Analysis of CO2 bubble growth detachment kinetics in direct methanol fuel cell flow channels

Abstract

CO₂ bubbles flow in the anode flow channel is an important issue in the commercialization process of direct methanol fuel cells (DMFC). A T-channel model is built in COMSOL using the phase field method to investigate the CO₂ bubbles flow at the anode side of the DMFC. The factors and mechanisms of single bubble detachment are discussed by analyzing the methanol inlet velocity (U_l), CO₂ inlet velocity (U_g), contact angle of the diffusion layer, and the Weber number during bubble detachment. The research findings indicate that increasing the liquid flow rate leads to the generation of smaller bubbles that detach more rapidly due to the increases of drag force (F_D) and the shear-life force (F_SL) to overcome the surface tension on the bubble. The CO₂ inlet velocity can promote the bubble detachment due to the increase in F_SL, but also leads to a larger detachment diameter. Compared to hydrophobic surfaces, hydrophilic surfaces are more conducive to bubble detachment and removal. In all case We (Weber number) is significantly less than 0.6, indicating that liquid momentum dominated the bubble detachment process. Once the ratio of the gas momentum to the liquid one is greater than 1, the bubble is hard to detach. The contour map of bubble flow patterns and the bubble detachment diameters distribute with the ratio of U_g/U_l can further indicate that the bubble detachment is connected with the ratio of U_g/U_l closely, which will have guiding significance for the selection of inlet velocity of DMFC.