Experimental Investigation of Acoustic Instabilities in Laminar Premixed Flames

Abstract

It has been known for more than a century that flames can spontaneously produce acoustic oscillations in tubes and other enclosures (Mallard and Le Chatelier¹). These self-excited acoustic oscillations can build up to very high intensities and their control is still a major problem in the design of modern liquid propellant rocket motors. Rayleigh² first gave the general criterion for acoustic amplification by any local heat source. This criterion states that an acoustic wave will be amplified if the time integral of the product of the pressure and heat release fluctuations is positive over a pressure cycle:

$$\int {_{cycle}} \delta p \delta q > 0$$

For this criterion to be fulfilled, there must exist a coupling between the acoustic wave and the heat source which modulates the instantaneous heat release in phase with the pressure. This paper presents an experimental and theoretical description of some aspects of the interaction of premixed methane flames with an acoustic wave. The coupling mechanism that we propose to explain the instability proceeds via the acoustic velocity field and is parametric in nature. It is shown that the observed characteristics of the interaction are well described by the proposed mechanism. Similar observations on propane flames have been published recently by Searby³ and by Searby and Rochwerger⁴ where the reader may find additional details.