Blowout and Blowoff Limits of Confined Coaxial Ammonia/Hydrogen/Nitrogen-Air Flames with Variable Ammonia Fraction

Abstract

The present experimental study reports first observations of stability, blowout, and blowoff characteristics of ammonia–hydrogen–nitrogen fuel blend flames with varying volumetric ammonia fractions (\({x}_{{NH}_{3}}\)) in a coaxial combustor. The \({x}_{{NH}_{3}}\) is varied from 20 to 80%. For flames of ammonia fraction equal to 70% (\({x}_{{NH}_{3}}=0.7\)), three types of flame transitions are observed within fuel flow Reynolds number (\({Re}_{f}\)) of 40–575 as a coflow Reynolds number (\({Re}_{a}\)) is increased in steps. Initially, the coflow air remains laminar and \({Re}_{a}\) is increased gradually from laminar to turbulent limit. Different flame stabilization modes are characterized as burner-attached and lifted flame. The flame extinction modes are classified as lifted-blowoff, attached-blowoff and attached-blowout types. These flame transitions and stabilization characteristics are shown to be similar to methane flames. However, the flame height and liftoff height are shown to be different. The flames of fuel blends with ammonia fraction less than or equal to 60% (\({x}_{{NH}_{3}}\le 0.6\)) are shown to behave fundamentally different from that of flames with \({x}_{{NH}_{3}}>0.6\) (and also methane flames). Specifically, within the tested \({Re}_{f}\) range, only one type of flame transition is observed as \({Re}_{a}\) is systematically varied in the former as compared to three types observed in the latter. Also, with a decrease in ammonia fraction (and a corresponding increase in hydrogen percentage), the liftoff limit, reattachment limit, and blowout limits all are observed to increase. The effect of ammonia composition on flame height and liftoff height is also elaborated. The present study also provides empirical correlations (particularly for the low power flames) for predicting blowout and blowoff limits in both lifted and attached conditions for ammonia-hydrogen–nitrogen fuel blend flames.