Abstract

Lean blowoff in distributed combustion was investigated at moderate heat release intensities of 5.72, 7.63, and 9.53 MW/m3-atm to characterize the blowoff phenomenon. Distributed combustion conditions were established from a conventional swirl flame at an equivalence ratio of 0.9 using carbon dioxide as the diluent to the inlet airstream. A gradual increase in the air flowrate provided a reduction of equivalence ratio that eventually resulted in the lean blowoff limit. Blowoff occurred at relatively higher equivalence ratios for higher heat release intensities, which was attributed to higher inlet turbulence leading to the early introduction of flame instabilities and blowoff. High-speed chemiluminescence imaging (at 500 frames/second) performed near blowoff moments demonstrated the transition of distributed reaction zone to a near V-shape zone due to quenching of flame surface along the sides. A closer examination of the reduction in equivalence ratio in small steps near the global blowoff showed the presence of a very thin thread-like rotating reaction zone. The observations of blowoff were further supported by the analysis of chemiluminescence signals in each case. The effect of inlet air preheats on blowoff was also investigated. Air preheats broadened the lean blowoff to a lower equivalence ratio which was attributed to enhanced flame speed, providing additional flame stability and reduction of flowfield instabilities. The laminar flame speeds obtained at each preheats case using Chemkin-Pro© simulation with GRI-Mech 3.0 reaction mechanisms supported such a hypothesis of gradually enhanced flame speed, providing additional flame stability.

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