Comparative study on microwave plasma-assisted combustion of premixed and nonpremixed methane-air flows

A novel microwave plasma-assisted combustion (PAC) system, which consists of a microwave plasma-assisted combustor, a gas flow control manifold, and a set of optical diagnostic system, was developed as a new test platform to study plasma enhancement of combustion. Employing this PAC system, a comparative study on microwave plasma-assisted combustion of premixed and nonpremixed methane-air flows was conducted. Experimental results showed that three different reaction zones (plasma zone, hybrid plasma-flame zone, and flame zone) exist regardless of the fuel injection pattern, namely premixed or nonpremixed. Both plasma zones were characterized with typical atmospheric microwave argon plasma emissions such as OH, NH, and atomic Ar and H lines. However, different optical emission spectra were observed in hybrid zone and flame zone. In hybrid zones, premixed PAC spectra featured with strong emissions from OH, NH, and relatively weak emissions from CN; while in nonpremixed PAC spectra, strong OH, NH, CN, and C₂ emissions were observed simultaneously. Within flame zones, only OH emission presented in premixed PAC, yet in nonpremixed PAC, C₂ emissions dominated over CN and OH emissions. In this work, ignitions at fuel (lean and rich) flammability limits at different plasma powers were also investigated, and results showed that ignition curve of plasma power versus φ_FL revealed a U-shape in premixed PAC whereas a curve formed as an approximately linear increasing line reaching a plateau in nonpremixed PAC. In both premixed and nonpremixed PAC, fuel lean flammability limit (φ_LFL) were extended to φ = 0.2, as compare to φ = 0.6 at the same combustion parameters except with no plasma, showing clear evidences of plasma enhancement effects on ignition and flame stabilization. In addition to optical emission spectroscopy (OES) that was employed to characterize the excited state of species such as OH, NH, - N, CH, C₂, etc., pulsed cavity ringdown spectroscopy (CRDS) was utilized to measure absolute number densities of ground state OH(X) radicals in flame zones of premixed and nonpremixed PACs. Results showed different number density profiles along the flame propagation direction between premixed PAC and nonpremixed PAC.