Intrinsic and Acoustic Instabilities in Flames Fueled by Multiphase Mixtures

ABSTRACT

We describe linear stability results for flames fueled by mixtures of air and particles. Nonsimilarity between the temperature and fuel concentration fields gives rise to an intrinsic pulsating instability for unconfined flames (a single mode). Acoustic interactions for confined propagating flames generate acoustic instabilities (an infinite number of modes) when the gas-phase velocity is different from that of the solid phase (i.e. there is slip). The confinement modes can not, in general, be classified as intrinsic, fundamental, first-harmonic, etc. For example, for propagation in a tube with flame initiation at the open end, a mode can start as the second harmonic but finish, when the flame is at the closed end, as the fundamental. Instabilities tend to be suppressed in spherical flames generated by point ignition in a confinement vessel. The triggering of acoustic instabilities in gas turbines by slip between fuel drops and air is discussed, and we show that the role of slip is quite different when the condensed phase is injected at a finite point rather than being dispersed throughout the gas phase.