Soot formation in fluctuating flows

Effects of sinusoidal velocity oscillation on soot formation in counterflow ethylene diffusion flames were experimentally and computationally investigated. Experimentally, the unsteady flow characteristics as well as the transient sooting limits were measured over extensive ranges in frequency and amplitude of the flow oscillation by using laser Doppler velocimetry and light scattering techniques. Both in-phase and out-of-phase oscillations were studied. Computationally, the flame processes were simulated with detailed descriptions of chemistry and transport, but without soot modeling. Results show that the hydrodynamic flow field responds differently upon imposition of either the in-phase or the out-of-phase velocity perturbations at the nozzle exits. In particular, the locations of flame and soot layer of the latter can vary quite substantially, while those of the former are almost unchanged. It is also demonstrated that permanent soot extinction is not possible for low frequency oscillations because the system basically behaves quasi-steadily such that soot formation can always revive when the local condition becomes favorable. At higher frequency oscillations, the effects of unsteadiness on the flame response are important. As a result, with increasing oscillation amplitude soot formation in the oscillating flow can be entirely suppressed. However, increasingly larger amplitudes are needed to effect complete soot extinction as the frequency increases. In addition, the required maximum stretch rate leading to transient soot extinction can be higher than the steady-state inception limit. Furthermore, this critical maximum stretch rate for the in-phase oscillation is larger than that of the out-of-phase oscillation because the former has larger response amplitude of stretch rate variations. In general, the present results substantiate the concept that soot inception is a phenomenon controlled by residence times. Therefore, during transient processes, such as those with sufficiently rapid oscillations, soot formation may not have enough time to recover even after the flow condition becomes favorable for sooting.