Beat: A Nonlinear Thermoacoustic Instability in Rijke Burners

Abstract

A beat phenomenon has been observed in Rijke burners with premixed flames. This oscillation is distinguished from the stable limit cycles and has very low frequency (~ 1 Hz) amplitude modulation in acoustic pressure fluctuations. The beat in the thermo-acoustic system is a nonlinear and self-excited oscillation. To understand the mechanisms of self-excited beat, a forced beat by superimposing external acoustic perturbations on a stable limit cycle has been measured, and the pressure oscillations and heat release rate fluctuations in the forced beat are compared with those in the self-excited beats. The power spectrums and the joint time-frequency diagrams highlight the differences of these two types of beats. For the self-excited beat, the low frequency flames pulsations and the corresponding heat release rates have been investigated. The pulsation effects on the acoustic pressure amplitude variation and the fundamental frequency variation are discussed. Finally, the effects of heat powers and equivalence...