SHORT-DURATION AUTOIGNITION TEMPERATURE-MEASUREMENTS FOR HYDROCARBON FUELS NEAR HEATED METAL-SURFACES

An apparatus has been designed, built, and extensively tested For maki ng short-duration autoignition temperature measurements of hydrocarbon fuels under atmospheric pressure conditions where the fuel/air stoich iometry, the nature of the hot metal surface, and the contact time bet ween the fuel/air mixture and the heated surface are well controlled. This approach provides a much more reliable database to establish the importance of fuel structure and surface effects on measured autoignit ion temperatures than the current ASTM E659 procedure, which involves variable ignition delay times and unspecified stoichiometries for igni tion in a heated glass flask. Two series of tests have been conducted: (1) over 1100 individual autoignition temperature determinations for the ignition of 15 hydrocarbon fuels containing 1 to 8 carbon atoms on heated nickel, stainless steel, and titanium surfaces for three diffe rent stoichiometries (phi = 0.7, 1.0 and 1.3); and (2) similar to 190 determinations for 10 linear and branched alkanes on heated nickel for stoichiometric conditions. Excellent repeatability has been achieved within a given series of measurements, and good replicate values have been obtained for data collected on Separate days. Autoignition temper atures measured under short-contact time conditions are much higher (b y typically 500 K or more) than found in most prior investigations, wh ere exposure times were longer and test conditions less well controlle d. The autoignition temperatures generally decrease for the larger hyd rocarbons and for richer mixtures, although the C-2 hydrocarbons (etha ne, ethylene and acetylene) have particularly low values. The highest autoignition temperatures are observed for nickel surfaces and the low est for stainless steel, with titanium being an intermediate case. Ove rall, the different metal sufaces exhibit a moderate influence on the observed autoignition temperatures. Prior experimental and modeling in vestigations indicate that the branched alkanes should be more resista nt to autoignition than the linear isomers, and thus present a reduced hazard. Data obtained in the present study are consistent with this p rediction, although the differences in measured autoignition temperatu res are typically less than 100 K for isomers containing the same numb er of carbon atoms.