(6f) AC Electrohydrodynamic Flows In Flame Plasmas

The combustion of hydrocarbon flames generates significant amounts of charged species including positive and negatively charged ions, electrons, as well as charged carbonaceous particles (“soot”).  Importantly, the movement of these species under the influence of an external electric field can induce hydrodynamic flows with the potential to enhance, control, or even extinguish combustion processes.   In this context, the effects of static (DC) fields have been well-documented and applied in practice (e.g., to reduce pollutant production by electrostatic precipitation or extend the limits of operation in fuel-lean combustion).  By contrast, time-varying (AC) electric fields (typical frequencies <10 kHz) have yet to be exploited and offer several advantages over DC fields such as the frequency-selective manipulation of charged species and the reduction of ion screening effects.  Here, we describe the frequency and voltage-dependent response of methane flames subjected to spatially-uniform but time-varying electric fields.  The response of the flame is characterized by two methods.  AC electrohydrodynamic gas flows resulting from the collisional momentum transfer between charged and neutral species are characterized using schlieren particle image velocimetry.  Additionally, the electrical response of the flame plasma is quantified using impedance spectroscopy.  The experimental results are explained by a charge transport model which accounts for the production and consumption of charged species within the flame, their movement in the applied field, and their influence on the surrounding fluid.  In closing, we highlight emerging opportunities for the application of electrokinetic/electrohydrodynamic concepts to combustion-related applications.