Effects of electric field waveforms on a lifted non-premixed jet flame

This study systematically investigates the behavior of lifted non-premixed jet flame under the influence of various electric field waveforms. In our experimental setup, high voltage is applied to the fuel nozzle, which acts as an electrode, while the flame serves as a floating electrode. The primary areas of interest are the responses of flame lift-off height, flame lift-off velocity, ion current and flow structure to electric fields. Results reveal that the flame lift-off height is predominantly influenced by the charging voltage, with the sequence of effectiveness being AC pulse >AC sine >DC pulse = DC. Furthermore, an increase in voltage frequency significantly enhances the flame lift-off velocity. A simplified model, based on charge transfer in corona wind, has been utilized to derive the electric force acting on the flame. This model establishes a scaling relation that correlates flame lift-off velocity with charging voltage, frequency, waveform and duty cycle. The ion current response in our system exhibits characteristics similar to those of an electrical RC circuit, where the charging voltage has a more significant impact on charge transfer and, subsequently, ion current enhancement compared to frequency. Elevated ion current values correspond to increased flame displacement speeds and reduced flame lift-off heights. The alternation of electric fields introduces a greater degree of turbulence within flames. Near the nozzle exit, a vortex ring with a consistent rotation direction is formed. This vortex ring, driven by the induced ionic wind, facilitates flame propagation and enhances flame stabilization.