Generation of low frequency flame oscillations for the development of passive acoustic flame arresters

Abstract

The development of acoustic-based flame arresters needs a profound understanding of low frequency flame fluctuations. The present study attempts the synthesis and analysis of such low frequency flame oscillations in a Rijke tube combustor subjected to external acoustic forcing. A high frequency (fn = 437±3 Hz) self-excited laminar premixed conical flame is considered. This flame is forced (ff)around its natural frequency (fn) to generate beating (fb=|ff−fn|). The results show that the system falls into lock-in synchronization for a range of forcing frequencies (0.97≤ff/fn≤1.03) at a forcing amplitude (εf) of 10% wherein quenching of natural mode is achieved. A strong flame beating is observed on either side of this synchronization region. The flame transfer function (FTF) at fb is then estimated to delineate the strength of the beating. The FTF gain at fb shows different behavior on either side of the synchronization region. For ff >fn, the gain exhibits a local peak at low beating frequencies (∼20 Hz) which is consistent with the response of a stable laminar conical flame subjected to acoustic forcing at these frequencies. However, for ff<fn, negligible FTF gain is obtained at low beating frequencies, followed by a continuous increase as the beating frequency increases. This asymmetric behavior is further analyzed using proper orthogonal decomposition technique (POD). For ff>fn, the POD results at low fbshow that the flame base undergoes a strong flapping-like oscillation at the beating frequency. However for ff <fn, the flame base oscillates only at the natural mode of the combustor. This different beating behavior of the system can be attributed to the strong and weak thermoacoustic coupling, after and before the synchronization region, respectively.