Flame impingement to facilitate the stability of NH3-CH4 laminar diffusion flames: High-speed imaging and schlieren visualization

As a hydrogen-rich and carbon-free fuel with high energy density, ammonia is regarded as a promising substitute for fossil fuels. In this study, flame characteristics and flow field were investigated to explore the maximum substitution ratio of ammonia (critical substitution ratio, i.e.) in NH3-CH4 by high-speed direct imaging and schlieren method. To follow up the realistic scenario of flames in enclosed combustion systems, the stability of impinging flame was also investigated under the effect of water-cooled copper wall. The results showed that the critical ammonia substitution ratio for free laminar flames was less than 63 % for the tested cases. Meanwhile, the free flames exhibited periodic expansion and separation, with significant fluctuations in lift-off height and flame stretch rate, accompanying with the periodic motion of the shear layer between the flame and air. After the introduction of the impinging wall, the critical ammonia substitution ratio of the flame increased from 63 % to over 80 % (up to 94 %) in dependent upon the height of impinging wall. At low ammonia substitution ratio (40 %), reducing the impingement height enhanced the stability of the flow field by suppressing the development of shear layers and vortex around the flame. When the ammonia substitution ratio gradually increased to 63% and the critical substitution ratio, the flame anchored on the wall at different impingement heights, and no obvious vortex development was observed in the flow field. Higher ammonia substitution ratios can weaken the influence of impingement height on the flame flow field.