Experimental study on expanding hydrogen/air turbulent premixed flames under high turbulence intensities

This study measures the characteristics of expanding hydrogen/air premixed turbulent flames over a wide range of equivalence ratios, ϕ (0.5 to 1.5) and root-mean-square (r.m.s) turbulent velocities, uʹ (1 m/s to 9 m/s), using a fan-stirred combustion vessel with high-speed schlieren imaging. These experimental conditions enable the exploration of both Lewis number and turbulence intensity effects on turbulent flame propagation in hydrogen/air flames, addressing a research gap related to flame behavior under relatively high turbulent intensities (uʹ > 4m/s). Pressure oscillations were observed after peak pressure when uʹ exceeded 5 m/s, with their amplitude increasing with increasing uʹ. Hydrogen/air mixtures with lower effective Lewis numbers, Leeff exhibited enhanced turbulent flame acceleration compared to those with higher Leeff, as indicated by the increase in the normalized turbulent flame propagation speed (Ssch/Ss) with decreasing Leeff across all flame radii. The influence of Lewis number on turbulent flame acceleration diminished when Leeff large than unity. The scaling of these normalized turbulent flame speeds with the Reynolds number, ReT followed a power-law trend, with improved correlation when ReT was normalized by Leeff. The mean representative flame propagation velocity, uc=0.5 determined for flame radii between 15 mm and 50 mm, increased with both ϕ and uʹ, reaching a maximum of approximately 27 m/s at uʹ = 9 m/s and ϕ = 1.5. Based on the present experimental data and existing literature on hydrogen/air flames, two correlations are proposed to cover a wide range of Leeff and uʹ. One is correlated with the Karlovitz stretch factor, K and the other with turbulent flame Reynolds number normalized by effective Lewis number (ReT /Leeff) both showing excellent agreement with the data, with R2 values of 0.95.