Analysis of the mechanisms of ignition and combustion of $i$-$\mathrm{C}_8\mathrm{H}_{18}$–$\mathrm{H}_2$ and $n$-$\mathrm{C}_{10}\mathrm{H}_{22}$–$\mathrm{H}_2$ composite propellants in air

The processes of ignition and combustion of $i$-$\mathrm{C}_8\mathrm{H}_{18}$–$\mathrm{H}_2$ and $n$-$\mathrm{C}_{10}\mathrm{H}_{22}$–$\mathrm{H}_2$ composite propellants in air are analyzed numerically. It is demonstrated that addition of hydrogen both to standard alkane ($n$-$\mathrm{C}_{10}\mathrm{H}_{22}$) and to alkane with a branched structure ($i$-$\mathrm{C}_8\mathrm{H}_{18}$) leads to an increase in the ignition delay time $\tau_{ind}$ if the initial temperature of the mixture $T_0$ is lower than a certain value $T_l$ and, vice versa, to a decrease in $\tau_{ind}$ at $T_0>T_l$. The greater than fraction of hydrogen in the mixture, the greater the change in $\tau_{ind}$. At sufficiently high temperatures ($T_0>T_h$), addition of a small amount of alkane ($\approx 2$–$10\%$) to hydrogen reduces the ignition delay time. The value of $T_l$ depends on the pressure of the fuel-air mixture and, to a smaller extent, on the $n$-alkane type. The value of $T_h$ depends on the fraction of alkane in the composite propellant. If the initial pressure is sufficiently high (10 atm and more), addition of a small amount of $i$-$\mathrm{C}_8\mathrm{H}_{18}$ or $n$-$\mathrm{C}_{10}\mathrm{H}_{22}$ to the hydrogen-air mixture reduces the value of $\tau_{ind}$ for all values of $T_0$. These features are caused by close interaction of the alkane and hydrogen oxidation kinetics. It is demonstrated that composite propellants consisting of hydrogen and $n$-$\mathrm{C}_{10}\mathrm{H}_{22}$ ($i$-$\mathrm{C}_8\mathrm{H}_{18}$) 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 composite mixture greater than $50\%$. In this case, it becomes possible to ensure stable combustion with a smaller fraction of $\mathrm{NO}$ in combustion products.