Environmental Engineering Reference
In-Depth Information
2.3.1.3 Extinction and Re-ignition Temperatures of Laminar
Diffusion Flame
Figure 2.38 shows the extinction limit of laminar flames expressed as a function of
air temperature, T a , where the parameter is the dilution amount of air by nitrogen.
The larger the N 2 dilution amount, the lower the extinction strain rate becomes,
which increases along with the air-preheating temperature. As the air-preheating
temperature rises, the flame intensity increases. Whereas the critical strain rate at
flame extinction, (2 V / R ) c , shows mostly linear increase in the range below the
spontaneous ignition temperature, it increases exponentially in a temperature range
above the spontaneous ignition temperature. The velocity gradient at flame extinction
becomes infinite at 1320 K and flames become stable above this temperature. This
preheated air temperature is lower than the flame temperature at extinction by only
150 K. It is understood, accordingly, that the chemical reaction rate is faster than
the diffusion rate between fuel and air.
Figure 2.39 shows the flame temperatures at extinction and re-ignition plotted
against the flame strain rate. Flame strain rate increases and the flame temperature
at flame extinction rises slightly as the airflow velocity increases. Transport of the
reactants is accelerated as the flame strain rate increases and, consequently, the
chemical reaction intensifies further as the reactants are consumed. This means that
the flame temperature, T f , rises to maintain the flames. The change from a wake
flame to an envelope-type flame causes the flame temperature to rise to about 200 K
above the extinction temperature. For this reason, the extinction/re-ignition phenom-
enon shows a hysteresis. See Figure 2.39.
12,000
addition mass ratio
10,000
0% N 2
6.7% N 2
8000
12.9% N 2
6000
4000
2000
0
0
200
400
600
800
1000
1200
Ta C
FIGURE 2.38 Extinction limits.
 
 
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