Effects of Coaxial Air on Nitrogen-Diluted Hydrogen Jet Diffusion Flame Length and NO{sub}x Emission

摘要

Turbulent nitrogen-diluted hydrogen jet diffusion flames with high velocity coaxial air flows are investigated for their NO{sub}x emission levels. This study is motivated by the DOE turbine program's goal of achieving 2 ppm dry low NO{sub}x from turbine combustors running on nitrogen-diluted high-hydrogen fuels. In this study, effects of coaxial air velocity and momentum are varied while maintaining low overall equivalence ratios to eliminate the effects of recirculation of combustion products on flame lengths, flame temperatures, and resulting NO{sub}x emission levels. The nature of flame length and NO{sub}x emission scaling relationships are found to vary, depending on whether the combined fuel and coaxial air jet is fuel-rich or fuel-lean. In the absence of differential diffusion effects, flame lengths agree well with predicted trends, and NO{sub}x emissions levels are shown to decrease with increasing coaxial air velocity, as expected. Normalizing the NO{sub}x emission index with a flame residence time reveals some interesting trends, and indicates that a global flame strain based on the difference between the fuel and coaxial air velocities, as is traditionally used, is not a viable parameter for scaling the normalized NO{sub}x emissions of coaxial air jet diffusion flames.