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Numerical Analysis Of Transport Processes In Proton Exchange Membrane Fuel Cell

Posted on:2008-03-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:H T TuFull Text:PDF
GTID:1102360242484550Subject:Engineering Thermal Physics
Abstract/Summary:PDF Full Text Request
In this paper, a single-phase mathematical model is firstly developed for proton exchange membrane fuel cell (PEMFC) and the steady two- and three-dimensional transport properties of PEMFC are simulated considering heat and mass transfer, electrochemical reactions and water transport in membrane driven by electro-osmotic drag, concentration difference diffusion and pressure permeation. The effects of flow channel configuration and different physical parameters on cell performance and the transient response property of transport processes in PEMFC are analyzed and discussed. And then, a hybrid two-phase mathematical model is developed based on the forenamed single-phase model considering water phase change and capillary flow of liquid water in porous media. The simulation of two-phase effects and the parametric study of porous media are also carried out using this model. The main work and conclusions are listed as follows.1. Two- and three-dimensional steady simulation. The two-dimensional isothermal and the three-dimensional non-isothermal simulations of PEMFC for base parameters are carried out using the single-phase model. The multi-dimensional transport properties and the effects of water transport in membrane on species transport and ionic conductivity of membrane are analyzed. And the distributions of characteristic parameters are discussed and compared. The results indicate that there is a large difference between the two-dimensional and three-dimensional transport properties. The two-dimensional results underestimate the activation and ohmic losses for ignoring the effects of the shoulder of current collector.2. Parametric analysis using single-phase model. The effects of different configurations of flow channel, geometric and physical parameters and operation conditions on cell performance are simulated and analyzed using the single-phase model. It can be concluded that the decrease of the thickness of membrane optimizes the water transport in membrane and improves the output performance of PEMFC, especially for large current densities. The interdigitated design of flow channel enhances the convection flow in porous media and improves the cell performance, which is especially obvious for low operation pressures and stoichiometric ratios. The change of averaged ionic conductivity of membrane vs. output current density is not sharp at the condition of zero operation pressure difference between anode and cathode. However, the anode side of membrane is easier to be dehydrated in that case. 3. Three-dimensional transient analysis of transport processes in PEMFC. The simulations of the transient responses under varying conditions are carried out for ignoring and considering the water accumulation process in membrane. The transient characteristics of species transport and the effect of water accumulation in membrane on cell's response are analyzed and discussed. The results indicate that the response characteristics of species transport for interdigitated design of flow channel are obviously better than that for straight channel, which shows the advantage of PEMFC system with interdigitated flow fields for vehicle usage. The response times of PEMFC with straight channels under varying conditions are all about 0.5~0.65 for ignoring water accumulation in membrane. And the response times of output cell voltage and O2 species transport are prolonged for about 1s and 0.5s respectively considering water accumulation in membrane. The temperature distribution of membrane has the longest response time, but it has little effect on cell response.4. Three-dimensional and two-phase simulation. Based on the single-phase model, a hybrid two-phase model is developed considering the phase change and capillary flow of liquid water in porous media, while ignoring the two-phase effect in flow channels. The transport processes under the effects of liquid water are simulated, and the effects of porous media's structure and physical parameters on transport characteristics and cell performance are analyzed and discussed in detail. The results indicate that the single-phase model overestimates the activation loss and underestimates the ohmic loss of membrane. The simulated temperatures of two-phase model are higher than those of single-phase model for the effect of the heat of vaporization. The increases of the diameter of carbon fiber, hydrophobility and porosity of porous media will benefit the species transport and cell performance in most cases. For two-layer diffusion layer, the decrease of carbon fiber's diameter of the first layer dfl improves the cell performance when the thickness of the first layer Ldl is small. With the increase of Ldl, the effects of Ldl and dfl on cell performance contradict each other and the two parameters need to be optimized.
Keywords/Search Tags:Proton Exchange Membrane Fuel Cell, Numerical Simulation, Heat And Mass Transfer, Transient Response, Two-phase Flow
PDF Full Text Request
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