Instability and electrical response of small laminar coflow diffusion flames under AC electric fields: Toroidal vortex formation and oscillating and spinning flames

Dynamical and electrical responses of a small coflow diffusion flame were studied using a high-voltage alternating current (AC) to a fuel jet nozzle fueled with propane and air as the oxidizer. High-speed imaging and electrical diagnostics were adopted to capture flame dynamics and electrical signals e.g. voltage frequency and current. In the voltage-frequency domain of 0-5 kV and 0-5 kHz AC-driven instabilities leading in various flame modes e.g. oscillation pinch-off and spinning of flames were identified. A visualization of near-nozzle flow structures displayed a close causality of initial counter-rotating vortices to the other observed flame. An axisymmetric inner toroidal vortex (ITV) shedding was identified within oscillating and pinch-off modes while asymmetric ITV shedding was identified with the spinning mode. Integrated electric power over several AC periods correlated well with variation in the flame surface area for these instabilities suggesting that measured electric power is a potential indicator of combustion instabilities in electric-field-assisted combustion.