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Abstract: The oxidation characteristics of several small methyl and ethyl esters with carbon number less than six were investigated in laminar flames. The kinetics of such fuels are subsets of those of larger alkyl esters that are constituents of practical biodiesel fuels. A total of seven fuels, namely methyl formate, methyl acetate, methyl propionate, methyl butanoate, ethyl formate, ethyl acetate, and ethyl propionate were considered. Experiments were conducted at atmospheric pressure, elevated reactant temperatures, and over a wide range of equivalence ratios. Laminar flame speeds were determined in the counterflow configuration in which flow velocities were measured using particle image velocimetry. Several detailed kinetic models were tested against the experimental data, and insight was provided into the high-temperature combustion kinetics of the aforementioned fuels. Based on comparisons between experimental and computed results it became apparent that the chemistry of alkyl-ester combustion chemistry is evolving and much needs to be done in order to derive improved rate constants for a wide range of elementary steps. [Copyright &y& Elsevier]

Abstract: The oxidation of three model biodiesel fuels, namely methyl butanoate (C5H10O2, CAS No. 623-42-7), methyl crotonate (C5H8O2, CAS No. 623-43-8), and methyl decanoate (C11H22O2, CAS No. 110-42-9) was investigated in laminar premixed and non-premixed flames. The experiments were conducted in the counterflow configuration at atmospheric pressure, for a wide range of equivalence or inert-dilution ratios, and elevated reactant temperatures. Laminar flame speeds and local extinction strain rates were determined by measuring the flow velocities using digital particle image velocimetry. The experimental data were compared against those derived for flames of n-alkanes of similar carbon number, in order to assess the effects of saturation, the length of carbon chain, and the presence of the ester group. Several recent chemical kinetic models were tested against the experimental data, and major differences were identified and assessed. The accuracy of the Lennard–Jones potential parameters assigned to the methyl esters in the transport databases of the different models was evaluated and new values were estimated. Insight was provided into the high-temperature kinetic pathways of methyl esters in flame environments. Additionally, the reduced sooting propensity of methyl ester flames compared to n-alkane flames was investigated computationally. [Copyright &y& Elsevier]

Abstract: The extinction characteristics of non-premixed dimethyl ether and ethanol flames were investigated in order to assess the effect of the molecular structure on the combustion behavior of two otherwise similar fuels. The experiments were conducted in the stagnation flow configuration by counterflowing streams of fuel diluted in nitrogen against pure oxygen. Extinction strain rates were determined at atmospheric pressure and elevated unburned mixture temperature, over a wide range of fuel to nitrogen ratios. Two recent chemical kinetic models were used to simulate the experiments. The Lennard-Jones potential parameters assigned to dimethyl ether in the transport databases of the models were evaluated, and the effect of their uncertainty on the prediction of extinction limits of non-premixed flames was assessed. Sensitivity analyses with respect to reaction rates and binary diffusion coefficients were conducted to provide insight into the controlling physico-chemical processes. Additionally, the high-temperature reaction pathways of dimethyl ether and ethanol oxidation in non-premixed flames were identified and were used to interpret the results. [ABSTRACT FROM AUTHOR]

Abstract: Laminar flame speeds and extinction strain rates of premixed methanol, ethanol, and n-butanol flames were determined experimentally in the counterflow configuration at atmospheric pressure and elevated unburned mixture temperatures. Additional measurements were conducted also to determine the laminar flame speeds of their n-alkane/air counterparts, namely methane, ethane, and n-butane in order to compare the effect of alkane and alcohol molecular structures on high-temperature flame kinetics. For both propagation and extinction experiments the flow velocities were determined using the digital particle image velocimetry method. Laminar flame speeds were derived through a non-linear extrapolation approach based on direct numerical simulations of the experiments. Two recently developed detailed kinetics models of n-butanol oxidation were used to simulate the experiments. The experimental results revealed that laminar flame speeds of ethanol/air and n-butanol/air flames are similar to those of their n-alkane/air counterparts, and that methane/air flames have consistently lower laminar flame speeds than methanol/air flames. The laminar flame speeds of methanol/air flames are considerably higher compared to both ethanol/air and n-butanol/air flames under fuel-rich conditions. Numerical simulations of n-butanol/air freely propagating flames, revealed discrepancies between the two kinetic models regarding the consumption pathways of n-butanol and its intermediates. [ABSTRACT FROM AUTHOR]

Abstract: Laminar flame speeds and extinction strain rates of premixed C5–C12 n-alkane flames were determined at atmospheric pressure and elevated unburned mixture temperatures, over a wide range of equivalence ratios. Experiments were performed in the counterflow configuration and flow velocities were measured using Laser Doppler Velocimetry. The laminar flame speeds were obtained using a non-linear extrapolation technique utilizing numerical simulations of the counterflow experiments with detailed descriptions of chemical kinetics and molecular transport. Compared to linearly extrapolated values, the laminar flame speeds obtained using non-linear extrapolations were found to be 1–4cm/s lower depending on the equivalence ratio. It was determined that the laminar flame speeds of all n-alkane/air mixtures considered in this investigation are similar to each other and sensitive largely to the H2/CO and C1–C4 hydrocarbon kinetics. Additionally, the resistance to extinction decreases as the fuel molecular weight increases. Simulations of the experiments were performed using the recently developed JetSurF 0.2 reaction model consisting of 194 species and 1459 reactions. The laminar flame speeds were predicted with good accuracy for all the n-alkane-air mixtures considered. The experimental extinction strain rates are well predicted by the model for fuel-lean mixtures. For stoichiometric and fuel-rich mixtures, the predicted extinction strain rates are approximately 10% lower than the experimental values. Insights into the physical and chemical processes that control the response of n-alkane flames are provided through detailed sensitivity analyses on both reaction rates and binary diffusion coefficients. [Copyright &y& Elsevier]

Abstract: Blends of n- and iso-alkane components are employed as surrogates for Fischer–Tropsch (F–T) and biomass-derived jet fuels. The composition of the blends has been determined based on data available for two F–T fuel samples obtained from different sources, using a systematic optimization approach. A detailed chemical kinetic mechanism for combustion of the surrogate blends has been assembled. The mechanism has been validated against fundamental experimental data. While drawing initially from other studies in the literature, the mechanism has been improved by enforcing self-consistency of the kinetic and thermodynamic data for the various surrogate-fuel components represented by the mechanism. These improvements have led to more accurate predictions of flame propagation, flame extinction, and NO x emissions. As part of the validation process, simulations were performed for a wide variety of experimental configurations, as well as for a wide range of temperatures and equivalence ratios for fuel/air mixtures. Comparison of the model predictions to the available literature data confirms the accuracy of the mechanism as well as of the approach for selecting the surrogate blends. [ABSTRACT FROM AUTHOR]