1. 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-Phase Flow in Micro Direct Methanol Fuel Cell Microchannels  Conclusions  Simulation Results 2007.6  Introduction Micro-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 Anode : Overall : Chemical Reaction Cathode : TOSHIBA, HITACHI, SAMSUNG, LG, SONY, NEC, PANASONIC, Sanyo Electric, IBM, and etc. DMFC Notebook Computer, Cellular Phone, PDA, MP3 Player, Video Game Console, Digital Camera, and etc.  Approach Two-phase flow (Methanol solution/CO 2 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. HotCold Bubble 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 Motivation Carbon dioxide (CO 2 ) 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. Bubble Methanol solution Microchannel  TLBM Lattice Boltzmann equation for flow field : c0c0 c8c8 c4c4 c3c3 c5c5 c1c1 c2c2 c6c6 c7c7 Surface tension : Fluid-solid interaction force : Buoyancy force : fluid-fluid interaction strength fluid-solid interaction strength Lattice Boltzmann equation for temperature field : Macroscopic mass density and momentum density : Macroscopic temperature : Hydrophilicity Effect Temperature effect (the Boussinesq approximation) : Contact Angle H+H+ H+H+ Proton Methanol solution AnodeCathode 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+ H+H+ H+H+ H+H+ H+H+ H+H+ CO 2 H2OH2O H2OH2O H2OH2O O2O2 O2O2 O2O2 e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- H2OH2O CH 3 OH Electron Fluid-solid interaction strength vs. contact angle Contact angle vs. bubble velocity Geometric effect Time=12.50ms Time=37.50ms Time=25.00ms Time= 1.75ms Gs = 0.007  -0.007Gs = -0.007  0.007 Bubble Velocity = 262.65  m/sBubble Velocity = 274.29  m/s 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 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 (Bubble Blockage) Inlet Velocity = 40.00  m/s (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 Divergent Microchannel Orificed Microchannel Straight Microchannel Geometric effect Straight channelBasic case Convergent channel Divergent channel Hydrophilicity effect Hydrophilic ( ) Less hydrophilic ( ) Increased hydrophilicity ( ) Decreased hydrophilicity ( ) Thermal 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 ( ↑ favorable ； ↓ adverse) 。