Spatially-resolved anchoring structure of laminar non-premixed flames of low-calorific-value fuels with different burner rim thicknesses

Understanding the stabilization of non-premixed flames anchored to the burner rim is essential for advancing the practical utilization of low-calorific-value alternative fuels. While it is known that a partially-premixed flame structure in a stratified fuel–air mixing layer stabilizes a lifted flame, little evidence confirms whether this structure also benefits the stabilization of a burner-rim-stabilized flame. Therefore, this study visualized the anchoring structure of non-premixed flames above the burner rim and evaluated its impact on low-calorific-value flame stabilization. Specifically, N 2 -diluted CH 4 mimicked low-calorific-value fuels, and quasi-two-dimensional non-premixed flames were formed above a slot burner (slot length 25 mm) with varying rim thicknesses (2, 4, and 6 mm). The stabilization characteristics of a low-calorific-value flame were observed as the dilution level increased, while the gas-injection velocity remained constant (0.2 m/s). Simultaneous CH* and OH* chemiluminescence imaging visualized the flame, and laser-induced breakdown spectroscopy (LIBS) measured the fuel–air distribution in the non-reacting zone under the burner-rim-stabilized. Experimental results indicate that, regardless of the rim thickness, a stratified fuel–air mixture forms under the burner-rim-stabilized flame. However, this mixing alone does not guarantee a partially-premixed flame structure that enhances flame stability. Instead, a sufficiently high local gas velocity, which enables the convective transport of the stratified fuel–air mixture towards the flame base, is an additional critical factor. In particular, an anchoring region above the burner rim was identified, where the local gas velocity is 0.01–0.1 times the gas injection velocity, enabling both burner-rim stabilization and the formation of a partially-premixed flame structure.