Non-premixed ignition of n-heptane and iso-octane in a laminar counterflow

The air temperature needed to ignite a prevaporized fuel/nitrogen mixture in a counterflow was determined experimentally over a range of strain rates and pressures for the reference fuels n-heptane and isooctane. The experiments were modeled with detailed transport and chemistry; using semiempirical reaction mechanisms. For both fuels, increasing strain rate increased the ignition temperature, increasing pressure decreased the ignition temperature, and the models overpredicted the ignition temperature by about 100 K. The ignition temperature of n-heptane is lower than that of iso-octane. These results were in qualitative agreement with previous data for C-2-C-4 hydrocarbons. A comparison of C-1 to C-8 ignition temperatures revealed the interplay between three main factors. The structure of the fuel molecule and the reactivity of the alkyl radical were responsible for the high ignition temperature for methane, isobutane, and isooctane. The reduced rate of diffusion as the fuel molecule became larger was responsible for an initial increase in ignition temperature for small alkanes. Finally: the general finite-rate kinetic mechanism of hydrocarbon oxidation was responsible for the somewhat uniform ignition temperatures for larger fuels. The change ill ignition temperature due to kinetic differences between the fuels was small in light of the uncertainties in measuring the ignition temperatures.