Measurements of Markstein numbers and laminar burning velocities for natural gas-air mixtures

Spherically expanding flames of Chinese natural gas-air mixtures have been used to measure the laminar flame speeds, at equivalence ratios of φ = 0.6-1.4, initial pressures of p = 0.05, 0.1. and 0.15 MPa, and preheat temperatures of T = 300-400 K. Following Markstein theory, one then obtains the corresponding unstretched laminar burning velocity after omitting the effect of stretch imposed on these flames. To study the effects of stretch on burning velocity, various Markstein numbers for both strain and curvature have been derived and the effects of initial temperature and pressure on these parameters have been discussed. Over the ranges studied, the laminar burning velocities are comparable with those previously reported for pure methane-air mixtures and fit by a functional form u_l = u_l0(T_u/T_u0)^α T(p_u/p_u0)^βp, where the dependencies of α_T and β_p on the φ value of the mixture are also deduced. Furthermore, it is presented that the extrapolation results are still in good agreement with the previous work at relatively high pressure. The effects of the dilution gases on the burning velocity have been studied at φ = 0.7-1.2, and the variations in burning velocities are plotted as functions of the dilution ratio and the equivalence ratio of the mixture.