Nitric oxide formation in a premixed flame with high-level plasma energy coupling

This paper presents quantitative planar laser- induced fluorescence (PLIF) imaging of nitric oxide (NO) in a transient-arc direct-current plasmatron igniter using premixed air/fuel mixtures. Quantitative measurements of NO are reported as a function of gas flow rate (20-50 standard cubic feet per hour), plasma power (100-900 mA, 150-750 W), and equivalence ratio (0.7-1.3). Images were corrected for temperature effects by using 2-D temperature field measurements obtained with infrared thermometry and calibrated by a more accurate multiline fitting technique. The signals were then quantified using an NO addition method and spectroscopic laser-induced fluorescence modeling of NO. NO PLIF images and single-point NO concentrations are presented for both plasma-discharge-only and methane/air plasma-enhanced combustion cases. NO formation occurs predominantly through N₂(v>) + O → NO + N for the plasma- discharge-only case without combustion. The NO concentration for the plasma-enhanced combustion case (500-3500 ppm) was an order of magnitude less than the plasma-discharge-only case (8000-15 000 ppm) due to the reduction of plasma reactions by the methane. Experiments show the linear decay of NO from equivalence ratio 0.8-1.2 under the same flow condition and discharge current.