Dept. of Power Mechanical Engineering, National Tsing Hua University

1. c6 c2 c5 c3 c0 c1 c7 c4 c8 CH3OH e- e- e- e- e- e- e- e- e- e- e- e- H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ O2 O2 O2 Electrochemical Reaction Methanol Solution + Oxygen Power + Carbon Dioxide + Water CO2 H2O H2O H2O H2O Electron Anode Cathode Methanol solution Oxygen Proton Water Carbon dioxide Proton exchange membrane Catalyst layer Diffusion layer Direct Methanol Fuel Cell ChemicalReaction Anode : Cathode : Overall : Notebook Computer, Cellular Phone, PDA, MP3 Player, Video Game Console, Digital Camera, and etc. TOSHIBA, HITACHI, SAMSUNG, LG, SONY, NEC, PANASONIC, Sanyo Electric, IBM, and etc. Research and Development Methanol solution Hot Cold Bubble Application Bubble Liquid Microchannel DMFC Time= 1.75ms Time=12.50ms Time=25.00ms Time=37.50ms Time= 1.75ms Time=12.50ms Time=25.00ms Time=37.50ms Time= 1.75ms Time=12.50ms Time=25.00ms Time=37.50ms 2007.6 Thermal Lattice Boltzmann Simulations of Two-Phase Flow in Micro Direct Methanol Fuel Cell Microchannels Dept. of Power Mechanical Engineering, National Tsing Hua University Kai Fei, Chao-Jen Tsai (Research students), Che-Wun Hong (Professor) • Introduction Micro-direct methanol fuel cells (mDMFC) 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. • Objective and Motivation Carbon 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. • TLBM Lattice Boltzmann equation for flow field : Surface tension : fluid-fluid interaction strength Fluid-solid interaction force : fluid-solid interaction strength Buoyancy force : • Approach • Two-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. Temperature effect (the Boussinesq approximation) : Macroscopic mass density and momentum density: Lattice Boltzmann equation for temperature field : 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. Macroscopic temperature : • Conclusions • Simulation Results Geometric effect Thermal effect Contact Angle Straight Microchannel Orificed Microchannel Convergent Microchannel Inlet Velocity = 250.00 mm/s Inlet Velocity = 40.00 mm/s (Bubble Blockage) Time= 1.75ms Time= 1.75ms Time=25.00ms Time=12.50ms Time=39.25ms Time=25.00ms Time= 1.75ms Time=34.50ms Time=25.00ms Bubble Velocity =319.62 mm/s Time=39.25ms Fluid-solid interaction strength vs. contact angle Divergent Microchannel Contact angle vs. bubble velocity Bubble Velocity =323.91 mm/s Inlet Velocity = 50.00 mm/s Inlet Velocity = 250.00 mm/s Hydrophilicity Effect Time= 1.75ms Time= 1.75ms Gs = 0.007  -0.007 Gs = -0.007  0.007 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 mDMFC. Time=29.75ms Time=12.50ms Time=71.25ms Time=25.00ms Time= 1.75ms Time=34.50ms Time=29.75ms Bubble Velocity =335.34 mm/s Bubble Velocity = 262.65 mm/s Bubble Velocity =274.29 mm/s Time=71.25ms Bubble velocity vs. temperature gradient