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Soot and NO formation in counterflow ethylene/oxygen/nitrogen
diffusion flames
Guo, H.; Liu, Fengshan; Smallwood, Gregory
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https://nrc-publications.canada.ca/eng/view/object/?id=c59b51fa-63d2-4107-8364-c10aba5fe7a1 https://publications-cnrc.canada.ca/fra/voir/objet/?id=c59b51fa-63d2-4107-8364-c10aba5fe7a1
Soot and NO Formation in Counterflow Ethylene/Oxygen/Nitrogen Diffusion
Flames
Hongsheng Guo
+, Fengshan Liu, and Gregory J. Smallwood
Combustion Research Group
National Research Council Canada
1200 Montreal Road, Ottawa, Ontario K1A 0R6
hongsheng.guo@nrc.ca
Key words: laminar flame, soot, NOx.
Introduction
Understanding the mechanisms of soot and NOX formation in combustion process is of great interest to
combustion scientists. Although many researchers have studied the formation mechanisms of soot and NOX in
various flames, fewer have dealt with the interaction of soot and NOx processes. It is the objective of this paper to
numerically investigate the soot and NO emission in counterflow ethylene diffusion flames, with the emphasis on
the interaction of soot and NO processes.
Numerical Model
The flames studied are atmospheric counterflow, axi-symmetric laminar diffusion flames, with fuel issuing
from one nozzle, and the mixture of oxygen and nitrogen from the opposite nozzle. The potential flow boundary
conditions were employed. The radiative heat transfer from the flames was calculated by an optically thin model.
Soot nucleation, growth and oxidation in the flames were described by an acetylene based semi-empirical
two-equation soot model [1].
The discretized governing equations were solved by Newton’s method. Adaptive refinement of meshes was
done. The chemical reaction mechanism used is essentially from GRI-Mech 3.0. The thermal and transport
properties were obtained by using the database of GRI-Mech 3.0.
Results and Discussions
Three flames were calculated. The fuel stream is pure ethylene for all the three flames, while the oxidant
streams are different mixtures of nitrogen and oxygen. The mole fractions of oxygen, called oxygen index (XO2), of
the oxidant stream for the three flames are, respectively, 0.20, 0.24 and 0.28.
The calculations were first conducted by two reaction schemes: the full GRI-Mech 3.0 and its revised version
obtained by simply removing all the reactions and species related to NOX formation. It was found that there is
almost no recognized difference in the predicted soot volume fraction profiles between the results obtained by the
two reaction schemes for all the three flames. Therefore the formation of NOX almost does not affect the soot
production and oxidation in these flames. Consequently the rest of the paper concentrates on the influence of soot on
NO emission. All the results hereafter were obtained by the full GRI-Mech 3.0 reaction scheme.
The predicted soot volume fractions are compared with those measured by [2] in Fig. 1. The global axial
velocity gradient in the experiment is the same as the stretch rate (27.5 1/s) in the simulations. The stagnation planes
of both experiment and simulations are at X = 0.0 cm in all the plots. The fuel stream comes from the right side, and
the oxidant stream from the left side. It is observed that the simulation captured the general soot characteristics in
these flames.
Soot may affect NO emission through the variations of radiation induced temperatures and the reaction induced
radical concentrations, i.e. effects of thermal and chemical reaction. To identify these effects, three simulations were
conducted for every flame. The first simulation (SIM1) was conducted by including both NOX and soot in the
model, while the inception and surface growth rates of soot were set as zero in the second (SIM2) and third (SIM3)
simulations for every flame. In addition, the temperatures of SIM2 were kept the same as those in SIM1, and were
calculated in SIM3. Therefore the result difference between SIM1 and SIM2 is mainly caused by chemical reaction
effect of soot, while the difference between SIM2 and SIM3 is mainly caused by the radiation induced thermal
(temperature) effect.
Soot does affect NO formation, and the influence is enhanced when the oxygen index in the oxidant stream is
increased, as shown in Figs. 2 and 3. This is apparently due to the increase of soot volume fraction in the flame
when XO2 is increased.
Moreover, Figs. 2 and 3 show that the influence of soot on NO formation is mainly through chemical reaction,
when XO2 is 0.20 (more clearly shown in the inset of Fig. 3). However, with the increase of XO2, the radiation
induced thermal effect increases. For the flame with XO2 of 0.28, thermal effect becomes more significant than
chemical effect. The variation of the relative effects of thermal and chemical reaction of soot on NO formation is
caused by disparities in soot volume fractions and the relative contribution of thermal and prompt routes to the total
through prompt route, while the thermal route contribution to NO formation is increased with the increase of XO2, as
shown in Fig. 4 by the rates of the initial thermal route reaction and the main prompt route reaction.
For the flame with XO2 of 0.2, the soot volume fraction in the flame is lower. Consequently radiation induced
temperature variation due to soot is negligible, as shown in Fig.5, and thus the thermal influence of soot process on
NO is very small. However, the chemical process of soot causes the variation in the concentration of radical CH in
the flame, as shown in Fig. 6, in spite of the lower soot volume fraction. This variation results in the disparity in the
reaction rates of the most important initial reaction, CH + N2 = HCN + N, of prompt route for NO formation.
Accordingly soot affects NO formation in this flame mainly through chemical reaction, since prompt route
dominates the production of NO in this flame and soot affects the prompt route initial reaction CH + N2 = HCN + N
through the variation in the concentration of radical CH.
When XO2 is increased to 0.28, the contribution of thermal route to NO formation increases due to the flame
temperature rise, Fig. 4. Because of the higher soot volume fraction in this flame, the soot induced temperature
variation increases, compared to the flame with XO2 of 0.20, Fig. 5. Meanwhile the variations in the concentrations
of radicals O and OH due to temperature effect also increase. These variations cause the influence of thermal effect
of soot on NO formation to increase. Although the high soot volume in this flame also results in the variations in the
concentration of radical CH and the reaction rate of the prompt initial reaction, this effect is smaller than the thermal
effect. Consequently the effect of soot on the NO emission in the flame with XO2 of 0.28 is mainly through the
thermal effect.
Conclusions
The NO emission has almost no effect on soot in counterflow ethylene diffusion flames. When the oxygen
index of the oxidant stream is lower, the relative influence of chemical reaction caused by soot production on NO
formation is more important, while the relative influence of radiation induced thermal effect becomes dominant for
the flame with higher oxygen index in the oxidant stream. The variation of the relative influence of thermal and
chemical reaction is caused by the variations in soot volume fraction and the relative contribution of thermal and
prompt routes to NO formation in the flame.
References
1. Guo, H., Liu, F., Smallwood, G.J., and Gülder, Ö.L., Combust. Theory Modelling 6: 173-187 (2002).
Fig. 1 Soot volume fractions obtained from the current Fig.2 NO profiles of the three flames. XO2
simulations and the experiment [2]. represents the oxygen index.
Fig. 3 Emission indices of NO. Fig. 4 Nitrogen consumption rates.
Fig. 5 Flame temperatures. Fig. 6 Mole fractions of radical CH.
X, cm -0.3 -0.2 -0.1 0.0 0.1 Soot vol u me fracti on , ppm 0.0 0.5 1.0 1.5 2.0 2.5 3.0 28% O2, simulation 24% O2, simulation 20% O2, simulation 28% O2, experiment 24% O2, experiment 20% O2, experiment X, cm -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 M o le fra c ti on of NO 0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008 SIM1 SIM2 SIM3 X O2 = 0.20 X O2 = 0.28 X O2 = 0.24 X, cm -0.3 -0.2 -0.1 0.0 M o le f ractio n of CH 0 1e-6 2e-6 3e-6 4e-6 5e-6 6e-6 SIM1 SIM2 SIM3 X O2 = 0.28 X O2 = 0.2 X, cm -0.4 -0.3 -0.2 -0.1 0.0 N2 c o nsumpti on by the re acti o n: N + NO = N 2 + O m o le /( cm 3s) -1.0e-6 -5.0e-7 0.0 5.0e-7 1.0e-6 1.5e-6 2.0e-6 N2 consu m pt io n by th e re acti o n : C H + N 2 = HCN + N m o le /( cm 3s) -4e-6 -3e-6 -2e-6 -1e-6 0 XO2 = 0.28 XO2 = 0.20 SIM1 SIM2 SIM3 X, cm -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 T e m peratu re , K 300 600 900 1200 1500 1800 2100 2400 SIM1 SIM3 X O2 = 0.20 X O2 = 0.24 X O2= 0.28
Oxygen mole fraction in oxidant stream
18 20 22 24 26 28 30 E m is si o n I nde x, g( N O )/k g (C H4 ) 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 SIM1 SIM2 SIM3 18 20 22 1.4 1.5
