Nitrogen oxides emissions in turbulent hydrogen jet non-premixed flames: Effects of coaxial air and flame radiation

Abstract

An experimental study was performed to investigate the effects of fuel-air mixing on NO x emissions in hydrogen non-premixed flames with coaxial air. The major parameters used to modify the fuel-air mixing were fuel jet velocity and coaxial air velocity. Measurements of NO x emission, flame length, and volume were made to investigate the relationship between flame residence time, global strain rate, and NO x emission scaling. Global strain rate and flame residence time from measured flame length and flame volume were used as parameters for the analysis of the experimental data to identify the relevant parameters that lead to the observed NO x scaling law in coaxial air flames. The overall half-power scaling was observed in coaxial air flames, irrespective of coaxial air conditions, but the degree of deviation from the half-slope curve differed in each case. Comparison of the results of pure hydrogen flames with those of helium-diluted hydrogen flames showed that flame radiation played a significant role in 100% hydrogen flames with coaxial air, and the deviation from half-power scaling was due to the difference in the flame radiation. Global strain rate rather than flame residence time is a more appropriate scaling parameter to predict the emission index of NO x /τ B in the turbulent hydrogen non-premixed flames with and without coaxial air. The half-power scaling law previously observed in hydrogen non-premixed jet flames with no coaxial air is applicable to NO x emission in hydrogen non-premixed jet flames with coaxial air, if flame radiation is taken into consideration.