Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 169

The surface temperature when a CO flame is formed increases with the velocity
gradient. A certain amount of CO on the surface of the solid carbon must be secured
for the formation of a CO flame. Since the amount of CO largely depends on the
flow field and more CO must be supplied for higher-velocity gradients, a high surface
temperature upon formation of a CO flame is indispensable. However, if the surface
Damköhler number is high and the reactivity on the surface is favorable, a CO flame
can be formed at lower surface temperatures. Regarding the conditions of CO flame
formation in high temperature airflow, it is revealed that the surface temperature is
lower by 100 to 200 K than the conditions of CO flame formation in room temper-
ature airflow. 69 This means that the conditions do not differ from the CO flame
formation in room temperature airflow if the velocity gradient is at about 1000 s -1
or higher.

2.5.4.8 Combustion Rate in High Temperature Airflow

The relationship between the surface temperature and combustion rate when CO
flame formation is expected was examined (see Figure 2.135a ) . With the velocity
gradient at 200 s -1 , a CO flame was formed at a surface temperature that is almost
equal to that in high temperature airflow. Because the combustion occurred with
the CO flame adhered to the surface of the solid, the combustion rate would not
increase. To increase the combustion rate in this case, the surface temperature
must be higher than 1700 K. With the velocity gradient at 640 s -1 , the combustion
rate increased with the surface temperature to 1550 K. It however decreased by
half with the surface temperature exceeding 1550 K as a CO flame was formed
thereafter. To recover the thus half-reduced combustion rate, the surface temper-
ature must be lowered slightly or increased to a level higher than 1800 K. With
the velocity gradient at 1300 s -1 , the surface temperature at which a CO flame is
formed is as high as 1950 K and the formed CO flame is separated from the surface
during combustion. In this case the combustion rate is larger, although by a minimal
margin, than that in the case where a CO flame is not present and an increase in
the combustion rate is expected.
With the velocity gradient at 6900 s -1 or higher, the surface temperature at which
a CO flame is formed is 2500 K or higher, and the formed CO flame cannot be
present in the gaseous phase during combustion. Figure 2.135b shows the relation-
ship between the surface temperature and combustion rate under the above-

mentioned conditions.

2.5.4.9 Summary

The behavior of the combustion of a solid carbon in high temperature air stagnation
flow was investigated. The study proved empirically that an increase in the flow
velocity resulted from thermal expansion increases in the velocity gradient to finally
raise the combustion rate when the mass flow rate of air is kept at the same level.
It was also found that the combustion rate in high temperature airflow is lower than
that in room temperature airflow. This was because the lowered concentration as a