Mechanism study on the auto-ignition and reaction kinetic characteristics of hydrogen/n-dodecane mixture under mid and high temperature conditions

Abstract

To acquire the ignition promotion methods for scramjet engine, the auto-ignition and reaction kinetic characteristics of hydrogen/n-dodecane mixture under mid and high temperatures were revealed by detailed chemical kinetic mechanism and experimental data. The results illuminated that the increases of n-dodecane's content and ϕ significantly declined the ignition delay time (IDT) of hydrogen mixture (by two orders of magnitude at most) at mid-temperature high-pressure (T = 950 K&p = 40 bar). The addition of n-dodecane induced the OH radical formation in initial stage and the advance of fuel's H-abstraction reactions (hydrogen: R3, n-dodecane: R2741/R2742/R2743/R2746). Whereas, the corresponding reduction in hydrogen content inhibited the establishment of active radical pool in dual-fuel reaction system before ignition time point. The ignition promoting effect of advanced initial reaction stage overweighted the ignition inhibiting effect of weakened rapid reaction stage in mid-temperature low-reactivity dual-fuel reaction system. Interestingly, at high-temperature high-pressure (T = 1300 K&p = 40 bar), the IDT firstly decreased (by 18.8%), and then continuously increased (by 105.3% at most) with the elevating n-dodecane's content. The minor addition of n-dodecane (1% of mole fraction) at high-temperature had a similar ignition promoting effect as that at mid-temperature. Whereas, the further reduction in hydrogen content dramatically suppressed the formation of active radical pool (H/OH/HO2/H2O2), which overweighted the ignition promoting effect of slightly advanced initial reaction stage. In other words, the hydrogen played a more critical role than the n-dodecane in high-temperature reaction system due to the originally high reactivity of hydrogen. The auto-ignition properties of hydrogen/n-dodecane mixture relied on the competition relationship of initial and rapid reaction stages under specific thermodynamic conditions and blending ratios. This investigation not only reveals internal interaction mechanism between fuels and active radicals in hydrogen/n-dodecane reaction system, but also provides fundamental insights for the precisely ignition promition methods of hydrogen/aviation-kerosene dual-fuel scramjet engine.