Date of Award

12-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Mechanical Engineering

Major Professor

Matthew M. Mench

Committee Members

Feng-Yuan Zhang, Subhadeep Chakraborty, Thomas A. Zawodzinski

Abstract

Thermally driven transport of water vapor in polymer electrolyte fuel cells, also known as the heat-pipe effect or phase-change-induced flow, can transport several times the generated amount of water given enough temperature differentials. Understanding this transport process is necessary to properly engineer the water balance in the fuel cell to ensure high performance and long operational life. Channel-land architecture, diffusion media heat and mass transport properties, and operational age can all have an influence on thermally driven flow. High resolution neutron imaging was used to determine the steady-state water accumulation in various cell configurations to understand the influence of these parameters. A novel non-dimensional parameter was proposed to predict the influence of engineering parameters on water balance. The thermal transport number (TTN) compares the strength of anode and cathode thermally driven flow to determine a bias for transport to the anode. Channel-land architecture, specifically asymmetric flow field patterns with larger anode lands, was found to pump water to the anode and allow for large accumulations of water with saturation approaching 60%. Water transport was facilitated to the anode by a delta-T inversion caused by the insulating effect of the cathode gas channel located opposite of the center of the large anode land. To limit anode water accumulation, a new experimental high diffusion resistance anode diffusion media was evaluated. This material was found to be effective at reducing anode water accumulation primarily due to the increased tortuosity of the material but also due to the higher thermal conductivity reducing the thermal transport effect. Material age was shown to reduce cell water content due to increased hydrophilic nature and increased thermal conductivity that increased from inlet to outlet with greater effects on the anode. A parametric study was performed to determine what parameters have the strongest influence on thermally driven transport. It was found that porosity and tortuosity of the diffusion media, and channel-land architecture can be engineered to drive water balance in a favorable direction. Temperature and thermal conductivity primarily influence the strength of thermal transport. This work demonstrated methods to properly design thermal management for optimized and predictive water transport.

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