Critical Conditions Of Extinction Research Articles

Rate-Ratio Asymptotic Analysis of Strained Premixed Laminar Methane Flame Under Nonadiabatic Conditions

ABSTRACT Motivated by the pioneering activation-energy asymptotic analysis of strained laminar premixed flames in counterflow by Libby and his coworkers, a rate-ratio asymptotic analysis is carried out to elucidate the structure and predict the critical conditions of extinction of strained premixed methane flames. Steady, axisymmetric, laminar flow of two counterflowing streams: a reactive mixture stream and a product stream toward a stagnation plane is considered. The temperature of the reactive mixture stream is and it is made up of methane (CH4), oxygen (O2) and nitrogen (N2), while the temperature of the product stream is , and it is made up of O2, carbon dioxide, water vapor and N2. The asymptotic flame structure is presumed to be made up of a thin reaction zone where all chemical reactions take place. On one side of the reaction-zone is an inert, preheat zone containing the reactants and on the other side a post-flame zone made up of products. Analysis of the preheat zone gives matching conditions that is required to analyze the structure of the reaction zone. A four-step, reduced mechanism is used to describe the chemical reactions. The reaction zone is presumed to be made up of an inner layer, where CH4 is consumed. The hydrogen (H2) and carbon monoxide that are formed in this layer are consumed in an oxidation layer that is made up of two layers: an H2-oxidation layer and a CO-oxidation layer. The results of the analysis are used to predict the flame location, , flame temperature, , and the speed of the convective flow, , in the reaction zone as a function of the strain-rate, . Classical C-shaped curves were obtained when , and are plotted as a function of and they were used to predict extinction. A key finding of this work is that is proportional to , where is the crossover temperature predicted by the rate-ratio asymptotic analysis. Whether abrupt extinction will take place or not was found to depend on the value of relative to , which is different from the predictions of activation-energy asymptotic analysis where must be compared with the value of the adiabatic temperature. Similar to the analysis of Libby and his coworkers, the rate-ratio asymptotic analysis predicts the existence of “negative flame speeds,” where the convective flow and the diffusive flow of reactants in the reaction zone, are in opposite directions. The predictions of the rate-ratio asymptotic analysis were found to agree with the results of computations with detailed chemistry and previous experimental data.

Experimental and computational investigation of extinction and autoignition of propane and n-heptane in nonpremixed flows

An experimental and computational investigation is carried out to characterize the influence of reactants on critical conditions for extinction and for autoignition of propane and n-heptane in nonpremixed counterflow configurations. Propane or vaporized n-heptane mixed with nitrogen is transported in one stream while the other stream is made up of air mixed with nitrogen. Measurements of the oxidizer stream temperature needed for autoignition are made at fixed values of the strain rate, either with the fuel mass fraction varied at a fixed oxygen mass fraction or with the oxygen mass fraction varied at a fixed fuel mass fraction. Extinction strain rates for propane are measured as a function of the oxygen mass fraction with room-temperature feed streams and the fuel mass fraction fixed and for n-heptane as a function of the fuel mass fraction with the oxygen mass fraction and feed-stream temperatures fixed. Predictions of critical conditions for extinction and autoignition are made employing detailed kinetic mechanisms. Predictions of critical conditions for extinction are in reasonable agreement with measurements, but there are significant discrepancies for autoignition. Measurements show that increasing the mass fraction of either fuel or oxygen increases the overall reactivity thereby reducing the autoignition temperature. The kinetic models predict the increase in reactivity of the mixing layer with increasing mass fraction of fuel but predict very little change in reactivity of the mixing layer with increasing mass fraction of oxygen, thus failing to predict the influence of oxygen on autoignition. It is concluded that there may exist kinetic pathways responsible for this disagreement that are yet to be discovered, and paths that fail to explain the results are identified.

Experimental and computational investigation of the influence of stoichiometric mixture fraction on structure and extinction of laminar, nonpremixed dimethyl ether flames

Experimental and computational investigation is carried out to elucidate the influence of stoichiometric mixture fraction, , on the structure and critical conditions of extinction of nonpremixed dimethyl ether (DME) flames. The stoichiometric mixture fraction represents the location of a thin reaction zone in terms of a conserved scalar quantity. The counterflow configuration is employed, wherein two reactant streams flow towards a stagnation plane. One stream is made up of DME and nitrogen (N) and the other stream is oxygen and N. Previous studies have shown that critical conditions of extinction depend on and the adiabatic temperature . Therefore, the present investigation is carried out with the composition of the reactants in the counterflowing streams so chosen that the adiabatic temperature is the same for different values of . The strain rate at extinction, , is measured for values of up to 0.8. Computations are performed using detailed kinetic mechanisms and critical conditions of extinction and flame structures are predicted. The measurements and predictions show that, with increasing , the strain rate at extinction first decreases and then increases. The predictions agree with measurements for , but significant deviations between measurements and predictions are observed at higher values of . The scalar dissipation rate at extinction, is calculated using measured and predicted values of . With increasing , the measured and predicted values of first increase and then decrease. It is noteworthy that changes in values of with for dimethyl ether flames are similar to those for methane flames, while the changes in values of with are remarkably different. Flame structures are predicted and they are found to be qualitatively similar to those for hydrocarbon fuels.

Rate-ratio asymptotic analysis of the influence of addition of carbon monoxide on the structure and mechanisms of extinction of nonpremixed methane flames with comparison to experiments

Rate-ratio asymptotic (RRA) analysis is carried out to elucidate the influence of carbon monoxide on the structure and critical conditions of extinction of nonpremixed methane flames. Steady, axisymmetric, laminar flow of two counterflowing streams toward a stagnation plane is considered. One stream, called the fuel stream is made up of a mixture of methane (CH4) and nitrogen (N2). The other stream, called the oxidizer stream, is a mixture of oxygen (O2), and N2. Carbon monoxide (CO) is added either to the oxidizer stream or to the fuel stream. Chemical reactions, represented by four global steps, are presumed to take place in a thin reaction zone. To the leading order the reactants, CH4, O2, and CO are completely consumed in the reaction zone. On either side of this thin reaction zone, the flow field is inert. These inert regions represent the outer structure of the flame. The outer structures provide matching conditions required for predicting the structure of the reaction zone. In the reaction zone, chemical reactions are presumed to take place in two layers—the inner layer and the oxidation layer. The scalar dissipation rate at extinction is predicted from results of the asymptotic analysis and compared with previous measurements and computational predictions using detailed chemistry. The predictions of the asymptotic analysis are found to agree well with the experimental data for CO addition to the fuel stream, and for small amounts of CO addition to the oxidizer stream. For large amounts of CO addition to the oxidizer stream, the approximations introduced in the asymptotic analysis become inaccurate. A key finding is that with increasing amounts of CO added to the oxidizer stream the scalar dissipation rate at extinction first increases and then decreases. It is attributed to changes in location of the inner layer within the reaction zone.

Rate-ratio asymptotic analysis of the influence of stoichiometric mixture fraction on structure and extinction of laminar, nonpremixed methane flames with comparison to experiments

Activation energy asymptotic analysis and rate-ratio asymptotic analysis of combustion in laminar, nonpremixed flames are often carried out using conserved scalar quantities as independent variables. One such representation of a conserved scalar quantity is the mixture fraction, ξ, based on thermal diffusivity. These analyses are carried out in the asymptotic limit of large Damköhler number, with chemical reactions presumed to take place in a thin reaction zone that is located at ξ=ξst. The quantity ξst is the stoichiometric mixture fraction. A characteristic diffusion time is given by the reciprocal of the scalar dissipation rate, χ. Previous computational studies have shown that the scalar dissipation rate at extinction depends on ξst and the maximum flame temperature, Tst. Here, a rate-ratio asymptotic analysis is carried out using reduced chemistry to elucidate the influence of ξst on critical conditions of extinction of methane flames. The scalar dissipation rate at extinction was predicted as a function of ξst with the mass fractions of reactants so chosen that the adiabatic flame temperature, Tst, is fixed. The predictions of the analysis show that with increasing values of ξst, the scalar dissipation rate at extinction first increases and then decreases. To test the predictions of the asymptotic analysis, critical conditions of extinction are measured on nonpremixed methane flames stabilized in the counterflow configuration. With increasing values of stoichiometric mixture fraction, the strain rate at extinction was found to increase, and the scalar dissipation rate at extinction was found to first increase and then decrease. The predictions of the asymptotic analysis agreed with experiments. A key outcome of the analysis is that with increasing ξst, the thickness of the regions where oxygen and fuel are consumed first increase and the decrease. This is responsible for the observed non-monotonic changes in the values of the scalar dissipation rate at extinction with changes in ξst.

Effects of Substitution on Counterflow Ignition and Extinction of C3 and C4 Alcohols

Dwindling reserves and inherent uncertainty in the price of conventional fuels necessitates a search for alternative fuels. Alcohols represent a potential source of energy for the future. The structural features of an alcohol fuel have a direct impact on combustion properties. In particular, substitution in alcohols can alter the global combustion reactivity. In this study, experiments and numerical simulations were conducted to investigate the critical conditions of extinction and autoignition of n-propanol, 1-butanol, iso-propanol, and iso-butanol in nonpremixed diffusion flames. Experiments were carried out in the counterflow configuration, while simulations were conducted using a skeletal chemical kinetic model for the C3 and C4 alcohols. The fuel stream consists of the prevaporized fuel diluted with nitrogen, while the oxidizer stream is air. The experimental results show that autoignition temperatures of the tested alcohols increase in the following order: iso-propanol > iso-butanol > 1-butanol ≈ n-propanol. The simulated results for the branched alcohols agree with the experiments, while the autoignition temperature of 1-butanol is slightly higher than that of n-propanol. For extinction, the experiments show that the extinction limits of the tested fuels increase in the following order: n-propanol ≈ 1-butanol > iso-butanol > iso-propanol. The model suggests that the extinction limits of 1-butanol are slightly higher than n-propanol with extinction strain rate of iso-butanol and iso-propanol maintaining the experimentally observed trend. The transport weighted enthalpy (TWE) and radical index (Ri) concepts were utilized to rationalize the observed reactivity trends for these fuels.

Nonpremixed and Premixed Combustion of Mixtures of Producer Gas and Methane

Abstract Experimental and kinetic modeling studies are carried out to characterize nonpremixed and premixed combustion of producer gas and mixtures of producer gas and methane. The producer gas, employed in the study, is made up of 55.00% carbon monoxide, 2.34% hydrogen, 12.72% methane, 5.24% ethene, and 24.7% carbon dioxide by mass. The primary focus is on characterizing the chemical influence of addition of producer gas on combustion of methane. The kinetic modeling studies are carried out employing a detailed chemical-kinetic mechanism, called the San Diego Mechanism, a skeletal mechanism, and a reduced mechanism made up of five global steps. Experiments on nonpremixed combustion are carried out employing the counterflow configuration. Critical conditions of extinction are measured for producer gas, methane, and mixtures of producer gas and methane. They are compared with predictions obtained using the detailed, skeletal, and reduced mechanism. Critical conditions of autoignition are measured for producer gas and compared with the predictions obtained employing the detailed mechanism. Experimental data and predictions show that with increasing amounts of producer gas in the mixture, the flame is more difficult to extinguish. Flame structures show that at a fixed value of the strain rate, leakage of oxygen from the reaction zone decreases with increasing amounts of producer gas in the combustible mixture. This is attributed to enhanced consumption of oxygen in an overall chain branching step that consumes hydrogen. Thus, the increase in the overall reactivity of the combustible mixture is attributed to presence of hydrogen in producer gas. Computations are performed, using the detailed mechanism and the skeletal mechanism, to investigate aspects of premixed combustion of stoichiometric mixtures of producer gas, methane, oxygen, and nitrogen at fixed values of adiabatic temperature. Burning velocities are calculated. They are found to be less sensitive to the amount of producer gas in the mixture. This is qualitatively different from that observed for nonpremixed combustion. It is attributed to complete consumption of all reactants including oxygen and combined influences of H2 and CO in producer gas on overall combustion of methane. A third set of computations was performed on strained premixed flames stabilized in counterflow between a stream of a stoichiometric mixture of producer gas, methane, oxygen, and nitrogen, and a stream of nitrogen at fixed adiabatic temperature. Critical conditions of extinction were obtained. The strain rate at extinction increased with increasing amounts of producer gas. The increase was comparable to that observed for nonpremixed combustion of these fuels.

Rate-Ratio Asymptotic Analysis of the Influence of Addition of Hydrogen on the Structure and Mechanisms of Extinction of Nonpremixed Methane Flames

Rate-ratio asymptotic analysis is carried out to elucidate the influence of hydrogen on the structure and critical conditions for extinction of nonpremixed methane flames. Steady, axisymmetric, laminar flow of two counterflowing streams toward a stagnation plane is considered. One stream, called the fuel stream is made up of a mixture of methane (CH4) and nitrogen (N2). The other stream, called the oxidizer stream, is a mixture of oxygen (O2), and N2. Hydrogen (H2) is added either to the oxidizer stream or to the fuel stream. A reduced mechanism of four global steps is employed in the analysis. Chemical reactions are presumed to take place in a thin reaction zone that is established in the vicinity of the stagnation plane. On either side of this thin reaction zone, the flow field is inert. These inert regions represent the outer structure of the flame. The reactants, CH4, O2, and H2 are completely consumed at the reaction zone. The outer structure is constructed employing a previously developed Burke-Schumann (flame-sheet) formulation. It provides matching conditions required for predicting the structure of the reaction zone. In the reaction zone, chemical reactions are presumed to take place in two layers—the inner layer and the oxidation layer. In the inner layer fuel (methane) is consumed and the intermediate species hydrogen and carbon monoxide are formed. These intermediate species and added hydrogen are oxidized in the oxidation layer to water vapor and carbon dioxide. Critical conditions of extinction were predicted from results of the asymptotic analysis and found to agree well with previous measurements. Addition of hydrogen to methane flames promotes combustion by delaying extinction. An important finding of the asymptotic analysis is that the mechanisms by which hydrogen promotes combustion when it is added to the oxidizer stream is different from that when it is added to the fuel stream.

Experimental investigations of the influence of pressure on critical extinction conditions of laminar nonpremixed flames burning condensed hydrocarbon fuels, jet fuels, and surrogates

Critical conditions of extinction are measured for high molecular weight hydrocarbon fuels, jet fuels and surrogates at pressures up to 0.4MPa. The hydrocarbon fuels tested are n-heptane, cyclohexane, n-octane, iso-octane, and n-decane. Jet fuels tested include JP-8 and Jet-A. The surrogates tested are the Aachen surrogate, consisting of 80% n-decane and 20% 1,3,5-trimethylbenzene by mass, and the 2nd generation POSF 4658 Princeton surrogate consisting of 49.6% n-dodecane, 24.3% iso-octane, 19.8% n-propylbenzene, and 6.3% 1,3,5-trimethylbenzene by mass. The counterflow, condensed-fuel configuration is employed. Air diluted with nitrogen at 298K is injected onto the surface of a pool of heptane. The mass fraction of oxygen in the oxidizer stream is represented by YO2,2. A flame is stabilized in the stagnation point boundary layer that is established above the liquid–gas interface. At a selected value of pressure p, and at a selected value of YO2,2, the flow velocity of the oxidizer stream is increased until extinction takes place. The strain rate at extinction is calculated. The experiment is repeated for a range of pressures. The general ordering of extinction strain rates of hydrocarbon fuels was observed and found to be in general agreement with the predictions of kinetic models and experiments conducted at atmospheric pressure in earlier experiments and computations. An initial linear increase of extinction strain rate with pressure is observed at pressures up to 0.175MPa, followed by a general flattening of the curves up to 0.35MPa. At pressures above 0.35MPa, extinction strain rates of some fuels begin to decrease with increased pressures. These general trends are consistent with results previously measured for n-heptane, n-hexane, and n-decane flames in a similar configuration.

Rate-ratio asymptotic analysis of the structure and mechanisms of extinction of nonpremixed CH4/N2–O2/N2O/N2 flames

Rate-ratio asymptotic analysis is carried out to elucidate the influence of nitrous oxide on the structure and critical conditions for extinction of nonpremixed methane flames. Steady, axisymmetric, laminar flow of two counterflowing streams toward a stagnation plane is considered. One stream is made up of a mixture of methane and nitrogen. The other stream is a mixture of oxygen, nitrous oxide, and nitrogen. A reduced mechanism of five global steps is employed in the analysis. Chemical reactions are presumed to take place in a thin reaction zone that is established in the vicinity of the stagnation plane. On either side of this thin reaction zone, the flow field is inert. These inert regions are called outer zones. Methane and nitrous oxide are completely consumed in the reaction zone, while oxygen is presumed to leak through the reaction zone. In the reaction zone, chemical reactions are presumed to take place in two layers—the inner layer and the oxidation layer. In the inner layer fuel (methane) is consumed and the intermediate species hydrogen and carbon monoxide are formed. These intermediate species are oxidized in the oxidation layer to water vapor and carbon dioxide. Radicals are produced in the oxidation layer from chain-branching reactions that consume hydrogen. Asymptotic analysis gives the scalar dissipation rate at extinction. Critical conditions for extinction predicted by the analysis agree well with previous experimental data. Nitrous oxide is found to have an inhibiting effect on the flame, promoting extinction. The inhibiting effect is attributed to the competition between the net reaction of nitrous oxide with hydrogen to form water vapor and nitrogen and the chain-branching reaction between oxygen and hydrogen that produces radicals.

Effect of pressure on structure and extinction of near-limit hydrogen counterflow diffusion flames

Results of measurements of critical conditions for extinction and of temperature profiles in counterflow diffusion flames are reported. The fuel was a hydrogen–nitrogen mixture with 14mole percent hydrogen, and the oxidizer was air. Pressures ranged from 0.1MPa to 1.5MPa; measurements were made in a facility especially constructed for carrying out counterflow combustion experiments at high pressures. With increasing pressure, the strain rate at extinction first increases and then decreases, in qualitative agreement with predictions, but there are observable quantitative differences. Temperature profiles, obtained employing an R-type thermocouple at a fixed strain rate of 100/s, agree well with predictions, within experimental uncertainty. The results may help to improve knowledge of underlying chemical-kinetic and transport parameters at elevated pressures.

The structure and extinction of nonpremixed methane/nitrous oxide and ethane/nitrous oxide flames

Knowledge of combustion of hydrocarbon fuels with nitrogen-containing oxidizers is a first step in understanding key aspects of combustion of hypergolic and gun propellants. Here an experimental and kinetic-modeling study is carried out to elucidate aspects of nonpremixed combustion of methane (CH4) and nitrous oxide (N2O), and ethane (C2H6) and N2O. Experiments are conducted, at a pressure of 1atm, on flames stabilized between two opposing streams. One stream is a mixture of oxygen (O2), nitrogen (N2), and N2O, and the other a mixture of CH4 and N2 or C2H6 and N2. Critical conditions for extinction are measured. Kinetic-modeling studies are performed with the San Diego Mechanism. Experimental data and results of kinetic-modeling show that N2O inhibits the flame by promoting extinction. Analysis of the flame structure shows that H radicals are produced in the overall chain-branching step 3H2+O2⇌2H2O+2H, in which molecular hydrogen is consumed. Hydrogen is also consumed in the overall step N2O+H2⇌N2+H2O where stable products are formed. Inhibition of the flames by N2O is attributed to competition between these two overall steps.

Kinetic modelling of extinction and autoignition of condensed hydrocarbon fuels in non-premixed flows with comparison to experiment

A detailed chemical–kinetic mechanism is used to predict critical conditions of extinction and autoignition of condensed hydrocarbon fuels in non-premixed flows. The mechanism includes reactions that describe “low temperature chemistry” as well as “high temperature chemistry” for many high molecular weight fuels. The fuels considered here are n-heptane, n-decane, n-dodecane, n-hexadecane, and iso-octane. The kinetic model is validated by comparing its predictions of critical conditions of extinction and autoignition for these fuels with experimental data obtained in a counterflow configuration where a steady laminar flow of an oxidizer is directed over the vaporizing surface of a condensed fuel. The residence time in this configuration is given by the strain rate. The kinetic model predicts that n-heptane is most difficult to extinguish followed by n-decane, n-dodecane, and n-hexadecane. This is in agreement with experimental data. Computations show that the influence of low temperature chemistry on critical conditions of extinction is small. The kinetic model predicts that at low strain rates n-hexadecane is most easy to ignite followed by n-dodecane, n-decane, and n-heptane. At high values of strain rate, n-heptane is more easy to ignite in comparison to n-decane. This is again in agreement with experimental data, including the “cross-over” in relative reactivities of n-heptane and n-decane. Sensitivity analysis shows that at low strain rates autoignition is promoted by low temperature chemistry for all fuels. At high strain rates, autoignition for n-heptane is promoted by high temperature chemistry, whilst low temperature chemistry continues to play a significant role in promoting autoignition for the other straight-chain hydrocarbon fuels. The “cross-over” in relative reactivities of n-heptane and n-decane is attributed to competition between the rates of low temperature chemistry, rates of high temperature chemistry, and rates of molecular transport for these fuels.

Experimental and kinetic modeling study of combustion of JP-8, its surrogates and components in laminar premixed flows

Experimental and kinetic modeling studies are carried out to characterize premixed combustion of jet fuels, their surrogates, and reference components in laminar nonuniform flows. In previous studies, it was established that the Aachen surrogate made up of 80 % n-decane and 20 % trimethylbenzene by weight, and surrogate C made up of 57 % n-dodecane, 21 % methylcyclohexane and 22 % o-xylene by weight, reproduce key aspects of combustion of jet fuels in laminar nonpremixed flows. Here, these surrogates and a jet fuel are tested in premixed, nonuniform flows. The counterflow configuration is employed, and critical conditions of extinction are measured. In addition, the reference components tested are n-heptane, n-decane, n-dodecane, methylcyclohexane, trimethylbenzene, and o-xylene. Measured critical conditions of extinction of the Aachen surrogate and surrogate C are compared with those for the jet fuel. In general the alkanes n-heptane, n-decane, and n-dodecane, and methylcyclohexane are found to be more reactive than the aromatics o-xylene and trimethylbenzene. Flame structure and critical conditions of extinction are predicted for the reference components and the surrogates using a semi-detailed kinetic model. The predicted values are compared with experimental data. Sensitivity analysis shows that the lower reactivity of the aromatic species arises from the formation of resonantly stabilized radicals. These radicals are found to have a scavenging effect. The present study on premixed flows together with previous studies on nonpremixed flows show that the Aachen surrogate and surrogate C reproduce many aspects of premixed and nonpremixed combustion of jet fuels.

Ignition and extinction of low molecular weight esters in nonpremixed flows

An experimental and kinetic modeling study is carried out to characterize combustion of low molecular weight esters in nonpremixed, nonuniform flows. An improved understanding of the combustion characteristics of low molecular weight esters will provide insights on combustion of high molecular weight esters and biodiesel. The fuels tested are methyl butanoate, methyl crotonate, ethyl propionate, biodiesel, and diesel. Two types of configuration – the condensed fuel configuration and the prevaporized fuel configuration – are employed. The condensed fuel configuration is particularly useful for studies on those liquid fuels that have high boiling points, for example biodiesel and diesel, where prevaporization, without thermal breakdown of the fuel, is difficult to achieve. In the condensed fuel configuration, an oxidizer, made up of a mixture of oxygen and nitrogen, flows over the vaporizing surface of a pool of liquid fuel. A stagnation-point boundary layer flow is established over the surface of the liquid pool. The flame is stabilized in the boundary layer. In the prevaporized fuel configuration, the flame is established in the mixing layer formed between two streams. One stream is a mixture of oxygen and nitrogen and the other is a mixture of prevaporized fuel and nitrogen. Critical conditions of extinction and ignition are measured. The results show that the critical conditions of extinction of diesel and biodiesel are nearly the same. Experimental data show that in general flames burning the esters are more difficult to extinguish in comparison to those for biodiesel. At the same value of a characteristic flow time, the ignition temperature for biodiesel is lower than that for diesel. The ignition temperatures for biodiesel are lower than those for the methyl esters tested here. Critical conditions of extinction and ignition for methyl butanoate were calculated using a detailed chemical kinetic mechanism. The results agreed well with the experimental data. The asymptotic structure of a methyl butanoate flame is found to be similar to that for many hydrocarbon flames. This will facilitate analytical modeling, of structures of ester flames, using rate-ratio asymptotic techniques, developed previously for hydrocarbon flames.

Experimental study of combustion of n-decane and JP-10 in non-premixed flows

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 H2,O2,CO2,H2O,CO,CH4, and hydrocarbons from C2 to C6 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.

Critical ignition in rapidly expanding self-similar flows

The generic problem of ignition of a particle undergoing an expansion given by a power law rate of decay behind a decaying shock is addressed in the present study. It is demonstrated, using a one-step Arrhenius irreversible reaction, that a sufficiently strong expansion wave can quench the reaction. The critical conditions for extinction are obtained in closed form in terms of the time scale for the expansion process and the thermochemical properties of the gas, yielding a critical Damkohler number, i.e., the ratio of the expansion time scale to the homogeneous ignition time scale, given by (γ−1)(Ea/RT)−1/n, where n is the power law exponent of the self-similar expansion. The critical ignition criteria, which are valid in the asymptotic limit n(γ−1)(Ea/RT)=O(1), were found in excellent agreement with numerical results. The applicability of the results obtained are discussed for ignition in rapidly expanding flows which occur behind decaying shock waves, as encountered in problems of detonation initiation by a Taylor–Sedov blast wave, and reacting jet startup, and for reactions in steady hypersonic flows around projectiles.

A surrogate fuel for kerosene

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.

Experimental and kinetic modeling study of extinction and ignition of methyl decanoate in laminar non-premixed flows

Methyl decanoate is a large methyl ester that can be used as a surrogate for biodiesel. In this experimental and computational study, the combustion of methyl decanoate was investigated in non-premixed, nonuniform flows. Experiments were performed employing the counterflow configuration with a fuel stream made up of vaporized methyl decanoate and nitrogen, and an oxidizer stream of air. The mass fraction of fuel in the fuel stream was measured as a function of the strain rate at extinction, and critical conditions of ignition were measured in terms of the temperature of the oxidizer stream as a function of the strain rate. A detailed mechanism of 8555 elementary reactions and 3036 species has been developed previously to describe combustion of methyl decanoate. Since it is not possible to use this detailed mechanism to simulate the counterflow flames because the number of species and reactions is too large to employ with current flame codes and computer resources, a skeletal mechanism was deduced from this detailed mechanism using the “directed relation graph” method. This skeletal mechanism has only 713 elementary reactions and 125 species. Critical conditions of extinction and critical conditions of ignition were calculated using this skeletal mechanism and they were found to agree well with experimental data. In general, the methyl decanoate mechanism provides a realistic kinetic tool for simulation of biodiesel fuels.

Experimental and kinetic modeling study of combustion of gasoline, its surrogates and components in laminar non-premixed flows

Experimental and numerical studies are carried out to construct surrogates that can reproduce selected aspects of combustion of gasoline in non premixed flows. Experiments are carried out employing the counterflow configuration. Critical conditions of extinction and autoignition are measured. The fuels tested are n-heptane, iso-octane, methylcyclohexane, toluene, three surrogates made up of these components, called surrogate A, surrogate B, and surrogate C, two commercial gasoline with octane numbers (ON) of 87 and 91, and two mixtures of the primary reference fuels, n-heptane and iso-octane, called PRF 87 and PRF 91. The combustion characteristics of the commercial gasolines, ON 87 and ON 91, are found to be nearly the same. Surrogate A and surrogate C are found to reproduce critical conditions of extinction and autoignition of gasoline: surrogate C is slightly better than surrogate A. Numerical calculations are carried out using a semi-detailed chemical-kinetic mechanism. The calculated values of the critical conditions of extinction and autoignition of the components of the surrogates agree well with experimental data. The octane numbers of the mixtures PRF 87 and PRF 91 are the same as those for the gasoline tested here. Experimental and numerical studies show that the critical conditions of extinction and autoignition for these fuels are not the same as those for gasoline. This confirms the need to include at least aromatic compounds in the surrogate mixtures. The present study shows that the semi-detailed chemical-kinetic mechanism developed here is able to predict key aspects of combustion of gasoline in non premixed flows, although further kinetic work is needed to improve the combustion chemistry of aromatic species, in particular toluene.