Experimental characterization of premixed turbulent flame dynamics in lean and ultra-lean hydrogen mixtures

• Rare measurements of lean ( ϕ < 0.6) and ultra-lean premixed turbulent H 2 /air flames at 1 - 4 bar. • Flame dynamics and morphology quantified from high-speed schlieren imaging over a wide turbulence range. • Ultra-lean flames show strong turbulence sensitivity with higher wrinkling and larger shot-to-shot variability. • Turbulent flame speed scaling includes Lewis number, turbulent Karlovitz, and Damköhler numbers. The transport sector’s dependence on fossil fuels demands an urgent shift towards sustainable alternatives such as hydrogen, a promising energy carrier for spark-ignition engines (SIEs). Achieving near-zero NO x emissions requires operation under lean and ultra-lean conditions (0.2 ≤ ϕ ≤ 0.6), where experimental data – especially at elevated pressures and moderate-to-high turbulence levels – remain limited. Furthermore, inconsistencies in the definition of key laminar flame parameters hinder the understanding of flame dynamics in this regime. This study investigates the influence of r.m.s. turbulence intensity ( u’ = 0.49–2.45 m/s), initial pressure ( p u = 1–4 bar), and equivalence ratio ( ϕ = 0.2–0.6) on the morphology and dynamics of outwardly propagating premixed hydrogen-air flames at an initial temperature of 373 K. Experiments were performed in a spherical combustion chamber using Z-type schlieren imaging. Results show that increasing r.m.s. turbulence intensity and initial pressure enhances flame propagation speed across all equivalence ratios, attributed to reduced Taylor and Kolmogorov scales and stronger interactions between turbulence and positively curved flame fronts. Under turbulent conditions, flame stretch behavior shifts markedly: the critical radius associated with flame acceleration progressively decreases with turbulence intensity and disappears at the highest u′ , indicating that acceleration occurs already from the earliest analysed radius ( R f ≥ 5 mm), i.e. once the self-sustained propagation regime is reached. Equivalent turbulent flame speeds were calculated, and a scaling accounting for preferential diffusion, turbulent Karlovitz numbers, and Damköhler numbers was developed and validated against both present and literature datasets. These results provide new experimental insight into ultra-lean hydrogen-air turbulent combustion and support the development of improved predictive models for practical low-emission combustion systems.