Impact of acoustic forcing on soot evolution and temperature in ethylene-air flames

Abstract

This combined numerical and experimental study assesses the transient coupling of soot formation, flame chemistry and fluid transport in ethylene-air coflow flames at acoustic forcing frequencies of 20 and 40 Hz. The measurements report soot volume fraction and flow velocity. For the computational analysis, the numerical code’s capability in modeling soot formation is first demonstrated in a steady coflow flame. Soot volume fraction and temperature measurements from different laboratories and optical techniques are used for validation. Then, acoustic forcing is applied to investigate the transient behavior of this multi-dimensional combustion problem. Forcing at different frequencies and amplitudes provokes very distinct transient soot, temperature, and flow conditions. The discussed steady ethylene-air flame is excited with 20 and 40 Hz, corresponding to Strouhal numbers of 0.23 and 0.46. For both frequencies, forcing amplitudes of 20, 50, and 60% are studied numerically and validated against measurements at 50%. With a start-up transient analysis, the computation time to reach a periodic state is evaluated and soot volume fraction predictions are then compared with the measurements. A reduction in maximum soot volume fraction for the increased forcing frequency is observed experimentally and numerically. The decrease in maximum soot volume fraction is explained by a residence time analysis revealing shorter maximum fluid parcel residence times for the 40 Hz than for the 20 Hz case. It is also found that at 40 Hz the transient evolution of maximum soot production and forced fuel velocity is almost synchronized, while for the 20 Hz case, a time lag of 32.5 ms is observed, corresponding to 65% of a full period.