High Frequency Pressure Wave Interaction with Premixed Flames

Abstract

The interaction of high frequency acoustic waves with premixed flames is discussed and the results presented for a simple one-dimensional theory involving the solution of the acoustic equation in the presence of a strong temperature gradient. The wavelength of the disturbance is very small on an acoustic scale, being on a level with a typical diffusion length. Thus one is considering effectively the interaction of “ultra sound” with flames (frequencies of the order of mHz).

The effect of the flame is to introduce a small but significant jump in the velocity perturbation as the pressure disturbance passes through the flame. Plots showing the effect on the waveform of transmitted and reflected waves are displayed and discussed. In particular one can observe the small but significant effect of the reaction zone on the amplitude and phase of the waves emitted from the flame. It is thus possible that ultra sound could be used as a diagnostic tool to analyse flame structure.

The theory can also be extended to larger amplitude pressure inputs where a non-linear wave equation must be invoked. In this case one can consider the effect of a pressure pulse passing through the flame with all the attending effects of wave steepening and non-linear growth or decay in the amplitude of the response.