Flame dynamics and combustion noise: progress and challenges

This article proposes a review of the state of knowledge in the field of combustion noise. The survey comprises an initial discussion of indirect and direct noise sources and their general characteristics, a summary of expressions devised to estimate combustion noise from turbulent flames, a discussion of the fundamental equations describing sound emission from a reactive region and an evaluation of scaling methods for combustion noise. An account is provided of a set of experiments on noise radiation from perturbed laminar flames. Sources of intense radiation of sound are identified and theoretical expressions of the pressure field are compared with detailed measurements from well controlled experiments. These experiments indicate that flame dynamics determine to a great extent the radiation of sound from flames. This is further demonstrated with experiments dealing with effects of confinement. Links between combustion noise and combustion instabilities are drawn on this basis. These two aspects are usually treated separately but they are manifestations of similar processes. Much of the current effort in the field of combustion noise focuses on numerical estimation techniques using modern computational tools. The state of the art is less advanced than in computational aeroacoustics (CAA) but it is possible to foresee that computational combustion acoustics (CCA) will progressively evolve into a well established scientific field.