A numerical study of localized swirling injection of oxidizer for homogeneous combustion with oxygen enrichment

Conceptually similar to MILD, FLOX, CDC, HiTAC, etc., Homogeneous Combustion (HC) continues to be pursued as an appealing technique towards minimizing NOx emissions. HC combustors are generally driven by high-momentum inlet jets which enable intense dilution of the reactants. This work numerically studies the effectiveness of localized swirling injection in enhancing reactant dilution for an HC combustor running with enriched oxidizers (X-O2 > 21%). Even though localized and low intensity, swirling injection is found to have far-field effects (over similar to 75 diameters). Effects of swirling injection on the flow field (near and far field) and on NOx emissions are explained. While swirl does help in reducing NOx, there exists an optimal swirl intensity beyond which NOx emissions increase. A mutual competition is seen between swirl assisted and entrainment driven dilution; and at higher swirl intensities, the reduction in the latter overwhelms the gains accrued by the former (in terms of NOx emissions). Along expected trends, thermal NOx is deduced to be the dominant pathway of NOx formation for oxy-enriched cases. Damkohler numbers in the reaction zone are low even for the highest oxygen content tested here (X-O2 = 40%). Volumetric standard deviation of Heat Release Rate (HRR) is seen to perform well as a measure of the tendency of transition to the conventional mode of combustion.