A compact microwave reentrant cavity applicator for plasma-assisted combustion

Summary form only given: advantages of combining an electrical discharge with combustion include a faster process, higher intensities, leaner combustion, pollutant reduction by altering by-products of combustion, improved fuel efficiency by achieving more complete combustion, more reliable ignition of combustion, and combustion across a wider range of pressures, temperatures and mixture stoichiometries. The benefits may also include the operation of combustion processes at extreme limits, such as aerospace applications at high speeds and altitudes. Recently we developed an experimental microwave system that enabled the repeatable, controlled coupling of microwave energy into a premixed flame. Here we describe an improved experimental apparatus that employs a more efficient and more compact microwave applicator that positions the flame in a region of high microwave electric field strength. The applicator is a tunable, reentrant coaxial cavity that excites the flame in a small, variable gap in a high electric field region of the applicator. The applicator has a length of 12 cm and a diameter of 3.5 cm which represents a considerable reduction (about 100) in applicator coupling volume over the earlier experimental system. The experimental performance of the applicator is evaluated by coupling 2.45 GHz microwave energy into an atmospheric pressure, premixed, CH₄/O₂ flame. The flame behavior is diagnosed by photography and optical emission spectroscopy measurements. The addition of only 2-15 Watt of microwave power alters the combustion process. This represents an improvement in coupling efficiency of a factor 3-5 over the earlier experiments. The addition of microwave energy extends the fuel lean burning limits, increases the flame length and intensity, and increases the number density and mixture of excited radical species in the flame vicinity. The downstream gas temperature also increases from 2200 to 3400 K as the coupled microwave energy incr- ases to 25 W. The applicator tuning and the improved microwave energy focus greatly enhances the flame coupling efficiency and thus allows the system to be excited with compact, low-power solid state power supplies and to be constructed with small coaxial cables resulting in a compact, portable microwave plasma-assisted combustion system.