Microwave effects on premixed flames

It has been proposed in the literature that microwave heating of combustion-generated plasmas in intenal-combustion engines can be used to increase the rate of combustion of dilute mixtures. Experiments were conducted on fuel-lean laminar flames held above a porous burner flowing premixed mixtures of fuel (propane, ethylene, or methane) and oxidizer (air or oxygen-argon mixtures). A flame was positioned in a cavity resonated with microwaves at a frequency of about 2.4 GHz, with electric field intensities ranging to over 10⁵ V/m. For the lean-mixture air flames (0.6 < equivalence ratio < 0.8) examined in this study, burning velocity enhancement increased with electric field intensity to a maximum value of 6%. We conclude that the enhancement can be explained in terms of simple microwave heating of the bulk gases in the flame zone, which yields a greater flame temperature.