after EOI

(d) 12ms after EOI

Figure 8. Spray development in exhaust pipe with different injection. Pressure

238 Page 8 of 13 Sådhanå (2021) 46:238

film deposition. When the injection pressure increases, the

area of liquid film decreases, so does the maximum liquid

film thickness. That is because the UWS atomization is

good and spray momentum is large when the injection

pressure is high. Lifting the injection pressure could help

improve atomization and increase the momentum of dro-

plets. Smaller droplets can reduce the prospect of

impingement as well. Big droplets tend to splash when they

hit the wall, leading to the reduction of the amount of liquid

film. The liquid film would not deposit broadly in high

injection pressure when the distribution of the film is small

and sparse. Meanwhile, the evaporation and thermolysis of

UWS becomes better, and the liquid film is decomposed

swiftly with rising exhaust temperature, and the pipe is then

less likely to be blocked.

3.3 Effect of injection angle on UWS atomizationmixing characteristics

The injection angles have been set as 30�, 35�, 40�, and 45�,respectively, and the injection pressure is 0.75 MPa. in

those conditions, the atomization of UWS droplets is

studied. Figure 13 shows spray pattern with varied injection

angle. The spray is compressed by its own momentum and

the airflow in the process of injection. With the increase of

injection angle, spray angle decreases. The heat transfer

among droplets and the gas flow would be blocked when

large numbers of droplets gather together. While spray is

injected nearly perpendicular to the wall, it will hit the wall

directly. The liquid film tends to form at low temperatures.

The spray tends to move in the direction of the gas flow as

the injection angle is reduced to the level that injection

direction is close to the direction of the movement of the

gas flow.

Figure 9. Distribution and UI c of NH3 concentration.

Figure 10. SMD of varied cross sections.

Figure 11. Film thickness of exhaust pipe with different

injection pressure.

Sådhanå (2021) 46:238 Page 9 of 13 238

(a) EOI

(b) 4ms after EOI

(c) 8ms after EOI

(d) 12ms after EOI

Figure 12. Spray development at the injection pressure of 0.75MPa at varied injection angles.

238 Page 10 of 13 Sådhanå (2021) 46:238

The droplets are therefore being given a larger space to

develop and the spray angle is increased as well. Mean-

while, the formation of liquid film is also impeded, as large

numbers of big droplets are produced in the beginning, the

impingement is alleviated. One can see from figures 12 and

13 that the number of droplets is large, and the distribution

is broad, indicating that reducing the injection angle can

benefit the atomization. Meanwhile, it can be observed

from figure 13 that urea index decreased from 0.439 to

0.419, or 4.6%, when a increases from 30� to 45�. Theconcentration becomes more uniform as well. However, the

area of ammonia distribution decreases. The ammonia

decomposed from urea increases, owing to the increase in

the area of high concentration as the injection angle

decreases.

More ammonia is produced at low temperatures as the

urea solution evaporates slowly. The amount of liquid film

generated at low temperatures also decreases. Therefore,

smaller injection angle can help to improve the perfor-

mance of the SCR catalyst at low temperatures.

Figure 14 shows the SMD from inlet expansion to SCR

monolith face for varied injector locations when the

injection pressure is 0.75 MPa. Section aD and bD are the

circular sections nearest to the nozzle. The SMD from aD

and 1D are not among those counted at the injection

angle of 30�, as the shape of the nozzle varies at varied

injection angles. The mixing characteristics may not be

calculated precisely according to SMD, especially at a

large injection angle. The atomization is not good owing

to that more impingement could be expected in the low

injection pressure. The location where the impingement

emerges or liquid film is generated is calculated

according to the trend of SMD. The SMD decreases

rapidly at an injection angle of 30�at the location

between bD and 2.5D where the impingement emerges.

That location moved toward before the section 1.5D near

Figure 13. Distribution and uniform index c of NH3

concentration.

Figure 14. SMD of varied cross sections at varied injection

angles.

Figure 15. Liquid film thickness.

Sådhanå (2021) 46:238 Page 11 of 13 238

the nozzle as injection angle increased. Meanwhile, the

evaporation and thermolysis could be weakened at low

temperatures if droplets move from 2.5D to 5D. Changes

of SMD at varied injection angle could be seen as well.

Figure 15 shows the thickness of liquid film from inlet

expansion to SCR monolith face for varied injector

locations when the injection pressure is 0.75 MPa. The

liquid would concentrate toward where impingement

occurred as the injection angle increases. The larger

injection angle makes the spray tend to be more per-

pendicular to the monolith. The impingement and the

resulting generation of liquid film are reduced as the

space of the development for spray grows when the

injection angle is decreased.

4. Conclusion

The development process of UWS spray at varied temper-

atures was studied in this paper. Temperatures would

mainly affect evaporation and thermolysis of UWS. The

impingement is significant, and the evaporation and the

pyrolysis are impeded at lower temperatures.

Low temperatures would hinder the evaporation and

thermolysis, meanwhile the impingement also emerges.

The evaporation of droplets and the thermolysis of urea

could be accelerated with the help of low temperature of

180�C. The injection pressure is increased to let droplets

smaller. As a result, the specific surface area increases. The

distribution of the spray has been improved and the

impingement reduced, owing to the effect of the gas flow

when the injection pressure is high.

Reducing the injection angle could make the develop-

ment of the spray more complete and help the spray to

move in the direction of the gas flow. The generation of

liquid film would be hindered as the spray angle is

increased and the impingement reduced.

Acknowledgements

This work was supported by the Natural Science Founda-

tion of Jiangsu Province [BK20201166]; The Priority

Academic Program Development of Jiangsu Higher Edu-

cation Institutions [PAPD]; The Transformation Foundation

of scientific and technological achievements in Jiangsu

Province [BA2016096].

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