Response of curved premixed flames to single-frequency and wideband acoustic waves

The dynamic response of a premixed curved flame interacting with sinusoidal acoustic waves has been numerically studied in the present work. Flame/acoustic interactions are particularly important both from a theoretical point of view and for practical purposes, as a possible trigger mechanism for combustion instabilities. Flames found in practical devices show a complex geometry, far from the planar configuration usually considered in theoretical studies. The particular purpose of the current study is to assess quantitatively the effects of acoustic waves on curved premixed flames, considering both single and wideband frequencies in order to mimic the conditions encountered in practical systems. The interaction process is studied by using Direct Numerical Simulation (DNS) including detailed physicochemical processes and differential molecular diffusion. The chemical reactions are modeled by a 25-step skeletal scheme involving 16 species to describe methane oxidation. The numerical results show strong flame front oscillations back and forth during interaction of the wave with the curved premixed flame. Moreover, the results demonstrate that a single-frequency acoustic wave has a magnifying effect on the preexisting wrinkling of the flame. This extending flame front leads to increasing fuel consumption rate. The effect is found to be maximum at an intermediate excitation frequency of 500 Hz. Interestingly, a wideband excitation from 100 to 1000 Hz leads to significant flame oscillation and the fuel consumption rate is highly increased in that case. As a whole, this study shows that curved flames are much more sensitive to acoustic excitations compared to planar flames, due to the baroclinic torque in combination with other inherent instabilities. An oblique acoustic wave has a similar but slightly enhanced disturbance to the premixed flame. Moreover, non-unity Lewis numbers have significant effects on curved flame-acoustic interaction, even in the present stoichiometric methane flame. However, it presented highly sensitive to the interaction.