Mixture Preparation Effects on Distributed Combustion for Gas Turbine Application - Novel Combustion Concepts for Sustainable Energy Development - page 284

Fig. 9 Normalized NO emissions for different configuration at an equivalence ratio of 0.6

The data obtained here can be combined with precious data for non-premixed
combustion to reveal the impact of injection location. NO emissions under different
injection scenarios are normalized with respect to NO emission from premixed
combustion mode and the results are shown in Fig. 9 . These reported values are at an
equivalence ratio of 0.6. Early injection as in con

gurations CA and NP0 resulted in
early ignition without proper mixing with hot recirculated reactive gases. On the
other hand, late injection cases (NP2, NP3, and NP4) resulted in high emissions.
This is attributed to the diffusion

flame behavior for these cases were combustion
occurs without the fuel properly mixing with air, leading to a locally high equiva-
lence ratio and high emissions regardless of the overall equivalence ratio (Khalil and
Gupta 2011a ). Con

fl

guration NP1 offers a balance of both the issues. Injection of
fuel is delayed enough to allow for proper entrainment of recirculated reactive gases
and their subsequent mixing with air, yet it is not delayed such that the fuel ignites in
a diffusion-like

flame without mixing with the hot and diluted oxidizer (i.e., the fresh
air and the recirculated reactive species). Hence, con

fl

guration NP1 is chosen for
further mixing enhancement to reduce the emissions to premixed combustion levels
that can bring premixed combustion instability issues.

4.2 Air Dilution

As the emission behavior of the different scenarios explained with NP1 con gu-

ration resulted in lower emissions as compared to NP0 and CA, it was sought to
further decrease emissions to premixed emission levels through air dilution. Table 2
gives a comparisons between air and fuel distribution for premixed, non-premixed,
and dilution cases.