ディーゼルエンジン向け排気後処理SCRシステム用，低通気抵抗

技術紹介
ディーゼルエンジン向け排気後処理SCRシステム用，低通気抵抗
ミキサーデバイスの開発
Development of Low Pressure-drop Static Mixer Device for Diesel Engine SCR
After-treatment System
フルニエ　オリビエ *
今村　勉 *
大内　健 **
渡部　昇 **
Olivier FOURNIER
Tsutomu IMAMURA
Ken OOUCHI
Noboru WATANABE
要　旨
選択触媒還元（SCR）付の排気後処理システムに対して，尿素水からアンモニアを効率的に高く発生
する事が窒素酸化物（NOx）の浄化性能向上の一つである．理想的には，高い排ガス温度と尿素噴霧位
置から SCR 触媒までに十分長い距離を有していればアンモニアの発生には特に問題ない．但し，自動
車のレイアウト課題に於いては，短いガス通路と低い排気ガス温度でも高いアンモニアの発生率が必要
であり，様々なミキサーデバイス品での NOx 浄化性能向上が確認されているが同時に通気抵抗を悪化
させる要因となる．本内容は，排ガスの流れをスワールさせるミキサーを流れ解析（CFD）によって設
計し，圧損と NOx 浄化性能の実験で確認した結果を解説する．
Abstract
For exhaust after-treatment systems using Selective Catalytic Reduction (SCR) technology, efficiently
producing ammonia from a urea liquid feed is a key process to reduce Nitrogen Oxides (NOx). Ideally,
high exhaust gas temperature and a long distance between the urea feeding and the SCR catalyst
will be sufficient to get good results. However, due to automotive layout constraints, such results are
required even with short gas path and low running temperatures. This has led to a wide variety of
devices which increase NOx conversion by improving urea mixing but also bring important pressuredrop penalty. In this paper, a static mixer based on swirl generating concept is designed with the help
of Computer Fluid Dynamics (CFD) analysis to minimize pressure-drop and increase NOx conversion
as much as possible; then performances are verified by cold flow and engine bench tests.
Key Word : Heat engine, Post treatment system, Selective Catalytic Reduction NOx removal / Swirl, Static mixer
1.　INTRODUCTION
Faced with new challenging regulations such as Japan
2015 Fuel Economy Standard, Japan 2016 emission
legislation (post-PNLT) and US EPA 2010, diesel engines
are been required to improve their fuel consumption
while keeping reducing their exhaust pollutants.
Although several technologies and strategies are still
competing on the best way to satisfy the most stringent
emission limits; exhaust after-treatment systems like
those shown in Figure 1, using the Selective Catalytic
Reduction (hereafter refers as SCR) technology seems
to be the preferred way of eliminating Nitrogen Oxides
(NOx) with the minimum fuel consumption penalty.
Figure 1. Schematic of SCR After-treatment System
Layout Frequently Encountered on Truck Applications
*排気事業本部　排気システム開発グループ
**実験研究センター　実験技術グループ
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CALSONIC KANSEI TECHNICAL REVIEW vol.8 2011
The specificity of those systems is the use of a SCR
of the exhaust pressure-drop was measured to have 1%
catalyst which requires the presence of ammonia (NH3)
to 3% impact on the vehicle mileage; it is found that there
to reduce NOx pollutants. Technically and due to safety
issues, aqueous urea (frequently called Diesel Exhaust
is a non-negligible need for a mixer device design to be
Fluid – DEF in automotive application) is used instead
In the following study, mixers' performances are measured
of ammonia (1). With sufficient temperature, DEF will
by three criteria: the flow velocity Uniformity Index
evaporate and hydrolyze to form required ammonia.
(UI) calculated by CFD, the NOx conversion measured
The literature shows that urea mixing devices
minimize the pressure-drop penalty.
experimentally and the Pressure-drop which is both
generating swirl movement in the exhaust gas,
estimated from fluid analysis and measured on test bench.
represent efficient solutions to increase the NO x
conversion efficiency of the previously describe systems,
3.　CFD BASED MIXER DESIGN INVESTIGATION
by improving the gas flow uniformity ahead of the SCR
Static mixer devices design should satisfy several
catalyst (2),(3),(4).
targets and functions, sometimes leading to trade-off
In this study, the development of a static mixing device
characteristics. Mixer are here composed of several
that lowers the pressure-drop penalty and provides the
blades oriented at a specified angle as shown in Figure 3;
highest possible NOx conversion efficiency is presented.
and designed to mix urea droplets with the exhaust gas
2.　DEVELOPMENT TARGET
stream by generating swirl, a rotational movement of the
gas flow.
Analysis of typical SCR after-treatment system
From the low pressure-drop target point of view, sudden
backpressure shows in Figure 2 that mixer the device
variations in the gas flow cross-sections shall be avoided,
can accounts for almost half of the full system pressure-
while reducing wall surfaces will be beneficial for
drop.
lowering gas flow friction.
On the other hand, for the NOx conversion improvement,
mixer design will rely on two functions. The first is to
promote DEF evaporation by adding wall surfaces thus
increasing chance for droplets to hit a“hot”surface and
thermally breakup. And the second is to generate swirl
to achieve a uniform distribution of ammonia and NOx
upfront the SCR catalyst.
Figure 2. Benchmarked After-treatment System's Main
Components and Mixer Device Pressure-drop Ratios
It is known from benchmark activities, that pressure-drop
of SCR after-treatment system for Heavy Duty trucks
can range from 10 to 25kPa at engine full load. Thus
considering that from engine bench trials, a 10kPa variation
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Figure3. Mixer Device Design (Three Blades at 45 degrees)
ディーゼルエンジン向け排気後処理 SCR システム用，低通気抵抗ミキサーデバイスの開発
Summarized in Table 1 are six static mixers designed to
According to the mixer functions explained previously,
achieve various levels of swirl; their flow characteristics
the different designs are evaluated in regard of two
and pressure-drop are investigated by Computer Fluid
performance criteria: the flow velocity Uniformity Index
Dynamic (CFD) analysis and results are reported
(UI) ahead of the SCR catalyst, and the Pressure-drop.
hereafter.
Table 2 and 3 summarize the results obtained from the
CFD analysis.
Table 1. Static Mixer Design Variation Matrix
Figure 4 and 5 show how the mixer's blades design
affect the exhaust gas flow velocity. It is found that the
number of blades and the blades' angle have a similar
effect on the swirl velocity. A high number of blades
combine with an important deflection angle generating
higher swirl velocity.
Table 2. Flow Velocity and Pressure-drop Sensitivity to
Mixer's Blades Number (for 45 degrees Blades Angle)
Figure 4. Swirl Velocity vs. Mixer's Blades Number
(for Blades Angled at 45 degrees)
Table 3. Flow Velocity and Pressure-drop Sensitivity
to Mixer's Blades Angle (for Three-blade Design)
Figure 5. Swirl velocity vs. Mixer's Blade Angle (for
Three-blade Layout)
Regarding the overall mixer performance, the flow
velocity Uniformity Index (UI) ahead of the SCR
catalyst and the Pressure-drop are in Figure 6 plotted
against the Swirl Velocity. It is found that by inducing
high swirl movement, mixers also tend to degrade both
the flow velocity distribution and the pressure-drop.
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CALSONIC KANSEI TECHNICAL REVIEW vol.8 2011
4.2.　Mixing performance
Mixing performance is verified by measurement of
NOx conversion rates on an engine test bench using
a four cylinders, common rail equipped diesel engine.
The components' layout is similar to the one described
in Figure 1. The exhaust system includes an oxidation
catalyst, a catalyzed filter and a SCR catalyst, all sized
for truck application. The urea injector is located in
the elbow upfront the mixer device. Mixers' NO x
conversion performances are done for different gas
mass flow and temperatures as summarized in Table 3.
Figure 6. Trade-off between Flow Uniformity Index,…
Pressure-drop and Swirl Velocity
Table 4. Engine Testing Points for SCR System NOx
Conversion Verification
The high Swirl velocity, which is expected to increase
urea mixing with the gas flow, seems in opposition with
the need for higher flow velocity uniformity and lower
pressure-drop.
4.　EXPERIMENTAL RESULTS
4.1.　Correlation with CFD Pressure-drop results
Experimental verification of pressure-drop is done on
a cold flow bench and measurements are made up to a
mass flow of 2100 kg/h. The full SCR after-treatment
system is tested with and without the mixing devices.
Figure 7 shows that CFD results for pressure-drop are
well correlated with experimental ones, with an average
Firstly, all the NOx conversion results for the five testing
points are averaged and compared, in Figure 8, with the
Swirl velocity obtained from the CFD analysis. It shows a
noticeable decrease in the NOx conversion performance
for the mixer with the highest Swirl velocity. This is
believed to be related with the degradation of the flow
velocity distribution upfront the SCR catalyst.
error margin of 20% when using all tested mixers' data
at both low, medium and high mass flow rates. It can
be noted that prediction accuracy also greatly increases
to less than 10% error, when low mass flow data are
excluded.
Figure 8. Measured NOx Conversion Performance
versus Calculated Swirl Velocity
Secondly, the above conversion results are plotted
against the measured pressure-drop. Figure 9 shows a
Figure 7. Correlation between CFD and Test Results
for Full After-treatment System Pressure-drop
70
general trade-off pattern between emission performances
and pressure-drop.
ディーゼルエンジン向け排気後処理 SCR システム用，低通気抵抗ミキサーデバイスの開発
Figure 9. Trade-off between Measured NOx
Conversion Performance and Pressure-drop
It is noticed that the pressure-drop penalty worsen as
Figure 10. Urea Spray Pattern and Wall Film Injection Start
Exhaust Gas Temperature 200℃ (Top) and 300℃ (Bottom)
the conversion performance improves. It is also found
that the mixer designed with three blades oriented at
45 degrees against the gas flow, can be described as the
best performer of the tested mixers. The mixer presents
the higher NOx conversion rate increases, with nearly
30% compared to a pipe without mixer layout. Regarding
pressure-drop, it ranks second with a penalty of only
2.4kPa, which means less than 1% impact on the vehicle
mileage. Analyzing the previous data separately for each
gas mass flow rate reveals that the three blades at 45
degrees mixer design's improvement in NOx conversion
performances is stronger at low mass flow and further
increases when gas temperature increases.
However concerning the mixing mechanism, it is observed
from deposits on exhaust system parts that the urea
spray hits the wall before reaching the mixer and can
even flow backward depending on the testing conditions.
This emphasis the need to take into account more detailed
CFD analysis which can accurately simulate the urea
spraying and the ammonia production and distribution
mechanisms before and after the mixer device.
5.　UREA SPRAY CFD ANALYSIS TRIAL
To verify the urea spraying behavior at operating
exhaust temperatures, a trial CFD analysis is done using
the AVL company software FIRE®. Due to the trial nature
of this analysis, only one mixer, the three blades oriented at
45 degrees design, is studied at two different temperature
conditions, while other parameters such as gas mass flow
and urea injection parameters are kept the same.
Figure11. Urea Spray Pattern and Wall Film Injection End
Exhaust Gas Temperature 200℃ (Top) and 300℃ (Bottom)
Figure 10 and 11 show the results of the analysis; they
draw the urea droplets size distribution and trajectory,
and also visualize the urea film created by deposition of
the droplets on the walls. Spray patterns and wall film
thickness are presented for exhaust gas temperatures
of 200℃ and 300℃ ; and for two different points in time,
near the urea injection beginning (1.09s) and near the
injection end (1.82s).
Analysis results confirm that the urea spray is greatly
deflected toward the duct wall before it can hit the
mixer. Urea wallfilm illustrations also confirm that for
low temperature conditions, droplets hitting the exhaust
pipe before the mixer accumulate on the wall between
the injector and the mixer. It is also noticed that the urea
film tends to flow back towards the injector nozzle.
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6.　CONCLUSION
A static mixer based on swirl generating concept,
is designed with the help of CFD analysis to minimize
the pressure-drop penalty and increase NOx conversion
as much as possible. The design with three blades at
45 degrees achieves an increase of 30% in the NOx
conversion rate, while pressure-drop is only 2.4kPa.
While simple CFD tools are good enough to predict
the mixer pressure-drop levels, it is found that more
complex analysis of urea spraying are required to
correctly estimate the mixing performances.
The present study will be followed by further
investigations on the mechanism through which the
Swirl affects the NOx conversion rate; and also on the
feasible performance increase through the optimization
of the urea injector location and orientation, at different
exhaust gas flow conditions.
REFERENCES
(1) Diesel Emission and Their Control, Magdi K. Khair
and W. Addy Majewski, SAE International, 2006,
p.418
(2) Mixer Development for Urea SCR Applications, G.
Zheng and Al., SAE paper 2009-01-2879
(3) Urea systems in Focus – New Challenges and
Solutions in the Development of Car and Commercial
Vehicle Exhaust Systems, J. J. Oesterle and Al., SAE
paper 2008-01-1186
(4) D e v e l o p m e n t o f P M a n d N O x R e d u c t i o n
Aftertreatment System for Heavy-Duty Commercial
Vehicle (First Report), M. Kowada and Al., JSAE
paper 201005725
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フルニエ　オリビエ
今村　勉
大内　健
渡部　昇
論　文
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