Experimental study on flame structure, stability, and emission characteristics of CH4-H2 swirl-induced inverse diffusion flame

Inverse Diffusion Flame (IDF) is a flame with a central oxidizer jet surrounded by an annular fuel jet. In this study, a swirler was inserted into the oxidizer nozzle to increase the mixing intensity. The combustion and emission characteristics of the swirl-induced IDF (S-IDF) were investigated and compared with those of non-swirl IDF (N-IDF). The flame behavior, mixing, and radical distribution were visualized to compare the combustion characteristics of the S-IDFs and N-IDFs. The flame stabilities were evaluated by measuring the blow-out and the emission characteristics were examined by measuring the NOx and CO emissions. Although the S-IDF had 90% lower blow-out limits than N-IDF due to curtain effect induced by swirling radial flow near fuel nozzle, S-IDF showed cleaner emission performance by reducing the maximum 88% of CO emission. Here, hydrogen addition was suggested to enhance the operating range of S-IDF while maintaining clean emissions. The hydrogen-added S-IDF (50 vol.% of hydrogen addition) showed 7.5 times higher blow-out limits and 69% lower CO emission than methane S-IDF. Based on these findings, it was confirmed that S- IDF showed a cleaner emission performance by reducing CO emission, and inferior operation range can be supported by adding hydrogen as a fuel. • Mixing of swirl inverse diffusion flames is investigated using a schlieren system • Swirl promotes mixing and lowers CO emissions while reducing flame stability • Swirling flow generates a double-flame structure at low momentum ratios • Hydrogen addition enhances the stability of swirl inverse diffusion flames • Hydrogen addition increases NOx emissions while reducing CO emissions remarkably