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Abstract
A burner was developed to accommodate extensive limits of firing for lean and rich fuel/air mixtures via efficient vitiation of the reactive streams by a pilot premixed flame jet under swirl and cross-flow effects. The stability and combustion characteristics of the corresponding normal and inverse triple flames were addressed for vitiating a double-swirled flow through a concentric vane swirler by a pilot stream swirled due to a tangential entry. Increasing the vitiating stream heat input ratio to 0.5 increased the stability limit to eight times the laminar flame speed as the inner swirl number increased to 2.0. Increasing the cross-flow air momentum flux ratio from 1.0 to 3.0 further increased the normalized stability limit to 8.5, while employing double coswirl and triple coswirl increased such limit to respective values of 10.2 and 11.9. Vitiating a double-swirled rich fuel/air mixture stream by a tangential entry-swirled lean fuel/air mixture stream enlarged the stoichiometric zone to pronounce a peak turbulent kinetic energy of 3.5 m2/s2. Increasing the fuel mass fraction gradient increased the stability limit only with a vitiating lean mixture, while increasing the swirled portion of the vitiating stream increased the normalized stability limit to 19.6. Allocating the counterswirl across the vitiated stream was more efficient to increase the normalized stability limit to 26.7 as the relative swirl angle reached 120°. Increasing the cross-flow opposing jet strain rate enhanced the H and H2 preferential diffusion, increased the temperature homogeneity, and reduced the flame length. There was a corresponding reduction in NOx emissions to a minimum of 38 ppm as the strain rate increased to 13000 s−1, while the normalized stability limit increased to 27.3. Decreasing the longitudinal spacing between the cross-flow stages from 12.0 to 4.0 cm decreased the NOx emissions further to 18 ppm and increased the normalized stability limit to 30.5.
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