Page 109 - Introduction to Transfer Phenomena in PEM Fuel Cells

98 Introduction to Transfer Phenomena in PEM Fuel Cells
– at the cathode:
- the gas mixture is ternary, the species involved are oxygen, nitrogen
and water vapor; the equation system to solve is written as:


 c
 x O 2 + x N 2 + x = 1
 HO
2
  N c 
 dx N 1  N O H O 
 2 =⋅  2 + 2  x ⋅ N [3.32]
 dx c  D eff D eff  2
  O,N 2 H O,N 2 
2
2
  c   
 dx O 1  N O 2 + N H O  1 1  N O 
 2 =⋅  eff 2 x ⋅ O 2 +  eff − eff  ⋅ N O 2 x ⋅ N 2 − eff 2 
 dx c  D  D D  D O,H O 
 O,H O   O,H O O,N 2  2 2 
2
2
2
2
2
- at the diffuser inlet, the relative humidities (RH) are expressed as
follows:
 p
 x ce = HR ⋅ sat
c
2
 HO p
   p 
−
 x e O 2 = 0,21 1 HR ⋅ sat   [3.33]

c
p
 
  p 
 x e = 0,79 1 HR ⋅ sat 
−

 N 2   c p 
3.4.3. Some parametric laws
3.4.3.1. Saturation pressure
Different formulas have been proposed for determining the saturation
pressure of water (in [Pa]). Wöhr presented, in his work, the following
empirical formula [MON 06]:
  A− B  
P sat () 10=  C T 273  +− ⋅ 100 [3.34]
T
In this relation, the temperature is in Kelvin, and the coefficients have the
following values: A = 8.073; B = 1 656.39; C = 226.86 in K. Chun-Ying
Hsu et al. [WÖH 00] used another formula where the decimal logarithm of