Dilution effect of different combustion residuals on laminar burning velocities and burned gas Markstein lengths of premixed methane/air mixtures at elevated temperature

Abstract

Most of the experimental examinations on laminar flame characteristics of a diluted combustible mixture have simulated combustion residuals with one of the main exhaust gases (N2, H2O, and CO2) or a mixture of two. However, flue gases have quite different thermodynamic properties and chemical reactivities. Therefore, simulating the post combustion products used for dilution with only one or two of the main exhaust gases may yield erroneous laminar flame characteristic data. In the present study, the laminar burning velocities and burned gas Markstein lengths of diluted methane/air mixtures at 1 bar and 473 K were measured with spherically expanding flames under constant pressure in an optically accessible constant volume combustion chamber. The mixtures were diluted with N2, H2O, and CO2 individually as well as with a mixture of 71.49% N2 + 19.01% H2O + 9.50% CO2 by volume, which represents the main product concentrations from stoichiometric methane/air combustion. Experimental results show that the laminar flame speed values of methane/air mixtures diluted with actual combustion residuals are between those of N2 and H2O dilution, whereas the laminar burning velocities of methane flames diluted with CO2 are considerably slower. The effects of different combustion residuals on the burned gas Markstein length were not found to be significant. At the experimentally-investigated initial conditions, CHEMKIN analyses were performed with the GRI-Mech 3.0, San Diego, and USC Mech II mechanisms to inquire about the performances of the kinetic schemes. The GRI-Mech 3.0 results were the closest to the experimental data. This mechanism was also utilized to numerically quantify the dilution, thermal diffusion, and chemical effects of combustion residuals. The dilution effect was the leading effect in decreasing the laminar burning velocity, while the thermal diffusion effect had the smallest contribution. Nevertheless, the thermal diffusion effect changes the thermal and mass diffusivities, and, as a result, the Lewis number, making it a vital parameter for flame stability and stretch. As the thermodynamic properties and chemical reactivities of the combustion residuals vary with temperature, pressure, equivalence and dilution ratios, real combustion residuals cannot be accurately represented with only one or two of the main exhaust gases, as shown in the current study.