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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, T2, 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, Ta, and the frequency factor, B. A Zel'dovich number, β, is constructed that is proportional to the ratio Ta/T2. The analysis is performed in the asymptotic limit of large values of β. It predicts the value of the Damkohler number, at autoignition. The Damkohler 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 Damkohler 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. [ABSTRACT FROM AUTHOR]

Abstract: Experimental studies are conducted on extinction and autoignition of methanol and ethanol flames in laminar, nonuniform flows. Two flame types are considered: nonpremixed and premixed. The studies are performed in the counterflow configuration. The burner used in the experiments is made up of two ducts. Studies in the nonpremixed configuration are carried out by injecting a stream comprised of fuel vapors and nitrogen from one duct, and a stream of air from the other duct. In the premixed configuration a premixed-reactant stream made up of fuel vapors, air, and nitrogen, is injected from one duct, and a nitrogen stream from the other duct. Numerical calculations are performed using detailed chemistry at conditions corresponding to those used in the experiments. For the nonpremixed systems considered here, the calculated values of the critical conditions of extinction agree well with experimental data. At given values of the strain rate and temperature of the fuel stream, the calculated temperature of the oxidizer stream at autoignition is found to be higher than the measured values. In the premixed configuration the strain rate at extinction is measured for various values of the equivalence ratio of the mixture in the premixed-reactant stream, ϕ 1. The value of ϕ 1, at which the calculated extinction strain rate is the highest, is found to be larger than the value of ϕ 1 at which the measured extinction strain rate is the highest. Sensitivity analysis is carried out to test the influence of various elementary reactions on critical conditions of extinction. The structure of a nonpremixed methanol flame is investigated. Concentration profiles of stable species and temperature profiles are measured. The flame structure is calculated using detailed chemistry. The results of numerical calculations agree well with experimental data. [Copyright &y& Elsevier]

Abstract: The effects of unsteady strain on hydrogen (H2) ignition in nonpremixed flows are investigated with both experimental measurements and numerical computations. A mixing layer is established in a counterflow configuration with a fuel stream containing N2–diluted H2 flowing against heated air. A reproducible ignition process is initiated by introducing atomic oxygen into the mixing layer with a pulsed ArF excimer laser, which photodissociates heated O2 from the oxidizer stream. The temporal evolution of OH during ignition is measured by planar laser-induced fluorescence. Following the induction phase, the measured OH mole fraction increases rapidly to a super-equilibrium value that is 60% greater than the OH mole fraction in a steady diffusion flame. The peak OH mole fraction occurs at approximately 6ms after the excimer laser pulse. To study the OH time history under transient strain, the fuel stream is pulsed at a fixed time after the initiation of ignition. The response of the ignition kernel is extremely sensitive to the time delay of the flow transient. The unsteady strain can delay the ignition time or extinguish the kernel. Comparisons between computations and experiments are made for the evolution of OH during autoignition both for steady and unsteady strain. For both steady and unsteady strain, the transient one-dimensional counterflow computations show excellent agreement with the experiment in terms of predicting ignition delays and the rate of OH accumulation during the induction period. The computations also capture the super-equilibrium OH during the transition to the formation of a steady flame, although not to the degree observed experimentally. The computations are further used to understand the influence of unsteady strain on the kernel evolution. It is found that the degree of super-equilibrium OH is sensitive to strain transients applied close to the time of thermal runaway. [Copyright &y& Elsevier]

Abstract: The influence of water vapor on critical conditions of extinction and autoignition of premixed and nonpremixed flames is investigated. The fuels tested are hydrogen (H2) and methane (CH4). Studies on premixed systems are carried out by injecting a premixed reactant stream made up of fuel, oxygen (O2), and nitrogen (N2) from one duct, and an inert-gas stream of N2 from the other duct. Critical conditions of extinction are measured for various amounts of water vapor added to the premixed reactant stream. The ratio of fuel to oxygen is maintained at a constant value, and the amounts of water vapor and nitrogen are so chosen that the adiabatic temperature remains the same. This ensures that the physical influence of water is the same for all cases. Therefore, changes in values for the critical conditions of extinction are attributed to the chemical influence of water vapor. Studies on nonpremixed systems are carried out by injecting a fuel stream made up of fuel and N2 from one duct ,and an oxidizer stream made up of O2 and N2 from the other duct. Critical conditions of extinction are measured with water vapor added to the oxidizer stream. The concentrations of reactants are so chosen that the adiabatic temperature and the flame position stay the same for all cases. Critical conditions of autoignition are measured by preheating the oxidizer stream of the nonpremixed system. Water vapor is added to the oxidizer stream. Numerical calculations are performed using a detailed chemical-kinetic mechanism and compared with measurements. Experimental and numerical studies show that addition of water makes the premixed and nonpremixed flames easier to extinguish and harder to ignite. The chemical influence of water is attributed to its enhanced chaperon efficiency in three body reactions. [Copyright &y& Elsevier]

Abstract: Reduced chemical kinetic mechanisms for methanol combustion were investigated by evaluating ignition delay magnitudes and combustion in a continuously stirred reactor. Unsteady computations were made to study the characteristics of the kinetic mechanisms proposed in the literature and to compare the dependence of various parameters on methanol combustion. All computations were done under isobaric conditions, and, to capture the influence of all the reactions involved in the mechanism, a very small time step was used. Finite-difference methods were used to solve the coupled differential equations. The five-step mechanism developed by C.M. Mueller and N. Peters [in: N. Peters, B. Rogg (Eds.), Reduced Kinetic Mechanisms for Applications in Combustion Systems, Springer-Verlag, New York, 1993, pp. 143–155] for premixed flames and both the five-step mechanism and the four-step mechanisms developed by C.M. Mueller, K. Seshadri, J.Y. Chen [ibid, pp. 284–307] for non-premixed flames were considered. It was found that the Mueller et al. five-step mechanism, with some modifications, best supported the spontaneous ignition and continuous stirred reactor combustion. The results were validated by comparing calculated ignition delays with available experimental data of C.T. Bowman [Combust. Flame 25 (1975) 343–354], and calculated final steady-state concentrations with chemical equilibrium calculations [J.-Y. Chen, Combust. Sci. Technol. 78 (1991) 127]. Initial temperature and concentration and the operating pressure of the system have a major effect on the delay of methanol ignition. The residence time of the continuous stirred reactor affects ignition delay and also changes the transient characteristic of chemical composition of the fuel–vapor mixture. The computations are intended to guide and explain many combustion studies that require a methanol kinetic mechanism. [Copyright &y& Elsevier]