Low-frequency AC electric field induced thermoacoustic oscillation of a premixed stagnation flame

Abstract

We present a novel phenomenon of low-frequency AC electric field induced thermoacoustic oscillations by employing a wall-jet premixed stagnation flame configuration. A high speed camera and a microphone are used to record the instantaneous flame topographies and the acoustic oscillations, respectively. We find that, under manipulations of the AC electric field, the acoustic oscillation occurs after the flame transits from a ‘disc’ mode to an ‘envelope’ mode. The observed correlation between the changing rates of the flame surface area and the pressure fluctuations in the time domain indicates a significant contribution from the unsteady heat release to the acoustic oscillation; meanwhile, the other possible mechanism, i.e. the direct impact of oscillating electric body force on the generated pressure wave, can be ruled out based on a theoretical analysis of a pressure wave equation. The oscillating behaviors of the flame front show that the electric field drives the movement of the flame cusp with a large stretch rate. Assessment of the current density distribution of the weakly-ionized flame plasma is conducted by non-dimensionalizing the convective–diffusive–reactive equation, estimating the electric conductivity, and solving Ohm's Law. This simplified model divulges that the non-uniform current density induces large perturbations at the corner positions of the flame front. The unsteady flame fronts strongly influence the current response of the flame, which indicates a self-looping process among the flame current, the electric body force and the flame reaction rate.