Page 102 - Introduction to Transfer Phenomena in PEM Fuel Cells

The value of (h m) depends on channel geometry and physical properties
of species (i) and (j), it can be determined from the Sherwood number
[SPI 07]: Mass Transfer Phenomena 91
D
h = Sh i,j [3.14]
m
D h
where:

– D h is the hydraulic diameter;
– D i,j is the binary diffusion coefficient of species (i) and (j);

– Sh is the Sherwood number.

The latter depends on the geometry of the channels, it can have the value
of (Sh = 5.39) for a uniform mass flux on the surface (ṁ = constant), and the
value of (Sh = 4.86) for a uniform concentration on the surface of the
electrodes (C S = Constant).

The dependence of the binary diffusion coefficient (D i,j) on temperature
can be expressed as [BAR 05]:

3
 T  2
T
D i,j () = D i, ( j T ref )⋅   [3.15]
 T ref 
where:

– T ref is the temperature with which the binary diffusion coefficient is
given;
– T is the fuel temperature used in the fuel cell.

The production of water at the cathode is shown in Figure 3.4 [SPI 07].
The water produced at the cathode will be eliminated by convective airflow.