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Abstract: Experiments are conducted on n-decane and JP-10 flames, stabilized between two counterflowing streams—fuel stream and oxidizer stream. The fuel stream is a mixture of prevaporized fuel and nitrogen, and the oxidizer stream is air. Critical conditions of extinction are measured for n-decane and JP-10. Concentration profiles of , and hydrocarbons from to are measured. These measurements are made by removing gas samples from the flame using a quartz microprobe and analyzing the samples using a gas chromatograph. Temperature profiles are measured using a thermocouple. Numerical calculations are performed using detailed kinetic models to predict the flame structure and critical conditions of extinction and autoignition. The predicted values of the critical conditions of extinction and autoignition for n-decane agree well with experimental data for extinction and with previously measured autoignition data. For JP-10, for a given mass fraction of fuel in the fuel stream, the predicted strain rates at extinction are higher than the measurements. The predicted autoignition temperatures, however, agree well with previously measured experimental data. The predicted profiles and maximum mass fractions of the major species are found to agree with the measurements. There are, however, differences between the predicted and measured mass fractions of those species that have more than three carbon atoms. In general, the concentrations of saturated hydrocarbons in the n-decane flame are higher than those in the JP-10 flame. [ABSTRACT FROM AUTHOR]

Abstract: Experimental and numerical studies are carried out to develop a surrogate that can reproduce selected aspects of combustion of kerosene. Jet fuels, in particular Jet-A1, Jet-A, and JP-8 are kerosene type fuels. Surrogate fuels are defined as mixtures of few hydrocarbon compounds with combustion characteristics similar to those of commercial fuels. A mixture of n-decane 80% and 1,2,4-trimethylbenzene 20% by weight, called the Aachen surrogate, is selected for consideration as a possible surrogate of kerosene. Experiments are carried out employing the counterflow configuration. The fuels tested are kerosene and the Aachen surrogate. Critical conditions of extinction, autoignition, and volume fraction of soot measured in laminar non premixed flows burning the Aachen surrogate are found to be similar to those in flames burning kerosene. A chemical-kinetic mechanism is developed to describe the combustion of the Aachen surrogate. This mechanism is assembled using previously developed chemical-kinetic mechanisms for the components: n-decane and 1,2,4-trimethylbenzene. Improvements are made to the previously developed chemical-kinetic mechanism for n-decane. The combined mechanisms are validated using experimental data obtained from shock tubes, rapid compression machines, jet stirred reactor, burner stabilized premixed flames, and a freely propagating premixed flame. Numerical calculations are performed using the chemical-kinetic mechanism for the Aachen surrogate. The calculated values of the critical conditions of autoignition and soot volume fraction agree well with experimental data. The present study shows that the chemical-kinetic mechanism for the Aachen surrogate can be employed to predict non premixed combustion of kerosene. [Copyright &y& Elsevier]

Abstract: Rate-ratio asymptotic analysis is carried out to elucidate the structure and mechanisms of extinction of laminar, partially premixed methane flames. A reduced chemical-kinetic mechanism made up of four global steps is used. The counterflow configuration is employed. This configuration considers a flame established between a stream of premixed fuel-rich mixture of methane (CH4), oxygen (O2), and nitrogen (N2) and a stream of fuel-lean mixture of CH4, O2, and N2. The levels of premixing are given by the equivalence ratios ϕ r of the fuel-rich mixture and ϕ l of the fuel-lean mixture. The value of ϕ r depends on the mass fraction of oxygen, , in the fuel-rich mixture, and the value of ϕ l depends on the mass fraction of fuel, Y F,l in the fuel-lean mixture. The mass fraction of the reactants at the boundaries are so chosen that the diffusive flux of reactants entering the reaction zone is the same for all values of ϕ r and ϕ l. The analysis shows that the value of the scalar dissipation rate at extinction increases with increasing Y F,l for , while it decreases with increasing for Y F,l =0. The results of the analysis agree with numerical calculations carried out using a chemical-kinetic mechanism made up of elementary reactions. They also agree with experimental data. A key finding is that at fixed values of the scalar dissipation rate, the leakage of oxygen from the reaction zone decreases with increasing Y F,l. As a consequence the flame temperature increases. This makes the flame more resistant to strain. At fixed values of the scalar dissipation rate, the leakage of oxygen from the reaction zone increases with increasing values of . This makes the flame less resistant to strain. [Copyright &y& Elsevier]

Abstract: Experimental and numerical studies are carried out to construct reliable surrogates that can reproduce aspects of combustion of JP-8 and Jet-A. Surrogate fuels are defined as mixtures of few hydrocarbon compounds with combustion characteristics similar to those of commercial fuels. The combustion characteristics considered here are extinction and autoignition in laminar non premixed flows. The “reference” fuels used as components for the surrogates of jet fuels are n-decane, n-dodecane, methylcyclohexane, toluene, and o-xylene. Three surrogates are constructed by mixing these components in proportions to their chemical types found in jet fuels. Experiments are conducted in the counterflow system. The fuels tested are the components of the surrogates, the surrogates, and the jet fuels. A fuel stream made up of a mixture of fuel vapors and nitrogen is injected into a mixing layer from one duct of a counterflow burner. Air is injected from the other duct into the same mixing layer. The strain rate at extinction is measured as a function of the mass fraction of fuel in the fuel stream. The temperature of the air at autoignition is measured as a function of the strain rate at a fixed value of the mass fraction of fuel in the fuel stream. The measured values of the critical conditions of extinction and autoignition for the surrogates show that they are slightly more reactive than the jet fuels. Numerical calculations are carried out using a semi-detailed chemical-kinetic mechanism. The calculated values of the critical conditions of extinction and autoignition for the reference fuels and for the surrogates are found to agree well with experimental data. Sensitivity analysis is used to highlight key elementary reactions that influence the critical conditions of autoignition of an alkane fuel and an aromatic fuel. [Copyright &y& Elsevier]

Experimental, numerical, and analytical studies are carried out to elucidate the mechanisms of inhibition of nonpremixed methane flames by bromotrifluoromethane (CF 3 Br). Experiments are performed in the counterflow configuration. In this configuration the reactants are injected into a mixing layer from two ducts. A fuel stream made up of methane and nitrogen (N 2 ) is injected from one duct, and an oxidizer stream of oxygen and N 2 is injected from the other duct. The inhibitor CF 3 Br is either added to the oxidizer stream or the fuel stream. A key objective of this study is to compare chemical inhibition of methane flames when CF 3 Br is added to the oxidizer stream with that when this inhibitor is added to the fuel stream. Critical conditions of extinction are measured. The concentrations of reactants at the boundaries are so chosen that the values of stoichiometric mixture, ? st , and adiabatic temperature, T st , are the same. Numerical calculations are performed using a detailed chemical-kinetic mechanism. A reduced chemical-kinetic mechanism made up of eight global steps is derived from this detailed mechanism. Rate-ratio asymptotic analysis is carried out using this reduced mechanism. Experimental data, numerical calculations, and results of asymptotic analysis show that CF 3 Br is more effective in inhibiting methane flames when it is added to the oxidizer stream when compared with its effectiveness when it is added to the fuel stream. The elementary reaction H + Br 2 ? HBr + Br plays a central role in the inhibition when CF 3 Br is added to the oxidizer stream. This reaction, however, has negligible influence on critical conditions of extinction when CF 3 Br is added to the fuel stream. [ABSTRACT FROM AUTHOR]

The diesel community has used one-equation models for ignition delay in engines for decades. The inverse Arrhenius forms characterizing many of these expressions thus lump together liquid fuel evaporation, mixing of the vapour with the charge and the chemical delay. As a result, correlations apply only to the injector, engine and operating conditions tested. Additionally, they typically exhibit energies or temperatures of activation that can be as much as ten times lower than those obtained in well-mixed, controlled systems that isolate and study the chemical delay. Here the latter system is modelled first, using classical analysis techniques. The result is then extended to the relatively straightforward fluid mechanic environment of a laminar counterflow burner to clarify how both chemistry and mixing can be included in a more general one-equation model. This new model is validated with ignition limit data for ethane or ethene with O2/N2 either premixed or unmixed in the counterflow arrangement and liquid diesel fuel in a pool configuration. Since the overall mixing time in this burner is known a priori, the data can be manipulated to yield global chemical ignition delay activation temperatures for these fuels. As expected, the results are equivalent to those measured in shock tubes and combustion tunnels where reactants are premixed before ignition. [ABSTRACT FROM AUTHOR]

A rate-ratio asymptotic analysis of nonpremixed n -heptane (C 7 H 16 ) flames stabilized in the counterflow configuration is carried out. A reduced chemical-kinetic mechanism made up of three global reactions is employed in the analysis. In the first global step, fuel reacts with radicals to form the intermediate compounds hydrogen (H 2 ) and carbon monoxide (CO). The second and third global steps describe oxidation of these intermediate compounds to water vapor (H 2 O) and carbon dioxide (CO 2 ). The number of moles of radicals, α, that react with one mole of n -heptane in the first global step depends on the principal path of oxidation of the fuel in the detailed mechanism from which the reduced mechanism is deduced. The value of α obtained from different detailed chemical-kinetic mechanisms is not the same. In view of this uncertainty, the present study is focused on obtaining an improved understanding of the influence of α on the critical conditions of extinction. The asymptotic flame structure is presumed to comprise outer transport zones and a thin reaction zone. The outer transport zones are inert. Improvements in the present asymptotic analysis over previous asymptotic analyses are that in the transport zones the Lewis number of reactants and products is not presumed to be equal to unity, and heat losses by radiation are included. The Lewis number of any species i is defined as the ratio of thermal diffusivity to the coefficient of diffusion of species i . All chemical reactions are presumed to take place in the reaction zone. The reaction zone is presumed to be made up of an inner layer and an oxidation layer. The structure of the inner layer is obtained by introducing the approximation that only the first global reaction takes place in this layer. The second and third global reactions are presumed to take place in the oxidation layer. Analysis of the outer transport zones and the reaction zone gives the scalar dissipation rate at extinction χ f,q . For a given value of α, radiative extinction is observed when the scalar dissipation rate, χ f , is decreased below a critical value and diffusive extinction is observed when χ f is increased above a critical value. A flame cannot be stabilized for values of α larger than a critical value. The existence of flammable boundaries is a key finding of the present study. [ABSTRACT FROM AUTHOR]