Morphology and turbulent burning velocity of n-decane/air expanding flames at constant turbulent Reynolds numbers

In this work we experimentally investigated the propagation and the turbulent burning velocity of n-decane/air turbulent expanding flames at 423 K and 0.5-3.0 atm, with constant turbulent Reynold number, Re_T,flow = (u_rmsL_I)/ν, ranging from 174 to 4712, using a medium-scale, fan-stirred combustion chamber, where u_rms and L_I are respectively the turbulence intensity and the integral length scale, and ν is the kinematic viscosity of a fresh mixture. It was found that the n-decane/air turbulent expanding flame is self-similar, and the normalized turbulent flame propagation speed scales with the turbulent flame Reynold number, Re_T,flame = (u_rms/S_L)(〈r〉/δ_L), to the one-half power for mixtures characterized by non-unity Lewis numbers, where the mean flame radius (〈r〉) is the length scale, and the thermal diffusivity (α = S_Lδ_L) is the transport property. The turbulent burning velocity (S_T,c=0.5) at the mean progress variable (〈c〉) of 0.5 decreases with the pressure at a constant Re_T,flow, following the similar pressure dependence of the laminar burning velocity (S_L). The S_T,c=0.5/S_L of the fuel-rich n-decane/air flame (ϕ = 1.4) is twice larger than that of the fuel-lean mixture (ϕ = 0.8), which would be attributed to the promotion of local burning rates by coupling between the differential diffusion and the turbulent flame stretch on the local flame fronts. In addition, five useful modified correlations for the turbulent burning velocity with the experimental Lewis number (Le_BM) consideration were obtained based on the present experimental data of n-decane/air turbulent expanding flames. Particularly, the successful scaling of the modified correlation based on Damköhler's second hypothesis, S_T,c=0.5/S_L = 1 + 0.115(Re_T,flowLe_BM⁻²)^0.5, suggesting that the S_T,c=0.5/S_L is inversely proportional to the Le_BM. The importance of using experimental determined values of the Lewis number in the scaling of turbulent burning velocity with the differential diffusion consideration should be emphasized.