A Non-Linear Acoustic Source Mechanism In Gaseous Combustion

A brief review of previous work on mechanisms of sound generation in turbulent gaseous combustion is presented and concluded that current literature does not adequately describe the noise generating process. The sound emission from acoustically perturbed premixed laminar flames is investigated and related to the flame geometry. An experimental arrangement is described which has enabled three stages in the combustion of small, isolated pockets of a combustible mixture to be distinguished. A theoretical model of the processes involved is given which shows that the three stages may be accounted for in terms of the changing acceleration of the flame front as the geometry of the combustible mixture alters. The model considers an elementary volume of burning gas as a hydrodynamic system with a steadily decreasing volume and surface area. By considering the conservation equations and the Arrhenius equation, and making certain simplifying assumptions, the model predicts that in the final stages of burning, when the flame preheat layer is approximately equal to the radius of the volume, the gas will undergo catastrophic collapse in a small thermal explosion. The model also gives, an expression for the acoustic pressure pulse associated with this thermal explosion. The investigation adds to the mechanisms proposed by other workers for sound emission from turbulent gaseous combustion, describing the final stages of the combustion process with the experimental findings in good qualitative agreement with the theoretical predictions. The limitations of the experiments are discussed. The experimental results themselves lead to the conclusion that the method described may also form the basis for the measurement of physical parameters such as effective activation energies using acoustic techniques.