Study of the high-temperature autoignition of n-alkane/O2/Ar mixtures

Ignition time measurements of propane, n-butane, n-heptane, and n-decane have been studied behind reflected shock waves over the temperature range of 1300-1700 K and pressure range of 1-6 atm. The test mixture composition varied from approximately 2-20% O-2, and the equivalence ratio ranged from 0.5 to 2.0. To determine more precisely the fuel mole fraction of the test mixture, a new technique has been employed in which a 3.39-mum HeNe laser and multiple-pass setup is utilized to measure the fuel in situ by absorption. Ignition delay times were measured at the shock tube endwall by a CH emission diagnostic (431 nm) that viewed the shock-heated mixture through a window in the endwall. This enabled the ignition time at the unperturbed endwall conditions to be determined accurately, thereby avoiding problems inherent in measuring ignition times from the shock tube sidewall. A parametric study of the experimental data reveals marked similarity of the ignition delay time characteristics among these four n-alkanes, and a unique correlation is presented in which the stoichiometric ignition time data for all four n-alkanes has been correlated into a single expression with an R-2 value of 0.992: tau = 9.4 X 10(-12) P-0.55XO2-0.63C-0.50 exp(46,550/RT) where the ignition time is in seconds, pressure in atmospheres, the activation energy in calories per mole, X-O2 is the mole fraction of oxygen in the test mixture, and C is the number of carbons atoms in the n-alkane. Comparisons to past ignition time studies and detailed kinetic mechanisms further validate the correlations presented here.