On the Formation of Nitrogen Oxides During the Combustion of ...

2.3 Kinetic Modeling
1.0
1
2000 K
m s −1
x
1.00
Temperature T
Axial velocity u
0.5
0.0
0.75
0.50
Temperature
Axial velocity
−0.5 CH 4 mass frac.
0.25
O 2 mass frac.
CO 2 mass frac.
H 2 O mass frac.
−1.0
0.000 0.004 0.008 0.012 0.016
0.00
0.020 m
Spatial coordinate x
Figure 2.12: Profiles of Temperature, Axial Velocity, and Mass Fractions over a Counterflow
Diffusion Flame of Methane. Apart from the fuel CH 4 displayed, similar studies
were conducted on C 2 H 6 , CO, and H 2 [298].
Mass fraction Y
tion point can be located where the axial velocity is u = 0. In the flame zone,
the bulk velocity is u < 0, a characteristic of diffusion flames also described,
for instance, in Nishioka et al. [314]. As a consequence, molecules of the fuel
CH 4 need to overcome this convective flow by diffusion [298].
Depicting a counterflow diffusion flame of CH 4 , Figure 2.13 is representative
of the NO production of CH 4 and C 2 H 6 flames. Both GRI mechanisms converge
for this flame type. However, the spatial profile of GRI 2.11 is only half
of that of GRI 3.0. Nonetheless, the low values of the GRI 2.11 mechanism are
consistent with the work of Ravikrishna and Laurendeau [345, 346], in which a
modified rate coefficient is proposed for the initiating reaction of prompt NO
based on a comparison of experimental data and numerical calculations. The
Leeds methane mechanism cannot be used as a reference here, as simulations
converged only in one out of four cases. Figure 2.13 unveils the best agreement
between GRI 3.0 and the combination “n-Decane (Princeton) + NO x (Li)”. The
normal relative difference ǫ NO,abs is around 10% for hydrocarbon fuels (CH 4 ,
C 2 H 6 ), and the total relative difference ǫ NO,tot is 2% for CH 4 and 8% for C 2 H 6 .
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