Critical Conditions Of Autoignition Research Articles

Experimental and computational investigation of the influence of iso-butanol on autoignition of n-decane and n-heptane in non-premixed flows

Experimental and computational investigation is carried out to elucidate the influence of iso-butanol on critical conditions of autoignition of n-decane and n-heptane. The counterflow configuration is employed. In this configuration an axisymmetric stream of air is directed over the surface of an evaporating pool of a liquid fuel. A stagnation plane is established. The temperature of the air is increased until autoignition is achieved in the mixing layer in the vicinity of the stagnation plane. The temperature of the air stream at autoignition, Tig, is measured at various values of the strain rate, which is defined as the axial gradient of the axial component of the flow velocity at the stagnation plane. Fuels tested are n-decane, n-heptane, iso-butanol and various mixtures of n-decane/iso-butanol and n-heptane/iso-butanol. Kinetic modeling is carried out using the recently updated version of the comprehensive CRECK chemical-kinetic mechanism. Critical conditions of autoignition are predicted for all fuels and fuel mixtures and the results are compared with the measurements. Computations show that low-temperature chemistry plays a significant role in promoting autoignition of n-decane and n-heptane, and the influence of low-temperature chemistry decreases with increasing strain rates because there is insufficient residence for the low-temperature reactions to take place. Experimental data and numerical simulations show that addition of even small amounts of iso-butanol to n-decane or n-heptane increases the value of Tig at low strain rates indicating that iso-butanol strongly inhibits the low-temperature chemistry of n-decane and n-heptane. Furthermore, the simulations show that when iso-butanol is present in the liquid fuel, only a small amount of n-decane is available in the gas phase and its low concentration cannot effectively sustain the low temperature degenerate branching oxidation process. Predicted flame structures show that the peak values of mole fraction of ketohydroperoxide are significantly reduced when iso-butanol is added to n-decane indicating that the kinetic pathway to low temperature ignition is blocked. This observation is confirmed from sensitivity analysis.

Critical autoignition conditions for arbitrary reaction order.

In this paper, the theoretical analysis of the critical autoignition conditions for exothermically reacting systems at any value of the reaction order was conducted. The calculated and approximate analytical dependencies for the relationship between the parameters at the ignition limit were obtained. On the basis of the obtained diagrams of critical parameters, the conditions of thermal explosion (TE) degeneration for reactions of arbitrary order were determined. It was established that the existing theory of TE gives the correct estimates of ignition temperatures for a given condition of exothermic reaction realization (heat transfer coefficient, specific surface area, initial concentration). However, the theory gives unsatisfactory predictions for the mentioned critical TE conditions at a given ambient temperature. Moreover, the classical theory cannot be applied in the intermediate case when the effect of reactant consumption is already significant but the reaction still proceeds with all the signs of a TE.

Experimental and computational investigation of autoignition of jet fuels and surrogates in nonpremixed flows at elevated pressures

Experimental and computational investigations are carried out to elucidate the fundamental mechanisms of autoignition of surrogates of jet-fuels at elevated pressures up to 6 bar. The jet-fuels tested are JP-8, Jet-A, and JP-5, and the surrogates tested are the Aachen Surrogate made up of 80 % n-decane and 20 % 1,3,5-trimethylbenzene by mass, Surrogate C made up of 60 % n-dodecane, 20 % methylcyclohexane and 20 % o-xylene by volume, and the 2nd generation Princeton Surrogate made up of 40.4 % n-dodecane, 29.5 % 2,2,4-trimethylpentane, 7.3 % 1,3,5-trimethylbenzene and 22.8 % n-propylbenzene by mole. Using the counterflow configuration, an axisymmetric flow of a gaseous oxidizer stream, made up of a mixture of oxygen and nitrogen, is directed over the surface of an evaporating pool of a liquid fuel. The experiments are conducted at a fixed value of mass fraction of oxygen in the oxidizer stream and at a fixed value of the strain rate. The temperature of the oxidizer stream at autoignition, Tig, is measured as a function of pressure, p. Experimental results show that the critical conditions, of autoignition of the surrogates are close to that of the jet-fuels. Overall the critical conditions of autoignition of Surrogate C agree best with those of the jet-fuels. Computations were performed using skeletal mechanisms constructed from a detailed mechanism. Predictions of the critical conditions of autoignition of the surrogates are found to agree well with measurements. Computations show that low-temperature chemistry plays a significant role in promoting autoignition for all surrogates. The low-temperature chemistry, of the component of the surrogate with the greatest volatility, was found to have the most influence on the critical conditions of autoignition.

Autoignited lifted flames of dimethyl ether in heated coflow air

Autoignited lifted flames of dimethyl ether (DME) in laminar nonpremixed jets with high-temperature coflow air have been studied experimentally. When the initial temperature was elevated to over 860 K, an autoignition occurred without requiring an external ignition source. A planar laser-induced fluorescence (PLIF) technique for formaldehyde (CH2O) visualized qualitatively the zone of low temperature kinetics in a premixed flame. Two flame configurations were investigated; (1) autoignited lifted flames with tribrachial edge having three distinct branches of a lean and a rich premixed flame wings with a trailing diffusion flame and (2) autoignited lifted flames with mild combustion when the fuel was highly diluted. For the autoignited tribrachial edge flames at critical autoignition conditions, exhibiting repetitive extinction and re-ignition phenomena near a blowout condition, the characteristic flow time (liftoff height scaled with jet velocity) was correlated with the square of the ignition delay time of the stoichiometric mixture. The liftoff heights were also correlated as a function of jet velocity times the square of ignition delay time. Formaldehydes were observed between the fuel nozzle and the lifted flame edge, emphasizing a low-temperature kinetics for autoignited lifted flames, while for a non-autoignited lifted flame, formaldehydes were observed near a thin luminous flame zone.For the autoignited lifted flames with mild combustion, especially at a high temperature, a unique non-monotonic liftoff height behavior was observed; decreasing and then increasing liftoff height with jet velocity. This behavior was similar to the binary mixture fuels of CH4/H2 and CO/H2 observed previously. A transient homogeneous autoignition analysis suggested that such decreasing behavior with jet velocity can be attributed to partial oxidation characteristics of DME in producing appreciable amounts of CH4/CO/H2 ahead of the edge flame region.

Determining role of the chain mechanism of hydrazine oxidation and decomposition in kinetically controlled combustion and explosion

It has been inferred from experimental data that the hydrazine oxidation and decomposition reactions occur via a branched-chain and a nonbranched-chain mechanism, respectively. Taking into account the chain nature of these processes helps explain the observed regularities under the critical autoignition conditions, including their “anomalous” character and the combustion-to-explosion transition.

Autoignition of condensed hydrocarbon fuels in non-premixed flows at elevated pressures

Experimental and computational investigation is carried out to elucidate the fundamental mechanism of autoignition of n-heptane, n-decane, and n-dodecane in non-premixed flows at elevated pressures up to 6 bar. The counterflow configuration is employed. In this configuration, an axisymmetric flow of a gaseous oxidizer stream is directed over the surface of an evaporating pool of liquid fuel. The oxidizer stream is a mixture of oxygen and nitrogen. The experiments are conducted at a fixed value of mass fraction of oxygen and at a fixed low value of strain rate. The temperature of the oxidizer stream at autoignition, Tig, is measured as a function of pressure, p. Computations are carried out using skeletal mechanisms constructed from a detailed mechanism and critical conditions of autoignition are predicted. The experimental data and predictions show that, for all fuels tested, Tig decreases with increasing p. At a fixed value of p, Tig for n-dodecane is the lowest, followed by n-decane and n-heptane. This indicates that n-dodecane is the most easily ignited, followed by n-decane and n-heptane. This is in agreement with previous experimental and computational studies at 1 atm, where a similar order of reactivities for these fuels was observed at low strain rates. Flame structures at conditions before and at conditions immediately after autoignition are calculated. A noteworthy finding is that low temperature chemistry is found to play a dominant role in promoting autoignition. The influence of low temperature chemistry is found to increase with increasing pressure.

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.

Autoignited laminar lifted flames of methane, ethylene, ethane, and n-butane jets in coflow air with elevated temperature

The autoignition characteristics of laminar lifted flames of methane, ethylene, ethane, and n-butane fuels have been investigated experimentally in coflow air with elevated temperature over 800 K. The lifted flames were categorized into three regimes depending on the initial temperature and fuel mole fraction: (1) non-autoignited lifted flame, (2) autoignited lifted flame with tribrachial (or triple) edge, and (3) autoignited lifted flame with mild combustion. For the non-autoignited lifted flames at relatively low temperature, the existence of lifted flame depended on the Schmidt number of fuel, such that only the fuels with Sc > 1 exhibited stationary lifted flames. The balance mechanism between the propagation speed of tribrachial flame and local flow velocity stabilized the lifted flames. At relatively high initial temperatures, either autoignited lifted flames having tribrachial edge or autoignited lifted flames with mild combustion existed regardless of the Schmidt number of fuel. The adiabatic ignition delay time played a crucial role for the stabilization of autoignited flames. Especially, heat loss during the ignition process should be accounted for, such that the characteristic convection time, defined by the autoignition height divided by jet velocity was correlated well with the square of the adiabatic ignition delay time for the critical autoignition conditions. The liftoff height was also correlated well with the square of the adiabatic ignition delay time.

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 combustion of JP-8, its surrogates and reference components in laminar nonpremixed flows

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.

Rate-ratio asymptotic analysis of autoignition of n-heptane in laminar nonpremixed flows

A rate-ratio asymptotic analysis is carried out to elucidate the mechanisms of autoignition of n-heptane (C 7H 16) in laminar, nonpremixed flows. It has been previously established that autoignition of n-heptane takes place in three distinct regimes. These regimes are called the low-temperature regime, the intermediate-temperature regime, and the high-temperature regime. The present analysis considers the high-temperature regime. A reduced chemical-kinetic mechanism made up of two global steps is used in the analysis. The reduced mechanism is deduced from a skeletal mechanism made up of 16 elementary reactions. The skeletal mechanism is derived from a short mechanism made up of 30 elementary reactions. The short mechanism is deduced from a detailed mechanism made up of 56 elementary reactions. In the reduced mechanism, the first global step represents a sequence of fast reactions starting from the rate-limiting elementary reaction between n-heptane and HO 2. In this global step C 7H 16 is consumed and hydrogen peroxide (H 2O 2) is formed. The second global step represents a sequence of fast reactions starting from the rate-limiting elementary reaction in which H 2O 2 is consumed and OH is formed. A key aspect of the second global step is that the sequence of fast reactions gives rise to consumption of fuel only without net consumption of H 2O 2. This makes the chemical system autocatalytic. The unsteady flamelet equations are used to predict the onset of autoignition. In the flamelet equations a conserved scalar quantity, Z, is used as the independent variable. On the oxidizer side of the mixing layer Z = 0 , and on the fuel side Z = 1 . The practical case where the temperature of the oxidizer stream, T 2 , is much greater than the temperature of the fuel stream is considered. Therefore autoignition is presumed to take place close to Z = 0 . Balance equations are written for C 7H 16 and H 2O 2. It is postulated that autoignition will take place when the gradient of mass fraction fuel with respect to Z, evaluated at Z = 0 , is zero. The value of T 2 when autoignition takes place is obtained as a function of the strain rate. These critical conditions of autoignition obtained from asymptotic analysis agree well with those calculated using the detailed mechanism and the skeletal mechanism.

The influence of water on extinction and ignition of hydrogen and methane flames

The influence of water vapor on critical conditions of extinction and autoignition of premixed and nonpremixed flames is investigated. The fuels tested are hydrogen (H 2) and methane (CH 4). Studies on premixed systems are carried out by injecting a premixed reactant stream made up of fuel, oxygen (O 2), and nitrogen (N 2) from one duct, and an inert-gas stream of N 2 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 N 2 from one duct ,and an oxidizer stream made up of O 2 and N 2 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.

Non-premixed and premixed extinction and autoignition of C 2H 4, C 2H 6, C 3H 6, and C 3H 8

Experimental studies are conducted on laminar non-premixed and premixed flames stabilized in the counterflow configuration. The fuels tested are ethene (C2H4), ethane (C2H6), propene (C3H6), and propane (C3H8). Studies on non-premixed systems are carried out by injecting a fuel stream made up of fuel and nitrogen (N2) from one duct and an oxidizer stream made up of air and N2 from the other duct. Studies on premixed systems are carried out by injecting a premixed reactant stream made up of fuel, oxygen, and nitrogen from one duct and an inert-gas stream of N2 from the other duct. Critical conditions of extinction are measured by increasing the flow rates of the counterflowing streams until the flame extinguishes. Critical conditions of autoignition are measured by preheating the oxidizer stream of the non-premixed system and the inert-gas stream of the premixed system. Experimental data for autoignition are obtained over a wide range of temperatures of the heated streams. In addition, for premixed systems, experimental data are obtained for a wide range of values of the equivalence ratio including fuel-lean and fuel-rich conditions. Numerical calculations are performed using a detailed chemical-kinetic mechanism and compared with measurements. The present study highlights the influences of non-uniform flow-feld on autoignition in premixed systems that were not available from previous studies using shock tubes. For the premixed system considered here, the changes in the strain rates at extinction with equivalence ratio are found to be similar to previous observations of changes in laminar burning velocities with equivalence ratio. The studies on autoignition in the premixed system show that the temperature of the inert-gas stream at autoignition reaches a minimum for a certain equivalence ratio. For premixed systems, abrupt extinction and autoignition are not observed if the value of the equivalence ratio is less than some critical value.

Critical Autoignition Conditions for Heterogeneous Condensed Systems in the Presence of Phase Transformations

The critical ignition conditions for a single particle which interacts with a melting medium to form an intermetallic compound on its surface were investigated using the model of a quasistationary melting front. For linear and parabolic drag laws, three characteristic regions, which transform with change of Biot's criterion, are distinguished on the diagram of critical parameters. It is shown that for small values of this criterion, the conditions of self‐heating of the particle are identical to the ignition conditions in a medium with constant temperature. For values of Biot's criterion greater than unity, the critical conditions are substantially affected by the parameters determining the melting kinetics. The characteristic features of self‐heating of a single particle can determine the qualitative pattern of self‐heating of the heterogeneous system as a whole.

Critical autoignition conditions for a polydisperse gas suspension of solidfuel particles

Critical autoignition conditions for a polydisperse gas suspension of solidfuel particles

Critical autoignition conditions for a system of particles

1. The critical conditions of autoignition for a system of particles have the same form as the critical conditions for the individual particle, but the place of the heat transfer coefficient α is taken by the coefficient αeff, for which an explicit expression has been found. 2. The cooperative effect of the particles is determined by the parameter A, which is inversely proportional to the square of the mean particle diameter. 3. This simple model, with the effective heat transfer coefficient, explains the qualitative aspects of the measured relation between ignition temperature and concentration [5] for particles of different diameter and, taking into consideration the remarks relating to supercritical heating, the complex (with maximum and minimum) dependence of ignition temperature on particle diameter.