K. Seshadri

Extinction and Autoignition of n-Heptane in Counterflow Configuration

A study is performed to elucidate the mechanisms of extinction and autoignition of n-heptane in strained laminar flows under nonpremixed conditions. A previously developed detailed mechanism made UP of 2540 reversible elementary reactions among 557 species is the starting point for the study. The detailed mechanism was previously used to calculate ignition delay times in homogeneous reactors, and concentration histories of a number of species in plug-flow and jet-stirred reactors. An intermediate mechanism made up of 1282 reversible elementary reactions among 282 species and a short mechanism made up of 770 reversible elementary reactions among 160 species are assembled from this detailed mechanism. Ignition delay times in an isochoric homogeneous reactor calculated using the intermediate and the short mechanism are found to agree well with those calculated using the detailed mechanism. The intermediate and the short mechanism are used to calculate extinction and autoignition of n-heptane in strained laminar flows. Steady laminar flow of two counter flowing Streams toward a stagnation plane is considered. One stream made up of prevaporized n-heptane and nitrogen is injected from the fuel boundary and the other stream made up of air and nitrogen is injected from the oxidizer boundary. Critical conditions of extinction and autoignition given by the strain rate, temperature and concentrations of the reactants at the boundaries, are calculated. The results are found to agree well with experiments. Sensitivity analysis is carried out to evaluate the influence of various elementary reactions on autoignition. At all values of the strain rate investigated here, high temperature chemical processes are found to control autoignition. In general, the influence of low temperature chemistry is found to increase with decreasing strain. A key finding of the present study is that strain has more influence on low temperature chemistry than the temperature of the reactants.

Asymptotic structure and extinction of methaneair diffusion flames

Abstract The asymptotic structure of a counterflow methaneair diffusion flame is analyzed using a three-step chemical kinetic mechanism, I CH 4 +O 2 →CO+H 2 +H 2 O, II CO+H 2 O⇄CO 2 +H 2 , III O 2 +2H 2 →2H 2 O , which was deduced in a systematic way through steady state and partial equilibrium assumptions from a detailed chemical kinetic mechanism for oxidation of methane. The rates for the three steps are related to the rates of elementary reactions. The outer structure of the diffusion flame is the classical Burke-Schumann structure governed by the overall one-step reaction CH4 + 2O2 → CO2 + 2H2O, with the flame sheet positioned at Z= Zst, where Z is the mixture fraction used as the independent variable in the analysis. The inner structure consists of a thin H2CO oxidation layer of thickness O(ϵ) toward the lean side, a thin nonequilibrium layer for the water gas shift reaction of thickness O(ν), and a thin fuel consumption layer of thickness O(δ) toward the rich side. These layers result, respectively, in the limit of large values for the Damkohler number characterizing the rate of reaction III, II, and I, while the ratios of activation temperature to gas temperature for the three reactions are assumed to be of order unity. We also find that ϵ > ν > δ. The results of the asymptotic analysis yield values of the temperature and the main species at the fuel consumption layer as a function of the scalar dissipation rate χst. We therefore obtain the upper branch and the turning of the classical S-shaped curve where the maximum flame temperature is plotted as a function of χst−1. The scalar dissipation rate at quenching χq is derived from the S-shaped plot and its relation to the laminar burning velocity is discussed. A comparison of the diffusion flame structure with that of a premixed flame shows that the rich part of the diffusion flame corresponds to the upstream part of the premixed flame while its lean part corresponds to the downstream part. First the kinetic scheme is based on the most important (principal) reactions to derive the basic structure. When a number of additional elementary chemical reactions are added the results of the asymptotic analysis are found to be in very good agreement with previous numerical calculations that used a complete kinetic mechanism, as well as with experiments.

Temperature cross-over and non-thermal runaway at two-stage ignition of n-heptane

Abstract To calculate ignition delay times a skeletal 56-step mechanism for n-heptane is further reduced to a short 30-step mechanism containing two isomers of the n-heptyl-redical and reactions describing both the high temperature and the low temperature chemistry. This mechanism reproduces ignition delay times at various pressures and temperatures reasonably well. Steady state assumptions for many of the intermediate species are introduced to derive separately two global mechanisms for the low temperature regime as well as for the intermediate and high temperature regime. In those formulations the OH radical is depleted by fast reactions with the fuel, as long as fuel is present. Its steady state relation shows that the OH concentration would blow up as soon as the fuel is depleted. Therefore the depletion of the fuel is used as a suitable criterion for ignition. In the intermediate temperature regime the first stage ignition is related to a change from chain-branching to chain-breaking as the temperature crosses a certain threshold. The chain branching reactions result in a build-up of ketohydroperoxides which dissociate to produce OH radicals. This is associated with a slight temperature rise which leads to a crossing of the threshold temperature with the consequence that the production of OH radicals by ketohydroperoxides suddenly ceases. The subsequent second stage is driven by the much slower production of OH radicals owing to the dissociation of hydrogen peroxide. The OH radicals react with the fuel at nearly constant temperature until the latter is fully depleted. In all three regimes analytical solutions for the ignition delay time are presented. The reduced 4-step mechanism of the low temperature regime leads with the assumption of constant temperature to linear differential equations, which are solved. The calculated ignition delay times at fuel depletion compares well with those of the 30-step mechanism. The analysis for the intermediate temperature regime starts from a 4-step subset of a 9-step reduced mechanism. It contains the cross-over dynamics in form of a temperature dependent stoichiometric coefficient which is analysed mathematically. The resulting closed form solutions describe the first stage ignition, the temperature cross-over and the second stage ignition. They also identify the rate determining reactions and quantify the influence of their rates on the first and the second ignition times. The high temperature regime is governed by a three-step mechanism leading to a nonlinear problem which is solved by asymptotic analysis. While the dissociation reaction of the ketohydroperoxide dominates the low temperature regime and the first stage ignition of the intermediate temperature regime, the hydrogen peroxide dissociation takes this role for the second stage of the intermediate and in the high temperature regime. The overall activation energy of the ignition delay time in the low temperature regime is the mean of the activation energies of two reactions only. The overall activation energy of the ignition delay time in the high temperature regime is shown to be related to the activation energies of only three but different rate determining reactions.

A comparison between numerical calculations and experimental measurements of the structure of a counterflow diffusion flame burning diluted methane in diluted air

Results of a theoretical and experimental study of the structure of a counterflow diffusion flame burning diluted methane in diluted air are reported. Concentration profiles of the stable species were measured using gas sampling techniques with quartz microprobes. The samples were analyzed with a gas chromatograph. Temperature profiles were measured using coated thermocouples. Numerical calculations of the structure of the flame were performed with an adaptive nonlinear boundary value method at conditions identical to those used in the experiment. The results are compared using both the physical coordinate and the mixture fraction as the independent variable. Excellent agreement is obtained for concentration profiles of CH4, O2, N2, CO2 and H2O and for the peak value of the temperature. The complete temperature profile and the H2 and CO profiles are not in as good agreement and the differences are attributed to the neglect of C2 chemistry in the numerical calculations.

Extinction of diffusion flames burning diluted methane and diluted propane in diluted air

Abstract A theoretical and experimental investigation of the extinction limits of counterflow diffusion flames burning methane and propane is outlined. A diffusion flame is stabilized between counterflowing streams of a fuel diluted with nitrogen and air diluted with nitrogen. Extinction limits for such flames were measured over a wide parametric range. Results for methane and propane were found to be in approximate agreement with previous measurements. The experimental results are interpreted by use of activation energy asymptotic theories developed previously. The gas-phase chemical reaction is approximated as a one step, irreversible process with a large value for the ratio of the activation energy characterizing the chemical reaction to the thermal energy in the flame. Equilibrium dissociation of products is neglected. The theoretical predictions are compared with experimental results, and the overall chemical kinetic rate parameters characterizing the gas-phase oxidation of methane and propane in a diffusion flame are deduced. The overall chemical kinetic rate parameters deduced by use of this procedure are valid only at flame temperatures where equilibrium dissociation is negligible. The scalar dissipation rate at extinction is predicted over a wide range.

A Comparison Between Numerical Calculations and Experimental Measurements of the Structure of a Counterflow Methane-Air Diffusion Flame

Abstract Results of a theoretical experimental study of the structure of a methane-air counterflow diffusion flame are reported. Concentration profiles of the stable species were measured using gas sampling techniques with quartz microprobes. The samples were analyzed with a gas chromatograph. Temperature profiles were measured using coated thermocouples. Numerical calculations including C2 chemistry were performed with an adaptive nonlinear boundary value solver at conditions identical to those used in the experiment. The results are compared using both the physical coordinate and the mixture fraction as the independent variable. Excellent agreement is obtained for concentration profiles of CH4, O2, N2, CO2, H2O, H2, CO, C2H2, C2H4, AND C2H6, for the peak value of the temperature and for flame standoff distances.

The inner structure of methaneair flames

The inner structure of a methaneair premixed flame is analyzed using a reduced four-step chemical kinetic mechanism ICO + H2O ⇌ CO2 + H2 IICH4 + 2H + H2O → CO + 4H2 IIIH + H + M → H2 + M IVO2 + 3H2 ⇌ 2H2O + 2H The rates of these four steps are related to the rates of elementary reactions appearing in the C1-chain mechanism for oxidation of methane. The inner layer is thin with reactions I–IV occurring in this layer, and is embedded between a chemically inert upstream layer and a broader (but asymptotically thin) downstream layer where reactions II, III, and IV occur and H2 and CO are oxidized. The analysis reported here extends a previous analysis by Peters and Williams of the structure of premixed methaneair flames, where a reduced three-step chemical kinetic mechanism was used. In the equations describing the structure of the inner layer, a parameter ω appears, which represents the ratio of the thickness of the fuel consumption layer to the thickness of the radical consumption layer of the previous analysis by Peters and Williams. Analytical solutions for the burning velocity eigenvalue L are obtained in the limit ω → 0 and ω → ∞, and by use of numerical integration, an approximation for L is obtained as a function of ω, which includes limiting expressions for ω → 0 and ω → ∞. The expression for L contains a number of parameters, which represent the influence of a number of elementary chemical reactions. In particular, a parameter defined as μ in this analysis is found to have a significant influence on the value of L, and consequently on the burning velocity, and the influence of this quantity increases with increasing pressure. The parameter μ represents the influence of the backward steps of the reactions CH4 + H ⇌ CH3 + H2 and CH4 + OH ⇌ CH3 + H2O. Using the results of the analysis, the burning velocity was calculated for a stoichiometric methaneair flame for values of the pressure p between 1 atm and 80 atm. At p = 1 atm, the calculated burning velocity was 35 cm/s in reasonable agreement with experimental results. The burning velocity decreased with increasing pressure, again in agreement with experimental measurements.

Asymptotic theory of diffusion-flame extinction in the stagnation-point boundary layer☆

Abstract An analysis is developed for predicting extinction of the diffusion flame that is established when an oxidizing gas flows about the nose of a vaporizing fuel body. Use is made of the limit of a large ratio of the activation energy to the thermal energy at the flame for the overall combustion process, since this limit encompasses all cases of practical interest. By revealing a correspondence with the asymptotic flame structure of a counterflow diffusion flame analyzed earlier, the theory makes available explicit formulas, in term of a Damkohler number, for study of gas-phase extinction in the present geometry. From these results a simplified but reasonably accurate method is developed for obtaining, from experimental data on extinction, kinetic information concerning the overall oxidation process occurring in the vicinity of extinction. Curves calculated from a parametric study are presented to facilitate application of the technique, and the procedure is illustrated for methanol burning in oxygennitrogen mixtures.

The structure of premixed particle-cloud flames

Abstract The structure of premixed flames propagating in combustible systems, containing uniformly distributed volatile fuel particles, in an oxidizing gas mixture, is analyzed. It is presumed that the fuel particles vaporize first to yield a gaseous fuel of known chemical structure, which is subsequently oxidized in the gas phase. The analysis is performed in the asymptotic limit, where the value of the characteristic Zeldovich number, based on the gas-phase oxidation of the gaseous fuel is large, and for values of φu ≥ 1.0, where φu is the equivalence ratio based on the fuel available in the fuel particles. The structure of the flame is presumed to consist of a preheat vaporization zone where the rate of the gas-phase chemical reaction is small, a reaction zone where convection and the rate of vaporization of the fuel particles are small and a convection zone where diffusive terms in the conservation equations are small. For given values φu the analysis yields results for the burning velocity and φg, where φg is the effective equivalence ratio in the reaction zone. The analysis shows that even though φu ≥ 1.0, for certain cases the calculated value of φg is less than unity. This prediction is in agreement with experimental observations.

Extinction of nonpremixed flames with halogenated fire suppressants

Abstract An experimental, analytical, and numerical study was performed to elucidate the influence of eleven gaseous agents, considered to be substitutes for CF3Br, on the structure and critical conditions of extinction of diffusion flames burning liquid hydrocarbon fuels. The effectiveness of these agents in quenching flames was compared to those of CF3Br and an inert diluent such as nitrogen. Experiments were performed on diffusion flames stabilized in the counterflowing as well as in the coflowing configuration. The fuels tested were heptane in the counterflowing configuration, and heptane, the jet fuels JP-8, and JP-5, and hydraulic fluids (military specifications 5606 and 83282) in the coflowing configuration. The oxidizing gas was a mixture of air and the agent. On a mass and mole basis CF3Br was found to be most effective in quenching the flames and the mass-based effectiveness of the other eleven agents was found to be nearly the same as that of nitrogen. Experimental results were interpreted using one-step, activation-energy asymptotic theories and the results were used to provide a rough indication of the thermal and chemical influence of these agents on the flame structure. To understand in some detail the influence of CF3Br on the structure and mechanisms of extinction of the flame, numerical calculations using detailed chemistry were performed. The calculated structure of counterflow heptane-air diffusion flames inhibited with CF3Br was found to consist of three distinct zones including a CF3Br consumption zone which appears to act as a sink for radicals. The calculated values of the critical conditions of extinction of counterflow heptane-air diffusion flames inhibited with CF3Br were found to agree fairly well with measurements. The study suggests the need for refinement of the inhibition chemistry.