(469h) Control of Flame Instabilities Via Oscillating Electric Fields

The flicker of a candle is a ubiquitous characteristic of (non-premixed) flames burning under normal gravity. The frequency of this oscillatory motion is almost universally found within the range of 10-20 Hz and is largely independent of the type of fuel, the size of the burner, and the fuel flow rate (e.g., for gaseous fuels). This weak dependence on externally controlled parameters is explained by the fact that flame flicker results as instability of the buoyancy induced flow field. The effect is similar to the Kelvin-Helmholtz instability, which occurs at the interface between two fluids flowing at sufficiently different velocities; however, in the context of flames, the velocity shear originates from steep thermal gradients coupled to buoyant convection. Given its ubiquity, it is interesting to ask if and how such flame instabilities can be controlled.

Here, we described one such approach based on oscillating electric fields applied to the flame. The approach is based on the perspective that flames can be considered not only as the hot, gaseous products of an exothermic oxidation process but also as weakly ionized, nonequilibrium plasmas ? which include electrons, molecular ions, and various charged carbonaceous particles (i.e., soot) created as chemical byproducts of the combustion process. The movement of these charged species under the action of an appropriate electric field can couple to the hydrodynamic flow field surrounding the flame boundary, thereby enabling one to shape, deflect, and even extinguish the flame. We demonstrate how such field-induced flows can be used to stabilize lab-scale flames and discuss the mechanism controlling flame stability.