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The interaction of non-premixed flamelets with acoustic pressure waves of large characteristic wavelength, central to the development of acoustic instabilities in liquid-propellant rocket engines, is investigated by evaluating the periodic response of a counterflow diffusion flame subject to a harmonic pressure variation. An irreversible step with an Arrhenius rate having large activation energy is used to model the exothermic reaction between the fuel and the oxidizer. The interactions of the chemistry with the prescribed time-dependent pressure variations are analyzed by numerical and asymptotic methods for large values of the Zel'dovich number β≫1 measuring the temperature dependence of the heat-release rate and small values of the relative amplitude ϵ≪1 of the pressure fluctuation, with the product βϵ assumed to be of order unity in the distinguished limit addressed. Evaluations of the local Rayleigh index for βϵ≪1 indicate that finite-rate chemical-kinetic effects dominate the acoustic pressure response of strained flamelets under conditions near diffusion-flame extinction. For robust, diffusion-controlled flames, on the other hand, unsteady modifications to the outer chemical-equilibrium transport regions flanking the reaction layer are more important but produce only moderate effects on acoustic instabilities. [ABSTRACT FROM AUTHOR]

Millimeter-size fuel droplets burning in microgravity show substantial thermal expansion at earlier times in their burning history. Here, we develop a simple model that accounts for thermal expansion of the liquid fuel and compare it against experimental measurements. The results show that excellent agreement with measured droplet-diameter histories throughout the hot-flame period of combustion is obtained when the effect of thermal expansion is included. [ABSTRACT FROM AUTHOR]

The Rayleigh index of counterflow hydrogen–air diffusion flames is employed as a vehicle for quantifying inaccuracies of predictions caused by the introduction of reduced chemistry to decrease computation times. Inaccuracies of a systematically reduced 2-step mechanism, derived from a detailed 12-step mechanism for hydrogen–air systems, are small at low strain rates but become appreciable as extinction is approached. [ABSTRACT FROM AUTHOR]

A scaling analysis of burner-supported spherical-diffusion-flame extinction brought about by radiative heat loss is presented. The results are compared against earlier microgravity experimental data and unsteady numerical computations with detailed chemistry available in the literature, for normal and inverse flames. The flame diameter at extinction is shown to correlate well with the present model, in which the extinction radius scales with one-third power of gas flow rate times reactant mass fraction divided by the Planck-mean absorption coefficient. [ABSTRACT FROM AUTHOR]

A C 1 –C 3 short chemical-kinetic mechanism (the San Diego mechanism), which involves 235 elementary reactions among 40 such species, is extended to C 4 by adding 22 chemical-kinetic steps, among 7 additional species, with their associated reaction-rate parameters, to include the ignition and combustion of n-butane over a range of conditions that encompasses both low-temperature and high-temperature chemistry, as well as both high and low pressures. Tests of predictions against measured ignition delays and laminar burning velocities are reported, as are comparisons with recent measurements in jet-stirred reactors, supporting the predictions of the mechanism, which may be useful in combustion computations, especially when larger mechanisms would be too time-consuming to be accommodated. [ABSTRACT FROM AUTHOR]

Counterflow configurations are useful for investigating the structures of premixed, non-premixed, and partially premixed flames. Ignition and extinction conditions also are readily measured in this configuration. There is a wide range of different possible designs of apparatus to be used in such measurements. The choices vary from opposing nozzle flows without any flow-smoothing screens to opposing flows through porous plates. It is desirable to select designs that correspond best to the conditions treated in available codes for calculating reacting flows because this facilitates comparisons of experimental and computational results. The most convenient codes to use are for steady laminar flows with one-dimensional scalar fields, and they often impose rotational plug-flow conditions at the boundaries. Accuracies of axisymmetric counterflow flame measurements in experiments intended to conform to these conditions are estimated here for designs of large aspect ratios with straight-duct feed streams that have multiple-screen flow-smoothing exits. Causes of departures from assumptions underlying computational programs are addressed by methods that involve theoretical analysis, experimental measurement, and axisymmetric computation. It is concluded that experimental results would not be expected to differ from predictions made with plug-flow boundary conditions by more than five percent for properly designed counterflow experiments of this straight-duct, multiple-screen type. [ABSTRACT FROM AUTHOR]

A one-step reduced chemical-kinetic mechanism describing near-critical hydrogen combustion, recently derived by assuming that all chemical intermediates maintain steady state, is used to investigate numerically the pulsating dynamics of fuel-rich hydrogen-air flames. The computations, considering pressures of up to 20 atmospheres, address the flame evolution for increasing values of the equivalence ratio ϕ , a relevant bifurcation parameter. Besides critical conditions associated with the Hopf bifurcation occurring at a critical value ϕ c of ϕ , supercritical dynamics for ϕ > ϕ c is investigated, including the nonlinear gradual growth of the oscillation amplitude with increasing ϕ , the occurrence of a period-doubling bifurcation, and the emergence of nonlinear relaxation oscillations. Comparisons with results of numerical calculations employing detailed chemistry reveal that the one-step description is able to predict with unexpectedly good accuracy the flame dynamics, including the critical conditions at the bifurcations as well as the nonlinear dynamics encountered for ϕ > ϕ c. [ABSTRACT FROM AUTHOR]

Starting with a detailed-chemistry description involving 20 elementary steps for hydrogen oxidation and 40 elementary steps for ammonia oxidation, it is shown that systematic application of sensitivity analyses of premixed flames under typical gas-turbine combustion conditions reduces the description to 12 elementary steps for hydrogen oxidation, 4 of them being reversible, and an additional 19 steps for ammonia oxidation, 6 of them being reversible, yielding reasonable predictions for auto-ignition and deflagration processes. Subsequent introduction of steady-state approximations for chemical intermediates, afforded by the high-pressure conditions existing in gas-turbine combustion chambers, effectively reduces the fuel-oxidation description in systems utilizing H 2 -NH 3 fuel mixtures to two global steps for deflagrations, namely, 2H 2 + O 2 ⇌ 2H 2 O and 4NH 3 ＋ 3O 2 ⇌ 2N 2 + 6 H 2 O. Analytical expressions for the associated overall rates, involving the local temperature and the O 2 , H 2 , NH 3 , N 2 , and H 2 O concentrations, are derived through selective truncation of the steady-state expressions, resulting in a simplified chemistry description that can facilitate future numerical analyses based on direct-numerical and large-eddy simulations. Novelty and significance statement A new short mechanism involving only 31 elementary reactions between 16 reactive species has been derived for hydrogen-ammonia oxidation under conditions of pressure, temperature and dilution typically found in gas-turbine burners. Introduction of steady-state assumptions for all intermediate species leads to a two-step mechanism that is shown to predict burning rates with sufficient accuracy. The proposed mechanism can significantly reduce computational times in future direct-numerical and large-eddy simulations. [ABSTRACT FROM AUTHOR]

Abstract: A newly designed high-pressure combustion facility was used to study the structures and extinction conditions of counterflow diffusion flames in air for nitrogen-diluted methane, ethane, and ethylene, from 0.1MPa to 2.0MPa. Besides employing thermocouples to measure temperature profiles, strain rates at extinction were measured and compared with predictions of two different chemical–kinetic mechanisms (San Diego and USC). In addition, the nitrogen in the fuel and oxidizer streams was replaced by helium for one of the methane tests of extinction strain rate as a function of pressure. In all cases, the strain rate at extinction was found to increase with pressure up to about 0.3–0.5MPa and to decrease with pressure thereafter, on up to 2.0MPa, although with helium there was a clear leveling tendency beyond 1.0MPa. While these behaviors were in qualitative agreement with most predictions of the chemical–kinetic mechanisms, in a number of cases the quantitative discrepancies were well beyond the experimental uncertainty. This underscores the desirability of improving chemical–kinetic descriptions for applications at elevated pressures. Such improvements for the San Diego mechanism are introduced here for two of the steps involving hydroperoxyl that become increasingly important with increasing pressure. [Copyright &y& Elsevier]

Abstract: The third explosion limit of hydrogen oxidation in closed vessels has always been thought to be the result of the competition between homogeneous gas-phase reactions and diffusion of hydroperoxyl radicals to the walls, where they are destroyed. It has recently been observed that this species actually follows a chemical-kinetic steady state in this regime, with the consequence that its diffusive rate toward the catalytic walls becomes irrelevant. Here we show that the critical explosion conditions are determined instead by the fate of hydrogen peroxide, which emerges as the controlling reactant for the resulting gas-phase chemistry. A simple, accurate analytic expression for the third explosion limit follows from identification of the critical conditions for existence of weakly reactive, diffusion–reaction solutions, thereby providing the answer to a long-standing problem that in early work was characterized as being hopelessly difficult. [Copyright &y& Elsevier]

Abstract: This paper addresses ignition histories of diffusion flames in unstrained hydrogen-air mixing layers for initial conditions of temperature and pressure that place the system below the crossover temperature associated with the second explosion limit of hydrogen–oxygen mixtures. It is seen that a two-step reduced chemical-kinetic mechanism involving as main species H2, O2, H2O, and H2O2, derived previously from a detailed mechanism by assuming all radicals to follow a steady-state approximation, suffices to describe accurately the ignition process. The strong temperature sensitivity of the corresponding overall rates enables activation-energy asymptotics to be employed for the analysis, following the ideas developed for mixing-layer ignition by Liñán and Crespo in 1976 on the basis of one-step Arrhenius model chemistry. When the initial temperatures of both reactants differ by a relative amount that is of the order of or smaller than the ratio of this temperature to the effective activation temperature, the chemical reaction is seen to occur at a significant rate all across the mixing layer. The ignition time is then determined as a thermal runaway in a parabolic problem describing the evolution of the temperature increment and the H2O2 concentration, with local accumulation, chemical reaction, and transverse convection and diffusion, all being important. By way of contrast, when the air side is sufficiently hotter than the hydrogen side, as often occurs in applications, ignition occurs in a thin layer close to the air-side boundary, enabling a simplified description to be developed in which the ignition time is determined by analyzing the existence of solutions to a two-point boundary-value problem involving quasi-steady diffusion–reaction ordinary differential equations. [Copyright &y& Elsevier]

Abstract: The feasibility of developing multipurpose reduced chemistry that is able to describe, with sufficient accuracy, premixed and non-premixed flames, one-dimensional detonations, high-temperature autoignition, and also low-temperature autoignition is explored. A four-step mechanism with O and OH in steady state is thoroughly tested and is shown to give satisfactory results under all conditions. The possibility of reducing this to a three-step mechanism, to decrease computation times without compromising the range of applicability is then investigated. The originality of this work resides in introducing a single species X, representing either HO2 for high-temperature ignition or H2O2 for low-temperature ignition. An algorithm is defined that covers the entire range without significant degradation of accuracy. Integrations show promising results for different laminar test cases, and applicability to turbulent flows is indicated. [Copyright &y& Elsevier]

Abstract: This paper addresses homogeneous ignition of hydrogen–oxygen mixtures when the initial conditions of temperature and pressure place the system below the crossover temperature associated with the second explosion limit. A three-step reduced mechanism involving H2, O2, H2O, H2O2 and HO2, derived previously from a skeletal mechanism of eight elementary steps by assuming O, OH and H to follow steady state, is seen to describe accurately the associated thermal explosion. At sufficiently low temperatures, HO2 consumption through HO2 +HO2 →H2O2 +O2 is fast enough to place this intermediate in steady state after a short build-up period, thereby reducing further the chemistry description to the two global steps 2H2 +O2 →2H2O and 2H2O→H2O2 +H2. The strong temperature sensitivity of the corresponding overall rates enables activation-energy asymptotics to be used in describing the resulting thermal runaway, yielding an explicit expression that predicts with excellent accuracy the ignition time for different conditions of initial temperature, composition, and pressure. [Copyright &y& Elsevier]

Abstract: Simplified chemical-kinetic mechanisms are sought that can provide agreement with measured shock-tube autoignition times and counterflow critical ignition conditions for methanol (CH3OH) oxidation. Existing detailed chemistry over-predicts measured counterflow ignition temperatures by 100K or more. It was found that the elementary step CH3OH+HO2 →CH2OH+H2O2 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 8×1013 cm3/(mols), 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 CH3 +OH for high-temperature shock-tube conditions) that predict ignition nearly as well as the detailed mechanism. In this mechanism, the intermediate species CH2OH, CH3O, HCO, H, O, and OH accurately obey steady states, while the intermediates CH2O, HO2, H2O2, CO, and H2 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 HO2 attack in ignition, are indicated to nevertheless possibly benefit from an increase of the rate of the corresponding step. [Copyright &y& Elsevier]

Abstract: The transient burning of an n-octane fuel droplet in a hot gas stream is numerically studied using a four-step reduced mechanism, with considerations of droplet surface regression, deceleration due to the drag of the droplet, internal circulation inside the droplet, variable properties, non-uniform surface temperature, and the effect of surface tension. Two different types of the four-step mechanism are examined and found almost identical. The four-step mechanism has earlier instant of the wake-to-envelope transition than the one-step mechanism at low ambient temperature, but this difference between the two mechanisms diminishes when the ambient temperature is increased. The four-step mechanism has smaller mass burning rate for a wake flame but greater mass burning rate for an envelope flame than the one-step mechanism. The two mechanisms have small differences in the critical initial Damkohler number. Lower ambient temperature yields later wake-to-envelope transition and smaller mass burning rate. Higher ambient pressure has greater overall mass burning rate because of greater gas density and thus greater concentrations of reactants for a major part of the lifetime. Greater ambient mass fraction of oxygen yields faster oxidation kinetics and greater Damkohler number. As the ambient mass fraction of oxygen increases, the instant of wake-to-envelope transition advances for an initial wake flame, and finally the initial flame becomes an envelope flame when the ambient mass fraction of oxygen exceeds some critical value. A correlation is developed for the critical initial Damkohler number in terms of the ambient temperature, ambient pressure, and ambient mass fraction of oxygen. [Copyright &y& Elsevier]

Abstract: This analysis addresses the propagation of spiral edge flames found in von Kármán swirling flows induced in rotating porous-disk burners. In this configuration, a porous disk is spun at a constant angular velocity in an otherwise quiescent oxidizing atmosphere. Gaseous methane is injected through the disk pores and burns in a flat diffusion flame adjacent to the disk. Among other flame patterns experimentally found, a stable, rotating spiral flame is observed for sufficiently large rotation velocities and small fuel flow rates as a result of partial extinction of the underlying diffusion flame. The tip of the spiral can undergo a steady rotation for sufficiently large rotational velocities or small fuel flow rates, whereas a meandering tip in an epicycloidal trajectory is observed for smaller rotational velocities and larger fuel flow rates. A formulation of this problem is presented in the equidiffusional and thermodiffusive limits within the framework of one-step chemistry with large activation energies. Edge-flame propagation regimes are obtained by scaling analyses of the conservation equations and exemplified by numerical simulations of straight two-dimensional edge flames near a cold porous wall, for which lateral heat losses to the disk and large strains induce extinction of the trailing diffusion flame but are relatively unimportant in the front region, consistent with the existence of the cooling tail found in the experiments. The propagation dynamics of a steadily rotating spiral edge is studied in the large-core limit, for which the characteristic Markstein length is much smaller than the distance from the center at which the spiral tip is anchored. An asymptotic description of the edge tangential structure is obtained, spiral edge shapes are calculated, and an expression is found that relates the spiral rotational velocity to the rest of the parameters. A quasiestatic stability analysis of the edge shows that the edge curvature at extinction in the tip region is responsible for the stable tip anchoring at the core radius. Finally, experimental results are analyzed, and theoretical predictions are tested. [Copyright &y& Elsevier]

Abstract: Diffusive extinction of methanol droplets with initial diameters between 1.25mm and 1.72mm, burning in a quiescent microgravity environment at one atmosphere pressure, was obtained experimentally for varying levels of ambient carbon-dioxide concentrations with a fixed oxygen concentration of 21% and a balance of nitrogen. These experiments serve as precursors to those which are beginning to be performed on the International Space Station and are motivated by the need to understand the effectiveness of carbon-dioxide as a fire suppressant in low-gravity environments. In these experiments, the flame standoff distance, droplet diameter, and flame radiation are measured as functions of time. The results show that the droplet extinction diameter depends on both the initial droplet diameter and the ambient concentration of carbon dioxide. Increasing the initial droplet diameter leads to an increased extinction diameter, while increasing the carbon-dioxide concentration leads to a slight decrease in the extinction diameter. These results are interpreted using a critical Damköhler number for extinction as predicted by an earlier theory, which is extended here to be applicable in the presence of effects of heat conduction along the droplet support fibers and of the volume occupied by the support beads. [Copyright &y& Elsevier]

Abstract: A laminar diffusion flame that is established over a spinning, thermoplastic, polymer fuel disk in a quiescent, oxidizing environment under microgravity is analyzed theoretically. The conservation equations for the polymer melt layer coupled to the gas-phase equations are solved numerically using similarity transformations. The polymer melting rate, the thickness of the melt layer, and the fraction of melted fuel that is burned in the gas-phase are predicted as functions of the ambient conditions and polymer property values. In these calculations the melt viscosity is assumed to vary with temperature in an Arrhenius form. Results are presented for polymethylmethacrylate (PMMA) disks burning in air at atmospheric pressure and compared against earlier experimental results. [Copyright &y& Elsevier]

Direct numerical simulations (DNS) of fuel-lean turbulent premixed hydrogen-enriched flames are analyzed to improve understanding of local effects of molecular diffusion of fast-diffusing, low-Lewis-number hydrogen species (H 2 and H) on the overall burning rates of reactant mixtures. Although it is often assumed that the importance of molecular diffusion decreases with increasing turbulence intensity, our analysis reveals that, on the contrary, diffusion of molecular and atomic hydrogen can remain the rate-controlling processes even at high Karlovitz numbers (Ka ≫ 1). Three-dimensional DNS with detailed chemical kinetics and species transport of turbulent premixed hydrogen-air and hydrogen/ammonia-air flames in the regimes of thin and distributed reaction zones are performed and analyzed at different reactant temperatures and pressure levels. The DNS data reveal a significant impact of H 2 and H diffusion in all flames analyzed. In particular, the magnitude of H 2 diffusion, occurring from low progress variable near the flame elements of the turbulent reaction front that exhibit convex (positive) curvature towards the fresh mixture, is greatly enhanced at high pressure through increased spatial gradients, resulting in localized equivalence-ratio enrichment and super-adiabatic conditions that accelerate and strengthen the flame front. Moreover, for higher reactant temperatures, back diffusion of atomic hydrogen, transported from the reaction zone into unburnt regions from negatively curved reaction layers surrounding them, facilitates the occurrence of localized spontaneous ignition events at the trailing edges of reaction fronts. For certain conditions these events, induced by differential diffusion, can become the dominant process controlling the overall rate of reactants consumption. [ABSTRACT FROM AUTHOR]

The present work exploits simplifications arising in weakly exothermic detonations when the postshock conditions are supersonic to investigate the structure of wedge-induced oblique detonations. These simplifications enable the linearized Euler equations (employed here in characteristic form) to be efficiently solved numerically, subject to the linearized Rankine-Hugoniot jump conditions across the leading oblique shock. A first set of computations employs one-step first-order Arrhenius chemistry appropriate for describing detonations when the postshock chemistry exhibits a thermal-explosion character. In that case, the relevant chemical-kinetic parameter of order unity ß is the product of the heat release and the activation energy divided by the square of the postshock thermal enthalpy. The transition from the shock to the detonation wave is continuous at small ß, begins to develop spatially decaying oscillations as ß increases, and develops a singularity at the shock at a critical value of ß; above which, the transition must become discontinuous and involve a triple point. Parametric results are presented in a plane of the wedge angle and the incident-flow Mach number: the two important controlling parameters. The triple point is found to develop when the incident-flow Mach number falls below a critical value that exhibits a U-shaped dependence on the wedge angle, becoming large at both high and low wedge angles and reflecting large differences between shock angles with and without heat release in those two extremes. Additional computations are performed for a three-step branched-chain scheme with the heat-release step having zero activation energy and for very fuel-lean hydrogen-air detonations with postshock temperatures above crossover. These cases, for which ignition develops as a chain-branching explosion, do not develop a singularity at the shock; although they display many of the features identified with the Arrhenius chemistry, including oscillations and appearance of a precursor point indicative of criticality. The results suggest a strong potential influence of the chemistry on the transition. [ABSTRACT FROM AUTHOR]

Droplet combustion experiments carried out in the International Space Station using n-alkane fuels have shown that large droplets, when ignited, first burn with visible hot flames and then extinguish, only to be followed later by quasi-steady cool-flame combustion. In a few of these experiments, some of the nitrogen in the nitrogen-oxygen ambient-gas mixture was replaced by helium, carbon dioxide, or xenon, for n-heptane, n-octane, n-decane, or n-dodecane droplets. Different diluents were observed to exert remarkably different influences on the cool-flame combustion. These initially unexpected differences are summarized here, with explanations offered for their causes. In particular, a simplified theoretical description of cool-flame-supported droplet combustion is employed to predict burning-rate constants and droplet diameters at cool-flame extinction, resulting in good agreement with much of the experimental data but disagreement with certain measurements, for reasons that are explained. The comparisons underscore the important influence of the diluent, especially on the Lewis numbers of cool-flame intermediate species derived from the fuel vapor. [ABSTRACT FROM AUTHOR]

Experimental data and detailed numerical modelling are presented on the burning characteristics of a model gasoline/biofuel mixture consisting of n-heptane and iso-butanol. A droplet burning in an environment that minimises the influence of buoyant and forced convective flows in the standard atmosphere is used to promote one-dimensional gas transport to facilitate numerical modelling of the droplet burning process. The numerical model includes a detailed combustion kinetic mechanism, unsteady gas and liquid transport, multicomponent diffusion inside the droplet, variable properties, and non-luminous radiative heat transfer from the flame. The numerical simulation was validated by experimental measurements in the standard atmosphere which showed good agreement with the evolutions of droplet and flame diameters. The iso-butanol concentration had a strong effect on formation of particulates. Above ~20% (volume) iso-butanol, flame luminosity was significantly diminished anddecreased with increasing iso-butanol concentration, while CO2 emissions as a representative greenhouse gas were not strongly influenced by the iso-butanol loading. The soot shell was located near a 1350 K isotherm for concentrations up to 20% (volume) iso-butanol, suggesting this value as a possible soot inception temperature for the mixture droplet. The combustion rate decreased with increasing iso-butanol concentration which was attributed to iso-butanol's higher liquid density. No evidence of a low temperature burning regime, or of extinction, was found (in experiments and simulations) for the small droplet sizes investigated. [ABSTRACT FROM AUTHOR]

The temporal evolution of the droplet radius is measured experimentally in high-temperature inert atmospheres for three different alcohols (ethanol, n-butanol, and glycerol) and three alkanes (n-heptane, n-dodecane, and n-hexadecane). It is shown that, while accompanying theoretical predictions of droplet-radius variations show excellent accuracy for the three alcohols, the three alkanes exhibit vaporization rates that are significantly smaller than those predicted theoretically. The accompanying observation of significant soot formation suggests that endothermic fuel pyrolysis may be responsible for the diminished vaporization rate. The quantification of this phenomenon is investigated here using a one-step irreversible reaction with an Arrhenius rate to model the fuel decomposition. It is shown how an analytical description developed on the basis of activation-energy asymptotics can be used in combination with the experimental measurements of the temporal droplet-radius evolution to adjust the fuel-pyrolysis kinetics, embodied at leading order in an effective pyrolysis temperature, which is obtained for n-heptane, n-dodecane, and n-hexadecane. [ABSTRACT FROM AUTHOR]

Numerical computations employing the relevant 9-step detailed chemistry are used to characterize the different combustion modes emerging in mixing layers separating nitrogen-diluted counterflowing planar streams of hydrogen and oxygen. Attention is focused on high degrees of dilution, resulting in near-limit flames, with peak temperatures close to the crossover temperature. A bifurcation diagram is presented in a plane, having the stoichiometric mixture fraction and normalized strain rate as coordinates, that identifies six different combustion regimes involving four different flame types, namely, diffusion-flame sheets, advancing and retreating edge flames, multiple flame tubes, and single isolated flame tubes. Multiple-tube flame configurations vary from small, round, widely separated flame strings at high strain rates to wide, flat, densely packed flame strips, with narrow flame-free gaps between them, at lower strain rates, and they are steady and stable in various arrays over a continuum of tube-separation distances. The observed flame behavior exhibits hysteresis in a certain range of parameters, with the structure that is established depending on the ignition mechanism, as it also does at high strain rates, and a continuum of different stable steady-state flame configurations exists, each accessed from a different initial condition. [ABSTRACT FROM AUTHOR]

A simplified formulation, based on treating the ratio of the heat release to the postshock thermal enthalpy as a small parameter, accommodating arbitrary chemistry descriptions, is shown to reproduce computationally the same variety of phenomena as more complex formulations for oblique detonations with supersonic postshock flow. The resulting small relative variations of velocity and thermodynamic properties across the reaction region are described by linearized Euler equations written in characteristic form, supplemented by the linearized Rankine-Hugoniot jump conditions across the leading shock. The simplified formulation is used to analyze the interaction of the oblique detonation with a weak vortex sheet, for an Arrhenius irreversible reaction with an activation energy large compared with the postshock thermal enthalpy. The analysis reveals that, as β, the product of the activation energy and the heat release divided by the square of the postshock thermal enthalpy, increases through values of order unity, decaying spatial oscillations, found for small values, are replaced by persistent nonlinear oscillations of finite amplitude for larger values. Beyond a critical value of β the growth of the oscillation amplitude leads to the development of a singularity at the shock, an explosion, consistent with the formation of a triple point. Many related problems can be clarified with this formulation. [ABSTRACT FROM AUTHOR]

A one-step reduced mechanism for hydrogen combustion, previously developed for fuel-lean flames, is extended to apply for all equivalence ratios under near-limit conditions by taking into consideration two additional recombination steps that are important under fuel-rich conditions. It is found that the crossover temperature that appears in the cutoff factor is smaller under fuel-rich conditions. Besides improving insights, the results can be beneficial in speeding computations. [ABSTRACT FROM AUTHOR]

The aim of this study is to eliminate unimportant steps from a detailed chemical-kinetic mechanism in order to identify a skeletal kinetic mechanism that can predict with sufficient accuracy ignition delay times and laminar premixed-flame velocities for H 2 - C H 4 mixtures under conditions of practical interest in gas-turbine applications, which pertain to high pressure, high reactant temperature, and primarily lean-to-stoichiometric mixture compositions (although somewhat rich conditions also are considered for completeness). The accuracy of selected detailed chemical-kinetic mechanisms that are suited to represent combustion of hydrogen-methane mixtures in air was evaluated through comparison of computed and measured ignition delay times and laminar flame velocities, and because of its relative simplicity and sufficient accuracy, the San Diego mechanism was selected for the needed chemical-kinetic reduction. Under the pressure and temperature conditions of the mixture composition addressed, thirty nine reversible elementary steps involving eighteen species were found to suffice to describe with acceptable accuracy both the ignition delay time and the laminar burning velocities. The skeletal mechanism is given here, along with discussion of its derivation and characteristics, as well as comparison of its predictions with those of the detailed mechanism and, where possible, with experiment. Image 1 • A skeletal mechanism is developed for CH 4 /H 2 combustion at gas turbine conditions. • The prediction is validated over a wide range of initial conditions. • Good agreement for ignition delay time and laminar burning velocity. [ABSTRACT FROM AUTHOR]

One dozen vertically oriented thin rectangular vanes, 62 cm tall and 15.2 cm wide, were placed 27 cm from the center of heptane and ethanol pool fires in continuously fed, floor-flush pans 3.2 cm and 5.1 cm in diameter in the laboratory. The vanes were all oriented at the same fixed angles from the radial direction, for 9 different angles, ranging from 0∘ to 85∘, thereby imparting 9 different levels of circulation to the air entrained by each pool fire. The different swirl levels were observed to engender dramatically different pool-fire structures. Moderate swirl suppresses the global puffing instability, replacing it by a global helical instability that generates a tall fire whirl, the height of which increases with increasing circulation. Except for the largest heptane pool, higher swirl levels produced vortex breakdown, resulting in the emergence of a bubble-like recirculation region with a ring vortex encircling the axis. Measured burning rates increase with increasing swirl levels as a consequence of the associated increasing inflow velocities reducing the thickness of the boundary layer within which combustion occurs right above the liquid surface, eventually forming detached edge flames in the boundary layer that move closer to the axis as the circulation is increased. Still higher circulation reduces the burning rate by decreasing the surface area of the liquid covered by the flame, thereby reducing the height of the fire whirl. Even higher circulation causes edge-flame detachment, resulting in formation of the blue whirl identified in recent literature, often meandering over the surface of the liquid in the present experiments. This sequence of events is documented herein. [ABSTRACT FROM AUTHOR]