4.3. Elevated pressure experiments
The results have been obtained to evaluate the performance of
the combustor at the elevated pressure of 2 atm. (15 psig). In order
to keep constant heat release intensity, air and fuel mass flow rates
were increased to double the value. Air injection diameter was increased
to increase the air flow rate. Elevated pressure will promote
the combustion kinetics to enhance the combustion
reactions. At low equivalence ratios, increase in pressure diminishes
CO by accelerating the rate of conversion of CO into CO2. At
high equivalence ratios, increase in combustion pressure reduces
CO emissions, albeit to a lesser extent, by suppressing chemical
dissociation. So in one aspect high pressures are beneficial, but
on the other hand, high pressure also accelerated NOx formation
leading to higher NO emissions.
The combustor experimental emissions were as expected; NO
emission increased due to the accelerated chemical kinetics of
the combustion process. However, CO emission decreased dramatically
which is a highly beneficial aspect. Fig. 9 shows a comparison
for NO and CO emission under elevated pressure condition and
these results are also compared to normal air inlet temperature
condition for non premixed combustion. At higher pressure
conditions the NO emissions show a slight increase. However, CO
emissions were dramatically reduced. Previously, the lowest demonstrated
CO emission was 70 PPM at normal pressure combustion
conditions. However, with combustor pressurized to only 2 atm,
this value was reduced to 8 PPM. The favorable operating pointfor this combustor design with preheated air was found to be at an
equivalence ratio of 0.6, resulting in NO and CO of 15 and 8 PPM,
respectively at a heat release intensity of 27 MW/m3-atm.
Fig. 10 shows a comparison for NO and CO emission under elevated
pressure condition and compared to normal air inlet temperature
condition for premixed combustion condition. The same
trend was found for both the cases under premixed combustion
condition. NO emissions were increased slightly while CO emissions
were reduced dramatically at elevated pressure. The lowest
demonstrated CO emission for the normal pressure case was50 PPM. However this value was reduced to 10 PPM at elevated
pressure condition. The favorable operating point for this combustor
design with normal temperature air pressurized to 2 atm. Pressure
was found to be at an equivalence ratio of 0.7, resulting in NO
and CO emission of 5 and 10 PPM, respectively at a heat release
intensity of 31.5 MW/m3-atm.
The radical intensity distribution of OH chemiluminescence
shows that the reaction zone is in the shape of a crescent formedopposite to fuel injection point for non-premixed combustion and
opposite to air/fuel injection point for premixed combustion. The
results reveal a decrease in OH intensity with decrease in equivalence
ratio. Fig. 11 shows the OH chemiluminescence intensity
distribution for the extended axial exit arrangement under elevated
pressure condition. Note that the intensity scale is different
in Fig. 11 than that used in Fig. 5 due to the increased OH intensity
with increase in pressure.