Some Characteristics of NOxEmission of Turbulent Nonpremixed Hydrogen-Air Flames Stabilized by Swirl-Generated Flow Recirculation

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Abstract
Oxides of nitrogen (nitric monoxide and nitrogen dioxide or NOx,) emission from swirl-stabilized hydrogen flames was systematically investigated. Effects of recirculation zone properties, overall equivalence ratio (φ0), flame length (Lf), Reynolds number (Re), and secondary air mixing on emission index (grams of NOx per kilogram of fuel, also denoted as EINOx) and the ratio of NO2 to NOx were demonstrated. Overall the fluid properties around the recirculation zone are shown to play significant roles in NOx emission characteristics. It was found that: (1) EINOx increased with φ0 and Lf (2) the fraction of NO2 of NOx increased with decreasing φ0 and with increasing Re until the flame length exceeded the recirculation zone length (LRZ); (3) secondary air eritrainment helped to convert NO into NO2 downstream of the flame for φ 0= 0.4-0.5 for which Lf ≈ LRZ; (4) the value of EINOx normalized by the recirculation zone (RZ) residence time (7tau;RZ) increased with decreasing Damkohler number (Da), revealing the nonequilibrium effect similar to that found in simple jet diffusion flames (Chen and Driscoll, 1990 and 1992). The present and previously published high ratios of NO2/NOx are explained with the help of results from temperature, velocity and turbulence measurements in the flame. The discussions lead to the consistency of the mixing and temperature window for NO to N02 conversion.