
265Heat and Mass Transfer in Fuel Cells
For slip condition at the interface of the diffusing particles and the liquid
medium, the force over the particles is given as
F
b
= 4πμ
b
U
a
R
a
, (6.116)
and the corresponding expression for diffusivity is given as
D
kT
R
ab
=
B
4πµ
.
(6.117)
Another empirical correlation based on the Stokes–Einstein equation for
low concentration of species i in liquid medium j is given by Wilke and
Chang (Sherwood et al., 1975),
D
v
ab
bb
ba
=
×
−
74 10
2
06
.()
/
.
ψ
µ
(6.118)
where
v
a
= molar volume of solute at normal boiling point (cm
3
/mol); molar vol-
umes at normal boiling point are listed in Table 6.5
μ
b
= viscosity of liquid (centipoise ...