Effects of CO2 dilution on partially premixed CH4-air flames in swirl and bluff body stabilized combustor

The effect of CO₂ dilution on the flame characteristics and pollutant emission of a partially premixed CH₄-air flame in a confined bluff body and swirl influenced flowfield is investigated using optical and laser diagnostic methods. The non-premixed burner produced a converging-diverging flowfield at the burner exit and a lifted flame is produced at all test cases, with an upstream movement of the flame with decreasing global equivalence ratios (?_g). Based on variations in ?_g, two flame stabilization modes – bluff body influenced and swirl stabilized – with a transition mode in-between is observed for the cases with (flame FB) and without dilution (flame FM). The characteristics of the heat release zone are influenced by dilution, with the FB flames being longer and also less intense when compared to FM flames. Pollutant measurement at 30 mm downstream from the combustor exit highlighted the ultra-low NO_x capability of the IIST-GS2 burner. CO₂ dilution leads to a reduction in NO_x emission due to both thermal and chemical effects. For ?_g ≥ 0.7 extreme low levels of CO and unburned hydrocarbons (UHC) are observed for both cases. For ?_g ≤ 0.6 the dramatic increase of both CO and UHC maybe due to the lower flame temperatures and shorter flame zone residence times, respectively.