Simulation Models for High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 180

3. A reaction model has to be worked out regarding which model is capable
of calculating ignition phenomena inside the furnace space (lifted flames)
instead of flames developing from a flame holder of a burner. Conventional
combustion simulation models form a flame on the assumptions that there
is a high temperature gas jet from the burner rim or that the high temper-
ature boundary conditions are set at a burner solid wall and so on. These
numerical techniques are based on the idea in which a stable flame is
always formed, and hence they cannot be applied to the prediction of
blow-off limit of a flame holder. Thus to calculate flames formed by
autoignition inside the furnace, conventional models are incapable of
predicting where the flames start to form. We must calculate the process
leading to ignition as a result of the coupled effects of mixing and reaction
in the furnace. Consequently, the selection of reaction mechanisms and
their rate constants is important in modeling of HiTAC flames.
4. Use of the full reaction mechanism is indispensable for taking all the
intermediates into account, but a practical calculation including a three-
dimensional flow with full reaction mechanism is far beyond the capability
of computers today. Hence, the most realistic solution would be to adopt
a set of greatly simplified reaction mechanisms covering some interme-
diates. For expressing the above-mentioned autoignition, a one-step global
reaction model is not appropriate and several steps of reduced reaction
mechanisms would be required.
3.2.3 T EMPERATURE C ORRECTION FOR T HERMAL D ISSOCIATION
As the flame temperature rises, thermal dissociation becomes significant, particularly
above 2000 K. Therefore, we examined the effects of thermal dissociation in a high
temperature range by use of numerical simulation. The maximum flame temperatures
in a counterflow diffusion flame were compared among three reaction models: the
full mechanism reaction model, the chemical equilibrium model, and theoretically
complete combustion model. The GRI (Gas Research Institute) reaction mechanism
comprising 48 chemical species and 275 elementary reactions was selected as the
full reaction mechanism for methane-air mixtures. 3 Temperature of combustion air
was changed from 300 to 1400 K. The stretch rate was kept constant at 45.5 s -1 .
Figure 3.3 shows the maximum temperatures of counterflow diffusion flames in
terms of air preheat temperature. Here, the shown theoretical complete combustion
temperature was obtained on the assumption of the complete combustion of stoichi-
ometric methane-air mixture, and the adiabatic equilibrium temperature of stoichi-
ometric methane-air mixture was calculated based on the chemical equilibrium.
Precise values of specific heat were used for calculating both theoretical and adiabatic
equilibrium temperatures. Consequently, the difference between the two may result
from the difference in species concentrations of burned product. The difference
between the adiabatic equilibrium temperature and that of counterflow diffusion
flames may be ascribed partly to the influence of flame stretch caused by the flow,
and partly to the difference in concentration of nonequilibrium intermediates. What
is clearly observed here is that the flame temperature is changed markedly by the