Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 168

The region below the ascending lines satisfies the condition expressed by expres-
sion (2.43) above. This means that a combustion rate higher than that in room
temperature airflow is obtained from the combination of the oxygen mass rate and
the carbon dioxide mass rate on the right side of the thick solid line. It should be
noted that the lower limit of the oxygen mass rate is on the order of 0.085 if the
surface temperature is 2000 K or lower.

2.5.4.7 Surface Temperature When a CO Flame Is Formed

It has been reported that the combustion rate is drastically lowered by the formation
of a CO flame when the velocity gradient is about 1000 s -1 or less in the case of
combustion in room temperature airflow. 66,67 With a similar case expected in high
temperature air combustion, we examined the conditions of CO flame formation by
asymptotic analysis method. As a result it was revealed that the conditional expres-
sion for ignition by Linan 70 can be applied. The conditional expression for ignition
contains velocity gradient and surface temperature, which are represented in Figure
2.134 . The abscissa shows the velocity gradient, while the ordinate shows the surface
temperature at which a CO flame is formed. The parameter is a surface Damköhler
number.

2500

Two-dimensional Flow

T =1280K
Yw =0.0008

Yo =0.085
Yp =0.187

2000

Yo =0.233
Yp =0.0

Da s =10 6

1500

Da s =10 7

Da s =10 8

1000

10 100 1000 10000

Velocity gradient, s -1

FIGURE 2.134 Relationship between velocity gradient and surface temperature when a CO
flame is formed.