Laminar burning velocities of CH_4/O_2/N_2 and oxygen-enriched CH_4/O_2/CO_2 flames at elevated pressures measured using the heat flux method

摘要

Laminar burning velocities (S-L) of CH4/O-2/N-2 and oxygen-enriched CH4/O-2/CO2 flames were measured at elevated pressures up to 0.5 MPa and equivalence ratios ranging from 0.6 to 1.6. The oxygen molar fraction was varied from 0.18 to 0.23 in the O-2/N-2 mixtures and from 0.31 to 0.42 in the O-2/CO2 mixtures. The experimental results showed good agreement with the results reported in previous works, validating the suitability and reliability of the present experimental method for measuring S-L at high pressure. Kinetic modelling was also performed using the GRI-Mech 3.0 and the HP-Mech mechanisms. Both mechanisms predict reasonably well S-L and a power factor beta that quantifies the dependence of S-L on pressure. Thermal-diffusion effects play a major role in the laminar burning velocity decrease due to CO2 dilution at normal and elevated pressures. Kinetic analysis indicated that the reverse of reaction CO + OH = CO2 + H retards the flame propagation in competition with H + O-2 = O + OH. Competition of the H consuming reaction H + O-2 = O + OH with the two CH3 consuming reactions 2CH(3) (+ M) = C2H6 and CH3 + H (+ M) = CH4 (+M) leads to a non-monotonic behavior of the overall reaction order for both the N-2- and CO2-diluted flames.