Experimental study on expanding hydrogen/air turbulent premixed flames under high turbulence intensities
Jinzhou Li, Huangwei Zhang, Junfeng Yang
Abstract
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, u ′ (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 ( u ′ > 4 m/s). Pressure oscillations were observed after peak pressure when u ′ exceeded 5 m/s, with their amplitude increasing with increasing u ′ . Hydrogen/air mixtures with lower effective Lewis numbers, L e e f f exhibited enhanced turbulent flame acceleration compared to those with higher L e e f f , as indicated by the increase in the normalized turbulent flame propagation speed ( S s c h / S s ) with decreasing L e e f f across all flame radii. The influence of Lewis number on turbulent flame acceleration diminished when L e e f f large than unity. The scaling of these normalized turbulent flame speeds with the Reynolds number, R e T followed a power-law trend, with improved correlation when R e T was normalized by L e e f f . The mean representative flame propagation velocity, u ¯ c ¯ = 0.5 determined for flame radii between 15 mm and 50 mm, increased with both ϕ and u ′ , reaching a maximum of approximately 27 m/s at u ′ = 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 L e e f f and u ′ . One is correlated with the Karlovitz stretch factor, K and the other with turbulent flame Reynolds number normalized by effective Lewis number ( R e T / L e e f f ) both showing excellent agreement with the data, with R 2 values of 0.95.