Abstract
Nano- and micro-fluidic simulation techniques have been developed in this thesis to analyze various issues inside micro direct methanol fuel cells (□DMFCs). Molecular dynamics simulations were first performed to investigate the hydrophilic nature of the platinum (Pt) catalyst. The contact angle between a nano-water droplet and a Pt surface is important for the design of porous catalyst layer in low temperature fuel cells. The measurement can generally be conducted using an atomic force microscope (AFM). However, the interaction force between the water droplet and the probe tip of the microscope may influence the measurement results. Molecular dynamics model was set up to investigate the offset of the contact angle measurement. Water molecules clustering on the platinum surface, and the original contact angle between the nano-scale water droplet and the platinum surface were predicted. The offset of the contact-angle measurement due to intrusion of the AFM probe was also evaluated. For engineering purposes, a correlation between the offset angle and the AFM measurement locations was presented. The removal of carbon dioxide (CO2) at the anode microchannels of a □DMFC is an emerging technique in micro engineering. The bubbles are generated at the anode and may block part of the catalyst/diffusion layer, causing the □DMFC malfunction. The second part of this thesis discusses the microfluidic CO2 bubble dynamics in a □DMFC using the lattice-Boltzmann method (LBM). The liquid-gas surface tension, the buoyancy force and the fluid-solid wall interaction force play the major roles in the bubble dynamics in a microchannel. They were treated as source terms in the lattice momentum equation. Simulation results indicated that the methanol stream flow rate, the pore size and the channel incline angle significantly affected the removal of CO2 bubbles. The incline angle effect is substantial at low stream flow rates. The critical pore size for removing bubbles at all angles under various flow conditions has been predicted for engineering purposes. A thermal lattice-Boltzmann model (TLBM) was further developed in the last part of this thesis. The main purpose is to investigate the thermal and geometric effects on the CO2 bubble dynamics at the anode microchannels. The simulation results show that the hydrophilic microchannel is favorable for the bubble removal. The plug bubble is larger in low temperature methanol solution since the surface tension decreases with the increasing temperature. Due to the Marangoni effect, the bubble transports more rapidly in the microchannel with an imposed positive temperature gradient. Comparing with the straight and converging microchannels, the bubble moves with less obstruction in the diverging microchannel. The thermal effect on the bubble transport is more significant than the hydrophilic and geometric effects. Hence, we can combine these effects with a local temperature control technique to remove the bubbles in the micro fuel cells.