Lewis number effects on laminar and turbulent expanding flames of NH3/H-2/air mixtures at elevated pressures

This study clarifies the effects of Lewis number ( Le ) on laminar and turbulent expanding flames of NH 3 /H 2 /air mixtures. The laminar burning velocity ( S L ) and turbulent burning velocity ( S T ) were measured using a medium-scale, fan-stirred combustion chamber with ammonia/hydrogen molar ratio (NH 3 /H 2 ) of 50/50 and 80/20 under the maximum pressures of 5 atm. The lean laminar flame with NH 3 /H 2 = 50/50 is significantly accelerated by the diffusional-thermal instability, which dominated the trend of S T , c = 0.1 with the equivalence ratio ( & phi;). The lean normalized turbulent burning velocity ( S T / S L ) increases with the decrease of hydrogen content due to the weakening effects of S L . However, the S T / S L reaches peak with hydrogen volumetric content less than 20% due to effects made by diffusional-thermal instability than S L did. The turbulent flame of NH 3 /H 2 /air mixtures is characterized by self-similar acceleration propagation, and propagation with Le < 1 is faster. A modified correlation considering the effects of Le was proposed, as (d < r > /d t )/ & sigma;S L = 0.118( Re T , flame Le -2 ) 0.57 , which was able to predict not only the self-similar propagation of NH 3 /H 2 /air but also the previous syngas/air flames. The Kobayashi correlations modified by three kinds of Le power exponents were used to clarify the effects of Le by comparing their fitting parameters and predictive powers on experimental data and literature data. Similar pre-factors, power exponents and the goodness of fit (R 2 ) were obtained with Le ranging from 0.58 to 1.62, which suggested that the determination of Le power exponent had no significant effect on the prediction accuracy of the S T / S L trend with data of Le near unity. This might be attributed to the fact that the variation ranges of the dimensionless number that characterizes & COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.