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Showing papers in "Combustion Science and Technology in 1991"


Journal ArticleDOI
TL;DR: In this article, a comprehensive reaction mechanism for the oxidation of carbon monoxide in the presence of hydrogen is described, and model predictions are compared with experimental data over wide ranges of physical conditions.
Abstract: A comprehensive reaction mechanism for the oxidation of carbon monoxide in the presence of hydrogen is described. Model predictions are compared with experimental data over wide ranges of physical conditions. The data, obtained from shock tube experiments and various types of reactor experiments, encompassed a combined temperature range of 823-2870 K, fuel-oxidizer equivalence ratios between 0.0005 and 6.0, C/H ratios between 0005 and 128, and pressures between 0.3 and 2.2 atmospheres, Validation of the reaction mechanism is obtained with reproduction of the experimental results by the model predictions and from sensitivity analysis calculations Results from the latter calculations provided a quantitative measure of the sensitivity of the model predictions to the imposed input parameters over the complete range of experimental parameters noted above.

423 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the validity of the assumed pdf model for the case of inert mixing of two scalars and extended the applicability of the model to multiple scalar mixing.
Abstract: Many engineering calculations of turbulent reacting flows employ the assumed-pdf approach. On the form of the assumed pdf, however, there has been little consensus and the choice has often been ad hoc. The objective of this work is to investigate the validity of the assumed β-pdf model for the case of the inert mixing of two scalars and extend the applicability of the model to multiple scalar mixing. By comparing the β-pdf model favorably with the two-scalar mixing data obtained from direct numerical simulations (DNS), it is demonstrated that the use of this model is justified for all stages of the mixing process in statistically-stationary, isotropic turbulence. It is also shown analytically that during the final stages of mixing the model β-pdf reduces to a Gaussian-pdf, consistent with the observations from experiments and DNS, The suggested multivariate β-pdf model for multiple-scalar mixing relates algebraically, The mean scalar concentrations and the turbulent scalar-energy—to the joint pdf...

261 citations


Journal ArticleDOI
TL;DR: In this article, a comprehensive analysis of multicomponent droplet vaporization at near critical conditions has been carried out, based on complete time-dependent conservation equations, with a full account of variable properties and vapor-liquid interfacial thermodynamics.
Abstract: A comprehensive analysis of multicomponent droplet vaporization at near critical conditions has been carried out. The model is based on complete time-dependent conservation equations, with a full account of variable properties and vapor-liquid interfacial thermodynamics. The influences of various high-pressure phenomena (including ambient gas solubility, property variation, thermodynamic non-ideality, and transient diffusion) on the vaporization mechanism are examined systematically. As a specific example, problems involving n-paraffin fuel droplets in nitrogen gas are studied, Results indicate that the ambient gas pressure has a profound impact on the vaporization process, especially for the conditions under which the droplet reaches its critical stale. Owing to its inability to accurately describe droplet behavior, the conventional low-pressure model may erroneously overpredict the evaporation rate significantly.

148 citations


Journal ArticleDOI
TL;DR: In this article, the large-eddy simulation (LES) model was extended to include a model for premixed fuel combustion, which explicitly uses a subgrid-scale local turbulent flame speed in the governing equation.
Abstract: The large-eddy simulation (LES) model previously developed for cold flow in an axisymmetric ramjet combustor is extended to include a model for premixed fuel combustion. The combustion model explicitly uses a subgrid-scale local turbulent flame speed in the governing equation. This model not only reduces the computational effort, when compared to a model with detailed finite-rate chemical kinetics, but also avoids the potential error in the amount of heat release causd by numerical diffusion. Both stable and unstable combustion are simulated with evidence of decaying and growing pressure oscillations, respectively. The observed flow features on unstable combustion. such as a propagating hooked flame and its phase relation with the pressure oscillation, agree with experimental observations. These simulations clearly establish that the LES model contains the essential physics of combustion instability in a ramjet. Although the simulations show general qualitative features of combustion instability, further ...

148 citations


Journal ArticleDOI
TL;DR: In this article, a triple-flame model is proposed for a dilffusion flame reaching an end at some position in a medium of non-premixed reactants, where the mixing of reactants that takes place ahead of the diffusion flame leads to the formation of a "triple-flame" structure, a structure which consists of a fuel-rich premixed flame, a fuellean premixed fire and a diffusion flame that starts where the two premixed flames meet, and the unique relationship between propagation speed and transverse mixture Traction gradient are computed numerically.
Abstract: A situation in which a dilffusion flame reaches an end at some position in a medium of non-premixed reactants is studied. The mixing of reactants that takes place ahead of the diffusion flame leads to the formation of a “triple-flame”, a structure which consists of a fuel-rich premixed flame, a fuel-lean premixed flame, and a diffusion flame that starts where the two premixed flames meet. An important property of such an end-point is its ability to propagate. The limits of low heat release, unit Lewis number and large Zeldovich number are considered. The structure of the triple-flame and the unique relationship between propagation speed and transverse mixture Traction gradient are computed numerically. For the range of values considered here, the end of the diffusion flame is shown to extend itself at a rate that can be substantially reduced, but that remains positive as the gradient of the mixture fraction is increased.

147 citations


Journal ArticleDOI
TL;DR: In this article, a flame spread map is presented which indicates three distinct regions where different mechanisms control the flame spread process: near-quenching region, very low characteristic relative velocities, a new controlling mechanism for flame spread - oxidizer transport-limited chemical reaction - is proposed.
Abstract: Microgravity tests varying oxygen concentration and forced flow velocity have examined the importance of transport processes on flame spread over very thin solid fuels. Flame spread rates, solid phase temperature profiles and flame appearance for these tests are measured. A flame spread map is presented which indicates three distinct regions where different mechanisms control the flame spread process. In the near-quenching region (very low characteristic relative velocities) a new controlling mechanism for flame spread - oxidizer transport-limited chemical reaction - is proposed. In the near-limit, blowoff region, high opposed flow velocities impose residence time limitations on the flame spread process. A critical characteristic relative velocity line between the two near-limit regions defines conditions which result in maximum flammability both in terms of a peak flame spread rate and minimum oxygen concentration for steady burning. In the third region, away from both near-limit regions, the flame spread behavior, which can accurately be described by a thermal theory, is controlled by gas-phase conduction.

142 citations


Journal ArticleDOI
TL;DR: In this article, a detailed kinetic reaction mechanism was proposed to model the reactions involved in the oxidation of C 1, C 4, C 2, H 2, CO, C02, CH 4 and allene.
Abstract: Methane oxidation in jet-stirred reactor has been investigated at high temperature (900–1300 K) in the pressure range 1–10 atm. Molecular species (H2, CO, C02, CH4. C2,H2, C2H4, C2,H6) concentration profiles were obtained by probe sampling and GC analysis. Methane oxidation was modeled using a detailed kinetic reaction mechanism including the most recent kinetic findings concerning the reactions involved in the oxidation of C1,-C4 hydrocarbons. The proposed mechanism is able to reproduce experimental data obtained in our high-pressure jet stirred reactor and ignition delay times measured in shock tube in the pressure range 1–13 atm, for temperatures extending from 900 to 2000 K and equivalence ratios of 0.1 to 2. It is able to correctly reproduce H and O atoms concentrations measured in shock tube at ≈ 2 atm at 1850–2500 K. The same kinetic mechanism can also be used to model the oxidation of ethylene, ethane, propyne and allene in various conditions.

121 citations


Journal ArticleDOI
TL;DR: The theory of large activation energy asymptotics is used in this article to model the interaction of pressure disturbances characterised by different length scales with conventional flames for medium to short wavelengths.
Abstract: The theory of large activation energy asymptotics is used to model the interaction of pressure disturbances characterised by different length scales with conventional flames. Byconventional flames it is understood that the deflagration Mach number is low and of the order of where 0 is the non dimensional activation energy and s is a positive power arising from the detail of a (global) kinetic scheme. By denoting the paper explores the coupling that exists between pressure disturbances and flames for medium to short wavelengths. that is with N in the range M −l to 1. The work complements that which has mainly been achieved assuming N ≫ M −1 (long wavelengths). As N becomes smaller the coupling from pre-heat and chemical zones becomes more pronounced. This work first deduces jump conditions across the flame zone for N ≈ M −1 with density variations included (up to now these have been ignored). The foundation is then laid for N ≈ 1/(02 M) (i.e. shorter wavelength) coupling and it is shown that a different un...

108 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that the orientation-averaged thermophoretic diffusivity of an aggregate containing N primary particles is usually within 8% of the (αTD)i−value for a single “primary” sphere in the free-molecule regime and within about 21 % in the continuum limit.
Abstract: There is now convincing theoretical and experimental evidence, assembled and discussed here, for the remarkable insensitivity of the orientation-averaged thermophoretic properties of aggregated particles to aggregate size and structure (morphology), as well as the nature of gas/surface scattering. Indeed, theoretical consideration of straight chains, and uniformly “packed” quasi-spherical agglomerates, as well as recent experimental data on soot aggregate transport in/from laminar flames at atmospheric pressure, indicates that the orientation-averaged thermophoretic diffusivity, (αTD)n, of an aggregate containing N primary particles is usually within about 8% of the (αTD)i,-value for a single “primary” sphere in the free-molecule regime and within about 21 % in the continuum limit. Among other things, this implies that, especially in the free-molecule regime, thermophoretically-dominated transport rates can be adequately predicted without a detailed knowledge of the size and morphology (-distribu...

107 citations


Journal ArticleDOI
TL;DR: In this article, the authors defined explosion limit temperatures for near stoichiometric mixtures of CO/H2O/O2 and H2O2 systems at atmospheric pressure without the presence of surfaces.
Abstract: New data on the kinetics of CO/H2O/O2 and H2/O2 mixtures in the temperature range of 852—1138 K and at I atmosphere pressure are reported. The data were obtained from adiabatic. constant pressure flow reactor experiments for variations in the initial carbon, hydrogen, and oxygen contents of the mixture and also for variations in the initial temperature. In particular, the results define explosion limit temperatures for near stoichiometric mixtures of CO/H2O/O2 and H2/O2 systems at atmospheric pressure without the presence of surfaces and also demonstrate the complex relationship between water vapor concentration and the carbon monoxide oxidation rate.

100 citations


Journal ArticleDOI
TL;DR: In this article, the early slages of soot formation on small flames (heights 2cm to 4cm) on a burner 1.6cm in diameter were studied and the more heavily sooting fuels were diluted with nitrogen (up to a nitrogen mole fraction of 90% for acetylene).
Abstract: Diffusion flames of methane, ethane, ethene, acetylene, propane, allene 1-butene, 1,3-butadiene and benzene in coflowing normal atmospheric air were investigated by analyzing samples of malerials collected on fine probes inserted into the flames. To focus attention on early slages of soot formation, small flames (heights 2cm to 4cm) on a burner 1.6cm in diameter were studied, and the more heavily sooting fuels were diluted with nitrogen (up to a nitrogen mole fraction of 90% for acetylene). Radiation-corrected thermocouples were employed to measure temperatures in the regions of earliest soot deposits, and the distances from the point of earliest deposit to that of the first visible soot emissions of radiation were also recorded. Scanning electron micrography (SEM) and X-ray photoelectric spectroscopy (ESCA) were applied lo the samples to investigate changes in soot morphology with increasing axial distance. The results showed the earliest deposils to be highly fuel-specific but later deposits to...

Journal ArticleDOI
TL;DR: The intensity and frequency dependence of sound emitted by a premixed flame are related to the dynamics of the flame surface within the corrugated Hamlet approximation for the turbulent flame brush, its velocity, thickness, and surface area are reviewed.
Abstract: The intensity and frequency dependence of sound emitted by a premixed flame are related to the dynamics of the flame surface within the corrugated Hamlet approximation For the turbulent flame brush, its velocity, thickness, and surface area are reviewed If the turbulence has a Kolmogorov spectrum the acoustic power spectrum varies as ω−5/2

Journal ArticleDOI
TL;DR: In this paper, the relative importance of four chemical pathways to NO formation in laminar, adiabatic, stoichiometric, and premixed CH4-air flames was calculated over a range of pressures from 0.1 to 20 atmospheres.
Abstract: Nitric oxide formation in laminar, adiabatic, stoichiometric. premixed CH4-air flames wascalculated over a range of pressures from 0.1 to 20 atmospheres. The detailed kinetics model (46 speciesand 212 reversible reactions) included four chemical pathways to NO formation, and the relative importanceof these pathways was found to vary with pressure and with position in the flame. In the flamefront,the rates of the Zeldovich mechanism and the N20 intermediate mechanism were increased substantiallyby the presence of superequilibrium °atoms. The Fenimore mechanism also contributed substantialamounts of NO. In the postflame gases, the calculations suggested that most of the NO was formed fromthe Zeldovich mechanism, but the N20 intermediate mechanism became important at high pressures whereit contributed as much as 10% of the postflame NO. Experimental measurements of superequilibriumradical concentrations and ffamefront NO formation are clearly essential to confirm the suggested importanceof superequilibrium (...

Journal ArticleDOI
TL;DR: In this paper, high speed visual observations of free turbulent jet diffusion flames are presented, where computer graphic volume rendering, whereby many frames from a movie sequence are displayed simultaneously, is used to provide a unique view of the flow evolution.
Abstract: High speed visual observations of free turbulent jet diffusion flames are presented. Computer graphic volume rendering, whereby many frames from a movie sequence are displayed simultaneously, is used to provide a unique view of the flow evolution. Two fuels were used, acetylene and ethylene, to span Ihe range from the momentum-driven to the buoyancy-driven regime. The data show that, for all flow regimes, the flames exhibit a large-scale organized motion in the far-field of the jet as manifested by a progression of organized structures up through the length of the visible flame. Occasional pairings of structures are also observed. These structures eventually become the flame tip, which burns out in a quasi-periodic manner. Under no conditions was a random, small-scale breakup and disappearance of the flame tip ever observed. In all cases the speed of the organized structure is seen to be constant and equal to 12 ± 2° of the jet exit velocity. The constancy of speed occurs in spite of the axial de...

Journal ArticleDOI
TL;DR: In this article, experiments on wind-aided firespread across an array of very small-diameter (1.3-4.4mm), discrete fuel elements were carried out in a specially designed wind tunnel.
Abstract: Laboratory-scale experiments on wind-aided firespread across an array of very-small-diameter (1.3-4.4-mm), discrete fuel elements were carried out in a specially designed wind tunnel. The rate of firespread, ty, is inferred from the output of streamwise-disiributed, near-bed-surface thermocouples. The fuel consists of an array of identical, regularly arranged, wooden, toothpick-like Tuel elements, positioned upright in shallow holes drilled in a ceramic substrate. Adoption of this well-defined fuel bed facilitates repetition. Extensive testing suggests that νy ∼ (U/m)1/2 over a wide range of the uniform ambient wind speed U and the fuel-mass distribution m (fuel, per unit planform area of the bed, consumed with firefront passage). The effects on the firespread rate of other parameters investigated and reported include the type of wood species; fuel-element length; fuel-element diameter; fuel-bed width, including streamwise-varyjng width; enhanced moisture content of the substratum and/or fuel; sm...

Journal ArticleDOI
TL;DR: In this article, a detailed mechanism for the oxidation of benzene is presented and used to compute experimentally obtained concentration profiles and ignition delay times over a wide range of equivalence ratio and temperature.
Abstract: A detailed mechanism for the oxidation of benzene is presented and used to compute experimentally obtained concentration profiles and ignition delay times over a wide range of equivalence ratio and temperature. The computed results agree qualitatively with all the experimental trends. Quantitative agreement is obtained with several of the composition profiles and for the temperature dependence of the ignition delay times. There are indications, however, that some important reactions are as yet undiscovered in this mechanism. Recent literature expressions have been used for the rate coefficients of most important reactions, except for some involving phenol. The discrepancy between the phenol pyrolysis rate coefficient used in this work and a recent literature expression remains to be explained.

Journal ArticleDOI
TL;DR: The CHEMACT as mentioned in this paper is a computer code that uses the QRRK treatment of chemical activation reactions to estimate apparent rate constants for the various channels that can result in addition, recombination and insertion reactions.
Abstract: CHEMACT is a computer code that uses the QRRK treatment of chemical activation reactions to estimate apparent rate constants for the various channels that can result in addition, recombination and insertion reactions. The working equations are developed, the necessary input file is described, and sources of required input data are discussed. Three applications of the code to representative examples of reactions relevant to combustion are presented. These include C2H5 + O2, H + C6H5Cl, and CH3 + OH. These reactions illustrate some of the expected behavior with different types of chemical activation and demonstrate the necessity of explicitly accounting for the effects of chemical activation in high temperature gas-phase reactions. Methods by which these results can be included in detailed kinetic mechanisms are discussed.

Journal ArticleDOI
TL;DR: The use of decomposed methanol in spark-ignition engines is discussed in this article, where an active and selective catalytic catalyst for the decomposition reaction is proposed.
Abstract: The use of decomposed methanol, i.e., hydrogen and carbon monoxide, in spark-ignition engines is reviewed. The emphasis is on onboard decomposition and the paper docs not deal with engines fueled by stored hydrogen. The paper concentrates mostly on the experimental work performed using catalytically decomposed methanol, where the methanol is used as a hydrogen carrier. The greatest potential for this type of engine, besides the low energy consumption, lies in the low emissions of nitrogen oxides when running at lean air/fuel ratios. An active and selective catalyst for the decomposition reaction is essential for the utilization of this concept. A literature review on catalyst investigations is included. Neat methanol engines arc not readily started at low ambient temperatures. The possibility of using decomposed methanol as a cold start fuel for methanol engines is reviewed. Decomposed methanol has been proposed as a fuel for gas turbines. A brief review of this subject is also included.

Journal ArticleDOI
TL;DR: In this article, the propagation of a fire front through either strewn debris in a heavily-damaged scenario or through brush-and-grass-type wildlands is studied.
Abstract: The propagation for spread rate lrofwind-aided firespreadacross an array of very-small-diarne-ter, discrete fuel elements is sought, as a step toward the objective of predicting the advance in time of a firefront through either strewn debris in a heavily-blast-damaged scenario or through brush-and-grass-type wildlands. Here, the quasisteady rate (if one exists) is sought for conditions under which there is both (1) a wind whose mean speed U is constant and whose direction is constant, and (2) a horizontal bed with a macroscopically uniform fuel distribution m. More precisely, m is the mass of fuel (per unit planform area of the bed) consumed with firefront passage; here the fuel elements are thin. so that, for conditions under which fire propagates at all, the fuel consumed is identical with the fuel loading initially present. In accord with laboratory-scale experiments in a specially dedicated firetunnel, to be reported separately, analysis suggests thai I'r varies with (U/m)1/2 over a wide (but obviousl...

Journal ArticleDOI
TL;DR: In this article, an experimental study of the surface structure of high Reynolds number turbulent premixed hydrogen/air jet flames is reported, which includes laser light sheet imaging to characterize flame surface properties and condilional laser velocimetry to characterize the turbulence properties of the unburned gas.
Abstract: An experimental study of the surface structure of high Reynolds number turbulent premixed hydrogen/air jet flames is reported. Test conditions involved various values of turbulence intensities relative to the laminar flame speed, and stable/neutral/unstable conditions for preferential diffusion, within the wrinkled and mixing-limited thin flamelet regimes. Measurements included laser light sheet imaging to characterize flame surface properties and condilional laser velocimetry to characterize the turbulence properties of the unburned gas. It was found that flame surface area (and thus the local turbulent burning velocity), flame brush thickness and the fractal dimension of the flame surface progressively increased with distance from the flameholder, with maximum values eventually limited by approach to the flame tip. Additionally, the rate of development of these properties with distance from the flameholder increased as turbulence intensities relative to the laminar flame speed increased. Finall...

Journal ArticleDOI
TL;DR: In this paper, an experimental and a theorietical study of transient radiation properties of strongly radiating turbulent diffusion flames is described, where simultaneous transient measurements of temperatures and volume fractions of soot obtained using a three wavelength emission-absorption probe are reported.
Abstract: An experimental and a theorietical study of transient radiation properties of strongly radiating turbulent diffusion flames is described. Simultaneous transient measurements of temperatures and volume fractions of soot obtained using a three wavelength emission-absorption probe are reported. Soot volume fractions inferred from absorption measurements are significantly higher than those inferred from emission data suggesting the presence oflarge quantities of relatively cold soot within the flames. The results show effects of progressive radiative cooling with distance from the injector exit. Reasonably good predictions of monochromatic intensities were obtained using a new bivariate stochastic analysis.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the mechanisms responsible for driving and damping of pressure oscillations in a laboratory combustor, which contained a turbulent methane/hydrogen/air premixed flame stabilized behind a rearward-facing step.
Abstract: The mechanisms responsible for driving and damping of pressure oscillations in a laboratory combustor have been investigated. The chamber contains a turbulent methane/hydrogen/air premixed flame stabilized behind a rearward-facing step. Shadowgraph cinematography reveals the shedding of large vortices from the step at frequencies of the system acoustic modes. Variations in the fuel equivalence ratio and the mean flow speed result in a wide variety of nonlinear dynamical behavior. Typically, large cycle-to-cycle variations are observed such that energy may be added or subtracted over one cycle of oscillation but zero net energy change occurs many cycles of oscillation. A quantitative version of Rayleigh's Criterion is constructed by using the cross-spectral-density of the measured flame radiation and pressure. The results under one set of operating conditions show that large driving near the flameholder is balanced by equally large damping further downstream. A second set of conditions results in large energy addition to an acoustic mode balanced by attenuation at the mode subharmonic.

Journal ArticleDOI
TL;DR: In this article, detailed numerical calculations are performed to determine the structure of heptane-air diffusion flames, and the results are compared with experimental measurements, using a chemical kinetic mechanism consisting of forty-two elementary reactions involving eighteen species.
Abstract: Detailed numerical calculations are performed to determine the structure of heptane-air diffusion flames, and the results are compared with experimental measurements. The configuration used is the diffusion flame stabilized in the vicinity of a stagnation plane, which is formed by directing an oxidi2ing gas flow onto the vaporizing surface of a pool of heptane. Profiles of the concentration of various stable species and of the temperature have been measured by gas chromatography and by thermocouples. respectively. To evaluate the influence of strain on the structure or the flame, the measurements taken at a fixed composition of the oxidizer stream and at two values of the strain rate were chosen for comparison with the calculated results. The computations were performed using a chemical kinetic mechanism consisting of forty-two elementary reactions involving eighteen species. To simplify the chemical kinetic mechanism, it was assumed that heptane is attacked by radicals to form the heptyl radical...

Journal ArticleDOI
TL;DR: In this article, a theoretical analysis of the active control of combustion instabilities with spatially distributed actuators is presented, based on a generalized wave equation which accomodates all influences of combustion, mean flow, and acoustic oscillation on the system behavior.
Abstract: A theoretical analysis of the active control of combustion instabilities with spatially distributed actuators is presented in this paper. The model simulates unsteady motions in a combustion chamber, with feedback control produced by the injection of secondary fuel into the chamber. The mass flow rate of the injected fuel is modulated by a proportional-plus-integral (PI) controller between the sensor and the fuel injector. The formulation is based on a generalized wave equation which accomodates all influences of combustion, mean flow, and acoustic oscillation on the system behavior. Control actions arising from the distributed combustion of the fuel are modeled by an assembly of point actuators; the power output of each acuator is determined by its position, the local burning characteristics of the fuel, and the time delay with respect to the instant of fuel injection. The analysis and design of both analog and digital PI controllers are presented. The stability of the system has been examined b...

Journal ArticleDOI
TL;DR: In this article, the authors describe a new turbulent flow system for the study of turbulent combustion processes, which produces very high levels of relative turbulence intensities and allow for the generation of an unstabilized, propagating flame.
Abstract: The purpose of this communication is to describe a new turbulent flow system for the study of turbulent combustion processes. The system produces very high levels of relative turbulence intensities and allow for the generation of an unstabilized , propagating flame

Journal ArticleDOI
TL;DR: In this article, the authors investigated methane/air explosions in closed vessels of large L/D with the aim of providing experimental data relevant to the explosion protection of such vessels, using a 76mm diameter tube of an L/d of 21.6 and a 162 mm diameter tube with variable L/Ds (6.2-18.4) to investigate the early stages of these explosions.
Abstract: The majority of experimental data on which current vent design practice is based, has been obtained in compact vessels of length lo diameter ratio (L/D) less than 3. This leads to great uncertainty when designing vents for larger L/D vessels. The need for more information on explosions in long enclosures has been noted in recent critical reviews and official guides. In this work methane/air explosions were investigated in closed vessels of large L/D with the aim of providing experimental data relevant to the explosion protection of such vessels. A 76mm diameter tube of an L/D of 21.6 and a 162 mm diameter tube of variable L/D (6.2-18.4) were employed. High rates of pressure rise, associated with fast flame speeds and significant overpressures, characterized the very early stages of these explosions, indicating the need for fast and effective pressure relief within the initial 10% of the total explosion time, Lower flame speeds and lower rates of pressure rise succeeded the fast initial phase. Sma...

Journal ArticleDOI
TL;DR: In this paper, the response of a premixed laminar flame to small perturbations in pressure, with characteristic times comparable to the natural time scale of the flame, is studied within the coniext of the NEF theory.
Abstract: The response of a premixed laminar flame lo small perturbations in pressure, with characteristic times comparable to the natural time scale of the flame, is studied within the coniext of the Near-Equidif-fusional Flame (NEF) theory. The main emphasis is on flame extinction. It is found that small dynamic variations in pressure, of the order of the reciprocal of the activation energy, have a substantial influence on the burning rate, and may even cause it to drop to zero (extinction). This is in contrast to an earlier study due to Peters and Ludford, which considered only slow variations in pressure, and concluded that substantially larger pressure variations, of the same order as the ambient pressure, would be needed to quench the flame.

Journal ArticleDOI
TL;DR: In this article, a simulation of a two-dimensional temporally developing high speed mixing layer under the influence of a second-order non-equilibrium chemical reaction of the type A + B→ Products + Heat is presented.
Abstract: Results are presented of direct numerical simulations of a two-dimensional temporally developing high speed mixing layer under the influence of a second-order non-equilibrium chemical reaction of the type A + B→ Products + Heat. Simulations arc performed with different magnitudes of the convective Mach number and with different chemical kinetics parameters for the purpose of examining the isolated effects of the compressibility and the heat released by the chemical reaction on the structure or the layer, A full compressible code is developed and utilized, so that the coupling between mixing and chemical reaction is captured in a realistic manner. The results of numerical experiments indicate that at the initial stages of the layers growth, the heat release results in a slight enhanced mixing, whereas at the intermediate and the final stages, it has a reverse influence. The effect of compressibility is the same in all stages of the development: increased compressibility results in a suppressed mix...

Journal ArticleDOI
TL;DR: In this paper, it was shown that at sufficiently low amplitudes of the velocity spatial variation, the effective speed of a passive flame has a global dependence on the underlying flow, while at high amplitudes, the effectiveness of flame speed is determined by the local features of the flow.
Abstract: Flame propagation through a periodic system or time independent eddies is studied. It is shown that at sufficiently low amplitudes of the velocity spatial variation, the effective speed of a passive flame has a global dependence on the underlying flow. However, at high amplitudes, the effective flame speed is determined by the local features of the flow. Simultaneously, the shape of the flame undergoes a transition from a smooth configuration to a cusped one.

Journal ArticleDOI
TL;DR: In this article, the oxidation of H-pentane in a jet-stirred flow reactor in the temperature range 950-1050K at atmospheric pressure for a wide range of fuel-oxygen equivalence ratios (0.2 to 2.0).
Abstract: The oxidation of H-pentane is studied in a jet-stirred flow reactor in the temperature range 950-1050K at atmospheric pressure for a wide range of fuel-oxygen equivalence ratios (0.2 to 2.0). A chemical kinetic reaction mechanism developed from previous studies on smaller hydrocarbons and extended to C5 species is used to reproduce the experimental data. Good agreement between computed and measured concentrations of major chemical species is obtained for the entire range of experimental conditions. The major reaction paths for H-pentane consumption and for the formation of the main products are identified. The same mechanism is used to model experiments performed by other investigators on n-pentane oxidation in reactors or in a shock tube. The experimental concentration profiles of the chemical species in a jet-stirred reactor between 1000 and 1250 K. are closely reproduced, as well as the ignition delays measured behind a reflected shock wave up to 1400K.