Experimental study of H₂/air turbulent expanding flames over wide equivalence ratios: Effects of molecular transport

The turbulent expanding flames of hydrogen/air mixtures are investigated over a wide range of equivalence ratios (ϕ = 0.4–5.0) using the fan-stirred combustion chamber. The effects of molecular transport on turbulent flame propagation are interpreted over a wide Lewis number range, including turbulent flame morphology, turbulent burning velocity, and its general correlation. The wrinkled degree of turbulent flame surface becomes strengthened at fuel-lean conditions than that of fuel-rich conditions at the same normalized turbulence intensity (u^’/S_L), and the flame morphology becomes more wrinkled with the flame propagating outwardly. The equivalence ratio of peak turbulent burning velocity is shifted towards fuel-lean side compared to that of laminar burning velocity. At fuel-lean conditions (ϕ = 0.6–1.0), the normalized turbulent burning velocity (S_T/S_L) decreases sharply with the equivalence ratio, while at fuel-rich conditions it first decreases slowly (ϕ = 1.0–2.0) and then nearly unchanged (ϕ = 2.0–5.0). The general correlations of turbulent burning velocity under different Lewis numbers can be obtained in different u^’/S_L ranges, and the effects of molecular transport on turbulent burning velocity are strengthened at the intense turbulent field. All of these indicate that the molecular transport has significant effects on hydrogen turbulent flames.