Effect of buoyancy on dynamical responses of coflow diffusion flame under low-frequency alternating current

The dynamical responses of a small coflow diffusion flame to low-frequency alternating current (AC) were investigated under voltages (V_ac) and frequencies (f_ac) in the range of 0–5 kV and of 0–200 Hz, respectively. As high voltages were applied to the fuel nozzle, a frequency-multiplication mode was identified from the flame oscillations at f_ac < 8 Hz using high-speed imaging. This mode was characterized by bulk flame oscillations at multiples of f_ac until f_ac = 12 Hz, close to the frequency of the natural buoyancy-driven oscillation with the burner configuration used in this study. As f_ac increased past 12 Hz, the bulk flame oscillated at f_ac, resulting in a ‘lock-in’ mode. The results of experiments using a counterflow diffusion flame configuration with negligible buoyancy confirmed that it was the coupling between buoyancy-driven flows and AC-driven ionic winds that caused the frequency-multiplication phenomenon. For f_ac > 32 Hz, the bulk flame ceased to oscillate, and a spectral analysis found that ionic winds dominated the dynamic flame responses. The distinctions between AC forcing and acoustic forcing were highlighted. Particle image velocimetry (PIV) experiments at a kHz repetition rate were conducted to reveal the time-resolved flow fields. Electrical diagnostics captured the electrical signals; the calculated power consumption of the applied AC, with respect to the flame-heating power, was about 10^–6.