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7/30/2019 08 Mf e&p - Microfluidic Design and Analysis II Cfd
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MF Effects & Phenomena: 08 Design & Analysis / slide 1www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Microfluidics 1: Effects & Phenomena
08 Design & Analys is8.4 CFD Computational Fluid Dynamics
8.5.1 Applications of CFD in Microfluidics
8.5.2 CFD solves transport equations
8.5.3 Numerically solving the transport equations
8.5.4 Modelling a complex problem
8.5 CFD Resources at the IMTEK
MF Effects & Phenomena: 08 Design & Analysis / slide 2www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
CFD is a Standard Tool in Microf ludicsWide range of applications
Dispensers
Capillary priming
Heat transfer
Movement of gas bubbles
Mixing
CFD is used to
Prove the principal of newsystems
Detailed insight into fluidicsystems
Find optimized parameters for asystem
detailed insight into coriolis mixing(IMTEK Laboratory for MEMS Applications)
optimization of droplet ejected from a nozzle
(IMTEK Laboratory for MEMS Applications)
prove of principlewater removal in fuel cell
(IMTEK Laboratory for MEMS Applications)
7/30/2019 08 Mf e&p - Microfluidic Design and Analysis II Cfd
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MF Effects & Phenomena: 08 Design & Analysis / slide 3www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
1 10 100 1000
0
5
10
15
20
CriticalPressure[kPa]
Density [g/cm3]
Surface Tension [mN/m]
Dynamic Viscosity [mPas]
Nozzle Diameter [m]
CFD allows to find optimal parameters fora system
Example: optimize pressurepulses for the dispensing of
different liquids
Varying interfacial tension
Varying viscosity
Varying density
Varying nozzle geometry
droplet ejected from a Top Spot nozzle
(IMTEK Laboratory for MEMS Applications)
critical pressure for droplet ejection in
dependence of material and nozzle parameter
MF Effects & Phenomena: 08 Design & Analysis / slide 4www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
CFD allows a detailed insight into fluidic
systemsExample: Is coriolis force
responsible for mixing on a rotating
disk?
Detailed insight by CFD made
visible:
Liquid is transported outwardsby centrifugal force
Coriolis force acts normal to flowdirection
Coriolis force and viscosity inducecomplex flow pattern: mixing
CoMix mixer CD
(IMTEK Laboratory for MEMS Applications)
simulated coriolis flow on a rotating disk
(IMTEK Laboratory for MEMSpplications)
liquid A
liquid Bcentrifugal force
movement
coriolis force
7/30/2019 08 Mf e&p - Microfluidic Design and Analysis II Cfd
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MF Effects & Phenomena: 08 Design & Analysis / slide 5www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
CFD allows a detailed insight into fluidicsystems
simulation of coriolis induced mixing pattern
(IMTEK Laboratory for MEMS Applications)
Coriolis force and viscosity induce
complex flow pattern: mixing
Fluid is folded
Fluid fluid interface increases
Increase of fluid fluid interface leads toan enhanced diffusive mixing
MF Effects & Phenomena: 08 Design & Analysis / slide 6www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
CFD allows to prove of principal of new
systems
Example: Can water be removed by capil lary forces
from the active area (MEA) of a fuel cell?
CFD simulation proved that it is possible tolift water droplets from a MEA
Usage of contrast in wetting behaviour andgeometry
droplets lifted from the MEA in a fuel
cell
during operation
CFD simulation of droplets lifted from a MEA by capillary forces
(IMTEK Laboratory for MEMS Applications)
hydrophobic MEA/GDL
hydrophillic
channel walls
1mm
realized test cell
(IMTEK Laboratory for MEMS Applications)
7/30/2019 08 Mf e&p - Microfluidic Design and Analysis II Cfd
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MF Effects & Phenomena: 08 Design & Analysis / slide 7www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
8. Methods of Microfluidic Design andAnalysis
08 Design & Analys is
8.4 CFD Computational Fluid Dynamics
8.5.1 Applications of CFD in Microfluidics
8.5.2 CFD solves transport equations
8.5.3 Numerically solving the transport equations
8.5.4 Modelling a complex problem
8.5 CFD Resources at the IMTEK
MF Effects & Phenomena: 08 Design & Analysis / slide 8www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
CFD solves Transport Equations
Transport phenomena in micro fluidics
Mass
Momentum
Heat
Dispersed particles
Phase boundaries,
Transport systems are coupled by the interaction of forces
7/30/2019 08 Mf e&p - Microfluidic Design and Analysis II Cfd
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MF Effects & Phenomena: 08 Design & Analysis / slide 9www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Generic Transport Equation (1)
Transport equations can be mapped to one generic transportequation
Generic transport equation for any transport variable :
temporalchange
explicitsources
( ) ( )( )ii i i j
v q ft
+ = + +
coupling toother fieldvariables
convection transport with moving liquid
diffusion
(generic parameter)
source termstemporal change
MF Effects & Phenomena: 08 Design & Analysis / slide 10www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Generic Transport Equation (2): Example 1
= velocities transport ofmomentum (Navier Stokes equations)
( ) ( )( )ii i i j
v q ft
+ = + +
2( )
volumep
t
+ = + v v v v f
generic transport equation:
Navier stokes Equation (->lecture fluid dynamics I)
i = vector of
momentum v
diffusion =
viscosity
=
sources of
moment = volume
forces
pressure
gradient
= coupling term
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MF Effects & Phenomena: 08 Design & Analysis / slide 11www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Generic Transport Equation (3): Example 2
= concentration transport of concentration: 2nd Ficks Law
( ) ( )( )ii i i j
v q ft
+ = + +
generic transport equation:
2
2
x
cDt
c
=
2nd Ficks law(-> lecture diffusion)
diffusion:
D =
i =
concentrationno convection no sources no couplings
MF Effects & Phenomena: 08 Design & Analysis / slide 12www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
= density conservation of mass(continuity)
= impulses transport ofmomentum(Navier Stokes equations)
= enthalpy (cpT) heat transport
= concentration chemical reactionsystems
= fluid fractions multiphase systems(volume of fluid)
= particle distributions
Overview of Transport Equations in
Microfluidics
simulation of coriolis induced mixing
pattern(IMTEK Laboratory for MEMS
Applications)
( )( )ii i i j
v q ft
+ = + +
7/30/2019 08 Mf e&p - Microfluidic Design and Analysis II Cfd
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MF Effects & Phenomena: 08 Design & Analysis / slide 13www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Example: apparent forces in a
rotational framework, source of
momentumother
Gravity on mass, source of momentum
Heating from radiation, heat source
External sources to the transportequations
simulated coriolis flow on a rotating disk
(IMTEK Laboratory for MEMS Applications)
liquid A
liquid Bcentrifugal force
movement
coriolis force
( )( )ii i i j
v q ft
+ = + +
MF Effects & Phenomena: 08 Design & Analysis / slide 14www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Any functions of other field variables jexample: capillary pressure is momentum
source as function ofcurvature of
interface (distribut ion o f phases) other:
chemical processes: coupling of eductconcentrations, temperature, pressure and
generation of products and heat
friction generates heat as a function of velocity
Coupling terms between transport
equations
water removal in fuel cell
(IMTEK Laboratory for MEMS Applications)
( )( )ii i i j
v q ft
+ = + +
7/30/2019 08 Mf e&p - Microfluidic Design and Analysis II Cfd
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MF Effects & Phenomena: 08 Design & Analysis / slide 15www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
What makes up the real strength of CFD?
Many different transport equations can be addressed in the sameway
Map to the generic transport equation
Complex systems can be addressed by coupling of dif ferent
transport equations
Only one numerical scheme will be necessary to solve an arbitrary
complex system of transport equations (-> next section)
CFD-codes can be applied straightforward to a wide range of
applications!
CFD-codes can be appl ied straightforward to a wide range of
applications!
MF Effects & Phenomena: 08 Design & Analysis / slide 16www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
8. Methods of Microfluidic Design and
Analysis
08 Design & Analys is
8.4 CFD Computational Fluid Dynamics
8.5.1 Applications of CFD in Microfluidics
8.5.2 CFD solves transport equations
8.5.3 Numerically solving the transport equations
8.5.4 Modelling a complex problem
8.5 CFD Resources at the IMTEK
7/30/2019 08 Mf e&p - Microfluidic Design and Analysis II Cfd
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MF Effects & Phenomena: 08 Design & Analysis / slide 17www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Al l equations can be solved in the sameway
( )( )ii i i j
v q ft
+ = + +
discretize geometry
map transport equationsto generic equation
matrix equations
solve numerically
discretization of
generic equation
problem dependent
genericgeneric
MF Effects & Phenomena: 08 Design & Analysis / slide 18www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Detailed view: Numerically solving the
transport equations
Three iteration loops
Solve field matrix for each variable(iteratively): inner iteration
Coupling + solution: outer iteration
Solve next time step
Convergence criteria
Breakdown criteria of iteration loops
Strong criteria: high quality of solution
long calculation time
Weak criteria: low quality
short calculation timesolution Strategy of the CFX Solver (CFX Manual)
7/30/2019 08 Mf e&p - Microfluidic Design and Analysis II Cfd
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MF Effects & Phenomena: 08 Design & Analysis / slide 19www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
General workflow in CFD
.
....
...
....
....
...
....
.........
.....
.....
01040497093
9323590162154019543208
954160703106920776002000901553052
8050244541192714332107591525819134
problem definition algorithm solves equations interpretation & analysis
developer & computeruser
postprocessingpreprocessing solving
user
MF Effects & Phenomena: 08 Design & Analysis / slide 20www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Verification and Validation
Like any sof tware a CFD model must be
verified and validated (Def. IEEE 1012)
Verification mathematical correctness of the model
done by the developer
user: are the error criterias in the verified
range?
Validation agreement of model results with experimental
results
done by the developer
always only a few cases are tested and
documented
user: is the simulation in a validated
parameter range?
All CFD simulations are
wrong!
All CFD simulations are
right!
A simulation is done by a model so it is a
question of the proper interpretation and
right usage wheter it is right or wrong!
7/30/2019 08 Mf e&p - Microfluidic Design and Analysis II Cfd
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MF Effects & Phenomena: 08 Design & Analysis / slide 21www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
8. Methods of Microfluidic Design andAnalysis
08 Design & Analys is
8.4 CFD Computational Fluid Dynamics
8.5.1 Applications of CFD in Microfluidics
8.5.2 CFD solves transport equations
8.5.3 Numerically solving the transport equations
8.5.4 Modelling a complex problem
8.5 CFD Resources at the IMTEK
MF Effects & Phenomena: 08 Design & Analysis / slide 22www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
The passive DMFC
1) Tapered channel structure forcapillary induced fluid flow
2) Hydrophobic venting membrane
3) Bubble fence
4) Membrane electrodeassembly, gas diffusion Layer
5) Cathode Flow Field
6) Flexible tubing
7) Fuel reservoir
8) Flow Sensor
9) Camera
1
2
3
4
5
7
6
O2 supply by diffusion
89
1.2 mm
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MF Effects & Phenomena: 08 Design & Analysis / slide 23www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Different pumping modes
Depending on bubble size twopumping modes can be realized:
A Non-Blocking and
B Blocking
Pump rate adjustable by
Geometry
Wetting properties
Gas flow rate
A B
MF Effects & Phenomena: 08 Design & Analysis / slide 24www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Modelling
Modeling and simulation issupposed to shed light on theinfluence of the gas venting andwetting phenomena on
The different pumping modes Blocking and
Non Blocking
Capillary induced fuel circulation
Requirements
Bubble generation
Capillary transport
Venting
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MF Effects & Phenomena: 08 Design & Analysis / slide 25www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Modelling
Standard models available inCFD-ACE+ for
Navier stokes finite volume grid
Free surfaces Volume of Fluid (VoF)
Surface tension ContinuumSurface Force (CSF)
Challenge
VoF and CSF requires very smalltime steps computationally very
demanding
Multiscale problem Nucleation of bubbles
Triple line phenomena
Venting process
MF Effects & Phenomena: 08 Design & Analysis / slide 26www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Modelling approach
Coarse grid modelling
Capillary transportresolved byfinite volume computational grid
Addi tional models implementedas user-sub-routines
Bubble Generation
Tripple line phenomena
Bubble Venting
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MF Effects & Phenomena: 08 Design & Analysis / slide 27www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Subdomains for bubble
generation
Bubble generation implemented as user-sub-routine
Approach
Domain is subdivided in controlvolumes
Control volume contains gas bubble growth continuously
Control volume contains no gas CO2 accumulates virtually up to a
critical mass
Bubble growth with a dynamic
based on stability criteria
Challenge
Small bubble and CSF unstablecapillary pressure
MF Effects & Phenomena: 08 Design & Analysis / slide 28www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Dynamic contact angle implemented as
user-sub-rout ine (1)
Contact Angle changes when
contact line moves
Pinning / contact angle hysteresis Contact line is pinned on rough
surfaces or chemical
heterogeneities
Dynamic contact angle Contact angle changes continuously
with the velocity of moving contact
linesadvancing
receding
vCa
=
static
hysteresis
0
ad rec
Bubble movement
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MF Effects & Phenomena: 08 Design & Analysis / slide 29www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Dynamic contact angle implemented asuser-sub-rout ine (2)
Dynamic contact angle as userdefined boundary condition
Challenge Discontinuity at = 0 leads to
instability
For a non moving contact line, is
free between rec and ad
Approach PI-Controller maintains Ca=0 in each
cell with hys as the maximum
correction variable range
ad>rec
ad rec
advancing
receding
vCa
=
static
hysteresis
0
MF Effects & Phenomena: 08 Design & Analysis / slide 30www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Venting implemented as user-sub-rout ine
Challenge
Resolution of a single pore of theporous membrane to time
consuming
Approach
Phase specific sink term as aboundary condition derived from
pressure-velocity characteristicsof the membrane
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MF Effects & Phenomena: 08 Design & Analysis / slide 31www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
The presentedmodel combination
allows for
Stable long-termsimulations
Studying bubbleconfigurations
during the pump
performance
Results (1)
Without Pinning ModelWith Pinning Model
side view
top view
MF Effects & Phenomena: 08 Design & Analysis / slide 32www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Results (2)
The presented model
combination allows for
Studying the influence of pinningon pump rates
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MF Effects & Phenomena: 08 Design & Analysis / slide 33www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Results (3)
MF Effects & Phenomena: 08 Design & Analysis / slide 34www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Conclusion
Challenges
Computationally demanding
Presented simulations require4-6 weeks on a single
processor
Achievements
Modelling and simulation of bubble generation
capillary movement
and membrane venting
Studying the influence of eachphenomena on the pump
performance is now possible
but
7/30/2019 08 Mf e&p - Microfluidic Design and Analysis II Cfd
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MF Effects & Phenomena: 08 Design & Analysis / slide 35www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Outlook
Requirements for quantitativeparameter studies
Further reduction of the calculationtime more efficient model
implementation
Parallelisation of User-Sub-Routines
MF Effects & Phenomena: 08 Design & Analysis / slide 36www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
8. Methods of Microfluidic Design and
Analysis
08 Design & Analys is
8.4 CFD Computational Fluid Dynamics
8.5.1 Applications of CFD in Microfluidics
8.5.2 CFD solves transport equations
8.5.3 Numerically solving the transport equations
8.5.4 Modelling a complex problem
8.5 CFD Resources at the IMTEK
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MF Effects & Phenomena: 08 Design & Analysis / slide 37www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
For Students: Cosmos Flow Works
Cosmos Flow Works included in Solidworkscampus license
Available forevery student inside the university
At home by vpn
License borrowing possible
Laminar & turbulent flow
Heat transfer
Heat transfer in solids included
Simple to use
MF Effects & Phenomena: 08 Design & Analysis / slide 38www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
For the researchers: ESI CFD-ACE+
Professional multipurpuse suite
Strong in free surface modelling
User friendly, systematic interfaces
Educational and research licenses
available at IMTEK
www.esi-group.com
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MF Effects & Phenomena: 08 Design & Analysis / slide 39www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
For the enthusiastic
Open source CFD code OpenFOAM
www.opencfd.co.uk
Linux platform only
Definition of own transport equations
Large suit of models
particles, hour glass
jet breakoff
world wide
wether
user interface
MF Effects & Phenomena: 08 Design & Analysis / slide 40www.imtek.de/anwendungen Roland Zengerle / 11.07.2008
Resources
CFD-Online: Sponsored information service for CFD-usershttp://www.cfd-online.com
http://www.cfd-online.com/Resources/homes.html#Company companies and suppliers
http://www.cfd-online.com/Forum/ discussion and information forum
http://www.cfd-online.com/Jobs/jobs for CFD Engineers and Researchers
Hompages of commercial suppliers of software http://www.software.aeat.com/cfx
http://www.fluent.com
http://www.flow-3D.com
http://www.cfdrc.com
...
Service providers for MEMS (not only CFD) http://www.memscap.com
http://www.coventor.com
...