Laminar burning speed of n-hexane-air mixtures - Chemical Kinetics in Shock and Detonation Wave

Hexane is an easy-to-use fuel for laboratory investigations of hydrocarbon vapor explosions and has been used widely as a surrogate for commodity fuels such as kerosene. As part of our ongoing studies into flammability hazards in aircraft environments, we have been carrying out experiments at reduced pressure, below 100 kPa, in order to measure ignition and flame propagation in hexane-air mixtures. The objectives of the present study were to study experimentally the effects of initial temperature and pressure on the burning speed of hexane-air mixtures. Our study expands on and complements existing data and compares the experimental measurements with numerical predictions from various chemical models. The laminar burning speed of n-hexane-air mixtures was measured experimentally using the spherically expanding flame technique. The effects of equivalence ratio, initial pressure and initial temperature were investigated in the ranges: Φ=0.75-1.7, P=40-100 kPa and T=295-380 K, respectively. A typical inverted U-shaped curve was obtained for the evolution of the burning speed as a function of equivalence ratio. The burning speed increases as the initial temperature increases and as the initial pressure decreases; this is in agreement with previous burning speed studies done using n-alkanes, from C5 to C8. Three detailed reaction models, the JetSurF model, the Dagaut model, and the Caltech model were evaluated with respect to the present data. The Caltech model which has been validated extensively for a wide range of hydrocarbons including n-heptane and n-dodecane, was extended to include the chemistry of n-hexane. The sub-mechanism for n-hexane was obtained by reducing the detailed n-alkane model from LLNL using the DRGEP method. It was found that the Dagaut model systematically underestimates the experimental data, whereas the JetSurF model and the Caltech model provide satisfactory estimates of the burning speeds.