Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 69

20

15

10

2 V/R = 3000 1/s
2 V/R = 6000 1/s

5

0

1000 1200 1400 1600

T f C

FIGURE 2.44 Relation between NO x concentration and flame temperature.

mechanism; therefore, NO x generated from the prompt NO mechanism is predom-
inant in this low temperature range. NO x concentration increases as the flame tem-
perature increases in a temperature range higher than 1625 K, which is very close
to the lowest temperature for maintaining flames where the surrounding is at room
temperature. This indicates that the Zel'dovich thermal mechanism, which is strongly
temperature dependent, becomes predominant in this temperature range.

2.3.1.6 Relationship between Flame Temperature and the
Critical Velocity Gradient

As seen in Figure 2.38 , above the spontaneous ignition temperature, the flame
extinction limit increases exponentially with respect to the air-preheat temperature.
This suggests that the overall chemical reaction rate can be expressed in the form
of an exponential function of the flame temperature T f . Flame extinction occurs
when the chemical reaction rate becomes slower than the rate of reactant transport
by diffusion. In the case of the porous cylindrical burner, the transport rate of
reactants by diffusion is estimated by velocity gradient, 2 V / R . Accordingly, the axis
of ordinate (2 V / R ) c is the critical velocity gradient and at the same time expresses
the overall chemical reaction rate. To express the overall chemical reaction rate of
propane in the form of the Arrhenius plot, it is necessary to know the flame tem-
perature. In the present study, the flame temperature was assumed to be an adiabatic
flame temperature, which is appropriate because the flame zone is so thin that it can
be regarded as a flame sheet without thickness. The adiabatic flame temperature