An experimental study of flame and autoignition interactions of iso-octane and air mixtures

Recent modifications to the University of Michigan rapid compression facility (UM RCF) were made to allow direct imaging of flame/autoignition interactions using compression to initiate autoignition chemistry and a spark plug to initiate simultaneous flame development. The experimental data in this study quantify the effects of spark-initiated flame propagation on autoignition of iso-octane/O2/inert gas mixtures at well-defined initial conditions. The work leveraged the controlled environment of the UM RCF, in which temperature, pressure, and composition are nominally uniform and well-known at the end of compression. Flame initiation by the spark plasma, flame propagation, and autoignition were monitored using high-speed optical imaging of chemiluminescence and in situ pressure time histories. End-of-compression temperatures from TEOC = 942–1012 K were considered, while the end-of-compression pressures were nominally constant within the range of PEOC = 7.8–9.5 atm. The fuel-to-O2 molar equivalence ratio was varied from ϕ = 0.20–0.99 and dilution, defined as the molar ratio of inert gases to O2 in the reactant mixture, was varied from inert:O2 = 3.76–7.47 to determine the effects on flame/autoignition interactions as well as to identify the lean flammability limit of the mixtures as a function of dilution. Flame propagation is generally expected to decrease autoignition delay times by compression heating the unburned portion of the mixture. The effect of flame propagation was maximized in these experiments by igniting the mixtures early during the autoignition process. Later spark timings had small to negligible effect on the autoignition delay time. Dilution had significant effect on the lean flammability limits, increasing from a lean limit of ϕ = 0.35 at air levels of dilution to ϕ = 0.65 at inert:O2 dilution of 7.5. The flammability limit was well correlated with the theoretical adiabatic flame temperature of each experiment. The propagation rates of flames successfully initiated by the spark plasma were determined from the imaging data and were ∼1 to 12 m/s. The magnitude of the propagation rates and the effect on the time integrated temperature scaled with the energy content of the mixtures as indicated by the theoretical adiabatic flame temperature.