Magnetic control of flame stability: Application to oxygen-enriched and carbon dioxide-diluted sooting flames

This present study explores possible stabilization mechanisms in flickering, sooting, ethylene flames burning in varying density coflow and exposed to different levels of an upward gradient of the square of the magnetic flux density (del(B-2)). In normal gravity, flame flickering defines a natural large scale and low frequency flame oscillation that is induced by a so called modified Kelvin-Helmholtz type instability. To assess the potential of the magnetically induced stabilization process, a range of coflow mixtures with varying N-2, O-2 , and CO2 contents in volume is studied. As a result, a domain of controllable flame stability is identified. Its extension depends on the maximum magnitude of del(B-2), i.e., 18.2 T-2/m for the present experimental setup. Spectral emission rate, spectral absorption coefficient, soot volume fraction, and soot temperature fields are measured in the flame by the Modulated Absorption/Emission technique (MAE). In agreement with former studies, the soot content is shown to play a key role in the stabilization process. Due to the magnetic force that is mainly acting on paramagnetic oxygen molecules, opposing gravity, and generated by del(B-2), the residence time of soot particles in the flame presumably increases with del(B-2). With growing soot volume fraction, radiative heat losses are enhanced leading to flame cooling. Therefore, flames exposed to the magnetic field exhibit both lower density gradients through the flame sheet and a weaker field of buoyant acceleration in the hot exhaust gas stream. Both mechanisms then reduce the flame vulnerability to the onset of oscillations due to modified Kelvin-Helmholtz type instabilities. The findings may be relevant for designing strategies to control the stability of oxyfuel combustion. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.