CHANNEL TWO-PHASE FLOW AND PHASE CHANGE IN POLYMER ELECTROLYTE FUEL CELLS

Open Access
Author:
Basu, Suman
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 25, 2008
Committee Members:
  • Chao Yang Wang, Committee Chair
  • Fan Bill B Cheung, Committee Member
  • Daniel Connell Haworth, Committee Member
  • Turgay Ertekin, Committee Member
Keywords:
  • Channel Flow
  • Phase Change
  • Two-Phase flow
  • PEFC
Abstract:
Gas-liquid two-phase flow in channels of a polymer electrolyte fuel cell (PEFC) remains a major challenge in water management. During the PEFC assembly the flexible gas diffusion layer (GDL), either carbon paper or carbon cloth, is compressed for excellent electronic and thermal contact with bipolar plates. Therefore, it intrudes into the end gas channels (i.e. channels near the tightening bolts) and cause non-uniform distribution of reactants in the PEFC. This maldistribution of flow in parallel channels decreases the ability of the flow to carry away liquid water in the intruded channels. Therefore more liquid water is accumulated in these channels and even less flow goes through, resulting in severe flow maldistribution among parallel channels A one-dimensional analytical model based on multiphase-mixture formulation is developed in this thesis to elucidate the two-phase flow maldistribution in the cathode channels. The channels of dimensions on the order of 0.5-1 mm are considered as structured porous media. Relative humidity (RH) at the inlet and flow stoichiometry are found to be the two key parameters strongly influencing the flow maldistribution. Interestingly, our analysis shows that decreasing the inlet RH worsens flow maldistribution. This model is then applied to a two-dimensional (extendable to three-dimension) numerical model for the cathode flow channels only. The effects of electrochemical reactions are considered in the model through uniform water injection along channels. This stand-alone channel model could simulate the liquid saturation distribution in the parallel channels as well as predict the two-phase pressure drop. The stand-alone channel two-phase model is then integrated with a two-phase PEFC model previously developed in our laboratory. The predicted surface coverage by liquid water at the GDL-channel interface is validated against experimental results for a wide range of parameters showing excellent agreement. The anode channels are modeled using the same two-phase model, capturing the dry-wet-dry transition existing in the anode. The effect of reactant maldistribution in the gas channels is found to have important effect on the current density distribution, implying non-uniform catalyst utilization. Therefore it would almost certainly accelerate the aging of the PEFC, exacerbating the already existing low durability issue. Interestingly, the amounts of liquid water in anode and cathode channels are found to be of the same order for the whole range of parameters. Another important physical phenomena occurring in a PEFC is liquid-vapor phase change, i.e. evaporation and condensation. To date, these phenomena have not been explored by the fuel cell community. In the present work, a two-phase, non-isothermal numerical model is used to elucidate the phase-change rate inside the cathode GDL of a PEFC. We could locate the major condensation and evaporation sites and quantify phase-change rate. Relative humidity at the inlet flow and thermal conductivity of the cathode GDL are found to have major influence on the rate of phase change. Condensation under the cooler land could be reduced substantially by decreasing the inlet relative humidity or increasing the GDL thermal conductivity. Inlet relative humidity effect is more pronounced near the inlet of the cell whereas the GDL thermal conductivity affects the phase change rate more uniformly throughout the flow length.