Mixture Preparation Effects on Distributed Combustion for Gas Turbine Application - Novel Combustion Concepts for Sustainable Energy Development - page 276

equivalence ratio that was well outside the fuel fl flammability limits. Consequently,

no flashback was possible. However, the study was limited to a single equivalence

ratio (0.8) on an ultra-high-intensity combustor for aviation application (Khalil et al.
2013 ). One of the air dilution cases (case 2) is further investigated here in detail to
evaluate its performance under different equivalence ratios.

fl

2 Experimental Investigations

Air/fuel mixture preparation and mixing with hot recirculated reactive gases are
investigated with swirling air flow created through tangential air injection.

A nominal combustion intensity of 36 MW/m 3 -atm at a constant heat load of
6.25 kW was used as an experimental condition to simulate stationary gas turbine
combustion conditions. For fresh streams (air and fuel), injection scenarios are
reported here to show the direct role of mixture preparation in achieving distributed
combustion conditions and ultra-low pollutants emission. Figure 1 shows a sche-
matic diagram of the arrangements on the fuel injection scenarios presented here
with their relative dimensions in terms of D, where D is the air injector diameter. In
the

fl

“ ”

fuel is injected in the air jet upstream of the
combustor, and the jet entering the combustor can be considered as perfectly mixed
air/fuel jet (see Fig. 1 a). The second con

rst con guration, named PR,

guration involves a coaxial nozzle, where
fuel is introduced as a center jet, while air is introduced through the annulus, named
“ CA ” (see Fig. 1 b). The third and fourth con guration had separate fuel jet

introduced in a behavior. Two different separation distances between

the air and fuel jets were investigated, namely

“ cross- fl ow ”

(see Fig. 1 c, d).
Changes in the fuel injection method are expected to have a signi

“ NP0 ” and “ NP1 ”

cant effect on
mixture preparation between air, fuel, and product gas. Adequate mixture prepa-
ration is critical to achieve distributed reactions as discussed earlier.
The con guration NP1 is further investigated here using air dilution to increase

the fuel jet momentum for better mixing based on fuel jet interaction with the core
fl

flow. Air dilution also offers mixing enhancement as portion of the air is already
mixed with fuel, which results in a less complex mixing issues as compared to non-
premixed combustion. In the dilution case, air and fuel fl flow rates are split into two

streams. The main “ top stream ” injects most of the air fl flow, while the other “ cross

stream ” injects most of the fuel. Fuel is added to the “ top stream, ” and air is added

to the “ cross stream ” such that the equivalence ratio for each stream is well outside

of the fl flammability limits for methane - air combustion (Turns 2006 ). The cross- fl ow

equivalence ratio was kept at 5. The top fl flow equivalence ratio varied from 0.2 to

0.12. Both air and fuel fl flow rates were varied to furnish the combustor with the

appropriate air and fuel flows for operation in the desired range of equivalence

ratios. Adding air to the fuel stream will increase the jet momentum of the cross-
fl

fl

ow
to result in better mixing process. In non-premixed case, the jet momentum ratio
(top to cross) was 20.4. In air dilution cases discussed here, this ratio varied

flow. Increased jet momentum will enhance the penetration of the jet in cross- fl