Environmental Engineering Reference
In-Depth Information
NO x formation in rich-dome combustors of propulsion engines can be described
reasonably well by assuming that it is represented well by an extended Zeldovich
mechanism (Gupta et al. 1991 ):
N 2 þ
O
,
NO
þ
N
;
ð
1
Þ
N
þ
O 2 ,
NO
þ
O
;
ð
2
Þ
OH
þ
N
,
NO
þ
H
ð
3
Þ
d d t ¼
Assume
0
; then generalized rate of NO formation rate is given by:
(
)
2
dNO
½
1 NO
½
= K C O 2
½
N 2
½
¼
2K 1 O
½
½
N 2
ð
4
dt
1
þ K 1 NO
½
= K 2 O 2
½
þ K 3 OH
½
Combining reactions 1 and 2 leads to an expression for K C given in above
equations, namely: N 2 þ
K 1 K 2
K 1
K 2 .
The next step of assumption (viz. O is in equilibrium with O 2 ) can generally be
considered over simpli
O 2 ,
2NO with equilibrium constant K C as K C ¼
cation. But if it were true, it leads to the rate of NO x
formation being proportional to p
0.5 because the generalized equation reduces to:
dNO
½
1 = 2
¼
2K 1 K C5 N 2
½
½
O 2
;
ð
4
Þ
dt
here, K C5 is chemical equilibrium constant of
1
2 O 2 ,
O
:
ð
5
Þ
flame front, better agreement with data has been
achieved by making the super O equilibrium assumption which is achieved by
assuming that the radicals can be calculated by regarding reactions ( 6 , 7 and 8 )as
fast reactions in addition to CO reaction ( 9 ) being equilibrated. This process leads to
the two expressions for [O] (viz. Equations 10 and 11 ) in terms of equilibrium
constants of reactions 6
In the regions close to the
fl
-
9 , namely K C6 , K C7 , K C8 , and K C9 . Either of these equa-
tions can be used for plugging [O] in Eq. 4
to calculate the rate of formation of
[NO] under super-equilibrium O condition.
H
þ
O 2 ,
OH
þ
O
ð
6
Þ
;
H 2 þ
O
,
OH
þ
H
ð
7
Þ
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