The dynamics of non-premixed flames subjected to a transverse acoustic mode

Abstract

A non-premixed methane/air flame under a transverse acoustic mode is studied by combining theoretical analysis and numerical simulations to understand the acoustic and flow mechanisms affecting the flame response. The experimental configuration under investigation consists of a combustion chamber, a coaxial injector with coflow, and speakers inducing stationary transverse acoustic waves. Numerical simulations reveal a strong response of the flow in the injector to transverse acoustic excitation, resulting in large fluctuations in the mass flow rates of the fuel and oxidizer streams. An analytical model is developed to describe the acoustic response of the flow in the injector to transverse excitation. This model is employed to describe unsteady inflow conditions in large-eddy simulations for different excitation amplitudes. The simulations show that the injector acoustic coupling must be considered to accurately predict the flame lift-off dynamics. Comparisons of numerical results with experimental measurements reveal similar flame structures arising from the longitudinal forcing and good agreement in the lift-off behavior. Further analyses of the flow field, mixing, and flame structure show that the coflow longitudinal forcing induces vortices that corrugate the stoichiometric surface. In addition, the jet forcing enhances the mixing at greater radial locations. The combination of these two mechanisms results in a periodically increased mixing at the stoichiometric surface, leading to significant heat release rate oscillations near the inflow. This work supports the hypothesis that strong acoustic/flame coupling can be present in non-premixed systems when the injectors are longitudinally excited by transverse acoustic waves.