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