Low-temperature oxidation of n-octane and n-decane in shock tubes: Differences in time histories of key intermediates

The impact of chain length on the time histories of key intermediate species that form upon first-stage ignition was studied experimentally in a shock tube using multi-color laser absorption diagnostics. CO, CH₂O, OH, and composite time histories of heavier carbonyls were measured simultaneously during the low temperature oxidation of n-octane & n-decane in the temperature range of 720–860 K, and pressure range of 4.1–5.8 atm. The test mixtures were 0.64% & 0.6% fuel in oxygen, respectively. A two-color diagnostic was also developed and used in tandem with other diagnostics to quantify the evolution of temperature during oxidation. To our knowledge, this work reports the first measurement of CH₂O, OH, and temperature during oxidation of C8 or heavier hydrocarbon fuels at low temperatures. The measured time histories were also compared against the predictions of two recent detailed kinetic models. Significant differences in the measured and predicted first-stage ignition delay times, absolute concentration of species post-ignition and the maximum rate of formation of species were observed. Our measurements will serve as targets for further refinement of these detailed kinetic models and in the development of rate rules for similar classes of fuels. The measurements also revealed significant differences in the relative reactivity of n-octane and n-decane. Taken together with previous speciation studies conducted in n-heptane, we observe a strong correlation between the species time histories, reactivity, and chain length of straight chain alkanes. This observation lends further support to the selection of CO and CH₂O as target species for development of Low-Temperature Hybrid Chemistry (HyChem) models for real transportation and aviation fuels in future.