Influence of sub-atmospheric pressure on flame shape and sooting propensity in ethylene laminar coflow non-premixed flame

Abstract

Sooting flames have been a longstanding research topic and an extensive literature has been developed at both atmospheric and high pressure. In contrast, studies of sooting flames at subatmospheric pressures are relatively scarce. As pressure decreases buoyancy, and consequently buoyancy-driven convective flow, decreases as well. So one could expect characteristic residence times to be longer. To assess the intuitive finding, steady coflow non-premixed ethylene/air flames were established at different pressure conditions, ranging from 0.2 to 1 bar. The configuration was documented by both numerical and experimental works. By the Modulated Absorption Emission (MAE) technique, fields of temperature, soot volume fraction, and dispersion exponent as a measure of soot maturity were extracted. Extending the MAE setup from 2 to 4 spectral ranges allows a more accurate evaluation of the dispersion exponent together with the temperature calibration factor. Numerical simulations were conducted using the CoFlame code, giving access to the flow topology and the governing characteristic times. According to numerical simulations, with increasing pressure, while the buoyancy-driven convective flow does increase, the flow velocities do decrease. It seems consistent with experimental results, finding higher maturity, expected with higher residence time, when increasing pressure. In addition to the important database produced for flames under sub-atmospheric conditions, this paper also couples originally experimental and numerical results, leading to (i) the reconstruction of the synthetic signals that a camera would capture, and (ii) the tracking of the quantities of interest experienced along the streamlines. Most of the global trends are well-captured by CoFlame, i.e. decreasing pressure leads to the decrease of soot volume fraction, maturity, and flame height, together with the increase in temperature. Meanwhile, significant discrepancies can be noticed, i.e. the numerical simulations overestimate the soot volume fraction, especially for the lower pressure levels, together with an underestimated flame temperature leading to an overestimation of the flame height.