Analysis of the mechanisms of ignition and combustion of i-C8H18–H2 and n-C10H22–H2 fuel blends in air

Resumo

The processes of ignition and combustion of i-C₈H₁₈–H₂ and n-C₁₀H₂₂–H₂ fuel blends in air are analyzed numerically. It is demonstrated that addition of hydrogen both to normal alkane (n-C₁₀H₂₂) and to alkane with a branched structure (i-C₈H₁₈) leads to an increase in the ignition delay time τ_ind if the initial temperature of the mixture _T0 is lower than a certain value T_l and, vice versa, to a decrease in τind at T₀ > T_l. The greater the fraction of hydrogen in the mixture, the greater the change in τind. At sufficiently high temperatures (T₀ > T_h), addition of a small amount of alkane (≈2–10%) to hydrogen reduces the ignition delay time. The value of Tl depends on the pressure of the fuel–air mixture and, to a smaller extent, on the n-alkane type. The value of T_h also depends on the fraction of alkane in the fuel blend. If the initial pressure is sufficiently high (10 atm and more), addition of a small amount of i-C₈H₁₈ or n-C₁₀H₂₂ to the hydrogen–air mixture reduces the value of τind for all values of T₀. These features are caused by intense interaction of alkane and hydrogen oxidation kinetics. It is demonstrated that fuel blends consisting of hydrogen and n-C₁₀H₂₂ (i-C₈H₁₈) have a higher velocity of the laminar flame and wider limits of stable combustion than the hydrocarbons themselves. Nevertheless, a noticeable increase in the laminar flame velocity is observed only for the molar fraction of hydrogen in the fuel blend greater than 50%. In this case, it becomes possible to ensure stable combustion with a smaller fraction of NO in combustion products.