Mechanisms of combustion instability

Abstract

This paper presents results of a study aimed at understanding mechanisms involved in triggering, amplification, and suppression of acoustic waves, due to presence of flames. Experiments performed demonstrate that strong interactions between acoustic and flame oscillations can exist through flow oscillations, and that these interactions are due to energy transport between various oscillation modes through both linear and nonlinear coupling. The intensity of the interactions can be altered by varying flow velocity through an organ pipe. Under reduced-flow conditions, the linear coupling is eliminated and the flame exhibits periodic self-sustained oscillations. However, with imposed acoustic excitation of sufficiently large amplitude, the interactions again become important, due to nonlinear effects. Nonlinear coupling between longitudinal acoustic oscillations and tangential or radial flow and flame oscillations is also observed. Supporting evidence is presented, which shows that the onset of the above-mentioned self-sustained oscillations of diffusion flames is due to instability of travelling Tollmien-Schlichting disturbance waves—the same mechanism which leads to onset of transition from laminar to turbulent flows. Effects of changes in the flow fied, due to the presence of interacting burning and nonburning simulated fuel droplets, on the oscillation characteristics are reported and interpreted in terms of the postulated mechanism of boundary-layer instability. Under some conditions, oscillations can actually be stopped by placing nonburning cylinders near an (otherwise) oscillating flame.