Sam Goroshin
McGill University, Mechganical Engineering, Department Member
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This paper critically reviews the theoretical and experimental literature regarding the fundamental aspects of flames in nonvolatile solid fuel suspensions. Unlike volatile fuels that form continuous premixed gaseous flame sheets, flame... more
This paper critically reviews the theoretical and experimental literature regarding the fundamental aspects of flames in nonvolatile solid fuel suspensions. Unlike volatile fuels that form continuous premixed gaseous flame sheets, flame fronts in nonvolatile suspensions are driven by heterogeneous reactions localized on the surface, or near the surface, of individual particles. Practically all peculiarities of heterogeneous flames can be linked to this "flame-inside-the-flame" combustion front structure. These localized reactions enable particles to self-heat and transition from kinetically to diffusion-limited heterogeneous reaction during the process of particle ignition. After ignition, burning particles behave as individual diffusion micro-reactors that are insensitive to the bulk gas temperature and overall heat loss from the system. Relatively small quenching distances of the flame in suspensions, long plateaus in the dependence of burning velocity on fuel concentration stretching to very fuel-rich mixtures, and the discrete flame propagation regime, where burning velocity is insensitive to particle combustion time and the flame-front structure is rough and nonuniform, are all manifestations of particle ignition and combustion in the diffusion-limited regime. This review summarizes the key experimental evidence of laminar flame structure and flame speed from a variety of experimental apparatus both in the laboratory and under microgravity conditions, and interprets these results in terms of relatively simple theoretical models. Heterogeneous flames are observed to exhibit many of the thermodiffusive and hydrodynamic instabilities of homogeneous flames, as well as several new instabilities that arise from the multiphase nature of the fuel and particle ignition and extinction. Flames of binary mixtures of heterogeneous fuels, or gaseous and solid fuel mixtures, are also reviewed and it is shown that a simple model based on matching the flame speed between thermally interacting fronts can capture the key physics. Finally, the last chapter of the review discusses why the important or even crucial role of radiation heat transfer predicted by theoretical models for flames in suspensions is not supported by the available experimental evidence. It is argued that large spatial scales of radiation heat transfer do not permit separation of the radiation transfer problem from boundary conditions and flow configuration, making one-dimensional flame models that include radiation inadequate for the description of flames in the laboratory and even in relatively large unconfined dust clouds.
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Experiments are carried out to determine the effects of particle size and mass loading on the free-field blast wave from spherical, constant volume metalized explosive charges. The charges are comprised of gelled nitromethane with... more
Experiments are carried out to determine the effects of particle size and mass loading on the free-field blast wave from spherical, constant volume metalized explosive charges. The charges are comprised of gelled nitromethane with uniformly embedded aluminum, magnesium, or glass particles. Particle sizes are varied over an order of magnitude with particle mass fractions up to 50%. Peak blast overpressures are directly measured within the fireball with piezoelectric pressure gauges and outside the fireball are inferred by tracking the velocity of the blast wave and using the Rankine–Hugoniot relation. With the addition of inert particles, the peak blast overpressure is initially mitigated, but then recovers in the far field. For charges with reactive particles, the particles react promptly with oxidizers in the detonation products and release energy as early as within the first few hundred microseconds in all cases. The particle energy release enhances the peak blast overpressures in...
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The ignition characteristics of boron particle agglomerates prepared by drying 2-3 mm water slurry droplets of crystalline or amorphous boron powders were investigated by introducing suspended agglomerates into a flow of high temperature... more
The ignition characteristics of boron particle agglomerates prepared by drying 2-3 mm water slurry droplets of crystalline or amorphous boron powders were investigated by introducing suspended agglomerates into a flow of high temperature oxidizing gases (oxygen/nitrogen/water vapor) obtained by a combination of combustion and electrical heating. Monitoring of the agglomerate luminosity, spectra, and temperature revealed a dual-stage ignition process. The first stage ignition was observed at flow temperatures as low as 715 K and was followed by agglomerate quenching if the oxygen concentration in the flow was below some critical value. As the oxygen concentration of the flow was increased beyond this critical value, the second stage ignition leading to full-fledge combustion was observed. The ignition characteristics of boron particle agglomerates were theoretically investigated by developing a numerical model that considers the oxygen concentration gradient inside the agglomerate in order to simulate the ignition process. The model results accurately captured the qualitative behavior of the first stage ignition and quenching as well as the dual-stage ignition processes. The model identified the closure of pores in the agglomerate due to the build-up of the boron oxide layer, which effectively seals off the large interior surface area of the agglomerate from the oxidizer, as being the mechanism responsible for the two-stage ignition phenomenon. The critical flow conditions in terms of flow temperature and ambient oxygen concentration for the second stage ignition were determined by this model. The experimental result for the critical ambient oxygen concentration for the second stage ignition in dry flow was determined to be 70%, which is fairly close to the model prediction of 79%.
The effect of spatially randomizing a system of pointlike sources on the propagation of reaction-diffusion fronts is investigated in multidimensions. The dynamics of the reactive front are modeled by superimposing the solutions for... more
The effect of spatially randomizing a system of pointlike sources on the propagation of reaction-diffusion fronts is investigated in multidimensions. The dynamics of the reactive front are modeled by superimposing the solutions for diffusion from a single point source. A nondimensional parameter is introduced to quantify the discreteness of the system, based on the characteristic reaction time of sources compared to the diffusion time between sources. The limits to propagation and the average velocity of propagation are expressed as probabilistic quantities to account for the influence of the randomly distributed sources. In random systems, two-and three-dimensional fronts are able to propagate beyond a limit previously found for systems with regularly distributed sources, while a propagation limit in one dimension that is independent of domain size cannot be defined. The dimensionality of the system is seen to have a strong influence on the front propagation velocity, with higher dimensional systems propagating faster than lower dimensional systems. In a three-dimensional system, both the limit to propagation and average front velocity revert to a solution that assumes a spatially continuous source function as the discreteness parameter is increased to the continuum limit. The results indicate that reactive systems are able to exploit local fluctuations in source concentration to extend propagation limits and increase the velocity in comparison to regularly spaced systems.
Metal sulfides are widely used in a variety of applications requiring high hardness and toughness. In this study, the microstructure and mechanical properties of chromium-chromium sulfide cermets are investigated. The chromium-chromium... more
Metal sulfides are widely used in a variety of applications requiring high hardness and toughness. In this study, the microstructure and mechanical properties of chromium-chromium sulfide cermets are investigated. The chromium-chromium sulfide cermet was manufactured using self-propagating high-temperature synthesis, a process where the material is created under a self-sustaining combustion reaction between the chromium and sulfur. This type of synthesis allows the creation of near-net shape structures and offers the possibility of tuning material properties and material behavior by changing the composition of the reactant. Microstructural characterization was performed using optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy. The mechanical properties of the cermet (Young's modulus, fracture toughness, flexural strength, and microhardness) have been measured and related to morphology and chemical composition of the samples. Results show that dense cermets (about 7 % porosity) with specific structure have been obtained. Pure CrS has a significant hardness, but its toughness was insufficient for tool applications. However, we found that the density and fracture toughness of the cermets increase with the addition of Cr. The addition of Cr also improved the flexural strength and hardness of the cermet by 60 % and almost 38 %, respectively.
This paper critically reviews the theoretical and experimental literature regarding the fundamental aspects of flames in nonvolatile solid fuel suspensions. Unlike volatile fuels that form continuous premixed gaseous flame sheets, flame... more
This paper critically reviews the theoretical and experimental literature regarding the fundamental aspects of flames in nonvolatile solid fuel suspensions. Unlike volatile fuels that form continuous premixed gaseous flame sheets, flame fronts in nonvolatile suspensions are driven by heterogeneous reactions localized on the surface, or near the surface, of individual particles. Practically all peculiarities of heterogeneous flames can be linked to this "flame-inside-the-flame" combustion front structure. These localized reactions enable particles to self-heat and transition from kinetically to diffusion-limited heterogeneous reaction during the process of particle ignition. After ignition, burning particles behave as individual diffusion micro-reactors that are insensitive to the bulk gas temperature and overall heat loss from the system. Relatively small quenching distances of the flame in suspensions, long plateaus in the dependence of burning velocity on fuel concentration stretching to very fuel-rich mixtures, and the discrete flame propagation regime, where burning velocity is insensitive to particle combustion time and the flame-front structure is rough and nonuniform, are all manifestations of particle ignition and combustion in the diffusion-limited regime. This review summarizes the key experimental evidence of laminar flame structure and flame speed from a variety of experimental apparatus both in the laboratory and under microgravity conditions, and interprets these results in terms of relatively simple theoretical models. Heterogeneous flames are observed to exhibit many of the thermodiffusive and hydrodynamic instabilities of homogeneous flames, as well as several new instabilities that arise from the multiphase nature of the fuel and particle ignition and extinction. Flames of binary mixtures of heterogeneous fuels, or gaseous and solid fuel mixtures, are also reviewed and it is shown that a simple model based on matching the flame speed between thermally interacting fronts can capture the key physics. Finally, the last chapter of the review discusses why the important or even crucial role of radiation heat transfer predicted by theoretical models for flames in suspensions is not supported by the available experimental evidence. It is argued that large spatial scales of radiation heat transfer do not permit separation of the radiation transfer problem from boundary conditions and flow configuration, making one-dimensional flame models that include radiation inadequate for the description of flames in the laboratory and even in relatively large unconfined dust clouds.
Experiments are carried out to determine the effects of particle size and mass loading on the free-field blast wave from spherical, constant volume metalized explosive charges. The charges are comprised of gelled nitromethane with... more
Experiments are carried out to determine the effects of particle size and mass loading on the free-field blast wave from spherical, constant volume metalized explosive charges. The charges are comprised of gelled nitromethane with uniformly embedded aluminum, magnesium, or glass particles. Particle sizes are varied over an order of magnitude with particle mass fractions up to 50%. Peak blast overpressures are directly measured within the fireball with piezoelectric pressure gauges and outside the fireball are inferred by tracking the velocity of the blast wave and using the Rankine-Hugoniot relation. With the addition of inert particles, the peak blast overpressure is initially mitigated, but then recovers in the far field. For charges with reactive particles, the particles react promptly with oxidizers in the detonation products and release energy as early as within the first few hundred microseconds in all cases. The particle energy release enhances the peak blast overpressures in the far field by up to twice the values for a constant volume charge of the baseline homogenous explosive. By plotting the peak blast overpressure decay as a function of energy-scaled distance, it is inferred that at least half of the particle energy release contributes to the blast overpressure in the far field of higher mass loadings, and nearly all of the particle energy for a particle mass fraction of 10%. For aluminum, the blast augmentation is not a systematic function of particle size. This observation implies that conventional models for particle combustion that depend on particle surface area are not appropriate for describing the rapid aluminum reaction that occurs in the extreme conditions within the detonation products, which influences the blast wave propagation.
Ceramic-metal composites (cermets) offer unique combinations of hardness and toughness, which make them attractive for a variety of applications. In this study, we propose a new method for the preparation of the metal-sulfur precursor... more
Ceramic-metal composites (cermets) offer unique combinations of hardness and toughness, which make them attractive for a variety of applications. In this study, we propose a new method for the preparation of the metal-sulfur precursor mixture based on the ability to melt-cast the precursor mixture. We have used self-propagating high-temperature synthesis to produce a chromium/chromium sulfide cermet, exploiting the fact that this mixture of metal and sulfur can support the propagation of reactive waves. This ability, together with the properties of the reaction products (low gas evolution and liquid sulfide products), enables the net-shape synthesis of dense, near theoretical density product with a relatively simple and low-cost setup. While the thermochemical calculations predict near-zero gas production for the chromium-sulfur system, the actual cermets showed a large amount of porosity (about 70 %), when synthesized at atmospheric pressure. The possible sources for porosity were identified, and the process improved to bring the porosity down to about 7 %. We also investigated the physical properties of the produced cermet with optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction techniques.
The exact solution for a reaction-diffusion front propagating in a heterogeneous system of discrete, point-like sources is obtained without resorting to a representation of the sources by a spatially continuous function. When the reaction... more
The exact solution for a reaction-diffusion front propagating in a heterogeneous system of discrete, point-like sources is obtained without resorting to a representation of the sources by a spatially continuous function. When the reaction time is smaller than the characteristic diffusion time between neighboring sources, the front speed predicted by this discrete source model differs from the continuum theory based on the spatial averaging of the heterogeneities. Furthermore, when the sources are regularly distributed in space, discreteness introduces a limit and propagation beyond this limit is only possible in a system with randomly distributed sources via local fluctuations of the concentration. The discrete regime of front propagation is observed experimentally in suspensions of iron particles burning in oxygen-xenon mixtures.
The rate of flame propagation is generally understood to directly reflect the burning rate of a combustible mixture, as it is proportional to the square root of the overall reaction rate in the flame. However, in the case of heterogeneous... more
The rate of flame propagation is generally understood to directly reflect the burning rate of a combustible mixture, as it is proportional to the square root of the overall reaction rate in the flame. However, in the case of heterogeneous mixtures composed of spatially separated discrete heat sources, such as suspensions of solid-fuel particles, theory suggests that the flame propagation speed may become insensitive to the reaction rate. Such a flame propagation regime, known as the "discrete flame" in the literature, is predicted to occur in suspensions of fast-burning metal particles and (or) low-conductivity gas media where the flame propagation becomes dominated by the heat transfer between fast-burning heat sources. In an attempt to experimentally confirm the existence of the discrete flame regime, flame fronts propagating in transparent glass tubes through iron particle (d 32 = 33 μm) suspensions in low-thermal-diffusivity oxygen/xenon mixtures were studied in a high-quality microgravity environment. The experiment was performed on board the European Space Agency MAXUS-9 sounding rocket that lifted off from the Esrange Space Center (Sweden) on April 7, 2017. In total, 41 combustion runs were completed in suspensions with 20% and 40% of oxygen content during the 12 min of microgravity. While the experimentally determined combustion time of a single iron particle differs by a factor of more than three in mixtures with 20% and 40% oxygen content, the flames in these mixtures were found to propagate with practically equal speed in accordance with the predictions of the discrete flame theory.
Imaging emission spectroscopy, spatially resolved laser-absorption spectroscopy, and particle image velocimetry (PIV) are applied to a flat flame stabilized in a suspension of micron-sized aluminum. The results from the combination of... more
Imaging emission spectroscopy, spatially resolved laser-absorption spectroscopy, and particle image velocimetry (PIV) are applied to a flat flame stabilized in a suspension of micron-sized aluminum. The results from the combination of diagnostics are used to infer the combustion regime of the particles and to estimate the characteristic combustion time of the suspension. It is observed that the reaction zone of the flame in stoichiometric aluminum-air suspensions exhibits strong self-reversal of the atomic aluminum emission lines. These lines also exhibit high optical depths in both emission and absorption spectroscopy. The strong self-reversal and high optical depths indicate high concentrations of aluminum vapor within the reaction zone of the flame at multiple temperatures. These features provide evidence of the formation of vapor-phase micro-diffusion flames around the individual particles in the suspension. In aluminum-methane-air flames, the lack of self-reversal and lower optical depths of the aluminum atomic lines indicate the absence of vapor-phase micro-diffusion flames, and point to a more heterogeneous, and likely kinetically-controlled, particle combustion regime. The reaction zone thickness is estimated from the spatially resolved profiles of aluminum resonance lines in both absorption and emission through the flame. The emission measurements yield a reaction zone thickness on the order of 1.7 ±0.3 mm in aluminum-air flames, and the absorption measurements yield a thickness on the order of 2.3 ±0.5. It is demonstrated that the combination of the combustion zone thickness measurement, flame temperatures determined from molecular AlO emission spectra, and particle velocity measurements from the PIV diagnostic permits an estimation of the burning time in the suspension. The burning time in stoichiometric aluminum-air suspensions using the suite of diagnostics is estimated to be on the order of 0.7 ms.
Metal powder combustion has traditionally been studied to mitigate the risk of industrial accidents and to determine the contributions of metals as additives to the performance of energetic materials. Recently, there has been growing... more
Metal powder combustion has traditionally been studied to mitigate the risk of industrial accidents and to determine the contributions of metals as additives to the performance of energetic materials. Recently, there has been growing interest in exploring the potential of metal powders as recyclable, zero-carbon energy carriers as an alternative to the hydrocarbons known to contribute to climate change. The present work introduces, for the first time, a stabilized flat iron flame. The counterflow burner used in this work is comprised of an inverted ceramic nozzle which sits above, and is aligned axially with, a lower nozzle producing a laminar flow of particles suspended in an oxidizing gas. A stabilized methane flame sits inside the top nozzle and the hot combustion products impinge upon the two-phase flow from the bottom nozzle, creating a stagnation plane. Spherical iron powder, with 90% of the particles less than 2.5 μm in size, is pre-loaded into a piston and dispersed using mixtures of 30% and 40% oxygen balanced in argon. Flame speeds are measured using particle image velocimetry (PIV), while flame temperatures are determined using multicolour pyrometry. It is found that flame speeds range between 30 cm/s and 45 cm/s for both oxidizing mixtures. Despite having fuel loadings below stoichiometric concentrations, the observed particle combustion temperatures are close to the adiabatic flame temperature of the stoichiometric mixture, indicating combustion in the diffusioncontrolled regime for these small particles. Finally, the independence of the flame speeds with respect to oxygen concentration suggests flame propagation in the discrete regime.
Experiments are carried out to determine the effects of particle size and mass loading on the free-field blast wave from spherical, constant volume metalized explosive charges. The charges are comprised of gelled nitromethane with... more
Experiments are carried out to determine the effects of particle size and mass loading on the free-field blast wave from spherical, constant volume metalized explosive charges. The charges are comprised of gelled nitromethane with uniformly embedded aluminum, magnesium, or glass particles. Particle sizes are varied over an order of magnitude with particle mass fractions up to 50%. Peak blast overpressures are directly measured within the fireball with piezoelectric pressure gauges and outside the fireball are inferred by tracking the velocity of the blast wave and using the Rankine-Hugoniot relation. With the addition of inert particles, the peak blast overpressure is initially mitigated, but then recovers in the far field. For charges with reactive particles, the particles react promptly with oxidizers in the detonation products and release energy as early as within the first few hundred microseconds in all cases. The particle energy release enhances the peak blast overpressures in the far field by up to twice the values for a constant volume charge of the baseline homogenous explosive. By plotting the peak blast overpressure decay as a function of energy-scaled distance, it is inferred that at least half of the particle energy release contributes to the blast overpressure in the far field of higher mass loadings, and nearly all of the particle energy for a particle mass fraction of 10%. For aluminum, the blast augmentation is not a systematic function of particle size. This observation implies that conventional models for particle combustion that depend on particle surface area are not appropriate for describing the rapid aluminum reaction that occurs in the extreme conditions within the detonation products, which influences the blast wave propagation.
Flame propagation through a non-volatile solid-fuel suspension is studied using a simplified, time-dependent numerical model that considers the influence of both diffusional and kinetic rates on the particle combustion process. It is... more
Flame propagation through a non-volatile solid-fuel suspension is studied using a simplified, time-dependent numerical model that considers the influence of both diffusional and kinetic rates on the particle combustion process. It is assumed that particles react via a single-step, first-order Arrhenius surface reaction with an oxidizer delivered to the particle surface through gas diffusion. Unlike the majority of models previously developed for flames in suspensions, no external parameters are imposed, such as particle ignition temperature, combustion time, or the assumption of either kinetic-or diffusion-limited particle combustion regimes. Instead, it is demonstrated that these parameters are characteristic values of the flame propagation problem that must be solved together with the burning velocity, and that the a priori imposition of these parameters from single-particle combustion data may result in erroneous predictions. It is also shown that both diffusive and kinetic reaction regimes can alternate within the same flame and that their interaction may result in nontrivial flame behavior. In fuel-lean mixtures, it is demonstrated that this interaction leads to certain particle size ranges where burning velocity increases with increasing particle size, opposite to the expected trend. For even leaner mixtures, the interplay between kinetic and diffusive reaction rates leads to the appearance of a new type of flame instability where kinetic and diffusive regimes alternate in time, resulting in a pulsating regime of flame propagation.
The novel concept of using gasless combustible mixtures as heating elements for materials processing in space and in ground-based microgravity facilities is presented. The unique properties of metal-sulfur combustible compositions (i.e.,... more
The novel concept of using gasless combustible mixtures as
heating elements for materials processing in space and in
ground-based microgravity facilities is presented. The unique
properties of metal-sulfur combustible compositions (i.e., high
flame temperatures, low ignition temperatures, liquid combustion products, nonporous charges, and gasless reactions) make
them ideally suited for such heating applications. Heating elements based on metal-sulfur combustion have an energy density more than order of magnitude greater than electrical batteries, can be easily integrated with processing samples, and can
operate under high pressures and in different gaseous environments. Demonstration prototypes of the gasless combustiondriven furnaces have already demonstrated peak temperatures
close to 2300 K and heating rates above 200 °C/s
heating elements for materials processing in space and in
ground-based microgravity facilities is presented. The unique
properties of metal-sulfur combustible compositions (i.e., high
flame temperatures, low ignition temperatures, liquid combustion products, nonporous charges, and gasless reactions) make
them ideally suited for such heating applications. Heating elements based on metal-sulfur combustion have an energy density more than order of magnitude greater than electrical batteries, can be easily integrated with processing samples, and can
operate under high pressures and in different gaseous environments. Demonstration prototypes of the gasless combustiondriven furnaces have already demonstrated peak temperatures
close to 2300 K and heating rates above 200 °C/s
Percolating reaction-diffusion waves in disordered random media are encountered in many branches of modern science, ranging from physics and biology to material science and combustion. Most disordered reactiondiffusion systems, however,... more
Percolating reaction-diffusion waves in disordered random media are encountered in many branches of modern science, ranging from physics and biology to material science and combustion. Most disordered reactiondiffusion systems, however, have complex morphologies and reaction kinetics that complicate the study of the dynamics. Flames in suspensions of heterogeneously reacting metal-fuel particles is a rare example of a reaction-diffusion wave with a simple structure formed by point-like heat sources having well-defined ignition temperature thresholds and combustion times. Particle sedimentation and natural convection can be suppressed in the free-fall conditions of sounding rocket experiments, enabling the properties of percolating flames in suspensions to be observed, studied, and compared with emerging theoretical models. The current paper describes the design of the European Space Agency PERWAVES microgravity combustion apparatus, built by the Airbus Defense and Space team from Bremen in collaboration with the scientific research teams from McGill University and the Technical University of Eindhoven, and discusses the results of two sounding-rocket flight experiments. The apparatus allows multiple flame experiments in quartz glass tubes filled with uniform suspensions of 25-micron iron particles in oxygen/xenon gas mixtures. The experiments performed during the MAXUS-9 (April 2017) and TEXUS-56 (November 2019) sounding rocket flights have confirmed flame propagation in the discrete mode, which is a prerequisite for percolating-flame behavior, and have allowed observation of the flame structure in the vicinity of the propagation threshold.
Stabilized aluminum flames are studied in the products of methane combustion. A premixed methane-air Bunsen flame is seeded with increasing concentrations of micron-size aluminum powder, and scanning emission spectroscopy is used to... more
Stabilized aluminum flames are studied in the products of methane combustion. A premixed methane-air Bunsen flame is seeded with increasing concentrations of micron-size aluminum powder, and scanning emission spectroscopy is used to determine the flame temperature via both the continuous and aluminum monoxide spectra. The flame burning velocity is measured and the condensed flame products are collected and analyzed for unburned metallic aluminum content. It was observed that, below a critical concentration of about 120 g∕m 3 , aluminum demonstrates incomplete oxidation with a flame temperature close to the methane-air flame. Below the critical concentration, the flame burning velocity also decreases similar to a flame seeded with inert silicon carbide particles. In contrast, at aluminum concentrations above the critical value, an aluminum flame front rapidly forms and is coupled to the methane flame. The flame temperature of the coupled methane-aluminum flame is close to equilibrium values with aluminum as a reactant, and the flame burning velocity remains flat for increasing aluminum concentrations. A simple theoretical estimation, which assumes that the aluminum reaction rate is controlled by the kinetic evaporation of aluminum, adequately predicts the critical concentration range at which the aluminum flame front can be coupled with the methane flame.
Several metals are reacted with supercritical water to produce hydrogen. Aluminum, aluminum alloys, and magnesium are found to be the most reactive. The solubility of the metal's oxide appears to be linked to the reactivity of the... more
Several metals are reacted with supercritical water to produce hydrogen. Aluminum, aluminum alloys, and magnesium are found to be the most reactive. The solubility of the metal's oxide appears to be linked to the reactivity of the metal.
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ABSTRACT Stabilized aluminum flames are studied in the products of methane combustion. A premixed methane–air Bunsen flame is seeded with increasing concentrations of micron-size aluminum powder, and scanning emission spectroscopy is used... more
ABSTRACT Stabilized aluminum flames are studied in the products of methane combustion. A premixed methane–air Bunsen flame is seeded with increasing concentrations of micron-size aluminum powder, and scanning emission spectroscopy is used to determine the flame temperature via both the continuous and aluminum monoxide spectra. The flame burning velocity is measured and the condensed flame products are collected and analyzed for unburned metallic aluminum content. It was observed that, below a critical concentration of about 120 g∕m3, aluminum demonstrates incomplete oxidation with a flame temperature close to the methane–air flame. Below the critical concentration, the flame burning velocity also decreases similar to a flame seeded with inert silicon carbide particles. In contrast, at aluminum concentrations above the critical value, an aluminum flame front rapidly forms and is coupled to the methane flame. The flame temperature of the coupled methane–aluminum flame is close to equilibrium values with aluminum as a reactant, and the flame burning velocity remains flat for increasing aluminum concentrations. A simple theoretical estimation, which assumes that the aluminum reaction rate is controlled by the kinetic evaporation of aluminum, adequately predicts the critical concentration range at which the aluminum flame front can be coupled with the methane flame.
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ABSTRACT Quenching distances for aluminum dust flames have been measured in an improved flow system which can yield stable, controlled, uniform dust mixtures. Experiments were performed with fine atomized aluminum dust (d32 = 5.4 μm). The... more
ABSTRACT Quenching distances for aluminum dust flames have been measured in an improved flow system which can yield stable, controlled, uniform dust mixtures. Experiments were performed with fine atomized aluminum dust (d32 = 5.4 μm). The dust dispersion technique uses an annular high-speed jet which disperses dust continuously supplied via a piston-type dust feeding system. Laminarized dust flow ascending in a vertical Pyrex tube (d = 0.05 m, L = 1.2 m) was ignited at the open tube end. Constant pressure flames propagating downwards were observed. A set of thin, evenly spaced steel plates was installed in the upper third part of the tube in order to determine the flame quenching distance. Three different stages of flame propagation were observed: laminar, oscillating, and turbulent accelerating flames. Flame speed and quenching distance as a function of dust concentration were determined during the laminar stage of flame propagation in dust-oxygen-nitrogen and in dust-oxygen-helium mixtures. It was found that the quenching distance and flame speed are very weak functions of dust concentration in rich mixtures. The minimum quenching distance is found to be about 5 mm in air and increases to 15 mm in mixtures of 11% O2. The substitution of nitrogen for helium in air increases the minimum quenching distance from 5 to 7 mm. A simple analytical model of a quasi one-dimensional dust flame with heat losses was developed with an assumption that the particle burning rate in the flame front is controlled by the process of oxygen diffusion. Algebraic equations defining flame speed were obtained in two limiting cases: lean and rich mixtures. The model predicts wide plateaus in the flame speed and quenching distance versus dust concentration plots in rich mixtures. These plateaus were observed experimentally. Calculated values of minimum quenching distances are in good agreement with experimental data.
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Spatially resolved emission spectra from Bunsen-type flames stabilized in aluminum suspensions in air and oxygenargon/helium mixtures were obtained using a mechanical-optical scanning system. A low resolution (1.5 nm) spectrometer was... more
Spatially resolved emission spectra from Bunsen-type flames stabilized in aluminum suspensions in air and oxygenargon/helium mixtures were obtained using a mechanical-optical scanning system. A low resolution (1.5 nm) spectrometer was used to acquire the broad spectra ...
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The exact solution for a reaction-diffusion front propagating in a heterogeneous system of discrete, point-like sources is obtained without resorting to a representation of the sources by a spatially continuous function. When the reaction... more
The exact solution for a reaction-diffusion front propagating in a heterogeneous system of discrete, point-like sources is obtained without resorting to a representation of the sources by a spatially continuous function. When the reaction time is smaller than the characteristic diffusion time between neighboring sources, the front speed predicted by this discrete source model differs from the continuum theory based on the spatial averaging of the heterogeneities. Furthermore, when the sources are regularly distributed in space, discreteness introduces a limit and propagation beyond this limit is only possible in a system with randomly distributed sources via local fluctuations of the concentration. The discrete regime of front propagation is observed experimentally in suspensions of iron particles burning in oxygen-xenon mixtures.
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Supercritical water is used to oxidize aluminum scrap and 3 mm aluminum slugs without employing catalysts, milling or specialized alloys.
The possibility of self-sustained solid-solid detonations (SSD) in Mn+S mixtures has been investigated. Charges 50 mm in diameter under different degrees of confinement were used. The initiation charges used include nitromethane, tetryl,... more
The possibility of self-sustained solid-solid detonations (SSD) in Mn+S mixtures has been investigated. Charges 50 mm in diameter under different degrees of confinement were used. The initiation charges used include nitromethane, tetryl, and C-4. No self-sustained SSD were observed in the present study. Neither did the degree of confinement nor the type or weight of the initiation charge appear to
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The critical conditions for the ignition of spherical aluminium particles dispersed during the detonation of long cylindrical explosive charges have been investigated experimentally. The charges consist of packed beds of aluminium... more
The critical conditions for the ignition of spherical aluminium particles dispersed during the detonation of long cylindrical explosive charges have been investigated experimentally. The charges consist of packed beds of aluminium particles, ranging in size from 3 - 114 mum in diameter, and saturated with sensitized liquid nitromethane (NM). The ignition conditions depend on both the charge and particle diameters
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The effect of the detonation of a spherical heterogeneous charge on the loading applied to a nearby structure has been investigated experimentally. The charge consists of a packed bed of solid reactive particles saturated with a liquid... more
The effect of the detonation of a spherical heterogeneous charge on the loading applied to a nearby structure has been investigated experimentally. The charge consists of a packed bed of solid reactive particles saturated with a liquid explosive. When the charge is detonated, the particles ignite while rapidly accelerating to high speeds, then impact either a rigid plate or a