Influence of lateral species diffusion and heat transfer on the evaluation of near-threshold sooting flames

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Premixed flat flames provide an ideal platform for investigation of soot formation by optical diagnostics. The local conditions can, however, be strongly affected by species diffusion and heat transfer perpendicular to the flow direction. This work investigates the influence of the burner geometry and shielding of the target flame by a nitrogen coflow or by a surrounding non-sooting fuel-rich methane/air flame. The analysis is based on 2D simulations as well as measurements of temperature and species concentration profiles at atmospheric pressure and 10 bar. The simulations indicate that the deviation from the ideal case caused by species diffusion and heat transfer is especially strong at near-threshold sooting conditions investigated in ethylene/air flames with an equivalence ratio ϕ = 2.1 and methane/air flames at ϕ = 1.82. The influence of species diffusion and heat transfer on the axial temperature and species concentration profiles is considerable at atmospheric pressure and becomes lower at 10 bar. Quasi unchanged axial profiles of temperature and soot volume fraction are observed for burner diameters of ≥ 40 mm shielded by a non-sooting flame and for ≥ 60 mm shielded by a nitrogen coflow. The measured soot volume fraction profiles of different ethylene/air ϕ = 2.1 flames with diameters of 20 and 60 mm and oxygen-containing or inert shielding coflows show almost no difference at low heights above the burner (i.e., ≤ 14 mm) but become significant at larger heights, especially for the low diameter 20 mm of flame with nitrogen as shielding coflow.