Measurement of laminar burning velocities of methane-air mixtures simultaneously at elevated pressures and elevated temperatures

Externally heated diverging channel method helps accurate and direct measurement of laminar burning velocities at elevated temperatures. In the present work, this method has been extended to higher pressures to evaluate the combined effect of pressure and temperature on the propagation of premixed methane-air flames. The experimental measurements of methane-air mixtures for different equivalence ratios are reported for a pressure range (1-5 atm), and elevated temperatures of 350-650 K. A non-monotonic behaviour for temperature exponent, alpha is obtained with a minimum value for slightly rich mixtures (phi=1.1). This non-monotonic behaviour of a continues even at higher pressures (2-5 atm) as well. Predictions from three widely used chemical kinetic mechanisms (GRI-Mech 3.0, Aramco 2, FFCM-1) are employed to compare with present experiments. LBV determined using Aramco 2 mechanism matches very well with the present measurements at various elevated pressures and mixture conditions. The variation of pressure exponent, beta follows a bell-shaped curve with maximum value for slightly rich mixtures (phi=1.1) and a peculiar non-linear behaviour for very rich mixtures (phi >= 1.3). Based on the detailed analysis of experimental results, the temperature exponent (alpha) is proposed as a function of pressure, and pressure exponent (beta) as a function of temperature at various equivalence ratios. A modified power-law correlation considering the alpha,beta variations is proposed: S-u = S-u,S-o(T-u/T-u,T-o)(alpha 0+alpha 1(1-Pu/Pu,o))(P-u/P-u,P-o)(beta 0+beta 1(1-Tu/Tu,o)). Analysis of the flame structure at high-pressure conditions indicates that the reaction layer thickness is reduced with an increase in pressure. A decrease in mixture thermal diffusivity with pressure contributes to a reduction in laminar burning velocity at elevated pressures.