Advances in the Lockhart-Martinelli Approach to Predict Pressure Drop in PEM Fuel Cells

The water produced in proton exchange membrane fuel cells can lead to performance reductions if not properly managed. Complex two-phase flows can emerge throughout the cell, including in the gas delivery channels where the two-phase system causes an increase in the pressure drop. The Lockhart-Martinelli (LM) approach to predict two-phase pressure drop has been updated previously for the unique water introduction method in active fuel cells where the water emerges from a gas diffusion layer perpendicular to the direction of gas flow. Recent work has studied the Chisholm parameter C in terms of key fuel cell operating variables (e.g. relative humidity, temperature, materials, gas stoichiometry). However, the results of the analysis are unclear. A new method is proposed by which C is determined as a function of the operating current density and the flow regime. To complete this analysis, a new flow regime map is presented with an accumulating flow regime in addition to the traditional fuel cell flow regimes of single-phase, film/droplet, and slug. An additional force balance is presented to corroborate the superficial gas and liquid velocity bounds of the accumulating regime. These results allow a fuel cell designer to quickly judge the impact of design and operating conditions on two-phase flow pressure drop.