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 (Vac) and frequencies (fac) 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 fac < 8 Hz using high-speed imaging. This mode was characterized by bulk flame oscillations at multiples of fac until fac = 12 Hz, close to the frequency of the natural buoyancy-driven oscillation with the burner configuration used in this study. As fac increased past 12 Hz, the bulk flame oscillated at fac, 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 fac > 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.