Insights into the flame flashback mechanism of ultra-lean hydrogen-air jet flames

• Flame flashback topology varies with burner diameter and H 2 concentration. • Soret diffusion enriches hydrogen near walls and accelerates flashback. • An analytical correlation was developed to quantify the Soret effect with burner hole sizes. • Soret effect increases with burner hole size in the presence of conjugate heat transfer. This study numerically investigates the mechanism of flame flashback in laminar lean premixed hydrogen-air jet flames with increasing burner diameter/flame thickness ratios ( D/ δ F = 3 – 7 ) and hydrogen concentration (15–65% H 2 v/v). For D/ δ F = 3.35, with an increase in hydrogen concentration (15–20% H 2 v/v), flame flashback topology transitions from a flat to an inverted V-shaped profile (20–60% H 2 v/v), and again back to a flat flame under highly rich conditions (> 60 H 2 % v/v). Under ultra-lean conditions 15% H 2 v/v), a premixed hydrogen-air jet flame exhibits two different flame front topologies during flashback depending on burner diameter size. The results showed that the smaller diameter burner ( D/ δ F = 3.35 ) exhibits a near-flat flame front profile or a central flame flashback, while the larger diameter burner ( D/ δ F = 7.01 ) maintains its inverted V-shaped profile during flame flashback under ultra-lean conditions, showing a boundary layer dominant flashback. However, premixed methane-air flames exhibit a flat-flame flashback in the range of burner diameters and equivalence ratios studied. A significant fuel accumulation close to the burner walls was observed in premixed-hydrogen flames, a phenomenon less profound in the premixed-methane flames. This accumulation increases with burner diameter, leading to two different flame topologies during flashback. Among different diffusion mechanisms studied, the Soret effect in combination with conjugate heat transfer was found to be the main reason leading to this phenomenon. Analytical correlations developed to quantify the Soret effect showed its direct relationship with the penetration distance, thermal diffusion coefficient, burner radius and average reactant velocity. The developed correlation showed that the Soret effect is directly proportional to the burner tube diameter and inversely proportional to the penetration distance. This leads to a 15% increase in the hydrogen fraction near walls for larger burners in comparison to the inlet hydrogen composition, leading to inverted V-shaped flame flashback.