Abstract
There is a lack of published soot measurement data in buoyant turbulent flames under reduced oxygen conditions. In this work, the laser induced incandescence (LII) technique is used to investigate the soot volume fraction (fv) and soot sheet thickness in buoyant turbulent diffusion flames in atmospheres with different oxygen concentrations (OC, oxygen molar fraction in oxidizer). The experiments establish an extensive database of quantitative fv spatial distribution in ethylene flames under three OCs (20.9%, 16.8%, and 15.2%) and two heat release rates (HRR = 10 kW and 15 kW). Mean, fluctuation, and probability density function of fv, as well as local soot intermittency are studied and compared across different conditions. The results show that the maximum instantaneous fv in normal air can be ~8 ppm; but the corresponding mean fv only reaches up to 0.58 ppm due to turbulence and flame intermittency. As the oxygen concentration is reduced from 20.9% to 15.2%, the mean fv decreases, and this effect is more significant at lower heights above the burner (e.g., ~75% reduction in the mean fv at a height of half a burner outer diameter). Soot is found to be distributed unevenly in the flame, appearing in sheet-like regions with irregular and complicated shapes. The soot sheet thickness is quantified to better understand soot-turbulence interactions. This soot dataset, along with other accompanying radiation and flow field measurements, provides a comprehensive understanding of the dynamics of turbulent buoyant flame dynamics under normal and vitiated conditions, and can be used to develop and validate models for soot formation and radiative heat transfer in such flames.
Published Version
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