Propagation, dissipation, and dispersion of disturbances on harmonically forced, non-premixed flames

Abstract

This paper describes the dynamics of non-premixed flames responding to bulk velocity fluctuations, and compares the dynamics of the flame sheet position and spatially integrated heat release to a similarly excited premixed flame. Bulk axial or transverse excitation, in either case, leads to the excitation of wrinkles on the flame that propagate axially. Inclusion of axial diffusion in the non-premixed case, and burning velocity stretch sensitivity in the premixed case, cause wrinkle dissipation and dispersion. There are important differences in spatially integrated unsteady heat release dynamics between premixed and non-premixed flames. For general Strouhal numbers, mass burning rate fluctuations are the dominant contributor to non-premixed flame heat release fluctuations, while area fluctuations are the dominant contributor to premixed flame heat release fluctuations. Moreover, the heat release response of non-premixed flames rolls off much slower with frequency, O(St−1/2) compared to O(St−1) for premixed flames, and, hence, are more sensitive to flow perturbations than premixed flames at high Strouhal numbers. The asymptotic tendencies of the non-premixed flame, however, are largely controlled by the near burner exit region with high transverse gradients and, thus, are expected to be quite sensitive to burner exit details and finite chemistry effects.