Frequency Response of Counter Flow Diffusion Flames to Strain Rate Harmonic Oscillations

Abstract

Turbulent diffusion flames are inherently complex due to the coupling of highly non-linear chemical kinetics and turbulence. It is necessary to understand this interdependency of transport and kinetic mechanisms in order to accurately predict non-equilibrium effects such as pollutant formation. Acoustically forced flames are useful because they exhibit a larger range of combustion conditions than those observed in steady flames. Thus, these flames give insights into a variety of chemistry-flow field interactions important in turbulent combustion. In this article, spatially and temporally resolved concentrations of PAH concentrations are presented for 1–500 Hz acoustically forced methane/air counter flow flames. Numerical results are compared with experimental measurements obtained in similar conditions. Both experimental and numerical results reveal a relevant net increase in aromatic concentrations accompanying flame forcing and PAHs appear to be a function of both flame strain rate and forcing frequency. Different size PAHs exhibit specific behaviour according to their characteristic chemical time scales that are longer than the one of the whole combustion process.

Keywords:

• Flamelets
• PAH
• Turbulence-chemistry interactions
• Unsteady diffusion flames

The authors would like to thank Prof. W. L. Roberts for his explanations about the experimental apparatus numerically simulated in the present work.