4. Results and discussion
4.1. Normal temperature and pressure experiments
Performance of the combustor under normal temperature and
pressure was previously investigated [18]. These results are used
here for the baseline case for the purposes of comparison of the results
at other operational conditions examined here. Fig. 3 shows
the emission of NO and CO from the combustor under non-premixed
conditions. This exit arrangement (ATF1), demonstrated ultra
low NO emission with values of 6.4, 3.4, 1.5 PPM of NO for U
= 0.8, 0.7 and 0.6, respectively. Also this arrangement gave low
CO emissions, for instance, at U = 0.7, CO emission was found to
be 70 PPM. Such low CO emission, while having such a high heat
release intensity, can be explained by the higher residence time
available for the burned gases leading to higher CO conversion (increased
combustion efficiency), as the gases have to travel a longer
path before exiting from the combustor. These results provide a direct
role of better fuel-air preparation on NO and CO emission and
outline the importance of residence time for complete combustion
to occur in the combustor. In all cases the focus is not to allow themixture to ignite until the mixture is diluted and uniformly distributed
in the entire combustion chamber.
Fig. 4 shows the results obtained for NO and CO emissions
under premixed combustion condition. The results show even
low pollutant emissions for this combustor design. For example,
at U = 0.7, NO and CO were found to be 2 PPM and 50 PPM,
respectively.
The OH chemiluminescence intensity distribution images for
cases ATF1 and ATP showed increased reaction zone distribution
with the extended product gas exit tube inside the combustor
(see, Fig. 5). The resulting reaction zone formed a crescent opposite
to the fuel injection location. For premixed combustion, the reaction
zone is distributed along the combustor, with the highest
intensity located opposite to the injection location of air/fuel.
Images of OH chemiluminescence intensity show a decrease in
intensity with decrease in equivalence ratio for both the premixed
and non-premixed cases. This is to be expected as OH intensity increases
as one approached close to stoichiometric conditions.
4.2. High temperature experiments
The results have been obtained to evaluate the performance of
the combustor with preheated air temperature of 600 K. In order
to maintain injection velocity associated with the change in air
density with temperature, the air injection diameter was increased.
It may be noted that higher air inlet temperature will result
in higher flame temperature. Such high flame temperature will
aid in the formation of thermal NOx which increases dramatically
with increase of flame temperature as NO emissions are very
dependent on flame temperature. On the other hand, increase in
the air inlet temperature will reduce CO as a result of increase in
the flame temperature, which accelerates the conversion of CO into
CO2. At equivalence ratios of stoichiometric and above, this higher
flame temperature promotes the formation of CO by dissociation
so that, at the highest equivalence ratios, the CO concentrations
are highest for the higher inlet air temperature [17].
The experimentally measured emissions from the combustor
were as expected; NO emission increased due to higher operational
temperature and is in agreement with NO dependence on flame
temperature. However, CO emission decreased dramatically which
is more favorable. CO decrease can be attributed to the fact that as
the flame temperature increases; conversion of CO into CO2 is
accelerated leading to less CO in product gases. This behavior
was demonstrated for both non-premixed and premixed combustion
conditions.
Fig. 6 shows a comparison for NO and CO under preheated air
combustion condition as compared to normal air inlet temperature
condition for non-premixed combustion condition. The results
show a slight increase of NO emission. However, CO emissions
were dramatically reduced. Previously, the lowest demonstrated
CO emission was 70 PPM with normal air temperature case. However,
with air preheats this value was reduced to 21 PPM. A favorable
operating condition for this combustor configuration using
preheated air was found to be at an equivalence ratio of 0.6 that resulted
in NO and CO emission of 10 and 21 PPM, respectively at a
heat release intensity of 27 MW/m3-atm.
Fig. 7 shows emission of NO and CO under preheated air condition
and the results are compared to those obtained with normal
air inlet temperature condition to the combustor for premixed
combustion case. The results showed a trend similar to that exhibited
in non-premixed combustion. NO emissions increased slightly.
The CO emissions were dramatically reduced with increase in air
preheat temperature. Previously, the lowest demonstrated CO
emission was 50 PPM. However, with air preheats to the combustion
air this value was reduced to 11 PPM. The favorable operating
point for this combustor design with preheated air was also found
to be at an equivalence ratio of 0.6, that resulted in NO and CO
emission of 4 and 11 PPM respectively at a heat release intensity
of 27 MW/m3-atm.
The radical intensity distribution of OH chemiluminescence
shows that the reaction zone is in the shape of a crescent formed
opposite to fuel injection point for non-premixed combustion
and opposite to air/fuel injection point for premixed combustion.
The results reveal that the OH intensity decreases with a decrease
in equivalence ratio. Fig. 8 shows the OH chemiluminescence
intensity distribution for the extended axial exit arrangement for
the preheated air temperature case. It is to be noted that the intensity
scale is different than that used in Fig. 5 due to the increased
OH intensity with increase in air preheat temperature that is expected
to increase the flame temperature.