Study On The Gas-liquid Transportation In μDMFC Anode Flow Channel Based On VOF Method

In recent years, Micro Direct Methanol Fuel Cell(μDMFC) is considered as a promising power source candidate for portable electronic devices due to its advantages such as friendly to environment, high energy conversion, simple structure and convenience for fuel storage and carrying. The block of reactant transportation induced by the CO2 bubbles in micro flow field is one of the key factors affecting the μDMFC performance. Hence the development of methodologies to characterize the gas-liquid transportation and to remove CO2 bubbles in the anode micro channels efficiently are vital for improving the μDMFC performance.In this paper, firstly a mathematical model was developed for predicting the CO2 bubble departure diameter from a micro pore in the microchannel of μDMFC with cross current methanol solution flow, where the gas velocity, the liquid cross-flow velocity and micro porous diameter were taken into account. Results indicate that the microbubble departure diameter decreases with the increasing liquid velocity, and increases with the increasing micro porous diameter and increasing gas velocity. Additionally, a kinematic model was developed by analyzing the forces acting on the single slug bubble in the μDMFC anode flow channel.The dynamic behavior of the single CO2 bubble in the microchannel of μDMFC was then simulated by using the VOF method. Simulations of the processes of the bubble growth and detachment were performed, which took the effects of CO2 gas velocity, liquid cross-flow velocity, wettability of the GDL, pore locations and micro porous diameter into account. The simulated results show that the inner pressure of the bubble decreases with the bubble growth. A higher gas velocity leads to a larger bubble departure diameter and an earlier bubble detachment, and a higher liquid cross-flow velocity will facilitate the bubble departure. The bubble departure diameter also increases with the increasing micro porous diameter. What’s more, the characteristics of bubble coalesce process and the interaction between the bubbles were explored. The VOF method was employed to simulate the movement of the slug bubbles in the channel as well. Moreover, the gas-liquid pressure drop in a rectangular micro channel was studied. The effects of channel dimension as well as the gas and liquid velocity were also obtained from the simulation results, which were compared with the theoretical results derived from a separated-phase flow model.At last, two effects including cross geometry of channel and wettability of sidewalls were mainly studied by numerical simulations. As to cross geometry of channel, simulations for microchannels with different cross-sections,including rectangle,trapezoid,semicircle and triangle were performed under two different conditions separately.One condition is keeping the width and depth of channel unchanged,and the other is keeping the channels with different cross-sections under the same area.Results indicate that the microchannel with triangle cross-section can mitigate the clogging phenomenon of the CO2 bubbles,and thus improve the performance of theμDMFC.As to wettability of sidewalls,the simulations for different wettabilities of sidewall show that the hydrophilic sidewalls can promote removal of CO2 bubbles.In addition,a double-layer channel with wettabilities of sidewalls changing with height was introduced and studied.A conclusion was drawn that sidewalls with hydrophobic upside and hydrophilic downside facilitated the CO2 bubble removal and liquid transporting to GDL.