urea injection and preparation in diesel applications · pdf filedipl.-phys. simon fischer ......

Institute for Powertrains

& Automotive Technology

Institute for Powertrains & Automotive Technology

Urea Injection and Preparation in

Diesel ApplicationsMultiphase Multicomponent Modeling using Star-CD

STAR European Conference 2010, London

Dipl.-Phys. Simon Fischer

Dr. Thomas Lauer

STAR European Conference 2010

Urea Injection and Preparation in Diesel Applications

March 23rd 2010 | London | Simon Fischer | Slide 2

Contents

The SCR-Process: Problem Introduction

Methodology and Analysis Setup

Thermodynamic Multicomponent Model of a Urea-Water-Solution

Droplet-Wall-Interaction and Liquid Film Dynamics

Conjugate Heat Transfer and Liquid Film Boiling

Ammonia Homogenization

Conclusion and Outlook




Contents











Problem Introduction: Future Diesel Emission Limits

exhaust aftertreatment of NOx is a major requirement for future Diesel engines

[1]

NOx- Emissions [g/km]

Pa

rtic

leE

mis

sio

ns

[g/k

m]

SC

R

source: Auto News




Principle of Selective Catalytic Reduction (SCR)

exhaust gas flow (NOx)

injection of a

water-urea mixture

32.5 mass-% urea

67.5 mass-% water

timescales of ammonia preparation

ammonia uniformity at catalyst frontal area

exhaust pipe

impingement film transport

heat transfer

ammonia (NH3) vapour

homogenization

preparation process

1) water evaporation

2) chemo-physical urea

decomposition to ammonia

(thermo-/hydrolysis)

CFD-simulation for efficient SCR-system designSCR-catalyst:„NOx+NH3 →N2 + H2O“




Contents











Methodology and Validation Setup

straight exhaust pipe

urea-water-

solution-injection

from engine

exhaust gas flow

SC

R-C

ata

lyst

CFD mesh according to

experimental setup

OP 3 - high temperature / mass flow / NOx raw emission

OP2 - intermediate temperature / mass flow / NOx raw emission

OP1 - low temperature / mass flow / NOx raw emission

Analysis of Representative Operating Points (OP):




Contents











Thermodynamic Multicomponent Model (Liquid Film, Droplets)

TBoil(Yurea)

Partial Pressures

(Raoult`s law):

AdBlue:

32.5 Mass.-% Urea,

67.5 Mass.-% Water

pure water:

Antoine

equation

implemented in

dropro.f, lqfpro.f

pure urea:

saturation

pressure curve

implemented in

dropro.f, lqfpro.f

)()( , TpxTp ipure

si

i

s

i

i

stot TpTp )()(

Total Pressure

(Dalton`s law):




Decomposition of a Single Urea-Water-Solution-Droplet

water evaporation urea thermolysis

two-step preparation process modeled

[1]

[1] Wang, Baek,Lee: Experimental Investigation on Evaporation of Urea-Water-Solution Droplet for SCR Applications, AICE 2009

heat flux

through fibre,

fine tuning of

urea-psat-curve




Contents











Multiphase Modeling: Challenge Of Timescales

Small Time Scales:

turbulent flow field

droplet dynamics

species (ammonia) transport

Steady-State Calculation (PISO)

fluid flow field

temperature field

no spray injection (liquid phase)

Large Time Scales:

liquid film formation and -dynamics

ammonia formation from film

wall cooling (CHT)

Transient Calculation (Simple)

thermodynamics (enthalpy)

droplets and liquid film

species (ammonia, water vap.)

time steps: 0.1 ms (droplets present)

1 ms (otherwise)

Solution Strategy:

Restart with

flow field

solvers

switched off

simulated interval of several minutes incl. spray dynamics and species transport




Droplet Dynamics: Injection

three hole nozzle

diameter

distribution

from

measurement

timescale miliseconds

droico.f

no primary break-up modelled




Droplet Impingement Regimes

depending on liquid properties, droplet dynamics, wall temperature,…

OP 3 – high temp.

OP2- medium temp.

OP1 - low temp

…complex behaviour

Bai-M

odel (S

w30)

Sp

rea

d-M

ode

l




Liquid Film Formation: Validation

Experiment

(Videoanalysis)

Simulation Spread-

Model

Simulation Bai-

Model- Switch 30

OP 1

(„cold“)

extensive liquid

film, flow to pipe

bottom x

OP 2

(„medium“)

thin film after

some delay, no

remarkable flowx

OP 3

(„hot“) no permanent film x

impingement model not yet predictive across whole temperature range

not observed

in simulation

FilmQuarzglas.wmv




Simulated Liquid Film Formation

OP3-Bai-Sw30

OP2-BaiSw30

OP1- Spread

…

OP1-BaiSw30massive liquid film with spread model in OP1

small extent liquid film with Bai model in OP1

no permanent film in OP3

transient behaviour in OP2




Droplet Impingement and Liquid Film Dynamics

timescale millisecondstimescale minutesliquid film spread after 120 s

high

Droplet

Diameter

low

Film

Thickness




Liquid Film Dynamics Validation

180°

90°

180°

90°

Expe

rim

ent

Sim

ula

tio

nV

ideo-A

naly

sis

[2]

Sim

ula

tion

Distribution of Liquid Film after 120 Seconds

OP1- Spread

exce

llen

td

escri

ptio

no

f film

dyn

am

ics




Contents











Thermal Wall Boundary Conditions In Star-CD

Adia

batic

Fix

ed

Tem

pera

ture

Fix

ed

with

Resis

tance

Conju

gate

Heat T

ransfe

r

Rele

vance

in

SC

R-M

odelin

g

wall as a heat source !!!cooling of wall due to

evaporation !!!

thermal inertia of wall !!

wall parallel heat conduction !!




Meshing For Conjugate Heat Transfer Approach

boundary Layer

extruded solid cells for CHT

single layer approach

fluid cells

trimmed from custom-grid




The Nukiyama Film Boiling Model (Star-CD 4.10)

OP 3

OP2

OP1

regime positions

from experiment

(IR-thermography)

Burnout Point

Leidenfrost Point

Film-boilingTransition-

Nucleate-Convection




Local Wall Spray CoolingOP 1-Spread

Temperature T[ C]

primary

impingement

position (water

evaporation)

Flow direction

IR-Thermography

Star-CD 4.10good qualitative representation




Wall Spray Cooling

OP3-Bai Sw30OP2-BaiSw30

Steady State Wall CoolingDynamics of Cooling Process

OP1-Spread

IR-Themography

Star-CD 4.06

Star-CD 4.10

numerical instability

in v4.10 Bai model

for thermal break-

up-regime

(currently fixed by

Switch 30 and

NOTHBREAK

in *.prob)




Contents











Film

Thickness

OP 1

Ammonia Vapour Homogenization

Ammonia-

Concentration

high

low




Ammonia Vapour Homogenization: Validation

FTIR- measurement [2]

Ammonia-

Concentration

OP 1

Simulation

high

low

concentrations

scaled by

mean value




Contents









Urea-injection and -preparation in Diesel applications


Conclusion

validated Star-CD-Model of the urea-water-solution preparation process:

simulation of several injection events and long time liquid film behaviour

via separated (steady) flow field and liquid-phase/thermodynamic calc.

two step ammonia preparation process modeled properly with Raoults` law

predictive impingement model for whole temperature range missing

→ massive film formation (OP1) currently forced by spread-model

→ break-up model in v4.10 (for high temp. impingement) not stable

liquid film dynamics well represented in simulation

implementation of Conjugate Heat Transfer for a correct prediction of liquid

film formation and preparation timescales (validated by IR-themography)

promising liquid film boiling model improvements from v4.06 to v4.10


Urea-injection and -preparation in Diesel applications


Outlook

check and adaption of improved Star-CDs` physical models (4.10/4.12)

NIST approach (internal calculation) for multicomp. thermodynamics properties

fix of Break-Up regime numerical instabilities of extended Bai-model (v4.10)

(currently NOTHBREAK in *.prob and Switch 30 → support)

underestimation of liquid film at low temperatures (OP1): tuning of critical

Regime Weber-Numbers to realize „one-fits-all“ Bai-impingement-model

adaption of liquid film boiling model (Nukiyama-curve) for urea-water solution

(currently standard Star-CD parameter set for pure water)

(thermography data available)

evaluation of post impingement droplet properties (e.g. size distribution)

laser-diffraction data available

towards a fully predictive SCR-process Star-CD-model

Institute for Powertrains

& Automotive Technology

Institute for Powertrains & Automotive Technology

Thank you for your attention!

Dipl.-Phys. Simon Fischer

[email protected]

urea injection and preparation in diesel applications · pdf filedipl.-phys. simon fischer ......

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