The usages electromagnetic energy and non-equilibrium plasma for enhancing ignition and combustion stability has been gaining more attention recently. The conventional technologies have adapted the electrical devices to make the electromagnetic field, due to various safety issues such as high-maintenance, additional high-cost system, electric shock, explosion, etc. Therefore, an electrodeless microwave technology has an advantage in term of efficiency economic and reliability as comparing with conventional one because of no oxidation. However, the application of microwave has been still limited because of the lack of interaction mechanism between flame and microwave. In this study, the experiments were performed with jet diffusion flames and lifted partially premixed flame induced by microwaves to clarify the effect of microwave. The burner used in this experiment consisted of coaxial tubes. Methane and compressed air were used as a fuel and a oxidizer. Waveguide system which designed to provide for more efficient coupling of the microwave power into the flame was used.
There was a noticeable difference on the flame stability according to the microwave intensity induced on the co-flow diffusion flames. When 1 kW microwave was applied, unstable flames such as rotating flames tended to be stabilized and the blowout limit was extended. The enhancement of stability is mainly due to the heating of combustion field and abundance of radical pool by microwave. NOx emissions increased according to microwave intensity in most cases. However, decreased at the conditions of high fuel velocity and low microwave intensity. It may be attributed to the competition of CH radical consumption during NOx and soot formation. Morphology of soot collected in the downstream of microwave induced flames was similar to the early soot, amorphous and not aggregates. This indicates that mature soot generated in flames completely may be oxidized along the longer yellow flame region. In the case of lifted partially premixed flame NOx emission was increased and lifted height was decreased with increasing microwave intensity. Those are mainly due to the heating of combustion field and abundance of radical pool by microwave,.i.e., increasing NOx emission by heating effect and decreasing lift-off height by increased burning velocity. When the induced microwave position was changed on the longitudinal axis of flame, NOx emission and lift-off height were maximized on the lifted partially premixed flame position corresponding to microwave, which implies that the primary reaction zone governs the overall flame characteristics under inducing a microwave.