Critical Ignition Conditions Research Articles

Detailed and reduced chemistry for methanol ignition

Simplified chemical-kinetic mechanisms are sought that can provide agreement with measured shock-tube autoignition times and counterflow critical ignition conditions for methanol (CH 3OH) oxidation. Existing detailed chemistry over-predicts measured counterflow ignition temperatures by 100 K or more. It was found that the elementary step CH 3OH + HO 2 → CH 2OH + H 2O 2 most strongly affects the predictions. Increasing the pre-factor in the Arrhenius expression for the rate of this step from different available literature values by a factor ranging from 2 to 13, namely to 8 × 10 13 cm 3/(mol s), within existing uncertainty, produces agreement of predictions with experiment. Using this revised rate, unimportant steps are deleted from the San Diego mechanism to obtain a set of 26 irreversible elementary steps (augmented to 27 by including fuel dissociation to CH 3 + OH for high-temperature shock-tube conditions) that predict ignition nearly as well as the detailed mechanism. In this mechanism, the intermediate species CH 2OH, CH 3O, HCO, H, O, and OH accurately obey steady states, while the intermediates CH 2O, HO 2, H 2O 2, CO, and H 2 do not. The result is a six-step overall reduced mechanism that describes ignition well at the lower temperatures. At higher temperatures, the aforementioned fuel decomposition becomes important, increasing the six-step mechanism to a seven-step mechanism. Expressions for the reaction rates, branching ratios, and steady-state species concentrations in the six-step reduced mechanism are given to facilitate future methanol ignition computations. Higher alcohols, which are less dependent on HO 2 attack in ignition, are indicated to nevertheless possibly benefit from an increase of the rate of the corresponding step.

Auto-ignition of methane–air mixtures flowing along an array of thin catalytic plates

In this paper, the heterogeneous ignition of a methane–air mixture flowing along an infinite array of catalytic parallel plates has been studied by inclusion of gas expansion effects and the finite heat conduction on the plates. The system of equations considers the full compressible Navier–Stokes equations coupled with the energy equations of the plates. The gas expansion effects which arise from temperature changes have been considered. The heterogeneous kinetics considers the adsorption and desorption reactions for both reactants. The limits of large and small longitudinal thermal conductance of the plate material are analyzed and the critical conditions for ignition are obtained in closed form. The governing equations are solved numerically using finite differences. The results show that ignition is more easily produced as the longitudinal wall thermal conductance increases, and the effects of the gas expansion on the catalytic ignition process are rather small due to the large value of the activation energy of the desorption reaction of adsorbed oxygen atoms.

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.

Critical conditions for wood ignition

Critical conditions for wood ignition

The dependence of the rules governing gas-phase combustion on the competition between chain propagation and chain termination reactions

The competition between chain propagation and chain termination reactions was shown to play an important role in gas-phase combustion. Conditions under which this competition to a substantial extent determined the critical ignition conditions and the rate of the process were analyzed. Examples that showed that ignoring the role played by chain propagation, chain termination, and chain branching reactions in branched-chain processes led to conclusions that contradicted experimental data were considered.

Critical conditions for nitrous oxide ignition

The minimum energies of ignition of gaseous nitrous oxide are measured at pressures up to 3 MPa. The minimum pressure at which nitrous oxide burnt out in the reactor completely was 0.5 MPa. It is shown that this pressure depends on the length of the vertically installed cylindrical reactor. At the optimal length, the minimum pressure was as low as 0.3 MPa. The effect of organic lubricants on the ignition energy is discovered. Liquid nitrous oxide failed to be ignited by a flame torch.

Modeling of the catalytic removal of CO and NO in dry combustion gases

Catalytic removal of pollutants in dry combustion gases in a planar stagnation-point flow over a platinum foil is studied using both numerical and analytical tools. The governing equations have been numerically integrated with the Newton technique, and the response curve has been obtained as functions of temperature and the mixture concentrations. Using the appropriate stoichiometry, the additional oxygen needed to reduce the NO and to achieve complete oxidation of CO has been obtained. The asymptotic analysis leads to an algebraic equation for the surface coverage of empty sites as a function of two nondimensional parameters: the mass transfer number, relating the residence time to the chemical time (sort of Damkohler number), and a parameter, which relates the desorption rate to the adsorption rate of carbon monoxide and depends strongly on temperature. Critical conditions of ignition (light-off) and extinction are identified and closed form solutions are obtained for these phenomena. © 2009 American Institute of Chemical Engineers AIChE J, 2010

NUMERICAL DETERMINATION OF CRITICAL CONDITIONS FOR THERMAL IGNITION

AbstractIgnition or thermal explosion in an oxidizing porous body of material can be described by a dimensionless reaction–diffusion equation of the form ∂tu=∇2u+λe−1/u. Here such equations will be formulated in symmetrically shaped bounded regions Ω, effectively reducing the mathematical formulation to that of one dimension. This is critically re-examined from a modern perspective using numerical methods. A computer algorithm is constructed and used to carry out a broad-ranging evaluation of the watershed critical initial temperature conditions for thermal ignition of nonuniform assemblies. It is then shown how the resulting mathematical structure for the ignition threshold curves can be correlated by a hyperbolic conic section with a high degree of accuracy over the full range of positive ambient temperature values. However, this sometimes over-predicts (which is bad) and sometimes under-predicts (which is good) the critical initial condition. The definition of additional dimensionless parameters is found to generate further simplification, leading to a universal correlating form capable of collapsing the entire solution space onto a single line in the plane of the new variables. In addition, this study considers the physically intuitive conjecture that spatial moments of the initial temperature profile ought to possess a direct mathematical link to the critical ignition threshold. As such, the mth-order spatial moment of the critical total energy content integrals is defined, and an empirical result is derived stating that certain orders of this moment should be insensitive to changes in ambient temperature and initial shape profile and may be considered functionally dependent on the dimensionless eigenvalue only, within some quantifiable error band. Spatial moment integrals, based on computed critical threshold conditions, are found to support this conjecture, with the best accuracy obtained for the second-order moments.

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.

Critical and Transition Conditions for Ignition of a Carbon Particles Dust Cloud in an Adiabatic Confined Vessel

Ignition and explosion characteristics of a carbon particles dust cloud in an adiabatic confined vessel have been investigated analytically and numerically in this study. The critical conditions as well as the transition conditions (at which criticality disappears) have been presented in mathematical expressions. Uniform particle temperature has been assumed in this work. The steady-state and unsteady-state solutions for the problem using the well-known definition of criticality has been offered for ignition of carbon particles cloud in diffusion, kinetic, and diffusion-kinetic regimes. It was found that the ignitability characteristics of carbon particles such as critical ignition temperatures and ignition delays are affected by the initial gas (oxidizer) temperature as well as the concentration of carbon particles in a gas. The temperature- time histories of both carbon particles and gas as well as the variation of particle diameter with time from the numerical study are presented and discussed.

Using the Method of Lines to determine critical conditions for thermal ignition

There are many situations in which the critical conditions for thermal ignition cannot be determined analytically. These include cases where the chemistry needs to be properly considered, where the geometry is not just the simplest and where other processes must be included. In these circumstances, numerical (or at least semi-analytical) means are used to determine critical conditions for thermal ignition. Once confronted with a numerical approach to solving a problem, it is necessary to be a little circumspect about the results and seek independent means to corroborate them. For this reason, the present paper reports on the Method of Lines to investigate a recent reactive hotspot problem which has previously been shown to display unexpected behaviour and demonstrates the use of sensitivity analysis to rigourously determine criticality in such a dissipative system.

Influence of Cr(CO) 6 and Mo(CO) 6 on the critical conditions for ignition and the velocities of flame propagation for the chain-branching oxidation of hydrogen and propylene

It has been found that small additions (∼0.1 vol%) of chromium hexacarbonyl and molybdenum hexacarbonyl promote the combustion of a stoichiometric mixture of H 2 + O 2 , resulting in a lowering of the lower limit of initiated ignition and an increase of the visible velocity of flame propagation; as this takes place, inhibition of propylene oxidation by the additives is observed indicating that hydrogen atoms do not play a noticeable role in chain branching in the initiated oxidation of hydrocarbons at room temperature; this indicates also that the kinetic mechanism of inhibition by carbonyls based on termination by the recombination of hydrogen atoms reported in the literature must be revised. Excited Cr atoms have been observed in a H 2 + O 2 flame in the presence of chromium hexacarbonyl.

05/01723 Molecular transport effects on turbulent flamepropagation and structure

Linear bifurcation and numerical techniques are employed to determine critical conditions for ignition in steady, counterflow, nonpremixed hydrogen-air systems, with varying degrees of nitrogen dilution of the fuel, at temperatures larger than the crossover temperature associated with the second explosion limit for hydrogen. Analysis of profiles of the radical pool at ignition reveals that, irrespective of the degree of dilution of the fuel or oxidizer streams, the O-atom steady state fails on the oxidizer side of the mixing layer. Therefore, at least three overall steps, with O and H atoms as the chain-branching species, are necessary to describe the ignition process. A simplified model with variable density, specific heat and transport properties, and with Stefan-Maxwell approximations for the diffusion velocities, is proposed to describe the structure of the H2O2N2 weakly reactive mixing layer. Results of bifurcation analysis with this flow-field model and a three-step reduced chemical-kinetic scheme show excellent agreement with results of numerical integration of the full conservation equations with detailed chemistry for all degrees of dilution of the fuel feed.

Analysis of the Critical Conditions for Ignition of Gas–Particle Mixtures by a Heated Body with Pulsed Energy Supply

The critical conditions for the ignition of solid particles suspended in a gas by a heated body with pulsed energy supply are determined using numerical and approximate methods. It is shownthat in the kinetic (one‐temperature) ignition regime, the critical duration of the thermal pulse is equal to the time of establishment of a zero gradient on the interface between the heaterand the gas–particle mixture. In the diffusive (two‐temperature) ignition regime (small coefficients of heat transfer between the particles and the gas), the critical duration of the thermal pulse is much shorter than the time of establishment of a zero gradient. It is established that the critical duration of the thermal pulse is determined from the condition that the time of complete particle burnout on the interface between the heater and the gas–particle mixture is equal to the time of repeated heating of the gas to the temperature of the transition of the particle oxidation reaction tothe diffusive reaction regime. An approximate method for calculating the critical duration of the thermal pulse for the diffusive ignition regime is proposed. Numerical calculations show that the minimum time of establishment of the high‐temperature combustion regime is reached when the thermal pulse duration is equal to the time of attainment of a zero pressure gradient on the interface between the heater and the gas–particle mixture.

Initiation of a Charge of a Gel-Like Reactive Substance by an Electric Explosion. Part 1. Thermal Processes

Ignition of a reactive substance by a high-temperature discharge cavity is analyzed by an asymptotic method. The effect of heat transfer into the lateral surface of the charge and decrease in the intensity of the radiant flux from the cavity on the moment and critical conditions of ignition is studied. Calculation results are compared with available experimental data.

Critical Conditions of Spark Ignition of a Mixture of Gases (Oxidizer and Fuel) and Reactive Particles

The problem of spark ignition of a gas suspension including a fuel, an oxidizer, and reactive particles is considered using a two-temperature thermodiffusion model of combustion. Dependences of the minimum ignition energy on disperse-phase parameters are obtained. It is shown that the presence of exothermically reacting disperse-phase particles has an ambiguous effect on the minimum spark energy leading to ignition of the gas suspension. The values of the lower and upper concentration limits for ignition of a methane–air mixture with suspended coal dust particles are in good agreement with experimental data. Key words: hybrid gas suspension, spark ignition, critical conditions, concentration limits.

Transient ignition and combustion of diluted hydrogen/air mixtures by a thin catalytic wire

This paper reports on a study of transient ignition and combustion of hydrogen/air mixtures by a heated, thin catalytic wire in a natural convection environment. Modeling of the process is accomplished via a reduced set of heterogeneous kinetic processes which include dissociative adsorption and desorption of both reactants, three fast surface reactions of the Langmiur-Hinshelwood type, and the desorption reaction of the adsorbed product, H2O(s). The overall surface reaction rate is found to be limited by the adsorption rate of molecular oxygen, which depends on the concentration of molecular oxygen close to the surface of the wire and the distribution of empty sites in the catalyst. The analysis allows the determination of the critical conditions for ignition and the ignition delay time as a function of the important physicochemical parameters. The resulting wire temperatures and the critical value of the external heating rate at ignition are computed. The analysis shows how the ignition temperature increases as the external heating rate increases. The configuration dependence of the ignition temperature is taken as an indication that the use of a critical Damkohler number provides a better ignition condition than the ignition temperature concept. A self-sustained combustion regime for strong diluted mixtures is described.

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.

The histeresis of heat and mass exchange and the critical conditions of ignition and extinction of aerosol of carbon particles

The histeresis of heat and mass exchange and the critical conditions of ignition and extinction of aerosol of carbon particles

Effect of longitudinal heat conduction on the catalytic ignition of carbon monoxide in a boundary layer

The catalytic ignition of dry carbon monoxide and air in a boundary layer flow over a palladium plate is studied in this paper. The heterogeneous reaction mechanism is modelled with the dissociative adsorption of the molecular oxygen and the non-dissociative adsorption of CO, together with a surface reaction of the Langmuir-Hinshelwood type and the desorption reaction of the adsorbed product, CO2(s). The critical condition for catalytic ignition, represented by the ignition Damköhler number, has been deduced using high activation energy asymptotics of the desorption kinetics of the most efficiently adsorbed reactant, CO(s). Longitudinal heat conduction along the plate has been considered and its influence on the ignition temperature has been evaluated. This influence is rather weak, indicating that the flat plate boundary layer flow configuration is a robust device to determine the critical conditions for catalytic ignition.