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

ABSTRACT

Oxides of nitrogen (nitric monoxide and nitrogen dioxide or NO_x,) emission from swirl-stabilized hydrogen flames was systematically investigated. Effects of recirculation zone properties, overall equivalence ratio (φ₀), flame length (L_f), Reynolds number (Re), and secondary air mixing on emission index (grams of NO_x per kilogram of fuel, also denoted as EINO_x) and the ratio of NO₂ to NO_x were demonstrated. Overall the fluid properties around the recirculation zone are shown to play significant roles in NO_x emission characteristics. It was found that: (1) EINO_x increased with φ₀ and L_f (2) the fraction of NO₂ of NO_x increased with decreasing φ₀ and with increasing Re until the flame length exceeded the recirculation zone length (L_RZ); (3) secondary air eritrainment helped to convert NO into NO₂ downstream of the flame for φ ₀= 0.4-0.5 for which L_f ≈ L_RZ; (4) the value of EINO_x 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/NO_x 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.

Key Words:

• Nitrogen oxides
• nonpremixed flames
• swirl
• turbulent flows
• laser doppler velocimetry
• flow visualization