Study on the stability of low heat release rate flames by laser-induced breakdown spectroscopy

• Instability mode of low heat release rate flames depended on burner rim thickness. • CH 4 , O 2 and N 2 distributions were measured by laser-induced breakdown spectroscopy. • Atomic spectral lines for achieving reliable N distribution measurements were found. • Thicker burner rim prevented flame liftoff and enhanced fuel-air mixing. • Thinner burner rim increased diffusion rate and reaction intensity at flame bases. This study investigated the effects of burner rim thickness on the stabilization characteristics of non-premixed jet flames with low heat release rate (HRR), which is crucial for expanding the application of low HRR fuels (e.g., ammonia) in practical burners. HRR of a flame is a significant parameter since flame stability is influenced by the balance of HRR and heat loss rate. In this study, low HRR fuels were simulated by CH 4 -N 2 mixtures and their flames were formed above a quadruple-slot burner with different rim thicknesses. OH* chemiluminescence imaging showed that flames with low CH 4 content in fuel lifted off and stabilized by triple flame structures above the thinner rim; thicker rim prevented flame liftoff but reduced the reaction intensity at flame bases. Mole fraction distributions of CH 4 , O 2 and N 2 in the vicinity of the rims were measured by laser-induced breakdown spectroscopy (LIBS), and the appropriate choice of nitrogen spectral lines was discussed. LIBS results indicated that fuel–air mixing was more effective near the thicker rim. However, species diffusion at the flame bases above the thinner rim was faster, resulting in higher reaction intensity. The results suggest that the flame stabilization mechanism above thinner rims and thicker rims may be different.