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Confidence in Modeling SCR Aftertreatment Systems

Rana Faltsi and Jayesh MutyalANSYS Inc.

Presentation co‐authors:

Markus Braun and Rolf Reinelt

Dimitris Papas

Theodoros Atmakidis andGrigoris Koltsakis

2Monday, October 08, 2012 2012 Automotive Simulation World Congress

Outline

• SCR• Single droplet validation• Urea spray validation• Effects of spray‐wall interaction and film formation• SCR catalyst modeling• Complete SCR system simulation


Selective Catalytic Reduction ‐ SCR

• Today, engine manufacturers are looking at selective catalytic reduction (SCR) as the answer to meet diesel engine emission norms.

• CFD modelling is increasingly used in the design process to ensure that a given SCR system achieves the required level of NOx reduction over the full operating cycle of a specific engine, however there are modelling challenges still to be met.


Typical SCR system

5

The SCR exhaust aftertreatment technology involves the injection into the exhaust system of aqueous urea, which decomposes into ammonia and iso‐cyanic acid and reacts with the nitrogen oxides (NOx) inside a SCR catalyst.

Increase NOx

Conversion Efficiency

Minimize Urea

Storage & Consumpti

on

No or Little

Ammonia Slip

Durable Design

Source Bosch

Requirements of an SRC System

Key simulation areasExisting simulation practice distinguishes two key simulation areas:• Process before the catalyst: mixture preparation CFD

– aims mainly at establishing ammonia uniformity at the SCR catalyst inlet.• SCR Catalyst: NOx reduction empirical 0‐D or physico‐chemical channel reactor models– aims at optimizing NOx conversion with minimum NH3 usage/slip

6

Exhaust gas

Aqueous urea injection

• Spray dynamics• Water evaporation• Urea decomposition• Wall film formation

• Thermolysis: CO(NH2)2 → NH3 + HNCO• Hydrolysis:HNCO + H2O→ NH3 + CO2

Exhaust pipe system

SCR catalyst

• Porous media description• Catalytic NOx reduction:NOx + NH3 + …→ N2 + …

NH3

NOxNOx

Monday, October 08, 2012 2012 Automotive Simulation World Congress

Mixer

CFD simulationsSpecialist models

Simulation Challenges• Complex physics

– Multiphase– Gas phase and heterogeneous reactions– Spray/wall interaction

• Multiple time scales– Spray dynamics: ms– Wall film formation: sec– Catalyst transients: min

• Lack of:– Validated physical models– Established best simulation practices– Methodology for complete SCR system simulation in

terms of NOx emissions in legislated test cyclesMonday, October 08, 2012 2012 Automotive Simulation World Congress 7

SCR process data

• Urea‐Water Solution (UWS) 32.5% wt., is also referred to as Diesel Exhaust Fluid (DEF), or with the brand name AdBlue.

• Exhaust gas composition:

• Typical operating temperature range: 423‐773 K8Monday, October 08, 2012 2012 Automotive Simulation World Congress

Species Range

NO 100...1500 ppm

NO2 50…500 ppm

O2 3…15%

CO2, H2O 5…10%

CO, Hydrocarbons ~ 100 ppm or less

Available sources of experimental data for model validation• Single urea‐water droplet experiments:

– T. J. Wang et al. “Experimental Investigation on Evaporation of Urea‐Water‐Solution Droplet for SCR applications” December 2009, AIChE J., Vol. 55, No. 12, p. 3267

– Experiment of Musa et al. as described and documented in F. Birkhold, “Selektive katalytische Reduktion von Stickoxiden in Kraftfahrzeugen : Untersuchung der Einspritzung von Harnstoffwasserlösung “ PhD Thesis, Universität Stuttgart, 2007.

• Urea‐water spray experiment:– J.Y. Kim, S. H. Ryu and J. S. Ha, Numerical Prediction on the Characteristics of Spray‐

Induced Mixing and thermal decomposition of Urea Solution in SCR System –Proceedings of ICEFA04, 2004 Fall Technical Conferences of the ASME International Combustion Engine Division October 24‐27, 2004, Long Beach, California USA

• Spray – wall interaction experiments:– F. Birkhold, et al. “Analysis of the Injection of Urea‐water‐solution for automotive

SCR DeNOx‐Systems: Modeling of the Two‐phase Flow and Spray/Wall‐Interaction” SAE 2006‐01‐0643.

– more details of the experiments are described in F. Birkhold, “Selektive katalytische Reduktion von Stickoxiden in Kraftfahrzeugen : Untersuchung der Einspritzung von Harnstoffwasserlösung “ PhD Thesis, Universität Stuttgart, 2007.

Monday, October 08, 2012 2012 Automotive Simulation World Congress 9

373 423 473 523 573 623 673 723 773 823 873

Available experimental data on UWS processes


Urea thermo‐chemical data

Meltingtemperature

408 K

Enthalpy of fusion

233 kJ/kg

Boiling point 483 K

Latent Heat 1400kJ/kg

SCR Operating conditions 423 K 823 K

Musa Single Droplet 473 K 773 K

Wang Single Droplet373 K 873 K

Kim Spray 573 K 673 K

613 KBirkhold plate

Temperature(K)

Liqu

id urea

Approach 2: wet solid combusting particle

• Solid urea + liquid water particle• water evaporates first, then follows urea decomposition

• convection/diffusion controlled vaporization and boiling for water

• Single kinetic rate devolatilization model for urea decomposition (Kim):

UWS droplet descriptionApproach 1: multi‐component particle

• Homogeneous urea‐water liquid mixture

• Convection/diffusion controlled vaporization and boiling

• Raoult’s law• Urea vapor pressure equation (Birkhold):

11

Liquid CO(NH2)2+ H2O

Solid CO(NH2)2 + H2O


where:A = 800 E = 2.94E7 kJ/kmol

Single UWS studies

• Single droplets suspended on the tip of a fiber in a heated environment.

• Dry stagnant air• Images of the evaporating droplet are recorded with high speed cameras.

• The studies reveal that the UWS droplet behavior differs depending on the ambient temperature.

• Phenomena such as bubble formation inside the droplet, micro‐explosions and solid residue formation are observed.

12From Wang et. al.


Fluent validation model setup

• The droplets are stationary, so they remain in the same location for the duration of the evaporation process.

• Only the evaporating droplet diameter/time history is evaluated and the continuous phase is not solved.

• A simple 2D planar mesh is created and the particle is positioned at the center.

• Multi‐component and wet‐solid particle approaches are used with material properties from Fluent’s property database.


0

0.2

0.4

0.6

0.8

1

0 25 50 75 100

(D/D

0)^2

t/D0^2 [s/mm2]

T=473 K

Single UWS studies – Simulation results


The slope change marks the end of water vaporization

Below urea boiling point above urea boiling point

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20

(D/D

0)^2

t/D0^2 [s/mm2]

T=673 K

experimental‐Musaexperimental‐WangMusa‐multicomponentMusa wet‐solidWang‐multicomponentWang wet‐solidsmall particle wet‐solid

• Considerable variability of the experimental data.

• The multi‐component and the wet‐solid models provide identical results for the water vaporization stage (D2 law), but different solutions for the urea decomposition.

Simulations:Musa: Dp=2.3 mmWang: Dp=0.81mmsmall particle: Dp=0.1mm

The D2 law does not apply for urea with the wet‐solid model

Urea spray validation ‐ Kim’s experiment

Sketch of the experimental setup of Kim

• Aqueous urea is injected in the hot gas stream flowing inside a circular duct and is converted to ammonia

• Sampling points exist at three downstream locations• Aqueous urea: 40% Urea by Wt

Exhaust gas conditions

1: J.Y. Kim, S. H. Ryu and J. S. Ha, Numerical Prediction on the Characteristics of Spray‐ Induced Mixing and thermal decomposition of Urea Solution in SCR System –Proceedings of ICEFA04, 2004 Fall Technical Conferences of the ASME International Combustion Engine Division October 24‐27, 2004, Long Beach, California USA


Kim’s experimental measurements • NH3 concentration at sampling point is recorded

• Urea conversion is plotted against residence time for three operating temperatures

Inlet gas temperature 573 K




Fluent validation model setup

• 2D axisymmetric model• Turbulent flow with k‐epsilon model• Injections setup with both multi‐component and wet‐solid approaches

• Physical properties are taken from Fluent’s property database

• Gas phase reactions: finite rate model


Reaction Reaction Rate ΔHreaction

Thermolysis CO(NH2)2 → NH3 + HNCO Very fast 85 kJ/mol(endothermic)

Hydrolysis HNCO + H2O→ NH3 + CO2‐93 kJ/mol(exothermic)

Kim’s experiment – Simulation results

experiment


0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

Conversion

NH3

573 K

0 0.2 0.4 0.6 0.8 1

Residence Time (s)

623 K

6.4m/s

9.1m/s

10.8m/s

00.20.40.60.81

0 0.2 0.4 0.6 0.8 1

673 K

wet‐solid

wet‐solid

wet‐solid

multi‐component

multi‐component

The multi‐component droplet setup over predicts the NH3 conversion, especially for small residence times, and low temperatures.The wet‐solid particle setup agrees with the experiment for the whole range of operating conditions.

simulation

simulation

simulation

experiment

experiment

Spray‐wall interaction and film formation ‐Birkhold metal plate experiments• Urea‐water solution is sprayed on a metal plate installed in the center of a flow channel. The transient temperature evolution due to spray induced cooling of the plate is measured with thermocouples. Wall film formation is visually monitored.


Fluent validation model setup considerations• 3D transient problem.• The time scale to observe the spray induced cooling and possible wall film formation in the experiment is 100‐200 seconds.

• In the simulation the time is scaled by 150, by reducing the metal plate thermal inertia (divide solid Cp by 150). This will reduce the time required to cool the wall, however the total amount of the injected urea‐water solution will not be correct.

• In the experiment urea‐water solution with a flowrate of 9.3 kg/h is injected every 0.5 s, for specified time duration (injection pulses).

• It is not clear how to define the injection flowrate in the simulation, given the fact that the simulation time is scaled. The injection flowrate was treated here as a parameter.


UWS droplet – wall interaction regimesApproach 1: for multi‐component particle we follow the droplet regimes

Tcrit = fcrit * Tbfcrit = critical transition factor, Tb = boiling temperature.According to Birkhold fcrit is close to 1.4 for the SCR application.


Tcrit

Approach 2: for wet solid combusting particle

• Water content > 0 same regimes as liquid particle.

• When all the water has evaporated, the particles stick upon impact to the wall

Fluent validation model setup• 3D transient with time step 2.E‐4 s

• Injection setup to match the droplet diameter distribution reported in Birkhold

• DPM with– Wave breakup model– Wall film

• 4 splashed drops• fcrit = 1.6• Hidden feature for wall‐film heat transfer enabled

• Planar wall conduction enabled on the plate


0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 50 100 150 200

X original data pointsFluent case

Results – particle paths and deposition


After 90 s:Particle paths colored by water mass fraction look very similar for multi‐component and wet‐solid approaches. However, with the wet‐solid approach solid urea particles are collected on the wall

multi‐component approach wet‐solid approach

Contours of solid film colored by height

Results – wall cooling


Contours of wall temperature after 90 s: With the wet‐solid approach the wall temperature is lower due to the endothermic decomposition of the deposited solid urea particles on the wall

Multi‐component approach wet‐solid approach

Increasing injection flowrate

Tmin = 450 KTmax = 613 K

Results – film formation


Multi‐component approach wet‐solid approach

No film formation

Increasing injection flowrate

Contours of wall film height after 90 s: The threshold for film formation and the location of the wall film differs for the multi‐component and the wet‐solid approaches.Experimental verification is required!

Maximum film height = 5E‐6 m

Plot with maximum film height = 5E‐5 m

No film formation

Particle Wall Interaction ‐ Discussion

• A methodology exists to include particle‐wall interaction and wall cooling in practical SCR studies, that uses a “speedup” factor to reduce the required runtime for the transient runs.

• Simulations following this methodology can only provide limited qualitative information, as the injection flowrate cannot be scaled consistently, and the film formation heavily depends on the injection flowrate.

• Both multi‐component and wet‐solid approaches predict wall cooling and film formation, however the location, shape and time of film formation are different for the two approaches.

• The simulations predict that the wall cooling takes place at and downstream of the main spray impact area. There are no experimental measurements in Birkhold’s work of the temperature downstream of the spray injection.

• The simulation results are in qualitative agreement with the reported behavior in literature, but further experimental validation is required. The experimental study of Birkhold does not provide sufficient data for an accurate reproduction of the experiment.


SCR simulation in the ‘V‐shape’ development process

Catalyst experts

CFD, FEA experts

Control department

System integration

Concept analysis

Model calibration: obtain the reaction rate parameters of each

coating technology

CFD, FEA experts use ‘general purpose’ CFD products. No need for recalibration of chemistry.

FasterSlower

1D1D+1D

2D2D+1D

3D 3D+1D

ReducedmodelsCFD Spreadsheet

calculationsMore detail

Lessdetail

SCR catalyst modeling scales

Dedicated simulation software for catalytic exhaust aftertreatment

Extensively validated and applied by major automotive OEMs and suppliers

Overview of flow‐through catalyst model equations in axisuite software

Koltsakis et al, Appl. Catal B., 1997. Pontikakis et al., Top. In Catal, 2001Tsinoglou & Koltsakis, Proc. IMechE, 2007

SCR Chemical reactions in axisuite

2012 Automotive Simulation World Congress 30Monday, October 08, 2012

Standard SCR reaction NH3 + NO + 1/4 O2 →N2 + 3/2 H2O

Fast SCR reaction 2 NH3 + NO + NO2 → 2 N2 + 3 H2O

NO2-SCR reaction NH3 + 3/4 NO2 → 7/8 N2 + 3/2 H2O

NH3 oxidation NH3 + 3/4 O2 → 1/2 N2 + 3/2 H2O

NO oxidation NO + 1/2 O2 → NO2

N2O production NH3 + NO2 → 1/2 N2 + 1/2 N2O+3/2 H2O

NH3 adsorption/desorption NH3 ↔ NH3 *

Reaction rate expressionsCalibration parameters

Reactions Rate Expressions Calibrationparameters

Description

NH3 adsorption reactionsNH3↔ NH3(l) A, E NH3 Adsorption

A, E NH3 Desorption

SCR reactionsNH3 + NO + 1/4 O2 N2 + 3/2 H2O A, E Standard SCR

NH3 + 1/2 NO + 1/2 NO2 N2 + 3/2 H2O A, E Fast SCR

NH3 + 3/4 NO2 7/8 N2 + 3/2 H2O A, E NO2‐SCR

NH3 + 5/4 O2NO + 3/2 H2O A, E NH3 oxidation

NH3 + NO2 1/2 N2 + 1/2 N2O + 3/2 H2O A, E N2O production


9

23RTE

GONONHeA

R

9

23RTE

GNONONHeA

R

9

23RTE

GNONHeA

R

2

3RTE

19 NHeA1TG1

9

23RTE

GONHeA

R

9

23RTE

GNONHeA

R

)1(WeAR j,eqj0RTE

b

freej,eq0RTE

f WeAR

Reaction Model calibration

• The calibrated model should be able to predict NOx conversion and NH3 slip as function of:– Temperature– NO2/NOx ratio– NH3/NOx ratio


0

10

20

30

40

50

60

70

80

90

100

0.7 0.8 0.9 1.0 1.1 1.2 1.3α (NH3:NO)

NO

Con

vers

ion

[%]

150°C

200°C

250°C

300°C

400°C

Experiment

Simulation

AdBlue spray & SCR reaction modelingaxisuite – ANSYS Fluent coupling approach

33

UDFs

Porous structureSRC catalyst reaction rates

TemperatureGas Concentrations

Particle ResidenceTime

• Detailed 3D Geometry representation• Turbulent flow-field• AdBlue spray dispersion/evaporation• Energy Balance• Gas phase reactions• SCR porous structure and detailed

chemistry are computed in axisuite and coupled to ANSYS Fluent through User Defined Functions

• Preliminary data for the surface reactions have been used

Full SCR system simulation example case 1


Geometry details:‐ SCR: diameter = 0.144 m, length = 0.250 m‐Canning and pipe thickness = 0.003 m‐Pipe internal diameter = 0.05 m‐Cone Angle Φ = 24ο‐ SCR Injection position = 0.500 m‐Total Exhaust length = 2.450 m‐Typical lengths before and after the device 1.2m and 1. m

Mesh : Hexa Sweep mesh 382800 Cells

Temperature Boundary Condition:Adiabatic Walls

Injection

SCR

Inlet

Velocity in the Exhaust System

Detailed View of SCR Axial Velocity

NO mole fraction

The NO conversion in the SCR unit is affected by the non‐uniform velocity distribution


Full SCR system simulation example case 2Geometry details:‐ SCR: diameter = 0.250 m, length = 0.250 m‐Canning and pipe thickness = 0.003 m‐Pipe internal diameter = 0.05 m‐ Injection position before device = 0.700 m‐Mixer position before device = 0.550 m‐Total Exhaust Length = 1.400 m‐Typical lengths before and after the device 0.850m and 0.200m

Mesh: Corse Mesh, Hexa Mesh with Tetra Mesh in the Mixer Region, 66976 Cells

Temperature Boundary Condition:Convection: Heat Transfer Coef.:10 , Free Stream Temp: 243.15, wall thickness: 0.0246m

Particle Tracks colored with residence time and contours of ammonia mass fraction


Temperature and NO mass fractions


The cold wall temperature of the SCR catalyst reduces the conversion rate of NO Changing the ambient temperature from 243 K to 333 K strongly affects NO conversion in the SCR

Difference in predicted NO mass fraction for a hot and a cold day

Conclusions


• Two possible approaches (multi‐component droplet and wet‐solid particle) for the representation of the urea‐water solution in a CFD simulation of an SCR system have been discussed and analyzed.

• Validation cases for single droplets and reacting spray with both approaches show that the wet‐solid option gives good results and is therefore recommended.

• In order to include spray‐wall interaction effects in practical simulations a “speedup” factor can be introduced to decrease the simulation time required; however further refinement to the methodology is required to provide appropriately scaled values of the spray flowrate.

• Finally for the complete SCR system simulation in terms of NOx emissions ANSYS‐Fluent has been coupled with the specialized third‐party tool axisuite. The axisuite software can be used in standalone mode for the NO chemistry calibration for the specific catalyst, and the calibrated chemistry library is made available subsequently for the coupled CFD simulation. The prototype code has been tested on two example configurations.

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