Showing papers on "Autoignition temperature published in 1998"

Supercharged Homogeneous Charge Compression Ignition

Abstract: The Homogeneous Charge Compression Ignition (HCCI) is the third alternative for combustion in the reciprocating engine. Here a homogeneous charge is used as in a spark-ignited engine, but the charge is compressed to autoignition as in a diesel. The main difference compared with the Spark Ignition (SI) engine is the lack of flame propagation and hence the independence from turbulence. Compared with the diesel engine, HCCI has a homogeneous charge and hence no problems associated with soot and NOdx formation. Earlier research on HCCI showed high efficiency and very low amounts of NOdx, but HC and CO were higher than in SI mode. It was not possible to achieve high IMEP values with HCCI, the limit being 5 bar. Supercharging is one way to dramatically increase IMEP. The influence of supercharging on HCCI was therefore experimentally investigated. Three different fuels were used during the experiments: iso-octane, ethanol and natural gas. Two different compression ratios were used, 17:1 and 19:1. The inlet pressure conditions were set to give 0, 1, or 2 bar of boost pressure. The highest attainable IMEP was 14 bar using natural gas as fuel at the lower compression ratio. The limit in achieving even higher IMEP was set by the high rate of combustion and a high peak pressure. Numerical calculations of the HCCI process have been performed for natural gas as fuel. The calculated ignition timings agreed well with the experimental findings. The numerical solution is, however, very sensitive to the composition of the natural gas. (Less)

Oxidation of Automotive Primary Reference Fuels at Elevated Pressures

Abstract: Automotive engine knock limits the maximum operating compression ratio and ultimate thermodynamic efficiency of spark-ignition (SI) engines. In compression-ignition (CI) or diesel cycle engines, the premixed burn phase, which occurs shortly after injection, determines the time it takes for autoignition to occur. In order to improve engine efficiency and to recommend more efficient, cleaner-burning alternative fuels, we must understand the chemical kinetic processes that lead to autoignition in both SI and CI engines. These engines burn large molecular-weight blended fuels, a class to which the primary reference fuels (PRF) n -heptane and iso-octane belong. In this study, experiments were performed under enginelike conditions in a high-pressure flow reactor using both the pure PRF fuels and their mixtures in the temperature range 550–880 K and at 12.5 atm pressure. These experiments not only provide information on the reactivity of each fuel but also identify the major intermediate products formed during the oxidation process. A detailed chemical kinetic mechanism is used to simulate these experiments, and comparisons of experimentally measured and model predicted profiles for O 2 , CO, CO 2 , H 2 O and temperature rise are presented. Intermediates identified in the flow reactor are compared with those present in the computations, and the kinetic pathways leading to their formation are discussed. In addition, autoignition delay times measured in a shock tube over the temperature range 690–1220 K and at 40 atm pressure were simulated. Good agreement between experiment and simulation was obtained for both the pure fuels and their mixtures. Finally, quantitative values of major intermediates measured in the exhaust gas of a cooperative fuels research engine operating under motored engine conditions are presented together with those predicted by the detailed model.

An augmented reduced mechanism for methane oxidation with comprehensive global parametric validation

Abstract: Using a computer algorithm for automatic generation of reduced chemistry, an augmented reduced mechanism, consisting of 16 species and 12 lumped reaction steps, has been developed for methane oxidation from GRI-Mech 1.2. Because the present mechanism consists of a larger number of non-steady-state intermediates than the conventional four- or five-step reduced mechanisms, it exhibits good to excellent performance in predicting a wide range of combustion phenomena under extensive thermodynamical parametric variations. Specifically, the phenomena tested include perfectly stirred reactor responses, autoignition and shock-tube ignition delay times, laminar flame propagation speeds, and ignition-extinction limits of counterflowing systems, whereas the thermodynamical parametric variations include those of temperature, pressure, and composition. It is recognized that, with the anticipated increase in computing capability in the foreseeable future, use of the present four- to five-step mechanisms will be unnecessarily limiting. Consequently, it is suggested that efforts should be expended toward development of augmented reduced mechanisms for more comprehensive description of combustion phenomena and for their potential implementation in the computational simulation of complex flows and systems.

Ignition of hydrogen-air mixing layer in turbulent flows

Abstract: Autoignition of a hydrogen-air scalar mixing layer in homogeneous turbulence is studied using direct numerical simulation (DNS). An initial counterflow of unmixed nitrogen-diluted hydrogen and heated air is perturbed by two-dimensional homogeneous turbulence. The temperature of the heated airstream is chosen to be 1100 K, which is substantially higher than the crossover temperature at which the rates of the chain-branching and termination reactions are equal. Three different turbulence intensities are tested in order to assess the effect of the characteristic flow time on the ignition delay. For each condition, a simulation without heat release is also performed. The ignition delay determined with and without heat release is shown to be almost identical up to the point of ignition for all of the turbulence intensities tested, and the predicted ignition delays agree well within a consistent error band. It is also observed that the ignition kernel always occurs where hydrogen is focused, and the peak concentration of HO2 is aligned well with the scalar dissipation rate. The dependence of the ignition delay on turbulence intensity is found to be nonmonotonic. For weak to moderate turbulence, the ignition is facilitated by turbulence via enhanced mixing, while for stronger turbulence, whose timescale is substantially smaller than the ignition delay, the ignition is retarded due to excessive scalar dissipation, and hence diffusive loss, at the ignition location. However, for the wide range of initial turbulence fields studied, the variation in ignition delay due to the corresponding variation in turbulence intensity appears to be quite small.

Second-order conditional moment closure for the autoignition of turbulent flows

Abstract: The conditional moment closure with second-order approximation for the reaction rate and an equation for the conditional fluctuations of the temperature increments before autoignition of a turbulent nonpremixed flow has been developed for one-step chemistry. The explicit incorporation of conditional variances is necessitated due to the temperature fluctuations induced by heat losses from the reaction zone before ignition, as indicated by recent direct numerical simulations (DNS). Predicted ignition times and reaction zone structure are in very good agreement with DNS data and the differences between the first- and second-order closure are discussed.

Experimental investigation of flame-holding capability of hydrogen transverse jet in supersonic cross-flow

Abstract: This paper describes an experimental effort to characterize the flame-holding process of a hydrogen jet injected into a high total enthalpy supersonic cross flow. An expansion tube is used to provide a correct simulation of true flight combustion chemistry, including ignition delay and reaction times. This approach permitted a number of unique experiments involving acceleration of radical-free air to high total enthalpies. The experiments were designed to map the near-field flow characteristics and autoignition process of an underexpanded transverse hydrogen jet injected into flight-Mach number 10 and 13 total enthalpy flow conditions. Flow visualization techniques included planar laser-induced fluorescence (PLIF) of OH and schlieren imaging applied simultaneously. Schlieren images show the shock structure around the jet and the periodically formed coherent structures in the jet-free-stream interface. Overlaid OH-PLIF and schlieren images allow characterization of the autoignition of a hydrogen jet in air cross flow for different jet-to-free-stream momentum flux ratios at both flow conditions. Transverse jet autoignition and flame-holding characteristics observed in both side view and top view images by OH-PLIF reveal differences with previous results in the literature. In the present experiments, the first OH signals are obtained in the recirculation region upstream of the jet exit and in the bow shock region, while in past experiments with similar geometry but lower total enthalpy conditions, no strong OH signal was observed within the first 10 jet diameters. The OH-PLIF results for Mach 10 conditions also show that the OH signal level decreases significantly as the mixture expands around the jet flow field, indicating a partial quenching of the ignition. This indicates that combustion of hydrogen and air in these high total enthalpy conditions is a mixing-limited process. It is evident from the results that improved injection schemes will be required for practical applications in seramjet engines.

Effect of dimethyl ether, NOx, and ethane on CH4 oxidation: High pressure, intermediate-temperature experiments and modeling

Abstract: The effects of small amounts of dimethyl ether (DME), NO, and NO2 on the autoignition and oxidation chemistry of methane, with an without small amounts of ethane present, were experimentally studied in a flow reactor at pressures and temperatures similar to those found in spark- and compression-ignition engines (under autoignition conditions). The reactions were studied at pressures from 10 to 18 atm, temperatures from 800 to 1060 K, and equivalence ratios from 0.5 to 2.0. It is found that 1% DME addition is as effective in stimulating the autoignition and oxidative behavior of methane as 3% C2H6 addition, and that NOx at even ppm levels is more effective than hydrocarbon additives. For the same reaction time and temperatures below 1200 K, addition of small amounts of NOx lowered the temperature at which reaction becomes significant by more than 200 K. Chemical kinetic mechanisms in the literature for the interactions of methane, ethane, and NOx do not predict the reported observations well. The most significant rate-controlling reactions for CH4 autoignition is found to be CH3+HO2=CH3O+OH. Good agreement, with and without NOx perturbations can be obtained by modifying the rate constants of three reactions (CH3+HO2=CH3O+OH; CH3+HO2=CH4+O2: CH2O+HO2=HCO+H2O2) and by adding the reaction CH3+NO2=CH3O+NO to the GRI-Mech v2.11 mechanism. These modifications do not significantly affect predictions for shock tube, flame, and other results used in developing GRI-Mech v2.11. Results strongly suggest that exhaust gas residuals and/or exhaust gas recirculation can have as profound an effect as natural gas contaminants on the apparent octane and cetane behavior.

The effects of pressure, temperature, and concentration on the reactivity of alkanes: Experiments and modeling in a rapid compression machine

Abstract: Experiments in a rapid compression machine have examined the influences of variations in pressure, temperature, and equivalence ratio on the autoignition of n-pentane. Equivalence ratios included values from 0.5 to � 2.0, compressed gas initial temperatures were varied between 675K and 980K, and compresed gas initial pressures varied from 8 to 20 bar. Numerical simulations of the same experiments were carried out using a detailed chemical kinetic reaction mechanism. The results are interpreted in terms of a low temperature oxidation mechanism involving addition of molecular oxygen to alkyl and hydroperoxyalkyl radicals. Idealized calculations are reported which identify the major reaction paths at each temperature. Results indicate that in most cases, the reactive gases experience a two-stage autoigni tion. The first stage follows a low temperature alkylperoxy radical isomerization pathway that is effectively quenched when the temperature reaches a level where dissociation reactions of alkylperoxy and hydroperoxyalkylperoxy radicals are more rapid than the reverse addition steps. The second stage is controlled by the onset of dissociation of hydrogen peroxide. Results also show that in some cases, the first stage ignition takes place during the compression stroke in the rapid compression machine, making the interpretation of the experiments somewhat more complex than generally assumed. At the highest compression temperatures achieved, little or no first stage ignition is observed.

Hydrogen autoignition at pressures above the second explosion limit (0.6-4.0 MPa)

Abstract: The investigation of high-pressure autoignition of combustible mixtures is of importance in providing both practical information in the design of combustion systems and fundamental measurements to verify and develop chemical kinetic models. The autoignition characteristics of hydrogen-oxygen mixtures at low pressures have been explored extensively, whereas few measurements have been made at high pressures. The present measurements extend the range of pressures up to 4 MPa, where few measurements have yet been reported. Using a rapid compression machine equipped with a specially designed piston head, hydrogen autoignition pressure traces were measured at pressures above the second explosion limit (p=0.6–4 MPa, T=950–1050 K). The measured pressure records show a more gradual pressure increase during induction time in this regime than in the low-pressure regime, indicating that the energy release becomes significant at conditions over the second explosion limit. By comparing the measurements and a thermodynamic model which incorporates the heat transfer and energy release, a modified reaction rate constant for H2O2+H=HO2+H2, one of the most important reactions for hydrogen oxidation at high pressure, and the reaction with the largest uncertainty, is suggested in this work as k17=2.3 . 1013exp(−4000/T) cm3/mol-s. The modeled pressure history with the modified reaction rate agrees well with the measured values during the induction period over the range of conditions tested. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 385–406, 1998

Autoignition of n-pentane and 1-pentene: Experimental data and kinetic modeling

Abstract: Autoignitions of n-pentane and 1-pentene are studied by rapid compression between 600 and 900 K at high pressure. Both hydrocarbons show a two-stage ignition and a negative temperature coefficient region (NTC). However, 1-pentene is less reactive. Ignition temperature limit is 50 K higher; cool flames and NTC are weaker and confined to a narrower temperature range. Chemical analyses are performed on the reacting mixture for fuel consumption and cyclic ethers. n-Pentane and 1-pentene give very different distribution patterns for cyclic ethers. 2-Methyltetrahydrofuran dominates the n-pentane pattern, whereas propyloxirane is by far the major cyclic ether formed by 1-pentene. Detailed mechanisms based on a common skeleton scheme are developed and used to simulate the experiments. They are validated for ignition delay times, cool flame intensities, and cyclic ether distributions. Good results are obtained for 1-pentene only if (1) direct addition channels of OH and HO2 to the double bond are included and (2) if a higher rate constant for the decomposition of the hydroperoxyalkyl radicals into cyclic ethers is used when this radical is formed by direct HO2 addition instead of isomerization of alkylperoxy radicals. The sensitivity analysis of the low-temperature scheme for 1-pentene points out that the total ignition delay time is dependent upon the competition between the decomposition channels of hydroperoxyalkyl radical into the branching sequence and into alkenes. The cool flame delay time is less sensitive but depends mainly upon the decomposition rate of unsaturated ketohydroperoxides.

Flameless combustor process heater

Abstract: A process heater is provided utilizing flameless combustion, the process heater having: an oxidation reaction chamber, the oxidation reaction chamber having an inlet for oxidant, an outlet for combustion products, and a flow path between the inlet and the outlet; a fuel conduit capable of transporting a fuel mixture to a plurality of fuel nozzles within the oxidation reaction chamber, each nozzle providing communication from within the fuel conduit to the oxidation chamber, with each nozzle along the flowpath between the inlet and the outlet; a preheater in communication with the oxidation chamber inlet, the preheater capable of increasing the temperature of the oxidant to a temperature resulting in the combined oxidant and fuel from the fuel nozzle closest to the oxidation chamber inlet being greater than the autoignition temperature of the combined oxidant and fuel from the fuel nozzle closest to the oxidation chamber inlet; and a process chamber in a heat exchange relationship to the oxidation reaction chamber wherein the heat transferred from the oxidation section does not causes the temperature of the mixture within the oxidation reaction chamber in the vicinity of each fuel nozzle to decrease below the auto ignition temperature of the combined mixture in the oxidation chamber in the vicinity of that fuel nozzle.

Investigation of flame broadening in turbulent premixed flames in the thin-reaction-zones regime

Abstract: A two-plane two-dimensional Rayleigh thermometry technique is applied to investigate, the three-dimensional flame structure in highly stretched turbulent premixed flames beyond the flamelet regime Experimental data of the preheat zone thickness, σ F , and flame temperature conditioned at the maximum temperature gradient, T o , are reported Broadening of the preheat zone thickness is observed and found to be correlated well with the increase of turbulence intensity This suggests a pure hydrodynamic straining effect in a chemically inert preheat zone in the thin-reaction-zones regime, which is consistent with the rate-ratio asymptotic analysis Although a general decrease of T o is also found with increasing flame stretch, this temperature drop is subject to the nonadiabatic condition in the postflame region Flame extinction would be expected when T o is approaching the ignition temperature ≥900 K

Mechanosynthesis mechanism of TiC powders

Abstract: During ball milling of a powder mixture of elemental titanium and graphite, TiC is synthesised by a combustion reaction. The factors controlling the reaction kinetics have been investigated using X-ray photoelectron spectroscopy and differential thermal analysis. In the incubation period before a combustion reaction, the carbon atoms diffuse along the grain boundaries of Ti, resulting in the mixing of the reactants on a nanometre scale. A transitional bonded state (Ti … C) is formed, reducing the ignition temperature for a combustion reaction. In addition, a minimum adiabatic temperature of 1800 K is necessary for the occurrence of combustion during the mechanosynthesis process.

Distributed injection process and apparatus for producing synthesis gas

Abstract: A novel injector/reactor apparatus and an efficient process for the partial oxidation of light hydrocarbon gases, such as methane, to convert such gases to useful synthesis gas for recovery and/or subsequent hydrocarbon synthesis. Sources comprising a light hydrocarbon gas, such as methane (23), and oxygen or an oxygen-containing gas (22), preheated and pressurized, are injected through an injector means at high velocity into admixture with each other in the desired relative proportions, at a plurality of mixing nozzles which are open to the partial oxidation zone of a reactor and are uniformly-spaced over the face of the injector means, to form a reactant gaseous premix having a pressure at least 3% lower than the lowest upstream pressure of either of the streams of the individual gases. The gaseous premix is injected in a time period which is less than its autoignition time, preferably less than 9 milliseconds, at a velocity between about 25 to 1000 feet/second, into the partial oxidation zone of the reactor. The gas mixture reacts before or simultaneously with the autoignition time delay of the mixture, to reduce the amounts of CO2, H2O and heat produced by the partial oxidation reaction to form a useful syngas which is cooled and recovered.

Evaluation of Ignition Improvers for Methane Autoignition

Abstract: The objective of this study was to estimate the efficiency of methane autoignition promotion by testing different ignition improvers including hydrogen, H2, hydrogen peroxide, H2O2, ozone,O3 and dimethyl ether, DME, (CH3)2O. This was accomplished by computing ignition delays for CH4/O2/Ar or N2 mixtures of various compositions, concentrations of the promoters, pressures and temperatures. Ignition delay times for additive-free mixtures were used for tuning a methane oxidation mechanism consisting of 185 reversible elementary reactions between 32 species. A selection of the reaction rate parameters available in the standard databases was made to optimize the agreement between simulation and experimental results for one particular set of test conditions (reference mixture) by refining the rate parameters of the most sensitive stages revealed by sensitivity analysis. The agreement achieved between model predictions and shock tube experimental data is very good. To investigate the effect of dimethyl ether on m...

Distributed injection catalytic partial oxidation process and apparatus for producing synthesis gas

Abstract: A novel injector/reactor apparatus and an efficient process for the partial oxidation of light hydrocarbon gases, such as methane, to convert such gases to useful synthesis gas for recovery and/or subsequent hydrocarbon synthesis. Sources of a light hydrocarbon gas, such as methane, and oxygen or an oxygen-containing gas are preheated and pressurized and injected through an injector means at high velocity into admixture with each other in the desired relative proportions, at a plurality of mixing nozzles which are open to the catalytic partial oxidation reaction zone and are uniformly spaced over the face of the injector means, to form a reactant gaseous premix having a pressure drop equal to at least about 3 % of the lowest upstream pressure of either of said gases. The gaseous premix is injected in a time period which is less than its autoignition time, preferably less than 9 milliseconds, at a velocity between about 25 to 1000 feet/second, into a catalytic partial oxidation zone so that the gaseous premix reacts in the presence of the fixed catalyst to reduce the amounts of CO2, H2O and heat produced by the partial oxidation reaction, to form a useful syngas which is cooled and recovered.

Effect of liftoff on NOx emission of turbulent jet flame in high-temperature coflowing air

Abstract: NO x emission levels of methane jet flames in high-temperature coflowing air up to 1470 K were measured. Special attention was paid to the relationship between the NO x emission levels and the lifted flames. In the coflowing air with temperature higher than the autoignition temperature, the flame was stabilized in far liftoff distance higher than the attached flame length. The NO x emission levels can be reduced remarkably by the far-lifted flames. When the liftoff height exceeded the attached flame length, the NO x emission levels rapidly decreased to 10% of that of the attached flames under the same coflowing air conditions. The low NO x emission level of the far-lifted flame was due to the decrease in flame temperature of the lean premixture formed at the flame base. The liftoff height changed with the air temperature and the fuel mass fraction of the jet. As the fuel was diluted by nitrogen, the rapid decrease of the NO x emission level by the far-lifted flame occurred in the higher temperature airflow. Also, the liftoff height at the rapid decrease became lower as the fuel mass fraction decreased. Compared with data in the literature, the local equivalence ratio at the critical liftoff base was between 1.0 and 1.3. From these results, the present mechanism of the low NO x emission can be applied in various combustion systems that utilize highly preheated air.

Effects of an ignition-enhancing, diesel-fuel additive on diesel-spray evaporation, mixing, ignition, and combustion

Abstract: The effects of an ignition-enhancing additive, 2-ethylhexyl nitrate, on diesel-spray evaporation, mixing, ignition, and combustion processes were investigated under direct-injection (DI) diesel-engine conditions in a constant-volume combustion vessel. Our objectives were to determine (a) how uniform the ignition enhancement is over a range of gas density and temperature conditions, (b) whether the ignition process of an ignition-enhanced fuel is similar to that of a non-ignition-enhanced fuel with the same cetane number (a measure of ignition quality), and (c) whether diesel-spray evaporation, mixing, or combustion processes are affected by the additive. Optical measurements of the liquid-phase fuel penetration, spray spreading angle, standoff distance from the injector to the flame, and evolution of preignition chemiluminescence were compared for three fuels over a wide range of ambient gas temperatures (700–1100 K) and densities (7.3–45 kg/m 3 ). The fuels considered were blends of three single-component fuels. Two of the fuels were blended to give cetane numbers (CN) of 45 and 55. The third fuel was made by adding 4000 ppm of 2-ethylhexyl nitrate to the CN-45 fuel blend, resulting in a second CN-55 fuel. The results show that the primary effect of the 2-ethylhexyl nitrate additive is to accelerate the preignition radical-pool formation, thus shortening the autoignition period. This effect, however, is not uniform. It is strongest at the lowest gas temperature-density conditions and weakest at the highest temperature-density conditions. The results also show that the development of the ignition process differed for the two CN-55 fuels. The additized-fuel ignition process was similar to the unadditized CN-45 parent fuel but shifted earlier in time. Finally, the effect of this ignition improver on parameters such as flame liftoff, maximum liquid-phase fuel penetration, and spray dispersion were negligible, indicating that neither the high-temperature combustion chemistry nor the mixing-controlled processes are affected by the additive.

Free-piston engine

Abstract: A combustion system which can utilize high compression ratios, short burn durations, and homogeneous fuel/air mixtures in conjunction with low equivalence ratios In particular, a free-piston, two-stroke autoignition internal combustion engine including an electrical generator having a linear alternator with a double-ended free piston that oscillates inside a closed cylinder is provided Fuel and air are introduced in a two-stroke cycle fashion on each end, where the cylinder charge is compressed to the point of autoignition without spark plugs The piston is driven in an oscillating motion as combustion occurs successively on each end This leads to rapid combustion at almost constant volume for any fuel/air equivalence ratio mixture at very high compression ratios The engine is characterized by high thermal efficiency and low NO x emissions The engine is particularly suited for generating electrical current in a hybrid automobile

An experimental and numerical study of kerosine spray evaporation in a premix duct for gas turbine Combustors at high pressure

Abstract: The evaporation and mixing of kerosine emerging from a flat prefilming airblast atomizer was studied experimentally and numerically in an optical accessible, straight rectangular duct at conditions relevant for lean premixed and prevaponzed combustion. Liquid phase properties were measured by Phase-Doppler anemometry and fuel vapour concentrations were determined by an infrared light extinction technique. Computations were based on the Lagrangian particle tracking technique, and captured the spray features sufficiently well with and without taking into account the spray feedback on the gas field. A degree of evaporation of 95% was measured after l00mm for 9 bar air pressure, 750K air temperature and 120m/s air velocity. No autoignition of the fuel occurred. Parametric variations of pressure, temperature and velocity of the air flow, as well as of the initial temperature of the spray and of fuel loading were conducted. A strong influence of the initial fuel temperature on evaporation was found. The air pre...

Turbulent ignition of non-premixed hydrogen by heated counterflowing atmospheric air

Abstract: Experiments were conducted to determine the effects of turbulence on the temperature of a heated air jet required to ignite a counterflowing cold hydrogen/nitrogen jet at atmospheric pressure. At high fuel concentration, the ignition temperature was found to be insesitive to the turbulent intensity. This is consistent with previous results on laminar flows, which found the dominant chemistry govering the second explosion limit, around atmospheric pressure, to be sufficiently rapid when compared with the transport rates for the ignition temperature to be insensitive to the local strain rate of the flow. The present results indicate that this is true whether the strain rate is manifested through the steady bulk strain rate of the flow or the unsteady straining dure to turbulent eddies. At lower hydrogen concentrations, however, ignition was found to be intermittent in that the flow is repeatedly ignited and extinguished over a range of temperatures. With decreasing fuel concentration, increasing turbulent intensity, or increasing bulk strain rate, the difference between the air temperatures when ignition events were first observed and when extinction was totally inhibited is increased, widening the range of intermittent ignition. This behavior can be explained using laminar flow calculations with detailed chemical kinetics and transport. These calculations show that there is only minimal S-curve hysteresis between ignition and extinction for low fuel concentrations. In turbulent flows, there is a fluctuating instantaneous strain rate experienced by the ignition kernel, which is the sum of the bulk and eddy strain rates. Consequently, in these flows the strain rate can alternately traverse beyond the ignition and extinction turning points, causing the flow to intermittently ignite and extinguish.