Modeling and Control of Two-Phase Flow in Direct Methanol Fuel Cells

Open Access
Author:
Jung, Seung Hun
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 07, 2010
Committee Members:
  • Chao Yang Wang, Dissertation Advisor
  • Chao Yang Wang, Committee Chair
  • John Michael Cimbala, Committee Member
  • Seungjin Kim, Committee Member
  • Michael Anthony Hickner, Committee Member
Keywords:
  • fuel cell
  • direct methanol fuel cell
  • two-phase flow
Abstract:
A direct methanol fuel cell (DMFC) involves two-phase flow on both anode and cathode sides. On the anode side, methanol-water solution is oxidized to produce carbon dioxide (CO2), whereas gaseous oxygen from air is reduced to form liquid water on the cathode side. Prediction and control of two-phase flow is of paramount importance for performance and fuel efficiency of DMFC in portable application. This dissertation aims to accurately predict and control two-phase flow in the channel and porous media of a DMFC to enable novel design and selection of components. CO2 gas produced during methanol oxidation reaction in a DMFC is the reason for the two-phase flow in the anode. This CO2 gas is typically removed through the anode channel for steady cell operation, which makes a strong two-phase flow in the anode channel. As this channel two-phase flow causes a large pressure drop which is not desired, the present work modeled a CO2 breathing DMFC which directly vents CO2 to the ambient through the porous surface film. Although the CO2 breathing DMFC shows similar cell performance with the conventional DMFC, the net power throughput and the system efficiency are improved since required pumping power is reduced due to reduced anode pressure. The role of CO2 in controlling water and methanol transport in a DMFC is elucidated with a computational method for the first time. It is found that the amount of CO2 in the anode (CO2 level) determines capillary diffusion which dominates water transport in a DMFC. In addition, the multi-D DMFC model explains that methanol is transported not only by molecular diffusion but also by capillary diffusion in the anode porous media, and both transport mechanisms are strong functions of the CO2 level. The present study predicted a significant cell performance loss due to severe non-uniform distribution of methanol concentration under ultra-low anode stoichiometry condition. After identifying the controlling parameters of the anode non-uniformity, two strategies to mitigate the anode non-uniformity and to boost cell performance as well as fuel efficiency are proposed. First, streamline-graded structures (SGS) which control methanol transfer resistance are devised and studied through a statistical analysis. Second, an interdigitated fuel distributor which converts the fuel transport mechanism from diffusive to convective is developed. It is found that cell performance and fuel efficiency are improved by mitigating fuel concentration non-uniformity due to reduced methanol crossover in the inlet region and improved fuel supply in the outlet region.