EXPERIMENTAL AND NUMERICAL INVESTIGATION ON THE LAMINAR FLAME SPEED OF CH_4/O_2 MIXTURES DILUTED WITH CO_2 AND H_2O

摘要

The effects of CO_2 and H_2O addition on premixed oxy-fuel combustion are investigated with experiments and numerical simulations on the laminar flame speed of CH_4/O_2/CO_2/H_2O~((v)) and CH_4/O_2/CO_2/H_2O~((v)) mixtures, at atmospheric pressure and for a reactants inlet temperature T_u = 373 K. Experiments are conducted with steady laminar conical premixed flames over a range of operating conditions representative ofoxy-fuel combustion with flue gas recirculation. The relative O_2-to-CO_2 and O_2-to-N_2 ratios, respectively defined as O_2/(O_2+CO_2) (mol.) and O_2/(O_2+N_2) (mol.), are varied from 0.21 to 1.0. The equivalence ratio of the mixtures ranges from 0.5 to 1.5, and the steam molar fraction in the reactive mixture is varied from 0 to 0.45. Laminar flame speeds are measured with the flame area method using a Schlieren apparatus. Experiments are completed by simulations with the PREMIX code using the detailed kinetic mechanism GRI-mech. 3.0. Numerical predictions are found in good agreement with experimental data for all cases explored. It is also shown that the laminar flame speed ofCH4/O2/N2 mixtures diluted with steam H_2O_((v)) features a quasi-linear decrease when increasing the diluent molar fraction, even at high dilution rates. Effects of N-2 replacement by CO_2 in wet reactive mixtures are then investigated. A similar quasi-linear decrease of the flame speed is observed for CH_4/O_2/CO_2 H_2O-diluted flames. For a similar flame speed in dry conditions, results show a larger reduction of the burning velocity for CH_4/O_2/N_2/H_2O mixtures than for CH_4/O_2/CO_2/H_2O mixtures, when the steam molar frac-tion is increased. Finally, it is observed that the laminar flame speed of weakly (CO2, H_2O-diluted CH_4/O_2 mixtures is underestimated by the GRI-mech 3.0 predictions.