Study On Fractal Tree-like Flow Field Of Proton Exchange Membrane Fuel Cell

Among various kinds of fuel cells, proton exchange membrane fuel cell (PEMFC) draw more attention because of its advantages such as high power density, low operation temperature, fast start up and zero emission. Flow field plate is one of the key components of fuel cell. Design of flow field is the vital issue of flow field plate for it determines the flow of reactants and product.Due to the relative long channel and many corners, pressure drop in traditional channel is high thus much pumping power is consumed and distribution of gas in channel is not uniform. This thesis attempts to design a kind of fractal tree-like channel which has multiple short channels and each channel with many branches. Intention of this design is to improve uniformity of gas distribution and decrease global flow resistance and pressure drop thus consumption of pumping power while fuel cell is working is saved. Main content of this thesis are:(1) Based on research of veins distribution in lotus leaf which is an excellent transportation system for it is the result of long term evolution and constructal theory, present work develops a novel flow field.3D models for PEMFCs with fractal tree-like channel are presented under the assumptions of steady and isothermal conditions. Numerical results show that PEMFC with fractal tree-like channel has better performance than traditional channel.(2) Performance of PEMFC with fractal tree-like channel is compared with that of fuel cell with traditional parallel channel and serpentine channel. To enhance performance, impact of four key geometrical characterizations on performance of fuel cell with fractal tree-like channel is investigated to get optimal geometrical characterizations. The four key geometrical characterizations include fractal level, porosity, channel height and channel cross-section shape. Numerical results suggest that performance of PEMFC improves with the increase of fractal level; optimal porosity which can balance mass transfer resistance and contact resistance for fractal tree-like channel is about0.5; fuel cell with channel height0.8mm shows the best performance; triangle shaped and trapezoid shaped cross-section channel provide slightly better performance compared with fuel cell with rectangle shaped cross-section channel. Increase operational pressure can improve performance of PEMFC, optimal temperature for PEMFC with fractal tree-like channel is around343K and optimal humidity is100%.(3) Single fuel cell with fractal tree-like channel is assembled and performance is investigated on test station. Experimental results show reasonable agreement with numerical simulation results. Experimental study is conducted to get optimal operational parameters for PEMFC with fractal tree-like channel.This thesis designs a kind of fractal tree-like channel for PEMFC by conducting numerical simulation and experimental study. Results of present work are instructive for design and manufacturing of PEMFC.