buletinul institutului politehnic din iaşi - Universitatea Tehnică ...

132 Adrian Sabău et al.
during fuel preparation, there is availability destruction due to mixing, which is
included in the calculation of afr. From Eqs. (4) and (5), it can be seen that in
order to calculate the combustion irreversibility, one has to determine the dead
state and calculate afr.
For the system to reach the dead state at P0=1 bar and T0=298 K, all of
the working medium constituents have to be either oxidized or reduced to N2,
O2, CO2 and H2O. Knowledge of the precise mechanism with which this
happens is not necessary for determination of the molar quantity of each of the
four species in the dead state.
This schematic representation has no meaning as a chemical reaction; it
simply shows the relation of molar quantities between the working medium
species and the molar quantities of N2 and O2 in the dead state. A similar
relation for the complete oxidation of an oxygenated fuel vapor gives
⎛ β γ ⎞
β
CHO α β γ + ⎜α+ − O2 αCO2
4 2
⎟ → + . (6)
⎝ ⎠
2H O
Finally, similar considerations for all species yield the following relations
for the composition of the dead state
0 NNO
NN = N
2 N + ,
2 2
0
NCO2 = NCO2 + αN
fv + N,
0 ⎛ β γ ⎞
NO = N
2 O −
2 ⎜α+ − ⎟N
fv +
⎝ 4 2⎠
.
NO + NNO −NH −N 2 CO
+
2
NOH − N H + ,
4
0 β
NHO= N
2 HO+ N N
2 fv + H2
2
NH + NOH
+ .
2
The differential variation of the G0 term is then given by
4 ⎡ ⎛ 0
0
N ⎞⎤
0 = ∑ i ⎢ i 0 0 + 0 ⎜ ⎥
⎜ 0 ⎟
i=
1 N ⎟
1
i
dG d N g ( T , P) RT ln . (8)
⎢⎣ ⎝∑⎠⎥⎦ The second term of this sum corresponds to the entropy of mixing in the
dead state.
2
(7)