Page 103 - Introduction to Transfer Phenomena in PEM Fuel Cells

92 Introduction to Transfer Phenomena in PEM Fuel Cells

Figure 3.4. Production of water at the cathode

3.4. Mass transfer in porous diffusers

3.4.1. Conservation of mass

The generalized mass conservation equation, valid for the description of
mass transfer phenomena inside a fuel cell (flows, diffusion, phase change
and electrochemical reactions) [LAL 12], is written as:


→
∂ (ρ⋅ε + ∇⋅ ρ⋅ →  0 [3.16]
)
u =
t ∂ s    
where:

– ε is the porosity of the medium (ratio between the pore volume and
s
the total volume);
–3
– ρ is the density of the gas mixture [kg.m ];
 –1
– u is the superficial rate [m.s ].