Study On Heat And Mass Transfer In Direct Methanol Proton Exchange Membrane Fuel Cells

Direct methanol fuel cells(DMFCs) are considered as highly promising power sources for electric vehicle in the 21st century. Aimed at some heat and mass transfer problems included by water, heat and gas management in DMFCs, profound theoretical exploration was performed in this paper. Additionally, a single cell with an active area of 100cm2 and its performance test system were both designed and assembled in-house.To study water transport in the PEM of a DMFC, on the first stage, the flow and phase change process as well as dehydration phenomenon in the PEM of a DMFC are both analyzed qualitatively and quantitatively. Based on the theory of fluid flow and heat transfer in porous media with inner heat source, the phenomenon of heat and mass transfer in a DMFC is studied quantitatively. The calculated results show that two dimensionless parameters D and N, which reflect the thermal content of water and heat production in the PEM respectively, are the most important factors for thermal balance in the PEM, and the small mass flow rate of liquid water and the high current density are the major contributions to the membrane dehydration.On the second stage, using the porous model, the mechanism of occurrence and development of a two-phase countercurrent flow with corresponding transport phenomenon in the PEM is analyzed qualitatively. A one-dimensional, steady state quantitative model of heat and mass transfer in internal volumetric ohmic heating porous media saturated by liquid and vapor phases is developed. The effects of capillarity, electro-osmotic drag and phase change are included. Two important formulas to calculate theoretical length of two-phase zone and to determine critical criterion for dry-out in PEM are deduced. By use of these two dimensionless parameters, dry-out of PEM can be easily predicted. Theoretical temperature, pressure and saturation profiles within the two-phase region are obtained numerically, which can help to explore the performance of a DMFC operating in its ohmic polarization region. The simulation results can be used to determine the catalyst content of cathode catalyst layer and the corresponding optimal length of PEM.The fact that waste heat removal from a stack is accomplished mainly by a combination of three mechanisms: sensible heating of reactants that pass through the stack,evaporation of water and cooling system is discovered through deducing and analyzing the thermal balance equation for a stack. The correlation between water balance and thermal balance was understood better by exploring the correlation between Q?e v,Q ? land Q?r c.Based on water and thermal managements of DMFCs, a novel cooling system with fractal tree-like net work channels is designed for the direct methanol fuel cell stacks. The heat exchange and pump requirement characteristics of this system are firstly analyzed, which shows that the performance of this structure as a cooling system is better than that of parallel and serpentine structure. The optimum structural parameters of the fractal net work channels are calculated for a DMFC whose size is chosen from a published paper, and the effect of flux of cooling water and heat load on temperature distribution in the stacks is analyzed quantitatively. The results show that this new structure is good at ensuring a less temperature gradient within its core area. The results can also be used to optimize the size of a MEA.By analogy with the flow boiling behavior in a traditional heated tube, gas-liquid two-phase flow characteristics in the anode flow channels of a DMFC was analyzed in this paper. The concept ofquasi-boiling was firstly presented, and the pressure drop model for the two-phase flow was developed. Based on the serpentine flow field on the bipolar plate of the DMFC, which have an active area of 100 cm2 and is under construction in our laboratory now, the total pressure drop in the channels was numerically calculated, and the effects of pressure components on it were also discussed in details. The results show that the small bubbles stagnating at some inactive points of the diffusion layer surface in channels have a biggish impact on the main flow through an additional frictional resistance when the DMFC has a small active area and works with a small current density. The model is helpful for a better understanding to pressure drop characteristics in the anode channels of a DMFC. It is also a useful tool to optimize flow field design, choose the proper feed pump and effectively accomplish the gas management.