Dept. of Power Mechanical Engineering, National Tsing Hua University
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Dept. of Power Mechanical Engineering, National Tsing Hua University Kai Fei, Chao-Jen Tsai (Research students), Che-Wun Hong (Professor) Thermal Lattice Boltzmann Simulations of Two-Phas Thermal Lattice Boltzmann Simulations of Two-Phas e Flow e Flow in Micro Direct Methanol Fuel Cell Microchannels in Micro Direct Methanol Fuel Cell Microchannels Conclusions Conclusions Simulation Results Simulation Results 2007.6 Introduction Introduction Micro-direct methanol fuel cells (DM FC) are considered a strong competito r of future power sources for portabl e equipment. The advantages are high efficiency, high power density, low o peration temperature and almost zero pollution. Electrochemical Reaction e H CO O H OH CH 6 6 2 2 3 2 2 3/2 6 6 3 O H e HO 3 2 2 2 3/2 2 CH OH O CO HO Anode : Overall : Chemical Reaction Cathode : TOSHIBA, HITACHI, SAMSUNG, LG, SONY, NEC, PANASONIC, Sanyo Electric, IBM, and etc. DMFC DMFC Notebook Computer, Cellular Phone, PDA, MP3 Player, Video Game Console, Digital Camera, and etc. Approach Approach Two-phase flow (Methanol solution/C O 2 bubble) in the microchannels is simulated with the lattice Boltzman n method (LBM) and the thermal latt ice Boltzmann method (TLBM) approac hes. Hydrophilic, geometric and the rmal (Marangoni effect) effects on the bubble dynamics are discussed. Hot Col d Bubble Liquid Marangoni effect : Marangoni effect : Liquid flows from a region of high temperature to a region of low temperature an d exerts an opposite reaction on the bubble to make it move from t he cold region to the warm region. Objective and Motivation Objective and Motivation Carbon dioxide (CO 2 ) bubbles flow i nto the diffusion layer and block the porous media if they cannot be removed efficiently, resulting in a decline of the cell performance. Hence, the bubble transport phenom enon in the microchannels is a maj or issue. Bubble Bubble Methanol solution Methanol solution Microchannel TLBM TLBM 1 , , , , i X c X X X eq i i i i f tt t f t f t f t Lattice Boltzmann equation for flow field : c 0 c 8 c 4 c 3 c 5 c 1 c 2 c 6 c 7 Surface tension : Fluid-solid interaction force : Buoyancy force : , X X X c a f a a a i a G F t tc , X X X c a s a a s w i i F t c G t , , X X c a a b i i F t G tt fluid-fluid interaction strength 0 0 exp X a a fluid-solid interaction strength 1 0 at the wall X c in the fluid w i t Lattice Boltzmann equation for temperatur e field : 1 , , , , i T X c X X X eq i i i i g tt t g t g t g t , , X X a a i i t f t , , , X U X X c a a a i i i t t f t Macroscopic mass density and momentum density Macroscopic temperature : Hydrophilicity Ef Hydrophilicity Ef fect fect , ( ) X j a T F t G T T Temperature effect (the Boussinesq approximation) : X, X, X, i i tT t g t Contact Contact Angle Angle H + H + Proton Methanol solution Anode Cathode Proton exchange membrane Catalyst layer Diffusion layer Carbon dioxide Water Direct Methanol Fuel Cell Oxygen H + H + H + H + H + H + H + H + H + H + CO 2 H 2 O H 2 O H 2 O O 2 O 2 O 2 e - e - e - e - e - e - e - e - e - e - e - e - H 2 O CH 3 OH Electro n Fluid-solid interaction strength vs. contact angle Contact angle vs. bubble velocity Geometric effect Geometric effect Time=12.50ms Time=37.50ms Time=25.00ms Time= 1.75ms Gs = 0.007 -0.007 Gs = -0.007 0.007 Bubble Velocity = 262.65 m/s Bubble Velocity = 274.29 m/s Thermal effect Thermal effect Time=12.50ms Time= 1.75ms Time=25.00ms Time=37.50ms Time=12.50ms Time= 1.75ms Time=25.00ms Time=37.50ms Bubble Velocity = 323.91 m/s Bubble velocity vs. temperature gradient A hydrophilic, divergent channe l with a positive temperature g radient is favorable for bubble removal in the microchannels. T he results provide important in formation for the design of the DMFC. Time=25.00ms Time= 1.75ms Time=39.25ms Inlet Velocity = 40.00 m/s (Bubble Blockage) (Bubble Blockage) Time=25.00ms Time= 1.75ms Time=39.25ms Inlet Velocity = 50.00 m/s Time=29.75ms Time= 1.75ms Time=71.25ms Time=29.75ms Time= 1.75ms Time=71.25ms Inlet Velocity = 250.00 m/s Time=12.50ms Time= 1.75ms Time=25.00ms Time=34.50ms Bubble Velocity = 319.62 m/s Inlet Velocity = 250.00 m/s Time=12.50ms Time= 1.75ms Time=25.00ms Time=34.50ms Bubble Velocity = 335.34 m/s Convergent Microchannel Convergent Microchannel Divergent Microchannel Divergent Microchannel Orificed Microchannel Orificed Microchannel Straight Microchannel Straight Microchannel Geometric Geometric effect effect Straight channel Basic case Convergent channel Divergent channel Hydrophilic Hydrophilic ity effect ity effect Hydrophilic ( ) Less hydrophilic ( ) Increased hydrophilicity ( ) Decreased hydrophilicity ( ) Thermal Thermal effect effect Low temperature ( ) Positive wall temperature gradient ( ) Negative wall temperature gradient ( ) Basic case ： hydrophilic walls with high temperature ( ) Arrow length represents the degree of the influence (↑favorabl e ；↓ adverse) 。 0.007 a s G 0.007 a s G 0.004, 333 a s G T K 0.007 0.007 a s G 0.007 0.007 a s G 298 T K 35 T K 35 T K K T 35
description
c 6. c 2. c 5. c 3. c 0. c 1. c 7. c 4. c 8. CH 3 OH. e -. e -. e -. e -. e -. e -. e -. e -. e -. e -. e -. e -. H +. H +. H +. H +. H +. H +. H +. H +. H +. H +. H +. H +. O 2. O 2. O 2. Electrochemical Reaction. - PowerPoint PPT Presentation
Transcript of Dept. of Power Mechanical Engineering, National Tsing Hua University
Dept. of Power Mechanical Engineering, National Tsing Hua UniversityKai Fei, Chao-Jen Tsai (Research students), Che-Wun Hong (Professor)
Thermal Lattice Boltzmann Simulations of Two-Phase Flow Thermal Lattice Boltzmann Simulations of Two-Phase Flow in Micro Direct Methanol Fuel Cell Microchannelsin Micro Direct Methanol Fuel Cell Microchannels
ConclusionsConclusionsSimulation ResultsSimulation Results
2007.6
IntroductionIntroductionMicro-direct methanol fuel cells (DMFC) are considered a strong competitor of future power sources for portable equipment. The advantages are high efficiency, high power density, low operation temperature and almost zero pollution.
Electrochemical Reaction
eHCOOHOHCH 66223
2 23/ 2 6 6 3O H e H O
3 2 2 23/ 2 2CH OH O CO H O
Anode :
Overall :
Chemical Reaction
Cathode :
TOSHIBA, HITACHI, SAMSUNG, LG,
SONY, NEC, PANASONIC, Sanyo Electric, IBM, and
etc.
DMFDMFCC
Notebook Computer, Cellular Phone, PDA, MP3
Player, Video Game Console, Digital Camera,
and etc.
ApproachApproachTwo-phase flow (Methanol solution/CO2 bubble) in the microchannels is simulated with the lattice Boltzmann method (LBM) and the thermal lattice Boltzmann method (TLBM) approaches. Hydrophilic, geometric and thermal (Marangoni effect) effects on the bubble dynamics are discussed.
Hot ColdBubble
Liquid
Marangoni effect :Marangoni effect : Liquid flows from a region of high temperature to a region of low temperature and exerts an opposite reaction on the bubble to make it move from the cold region to the warm region.
Objective and MotivationObjective and MotivationCarbon dioxide (CO2) bubbles flow into the diffusion layer and block the porous media if they cannot be removed efficiently, resulting in a decline of the cell performance. Hence, the bubble transport phenomenon in the microchannels is a major issue.
BubbleBubble
Methanol solutionMethanol solution
Microchannel
TLBMTLBM
1, , , ,iX c X X Xeq
i i i if t t t f t f t f t
Lattice Boltzmann equation for flow field :
c0
c8c4
c3
c5
c1
c2c6
c7
Surface tension :
Fluid-solid interaction force :
Buoyancy force :
,X X X caf
a a ai
aGF t t c
,X X X cas
a as w i
i
F t cG t
, ,X X ca ab i
i
F t G t t
fluid-fluid interaction strength
0 0expXa a
fluid-solid interaction strength
1
0
at the wallX c
in the fluidw i t
Lattice Boltzmann equation for temperature field : 1
, , , ,iT
X c X X Xeqi i i ig t t t g t g t g t
, ,X Xa ai
i
t f t , , ,X U X X ca a a
i ii
t t f t
Macroscopic mass density and momentum density :
Macroscopic temperature :
Hydrophilicity EffectHydrophilicity Effect
, ( )X jaTF t G T T
Temperature effect (the Boussinesq approximation) :
X, X, X,ii
t T t g t
Contact Contact AngleAngle
H+
H+
Proton
Methanol solution
Anode Cathode
Proton exchange membrane Catalyst layer Diffusion layer
Carbon dioxide
Water
Direct Methanol Fuel Cell
Oxygen
H+
H+ H+
H+
H+
H+
H+
H+
H+
H+
CO2H2O
H2OH2O
O2
O2
O2
e- e- e-e- e- e- e-e- e-e- e- e-
H2OCH3OH
Electron
Fluid-solid interaction strength vs. contact angle
Contact angle vs. bubble velocity
Geometric effectGeometric effect
Time=12.50ms
Time=37.50ms
Time=25.00ms
Time= 1.75ms
Gs = 0.007 -0.007 Gs = -0.007 0.007
Bubble Velocity = 262.65 m/s Bubble Velocity = 274.29 m/s
Thermal effectThermal effect
Time=12.50ms
Time= 1.75ms
Time=25.00ms
Time=37.50ms
Time=12.50ms
Time= 1.75ms
Time=25.00ms
Time=37.50ms
Bubble Velocity = 323.91 m/s
Bubble velocity vs. temperature gradient
A hydrophilic, divergent channel with a positive temperature gradient is favorable for bubble removal in the microchannels. The results provide important information for the design of the DMFC.
Time=25.00ms
Time= 1.75ms
Time=39.25ms
Inlet Velocity = 40.00 m/s (Bubble Blockage)(Bubble Blockage)
Time=25.00ms
Time= 1.75ms
Time=39.25ms
Inlet Velocity = 50.00 m/s
Time=29.75ms
Time= 1.75ms
Time=71.25ms
Time=29.75ms
Time= 1.75ms
Time=71.25ms
Inlet Velocity = 250.00 m/s
Time=12.50ms
Time= 1.75ms
Time=25.00ms
Time=34.50ms
Bubble Velocity = 319.62 m/s
Inlet Velocity = 250.00 m/s
Time=12.50ms
Time= 1.75ms
Time=25.00ms
Time=34.50ms
Bubble Velocity = 335.34 m/s
Convergent MicrochannelConvergent Microchannel
Divergent MicrochannelDivergent Microchannel
Orificed MicrochannelOrificed MicrochannelStraight MicrochannelStraight MicrochannelGeometric Geometric
effecteffect
Straight channelBasic case
Convergent channel
Divergent channel
HydrophilicitHydrophilicity effecty effect
Hydrophilic ( )
Less hydrophilic ( )
Increased hydrophilicity ( )Decreased hydrophilicity ( )
Thermal Thermal effecteffect
Low temperature ( )
Positive wall temperature gradient ( )
Negative wall temperature gradient ( )
Basic case： hydrophilic walls with high temperature ( ) Arrow length represents the degree of the influence (↑favorable；↓ adverse)。
0.007asG 0.007a
sG
0.004, 333asG T K
0.007 0.007asG
0.007 0.007asG
298T K
35T K 35T K
KT 35
