Effects of AC electric fields applied to radially on co-flow Bunsen flame

We investigated the impact of AC electric fields on a nonpremixed coflow-jet flame. To eliminate the azimuthal component of the ionic wind, we created a radially applied electric field using an axisymmetric electrode structure consisting of a nozzle and a cylindrical mesh surrounding the co-flow burner. By applying various voltages and frequencies, we identified four distinct flame behaviors: oscillating flame with applied frequency, fluctuating flame, instability-induced extinction, and blowout. Each behavior was analyzed and discussed using phase analysis, flame length variation over time synchronized with the polarity change of the applied voltage, and FFT analysis. When an AC voltage (VAC) of 2 kV was applied, the oscillating flame with applied frequency was observed regardless of the AC frequency (fAC). Notably, a double-peak oscillation was observed due to the asymmetric bidirectional ionic wind resulting from the polarity change. The fluctuating flame was exclusively observed at VDC = 5 kV with fAC = 1 Hz. In addition to the double-peak oscillation, an overlapping counter rotating vortex-induced oscillation was observed, which was confirmed to be caused by the positive ions directed towards the nozzle in the negative voltage section. At VAC = 5 and 7 kV with fAC = 10 Hz, the oscillation frequency of the counter-rotating vortex induced oscillation (approximate to 10 Hz) was almost identical to fAC, resulting in constructive interference, ultimately leading to the instability-induced extinction. Blowout occurred at VAC = 7 kV with fAC = 1 Hz, where the strong bi-directional ionic wind caused necking of the fuel stream and resulted in flame extinction. Finally, we derived a critical frequency for observing a stable flame regardless of the AC voltage by comparing and analyzing the collision reaction time with the applied frequency.