Effect of hydrogen addition on laminar burning velocity of CH4/DME mixtures by heat flux method and kinetic modeling

Laminar burning velocities were measured by the heat flux method for premixed methane dimethyl-ether mixtures (CH4/DME) with air, at various mixture fractions from 100% CH4 to 100% DME with hydrogen addition of 0%, 20% and 40%. Experiments were carried out at atmospheric pressure and room temperature within an equivalence ratio range between 0.6 and 1.7. Four chemical kinetic mechanisms specifically created for DME combustion were validated against measured laminar burning velocity data, with Zhao's mechanism providing accurate prediction for both CH4 and DME. Decreasing the DME/CH4 ratio in the fuel decreased the laminar burning velocity but the decreasing pattern behaved differently in rich flames. Enhancement of the laminar burning velocity by hydrogen addition was greater for 100% CH4 fuel than for 100% DME by 20-60% depending on the equivalence ratio and showed a non-monotonic behavior in rich CH4/DME flames. Reaction path and sensitivity analyses point at the increasing importance of CH3 than H and OH in rich flames with H-2 addition. A strong linear correlation was indicated between laminar burning velocity and maximum concentration of [H+ OH+ CH3] radicals. This result indicates the importance of the CH3 radical in promoting combustion at rich equivalence ratios which is the reason for the non-monotonic laminar burning velocity enhancement by hydrogen addition.