Since low-temperature PEM fuel cells operate under relatively low temperature (C), the water vapor may condense to liquid water, especially at high current densities. While the existence of the liquid water keeps the membrane hydrated, it also blocks the gas diffusion passage, reduces the diffusion rate and the effective reacting surface area and hence the cell performance. To model the formation and transport of liquid water, Ansys Fluent uses a saturation model based on [476], [469]. In this approach, the liquid water formation and transport is governed by the following conservation equation for the volume fraction of liquid water, , or the water saturation,

(20–83)

where is the porosity, the subscript stands for liquid water, and is the condensation rate that is modeled as,

(20–84)

where is added to the water vapor equation, as well as the pressure correction (mass source). This term is applied only inside the catalyst and gas diffusion layers. The model accounts for the phase change on the cathode side only. An excessive water vapor generation due to the electrochemical reaction can lead to condensation on the cathode side at higher loads. Condensation due to normal causes, such as lower temperatures and higher pressures on the anode side, is rare and not accounted for. When condensation considerations are important for your simulation, use a more general PEMFC model described in PEMFC Model Theory. The model accounts for condensation across all fuel cell layers including both gas channels. More details about the model implementation can be found in Water Transport and Mass Transfer in PEMFC.

The condensation rate constant is hardwired to . It is assumed that the liquid velocity, , is equivalent to the gas velocity inside the gas channel (that is, a fine mist). Inside the highly-resistant porous zones, the use of the capillary diffusion term allows us to replace the convective term in Equation 20–83:

(20–85)

Depending on the wetting phase, the capillary pressure is computed as a function of (the Leverett function) [358]:

(20–86)

where is the surface tension (N/m), the absolute permeability, and is the contact angle. Values of from 0 to 90 degrees represent a hydrophilic media, while values from 90 to 180 degrees represent a hydrophobic media.

Equation 20–83 models various physical processes such as condensation, vaporization, capillary diffusion, and surface tension.

The clogging of the porous media and the flooding of the reaction surface are modeled by multiplying the porosity and the active surface area by , respectively.