2. Geometry and configurations
Fig. 1 shows schematic diagrams of the CDC combustor examined
here. In one of our previous investigations [18], the effect of
product gas exit location and fuel injection location were investigated
with swirling air flow created through tangential air
injection into the combustor. A combustion intensity of 36 MW/
m3-atm at constant heat load of 6.25 kW was used to simulate
gas turbine combustion conditions. Three arrangements of product
gas exit were examined. One exit is normal to the cylinder axis, the
second exit is along the cylinder axis, and the third exit is also
along the cylinder axis but extended inside the combustion chamber
denoted as ‘AT’, see Fig. 1. Change in exit location demonstrated
a significant effect on the flowfield and gas recirculation
inside the combustor as well as on the pollutant formation and
emission, and flame stability. Fuel injection location relative to
the air inlet was examined for different product gas exits. This provided
a direct comparison between different hot product gas exits
and fuel injection locations. Arrangement demonstrating ultra low
pollutant emissions was found to be the one where hot product
gases exit through an axial exit that is extended inside the combustion
chamber; denoted ‘‘AT’’. Also, fuel injection location comparison
led to the adoption of a certain fuel injection location that
demonstrated ultra low pollutant emissions. Detailed information
about this comparison can be found in the previous publication
[18]. Consequently, this combination will be adopted herein. To
simulate gas turbine operational conditions that have preheated
air, the injected air into the combustor is preheated to 600 K which
corresponds to characteristic air temperature exiting from the
compressor of a gas turbine. Also, a valve is used at the combustor
exit to increase pressure inside the combustor through choking of
the hot product gases exiting the combustor. Preheating of the inlet
air along with increase in the combustor pressure affected the
combustion behavior to impact the pollutants emission. The higher
the inlet air temperature, the higher will be the flame temperature.
Such high flame temperature will affect the formation of pollutants,
especially NO and CO. Such temperature effect on pollutant
emission will be examined to obtain the more favorable operating
range of the combustor under CDC conditions. Increase in the combustor
pressure is expected to yield the same effect. Detailed investigation
for exhaust emissions and OH⁄ chemiluminescence
emission were performed under preheated air temperature to the
combustor under elevated combustor pressure conditions.
Theoretically premixed flame mode should provide minimum
thermal NOx (under lean conditions) and is considered as the baseline
case in comparison to the non-premixed cases. Since the
flames examined in CDC mode require the use of diluted air at high
temperatures, the configurations discussed here cannot be started
directly. The flame may be started in the conventional diffusion
mode and then transitioned to CDC mode of combustion. Alternatively,
a pilot flame (not shown here) may be used for initial heat
up, which can then be transitioned to the appropriate CDC flow
modes.