Flame Interactions as a Source of Noise and Combustion Instabilities

Abstract

Mechanisms giving rise to sound emission from flames are analyzed in this article. Using simple experiments on laminar flames it is shown that intense sound emission may originate from flame interactions. In a first case, a premixed laminar flame interacts with a cool plate. The flame movement is initiated by weak perturbations of the flow impinging on the flame from upstream. It is shown that this induces a strong flame motion and an intense emission of sound. Detailed analysis of the signals detected by a far field microphone, a photomultiplier recording the global emission from free radicals and phase conditioned images indicate that acoustic emission is related to the rate of change of the flame surface area. The flame-wall interaction amplifies the sound radiation by about 20 dB over a broad range of frequencies. In a second experiment, the flame is formed on an annular burner and the steady state takes the shape of a fountain. When this flame is submitted to weak acoustic perturbations from upstream it is set into a large amplitude periodic motion. Adjacent flame elements interact strongly inducing cyclic variations of the flame area and corresponding oscillations of heat release. The motion induces an intense emission of sound due to mutual flame annihilation. In the third configuration, an inverted conical flame is stabilized on a central rod. The flame interacts with the shear layer originating from the burner lip. Under low level upstream modulations, the flame executes a periodic motion involving interactions between vortices shed from the burner lip and the reactive front. This induces a rolling up of the front elements at the flame extremities and is accompanied by an intense emission of sound. These three examples indicate that rapid rates of change of flame surface area constitute a strong source of combustion noise, a mechanism which could be the source of much of the noise radiated by turbulent flames. Such interactions may also drive instabilities under resonant conditions.