Impact of soot on flame flicker

The impetus for the present study is a perceived anomaly in which a vertically mounted ethylene jet diffusion flame used to study soot is observed to be steady; whereas similar flames used in other studies have been observed to flicker or oscillate at a frequency of about 12 Hz. This difference raises a question as to why the sooting flame is steady. A well-validated, Navier-Stokes-based, time-dependent numerical code is used to provide an answer to this question. A prediction is made, and later confirmed by experiments, of a phenomenon that is new to the authors and perhaps to the combustion community. That is, soot radiation can influence flame flicker resulting from a buoyancy-induced instability to such a point that the oscillation is completely suppressed. Experiments are used to validate the prediction and the result of numerical studies of the impact of soot on flame flicker. It is found that in a jet diffusion flame, the magnitude of flame oscillations (flicker) decreases with the amount of soot generated in the flame; however, the frequency of the oscillations does not change. Good agreement is obtained between measurements and computations for the dynamic behavior of flames with different soot levels. Both experiments and calculations yield a steady flame when the generated soot is sufficiently high and then produce the initial dynamic flame when the soot formation is restricted. The impact of soot on flame flicker is further studied by performing a simulation for the sooty flame after neglecting radiation from soot.