Effect of methane-dimethyl ether fuel blends on flame stability, laminar flame speed, and Markstein length

Binary fuel blends provide challenges to chemical kinetics models not seen by testing only pure fuels, allowing them to be proven over a wider range of inputs. The effect of adding dimethyl ether (DME) to methane (CH4) on the laminar flame speed, Markstein length, and Lewis number was studied experimentally and numerically over a range of initial pressures from 1 to 10 atm, with the volumetric ratios of the fuel blends ranging from 60% CH4/40% DME to 80% CH4/20% DME. This data set includes high-pressure results that have never been published before. The experimental results were compared to an improved kinetics model, an ongoing effort spanning the past few years. Model results are in very good agreement with the experimental data. For the 80/20 blend of CH4/DME, the Lewis number remained close to unity as the equivalence ratio increased, in comparison to the large decrease in Lewis number for pure DME as equivalence ratio is increased. This small change in Lewis number, with the value remaining near unity, resulted in the 80/20 blend of CH4/DME remaining stable throughout the entire range of 5-atm experiments, while all other pure fuels and blends exhibited instabilities at initial pressures equal to or greater than 5 atm. In addition, the Markstein lengths were greatly affected by the blending of the fuels. A small amount of DME addition caused the Markstein lengths to change by a large value. Finally, a rigorous uncertainty analysis was performed on the experimental data, giving the error with respect to the true value rather than the standard deviation of repeated experiments.