Environmental Engineering Reference
In-Depth Information
the flame and the porous cylinder surface is narrower under the latter condition with
the higher flame strain rate than under the reference condition.
As seen from the temperature distributions in the two cases, the thermal diffusion
layer of the latter case is thinner than the reference case. The concentration mea-
surements were made on the air side of the flame because the measurements in the
fuel side of the flame were impossible because the diameter of the water-cooled
probe was too large. Note that the flame temperature is lower in the latter case than
in the reference case. NO
x
is produced mainly in the flame zone in the counterflow
diffusion flame. It is suspected that the NO
x
formation would be much larger in
practical burners because the residence time would be far longer. The higher the air-
preheat temperature becomes, the larger the maximum NO
x
concentration in the
flames, because thermal NO
x
produced through the Zel'dovich mechanism increases.
Comparing the two graphs, note that NO
x
concentration in the flame zone is
higher in the reference case than with the larger flame strain rate and that both
concentrations of NO and NO
2
are lower in the latter case. NO
2
concentration is
higher than that of NO in both cases. There is a possibility that NO was converted
to NO
2
in the probe because, in the present experiments, sample gas was aspirated
by the water-cooled probe made of stainless steel. The concentration of NO
x
(= NO
+ NO
2
), however, is not influenced by this conversion.
Concentrations of CO and CO
2
are far higher than the equilibrium values. This
indicates that there is a high possibility of incomplete combustion taking place due
to a large strain rate and a short residence time. A comparison of
Figure 2.42
and
Figure 2.43
shows that the concentrations in Figure 2.43 are higher than the corre-
sponding values in
Figure 2.42,
due to the influence of the shorter residence time.
2.3.1.5 Effect of Flame Temperature on NO
x
F
ormation
of flame temperature in the reference case and with the larger flame strain rate. The
flame temperature was measured with a thermocouple. The measurement procedure
was as follows: first, temperature distribution was measured along a stagnation
streamline; then NO
x
concentration corresponding to the flame temperature
T
f
was
measured. The results were plotted on the flame temperature vs. NO
x
concentration
plane for two different values of strain rate (2
V
/
R
) of 3000 and 6000 s
-1
as the
parameter. It is interesting that these two curves are similar to each other. An
important point is that the NO
x
concentration decreases as the flame strain rate
becomes larger. This observation result is in conformity with the result of the
numerical calculation made by Drake and Blint.
14
As stated above, the residence
time becomes shorter as the flame strain rate increases, and as a result incomplete
combustion takes place and the amount of NO
x
formed in the flames and the diffusion
layer becomes smaller. It is suspected, however, that NO
x
formation is accelerated
by the Zel'dovich mechanism when the combustion reaction proceeds to the equi-
librium state in the wake.
NO
x
concentration is almost constant in this temperature range from 1300 to
1625 K, which means that NO
x
concentration does not depend on the flame tem-
perature. This weak flame temperature dependency is inherent in the prompt NO
Search WWH ::
Custom Search