Numerical Analysis Of Flow And Mass Transfer Processes In Proton Exchange Membrane Fuel Cell

Proton exchange membrane fuel cell is the most promising, especially for the applications of spaceflight, military affairs, electric vehicle, communication and decentralized power station, because of high energy efficiency, low operating temperature, capable of quick start-up, clean and quiet, simplicity in design and operation. Thus, its applied foreground is very wide. But the detailed distributions of temperature, pressure, velocity, molar concentration and current density cannot be easily get by the experiment. So the simulations of flow and mass transfer processes for PEMFC is very important. Using universal Darcy law discribes the momentum conservation in the porous medium and non-porous medium. For the first time, derivative distribution of velocity in cell is presented and some new conclusions are obtained. The results of this paper provide theoretical evidence for the design of PEMFC. First, a two-dimension, steady-state, multi-component transport model is presented to investigate the cathode flow and mass transfer processes in proton exchange membrane fuel cell with an interdigitated gas distributor. The model can predict the pressure, velocity, mass fraction distributions in the electrode and the current density generated at the electrode and membrane interface. This paper discusses the effects of operating conditions, including differential pressure between the inlet and the outlet and mass fraction of inlet oxygen, on the performance of fuel cell. This paper also discusses the effects of electrode structure parameters, including electrode thickness, rib width, porosity and channel number, on the performance of fuel cell. Results from the model show that: (1) the current density increases with higher differential pressure between the inlet and outlet, but the increment is diminishing; differential pressure has an optimum value; (2) the local current density increases with higher mass fraction of inlet oxygen, but its distribution is going to heterogeneous; the relation of average current density and mass fraction of inlet oxygen is linear; (3) electrode thickness has an optimum value which depends on the electrode morphology and the gas distributor design parameters; (4) using narrower electrode rib can improve the performance of fuel cell; (5) in summary, the optimum value of electrode porosity is about 0.4; (6) with channel number increasing, the gas velocity, the molar concentration of oxygen at the reaction interface and the local current density are increscent; but the molar concentration of water vapor at the reaction interface is reduced; (7) according to the derivative distribution of velocity in electrode, the velocity gradient in electrode is very small; in this zone, inertia force can be ignored; this conclusion provides theoretical evidence for the using of Darcy equation in porous medium. Next, a two-dimension, steady-state, multi-component transport model is presented to investigate the anode flow and mass transfer processes in proton exchange membrane fuel cell with a traditional gas distributor. The model can predict the change of mass components in the direction of flow. This paper discussed the effects of inlet veloctiy, inlet hydrogen content and catalyst layer thickness on mass transfer of hydrogen. The results show that: (1) the velocity profile is a parabola in the gas channel; the gas velocity sharply reduces and is close to zero in porous medium; (2) the velocity in the middle of channel is not maximal; the maximal velocity leans to the interface between channel and diffusion layer; the velocity at the interface between channel and diffusion layer is not equal to zero; (3) with inlet velocity, inlet hydrogen content increasing and catalyst layer thickness decreasing, the mass transfer velocity of hydrogen and the concentration of hydrogen at the reaction interface is higher, thus the performance of fuel cell is improved; (4) the change of velocity derivatives in inlet is big; in channel and electrode ?u ?x, ?v?x,?v?y are very small, but near the channel wall and interface between channel and diffusion layer ?u ?y is very large, so inertia force cannot be ignored in these zones.