Environmental Engineering Reference
In-Depth Information
Reaction Rate Formula
x
a
b
c
E
cal/mol
3.12
17.3
2
0.5
0
1.0
7
0.40
49,600
3.13
14.7
0
0.9
0
1.1
8
-0.3
7
50,000
3.14
14.6
0
1.0
0
0.2
5
0.50
40,000
3.15
13.5
3
0.8
5
1.4
2
-0.5
6
41,000
3.2.5 C
OMPARISON
OF
R
EACTION
M
ODELS
A 1-mm-square fuel nozzle was placed at the center of a square duct (80 mm × 80
mm), and a region of 600 mm from the nozzle tip was considered as the flow field
of simulation. Flow velocity of air
u
a
in the duct was set to be 6 m/s, its standard
temperature
T
a
1300 K, and the fuel temperature
T
f
300 K. With regard to the oxygen
concentration, which is the key to NO
x
formation in HiTAC, atmospheric oxygen
concentration was adopted as the standard and lower-concentration cases were set
forth by dilution with nitrogen.
In the first place,
Figure 3.6
shows temperature distributions predicted by the
three reaction models: Coffee's one-step global reaction model, Jones' four-step
model, and Srivatsa's four-step model, respectively. Since Coffee's model was for-
mulated for the purpose of calculating the burning velocity, it is modeled focusing
on the exothermic reactions in relatively early stages among various elementary
reactions. Jones' model is divided into two groups of reactions, in which fuel turns
into intermediate species in the first half and oxidation-terminating reactions in the
second half, and thus the intermediates, such as CO and H
2
, exist due to serial
occurrence of these reactions. Judging from the simulation results, the reaction rate
of the first half of Jones' model is slightly faster than the one-step global reaction
model and the oxygen consumption is faster as a consequence. Its oxidation-termi-
nating reactions of CO in the second half are also faster than Coffee's model.
Srivatsa's model is characterized by the fact that the time required for starting the
first-half reactions to turn fuel into intermediate species is longer than the other
models. Since concentration of CH
3
is involved in those reactions, a small amount
of CH
3
continues to appear at considerably downstream positions.
3.2.5.1 Comparison of Flame Lifted Height by Different
Reaction Models
The height of the flame lift calculated by the three reaction models is shown in
Figure 3.7
.
The definition of lifted height here is the distance from the nozzle tip to
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