Abstract
An activation-energy asymptotic theory is developed that predicts autoignition of condensed hydrocarbon fuels in non-premixed flows. Steady, laminar, stagnation-point flow of an oxidizer stream, toward the vaporizing surface of a liquid fuel is considered. The analysis is restricted to the case where the temperature of the oxidizer stream, T 2, is greater than the normal boiling point of the liquid fuel. The gas-phase chemical reaction is described by a one-step overall process. The chemical-kinetic rate parameters are the activation temperature, T a, and the frequency factor, B. A Zel'dovich number, β, is constructed that is proportional to the ratio T a/T 2. The analysis is performed in the asymptotic limit of large values of β. It predicts the value of the Damköhler number, at autoignition. The Damköhler number is defined as the ratio of a characteristic flow time to a characteristic chemical reaction time. The flow time is the reciprocal of the strain-rate, and the chemical time depends on the chemical-kinetic rate parameters. To illustrate the application of the results of the analysis, experiments are conducted in the counterflow configuration. Fuels tested are n-heptane, n-octane, n-decane, n-dodecane, n-hexadecane, iso-octane, cyclohexane, methylcyclohexane, o-xylene, JP-10, JP-8, and diesel. The temperature of the oxidizer stream at autoignition is measured for various values of the strain-rate. The strain-rate, and the value of the Damköhler number at autoignition, obtained from the analysis, are used to obtain the chemical-kinetic rate parameters, for the fuels tested here. Critical conditions of extinction of flames burning these liquid fuels are also measured. A key finding of this work is that for the straight-chain alkanes tested here, at a given value of the strain-rate, n-heptane is the most difficult to ignite, followed by n-octane, n-decane, n-dodecane, and n-hexadecane. The order is reversed for extinction; here it is found that flames burning n-heptane and n-octane are the most difficult to extinguish followed by n-decane, n-dodecane, and n-hexadecane.
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