| Micro direct methanol fuel cells (?DMFC) has long been considered as a promising power source candidate for portable and transport applications due to their high energy density and efficiency, easy construction, facile refueling and storage of the fuel, and low pollutants emission. However, widespread applications of the ?DMFC are still hindered by several technological challenges. Among which, the anode flow field plate is mainly used to supply and distribute methanol solution to the anode electrode, exhaust the products and collect the released electrons. Hence, whether it's designed well or not will affect the performance of ?DMFC directly. This thesis focuses researches on the structures of conventional flow fields and novel flow fields with the superhydrophobic gas channels.The previous studies tended to draw a conclusion about the anodic flow field parametric impacts on the ?DMFC performance by several isolated samples. And the cross effects of the anodic flow field design parameters were ignored. The purpose of this paper is to study the global influence of the design parameters on the performance by introducing a response-surface-method (RSM) driven parametric analysis in its whole designing space. A three-dimensional model of micro direct methanol fuel cell was presented based on the control-equation of a coupling of continuity equation, Navier-Stokes equation and Butler-Volmer equation and so on. The peak power density of the ?DMFC were calculated. Then, the RSM equation was established and validated by experiments. The results indicated, ?DMFC with the open ratio of 40%?50% performed best, higher or lower anodic flow field open ratio would degrade the ?DMFC performance, mainly due to the increasing methanol crossover at high open ratio or high mass-transport impedance at low open ratio. For the same flow rate,30ml/h, the liquid velocity increased with decreasing depth of flow channel from 0.6mm to 0.3mm, which would lead to a higher mass transfer coefficient, which was helpful for reactants to get the interface between gas diffusion layer and catalyst layer. But a shallower flow channel would lead to a too small effective mass transfer area, hence a lower mass transfer rate, which in turn caused the cell performance to decline. For the same open ratio and 0.4mm channel depth, the increase in the total length of the flow channel led to a more sufficient and more uniform distribution of methanol solution, hence a better performance. At last, the optimal structure parameters of the flow channel were obtained.Three types of novel flow fields with nested arrangement of fuel channels and superhydrophobic gas channels were designed on the basis of conventional serpentine, spiral and parallel flow fields. Level Set methodology, a part of finite element technique, was used to establish the novel and conventional flow channel numerical models. Novel single batteries were assembled for watching the CO2 bubble behaviors and test of pressure drop between inlet and outlet of the liquid channels. Both simulation and experiments showed that, parts of bubbles would be exhausted through the gas channels after they crossed the gas diffusion layer. And the quantity of bubble in the liquid channels would decrease, and no gas slug was seen. Meanwhile, the average pressure drop of novel serpentine, novel spiral and novel parallel flow fields was reduced by 43.3%,50.5% and 62.5% respectively. The drop pressure amplitude was separately degraded by 20%,57% and 25%. All the above reflects that novel flow fields are helpful for the exhaust of CO2 bubbles and they can enhance the efficiency of the reactants transfer. |
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