In this study, a microwave re-entrant cavity discharge system and a direct current (DC) plasmatron are used to investigate flame enhancement and nitric oxide (NO) formation using laser and optical diagnostics. The uniqueness of this study lies in the direct coupling concept, a novel highly efficient strategy used here for the first time.To investigate combustion dynamics of direct microwave coupled combustion, an atmospheric high-Q re-entrant cavity applicator is used to couple microwave (2.45 GHz) electromagnetic energy directly into the reaction zone of a premixed laminar methane-oxygen flame using a compact torch. When microwave energy increases, a transition from electric field enhancement to microwave plasma discharge is observed. At 6 to 10 Watts, ionization and eventually break-down occurs. 2-D laser induced fluorescence (LIF) imaging of hydroxyl radicals (OH) and carbon monoxide (CO) is conducted in the reaction zone over this transition, as well as spectrally resolved flame emission measurements. These measurements serve to monitor excited state species and derive rotational temperatures using OH chemiluminescence for a range of equivalence ratios (both rich and lean) and total flow rates. Combustion dynamics is also investigated for plasma enhanced methane-air flames in premixed and nonpremixed configurations using a transient arc DC plasmatron. Results for OH and CO PLIF also indicate the differences in stability mechanism, and energy consumption for premixed and nonpremixed modes. It is shown that both configurations are significantly influenced by in-situ fuel reforming at higher plasma powers.Parametric studies are conducted in a plasma assisted methane/air premixed flame for quantitative NO production using a DC plasmatron with PLIF imaging. Quantitative measurements of NO are reported as a function of gas flow rate (20 to 50 SCFH), plasma power (100 to 900 mA, 150 to 750 W) and equivalence ratio (0.7 to 1.3). NO PLIF images and single point NO concentrations are presented for both plasma discharge only and for methane/air plasma enhanced combustion cases. NO formation occurs predominantly through N2(v)+O-->NO+N for the pure plasma discharge without combustion. The NO concentration for the plasma enhanced combustion case (500 to 3500 ppm) was an order of magnitude smaller than the pure plasma discharge (8000 to 15000 ppm) due to the reduction of nitrogen break up from plasma reactions by the methane. Experiments show the linear decay of NO between the equivalence ratio range 0.8 to 1.2 under the same flow condition and discharge current.The diagnostic methods performed in this study include: (1) species concentration by laser induced fluorescence (NO, OH and CO); (2) rotational temperature by optical emission spectroscopy; (3) excited species emission by optical emission spectroscopy; (4) IR thermometry; and (5) multi-line NO PLIF thermometry.
Read
