Samuel Goroshin

Quenching distance of laminar flame in aluminum dust clouds

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.

Particle jet formation during explosive dispersal of solid particles

Previous experimental studies have shown that when a layer of solid particles is explosively dispersed, the particles often develop a non-uniform spatial distribution. The instabilities within the particle bed and at the particle layer interface likely form on the timescale of the shock propagation through the particles. The mesoscale perturbations are manifested at later times in experiments by the formation of coherent clusters of particles or jet-like particle structures, which are aerodynamically stable. A number of different mechanisms likely contribute to the jet formation including shock fracturing of the particle bed and particle-particle interactions in the early stages of the dense gas-particle flow. Aerodynamic wake effects at later times contribute to maintaining the stability of the jets. The experiments shown in this fluid dynamics video were carried out in either spherical or cylindrical geometry and illustrate the formation of particle jets during the explosive dispersal process. The number of jet-like structures that are generated during the dispersal of a dry powder bed is compared with the number formed during the dispersal of the same volume of water. The liquid dispersal generates a larger number of jets, but they fragment and dissipate sooner. When the particle bed is saturated with water and explosively dispersed, the number of particle jets formed is larger than both the dry powder and pure water charges. More extensive experiments that explore the effect of particle size, density and the mass ratio of explosive to particles on the susceptibility for jet formation are reported in Frost et al. (Proc. of 23rd ICDERS, Irvine, CA, 2011).

Flame speed in a binary suspension of solid fuel particles

Most natural and industrial combustible dusts have a wide distribution of particle sizes. Yet, the majority of experimental data on flame propagation in dust clouds are given in relation to some average particle size, and all known theoretical models of dust combustion consider only monosize suspensions. Since the ignition temperature and combustion rate of an individual dust particle are functions of particle size, the flame in real dust suspensions has a complex, multistage structure. As a first step toward understanding multisize dust combustion, the combustion of a suspension of two monosize powders (that in general can be also of different chemical nature) is investigated in the present work theoretically and experimentally. A simple analytical model developed for the flame in a fuel-lean binary suspension permits flame speed and structure to be analyzed as a function of the dust composition and combustion properties of individual particles. The flame speeds predicted by the binary model were compared with flame speeds calculated from a model of monosize dust flame using various average particle size representations. It is shown that averaging of the particle size in general fails to correctly predict the flame speed over the wide range of the binary dust compositions. The flame propagation speed in a binary suspension of aluminum and manganese powders was investigated experimentally by observing the laminar stage of flame propagation in a semi-open vertical tube. The model correctly predicts dependence of the flame speed on mixture composition (mass ratio of manganese and aluminum dusts in suspension) and the mixture composition at the limit of flame propagation.

Powdered Metals as Fuel for Hypersonic Ramjets

The concept of a hypersonic ramjet fueled by powdered metals (B, Al, Mg, and MgB2) is considered. Thermodynamic calculations of the combustion heat release, specific impulse, and volumetric specific impulse show that metal fuels can exceed hydrocarbon fuels in volumetric energy content and, in the case of boron, exceed conventional fuels on a mass basis as well. The refractory nature of metal fuels and their combustion products also suggest they may permit ramjets utilizing subsonic combustion to extend their operation to hypersonic Mach numbers (greater than Mach 5). The feasibility of stabilizing combustion using a pure powdered-metal fuel without the use of hydrocarbons is investigated experimentally. The ability to effectively inject powdered metal into a combustor is demonstrated using a laboratory-sca le dispersion apparatus. This apparatus is then used to measure fundamental burning characteristics of aluminum powder suspensions in air. The burning rate of micron-size aluminum air suspensions is seen to be similar to gaseous hydrocarbons in air, but the dependence of burning rate on fuel equivalence ratio is very different from gas flames. A nearly constant plateau in burning velocity obtained with fiiel-rich mixtures suggests that the combustor must operate rich for stable operation. Thermodynamic calculations show that maximum specific impulse is obtained for lean metal-air mixtures. Thus, a conceptual design of the dust combustor is proposed that uses a preburner to stabilize a fiiel-rich metal flame followed by gradual mixing of the burning suspension with a secondary airflow to obtain a high air/fuel excess ratio.

Combustion of Aluminum Suspensions in Hydrocarbon Flame Products

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...

Aluminum Particle Combustion in High-Speed Detonation Products

Aluminum particles ranging from 2 to 100 μm were subjected to the flow of detonation products of a stoichiometric mixture of hydrogen and oxygen at atmospheric pressure. Luminosity emitted from the reacting particles was used to determine the reaction delay and duration. The reaction duration was found to increase as d n with n ≈ 0.5, which is more consistent with kinetically controlled reaction rather than the classical diffusion-controlled regime. Emission spectroscopy was used to estimate the combustion temperature, which was found to be well below the flow temperature. This fact also suggests combustion in the kinetic regime. Finally, the flow field was modeled with a CFD code, and the results were used to model analytically the behavior of the aluminum particles.

Effect of discreteness on heterogeneous flames: Propagation limits in regular and random particle arrays

In a system with discrete heat sources distributed in an inert, heat conducting medium, there exists two asymptotic regimes of flame propagation. When the flame thickness is much greater than the inter-particle spacing, the system approximates a homogeneous medium and the flame can be modeled as a continuum. In the other extreme, when the flame is very thin due to rapid reaction of particles, the heterogeneous flame can no longer be treated as a continuum since discrete effects become dominant. The effects of discreteness are characterized by a strong dependence on the spatial distribution of the sources. The present work investigates the effects of discreteness on flame propagation and demonstrates that these effects result in a propagation limit in the absence of losses. For a system of regularly spaced particles, this limit can be found analytically for one-, two-, and three-dimensional systems, although the flame exhibits a complex dynamic of bifurcations as it approaches this limit. Propagation of a flame beyond this limit is only possible through concentration fluctuations in a system with randomly distributed particles. Two-dimensional numerical simulations with randomly distributed particles show a strong dependence of the propagation limit on the size of the computational domain. A consequence of the random particle distribution is that the flammability limit can only be defined as a probabilistic outcome of the flame propagation simulations.

Powdered Magnesium-Carbon Dioxide Propulsion Concepts for Mars Missions

The use of a fuel that can be burned directly with carbon dioxide derived from the Martian atmosphere has recently received attention as an alternative to chemical in situ propellant production strategies. Prior studies of theoretical performance (specific impulse) have identified magnesium as being one of the more promising fuels to bum with carbon dioxide, and magnesium has the additional advantages of being a dense, noncryogenic, storable, and environmentally benign fuel. While magnesium has been shown to bum in carbon dioxide, studies to date have been conducted with single particles, with little attention being paid to the details of how the metal fuel would be used in an actual rocket engine. This paper reyiews various options for a magnesium/carbon dioxide rocket (slurry, hybrid, etc.), and the concept of using compacted powder and carbon dioxide as bi-propellants is selected as being the most promising. The use of small particle size powdered magnesium in combination with an effective dispersion system has the potential to give the highest burning rates while minimizing particle agglomeration and two-phase losses. In order to obtain the greatest mass leveraging by in situ utilization, the magnesium/ carbon dioxide engine must‘ be able to stabilize combustion without the use of additional hydrocarbon fuels. The issues associated with magnesium/carbon dioxide combustion are addressed theoretically and experimentally. A laboratory powder dispersion system demonstrates the feasibility of working with pure powdered metal and is used to obtain some preliminary data on flame pfopagation in powdered magnesium suspensions in gaseous carbon dioxide at standard pressure and temperature. A flame speed of 1 m/s was observed .for magnesium in carbon dioxide at concentrations of approximately 800 g/m3. Flame propagation could not be established at lower

METAL-SULFUR COMBUSTION

A number of metal-sulfur compounds have been synthesized in the present investigation via self-propagating, high-temperature synthesis (SHS) process. Equilibrium calculations indicate that a large number of metals react with sulfur energetically, yielding adiabatic flame temperatures sufficiently high to support self-sustained flame propagation. To permit the combustion process to proceed entirely in the condensed phase, the pressure of the environment has to be kept at elevated levels (1–20 M Pa) for most of the metal-sulfur mixtures studied. Computed adiabatic flame temperatures of some of the metal-sulfur reactions exceed 5000 K (Ca−S, Sr−S, Ba−S). The present experimental investigation was carried out to verify the equilibrium calculations. Seven metals have been studied (i.e., Mg, Si, Ti, Mn, Cr, n, and Mo). Samples are prepared by melting the metal-sulfur precursor mixture, which eliminates any trapped gas so that the density of the sample approaches the theoretical value. X-ray diffraction analysis of the products shows that the synthesized samples have a high purity and that the reactions are close to completion. Ignition temperatures for the metal-sulfur mixtures were found to be in the range of 350–600 °C. Flame speeds measured for several of the metal-sulfur mixtures range from 5 to 10 mm/s. The large-energy-release, low-ignition temperatures and high adiabatic flame temperatures associated with metal-sulfur combustion make this process also promissing for a variety of “energetic” applications.

In-situ measurements of the onset of bulk exothermicity in shock initiation of reactive powder mixtures

The shock initiation process was directly observed in different powder mixtures that produce little or no gas upon reaction. The samples of reactive powder were contained in recovery capsules that permitted the samples to be analyzed after being shocked and that allowed the initiation of reaction to be monitored using three different methods. The microsecond time-scale processes were observed via a fast two-color pyrometer. Light intensity detected from the bottom of reactive samples was slightly greater compared to inert simulants in the first 10 μs after shock arrival. However, this light was much less intense than that which would correspond to the bulk of the material reacting. Thus it seemed that only small, localized zones, or hot spots, had begun to react on a time scale of less than 30 μs. Light emissions were then recorded over longer time scales, and intense light appeared at the bottom of samples a few milliseconds to a few hundreds of milliseconds after shock arrival at the bottom of the test ...