Convective burning of an aluminum-water mixture

The burning of a stoichiometric mixture of aluminum (PAP-2 powder) with water in a constant-volume bomb is studied. It is shown that, depending on the charge diameter and igniter-generated pressure, three situations can arise: the mixture does not burn, burns slowly (in the layer-by-layer mode), or burns rapidly in the convective mode. The characteristics of the rapid burning, such as the effect of the igniter-generated burning, charge length, and initial charge density, are in general similar to those of the convective burning of mixtures of aluminum powder with an oxidizing agent (AP or PA), described in the literature. The difference lies in the fact that, due to a relatively low water activity as an oxidant, the convective burning of aluminum-water mixtures is harder to initiate, and it proceeds at a much lower velocity.     

Identification of droplet burning modes in lean, partially prevaporized swirl-stabilized spray flames

Experimental and numerical investigations of single droplet burning modes in a lean, partially prevaporized swirl-stabilized spray flame are reported. In the experiment single droplet flames have been visualized by CH-PLIF and simultaneous recording of the Mie signal. Two single droplet burning modes were identified: the envelope flame is a spherical diffusion flame burning at near-stoichiometric conditions. The wake flame is a potentially lean, partially premixed flame located downstream of the droplet. The droplet burning mode is of practical relevance, since it has significant impact on NO formation due to incomplete prevaporization.The droplet burning mode is determined by the ratio of chemical and convective time scales. The convective time scale is related to the droplet slip velocity. The impact of turbulent gas phase velocity fluctuations on droplet mechanics and droplet burning is discussed, based on a previous numerical investigation. In the present study the droplet slip velocity was measured with the 3D Phase Doppler (3D-PD) technique. For the measured slip velocities and ambient conditions in the hot gas region of the spray flame, simulations of single droplet burning were performed utilizing detailed models for chemical reaction, diffusive transport and vaporization. An agreement between the droplet burning modes predicted by the simulation and the droplet burning modes observed in the experiments was found.     

弱点火条件下炸药燃烧转爆轰的数值模拟

考察颗粒炸药从传导燃烧到对流燃烧再到爆轰的过程.对装填密度为85%的HMX颗粒炸药的燃烧转爆轰过程进行数值模拟,分析传导燃烧、对流燃烧和爆轰的发展过程.点火早期燃烧速度很低,火焰面在8.16 ms之内只前进了不到0.2 mm;形成对流燃烧之后燃烧速度快速增加,只用了0.1 ms就形成了速度为8 165 m·s^-1的稳定爆轰.当炸药颗粒直径或点火压力减小时,形成稳定爆轰所需的时间增加.     

Onset and development of convective burning in highly porous charges of ammonium perchlorate and its mixtures with aluminum

The onset and development of convective burning in the charges with a high porosity prepared from the finely dispersed ammonium perchlorate and its mixtures with aluminum ASD-4 is studied. The experiments were carried out in a constant volume bomb with the record of the pressure-time history and in the confinement with a slit, which makes it possible to perform simultaneously the photographic and piezometric recording of the process. Special attention is given to the mixtures with the increased aluminum content. The minimum lengths of samples are determined at which convective burning or explosion occur. The dependence of these lengths on the aluminum concentration in the mixture is determined. The possibility of convective burning and low-velocity detonation in ammonium perchlorate without the combustible additive is shown. It is established that the introduction of aluminum causes the ignition of the dispersed suspension behind the front of convective burning with the formation of the brightly glowing high-pressure zone (the secondary wave), which intensively expands in both sides from the place of origin. When the secondary wave overtakes the front of convective burning, the low-velocity detonation appears. The obtained results are of interest for explosion safety of the mixtures of ammonium perchlorate with aluminum and for designing generators of high-temperature suspensions with aluminum particles.     

High-rate combustion of cellulose nitrate in filled polymer systems

It was found that, at a certain external pressure, the conventional slow flameless combustion of nitrocellulose in filled compositions can go over into a high-rate low-temperature mode, with a burning velocity increasing by more than 40-fold. It was shown that the critical external pressure at which the changeover of the combustion modes occurs decreases with the sample porosity. It was assumed that the high-rate combustion of nitrocellulose in filled systems proceeds by the well-known mechanism of convective combustion of energetic condensed systems with some specific features.     

Convective burning of pressed aluminum-ammonium perchlorate charges

The convective burning of pressed aluminum-ammonium perchlorate (AP) charges with a porosity of 7 to 18% was studied. The experiments were performed at pressures of up to 300 MPa in a constant volume bomb provided with means for recording pressure-time diagrams, and in a nozzle setup equipped with a streak photocamera and piezoelectric pressure gauges. In contrast to loose-packed-density charges, which are highly explosive, the burning of pressed aluminum-AP charges propagates without marked acceleration, with a moderate velocity and a relatively slow rise in pressure in the bomb. The basic regularities were studied, and the key factors that determine the characteristics of convective burning, such as the aluminum particle shape (when a finely dispersed spherical-particle powder was replaced by a flaky aluminum powder with the same speciic surface area, the convective burning velocity decreased by more than an order of magnitude), ratio of mixture components, and charge porosity, were identified. The effects of the ammonium perchlorate particle size, an organic additive, and the ignitor mass were also studied. The experimental data were analyzed by invoking numerical modeling. The calculations were performed using a program developed earlier based on a model of the convective burning of aluminum-AP mixtures. The calculation results, which were in qualitative agreement with the available experimental data, made it possible to explain the main experimentally observed regularities. The compositions tested and the results obtained are of considerable interest for designing convective-burning charges for multipurpose pulse engines and thermo-and gas generators with operation durations from a few milliseconds to several tens of milliseconds.     

高密实含能颗粒床对流燃烧过程的实验研究

高密实含能颗粒床对流燃烧过程的实验研究张小兵，金志明，袁亚雄（南京理工大学动力学院南京２１００９４）关键词对流燃烧；含能颗粒床；实验研究１引言在高能燃料火箭推进系统和粒状火药炮膛内，火药颗粒燃烧的热量传递机理是强迫对流。当装填密度高到一定程度时，火焰...     

The effect of hydrodynamical instabilities and of convection in the conversion of an hadronic star to a more compact object

We study the hydrodynamical transition from an hadronic star into a quark or a hybrid star. We discuss the possible mode of burning, using a fully relativistic formalism and realistic Equations of State in which hyperons can be present. We take into account the possibility that quarks form a diquark condensate. We find that the conversion process always corresponds to a deflagration and never to a detonation. Hydrodynamical instabilities can develop on the front but the increase in the conversion’s velocity is not sufficient to transform the deflagration into a detonation. Concerning convection, it does not always develop. Instead, the process of conversion from ungapped quark matter to gapped quarks always allows the formation of a convective layer. The text was submitted by the authors in English.     

Electrical Conductivity and Temperature Distribution of Arc Plasma in Narrow Insulating Channels

Having examined electrical conductivity and temperature distribution of a cross-section of an arc plasma column burning in the narrow channel between insulating walls. It was shown that arc pressure created by intrinsic magnetic field has little effect at subsonic velocity. In contrast with an open arc with convective cooling, mean electrical conductivity of an arc in a narrow channel is significantly dependent on the current passing through it.     

An Earth-based equivalent low stretch apparatus for material flammability assessment in microgravity and extraterrestrial environments

The standard oxygen consumption (cone) calorimeter (described in ASTM E 1354 and NASA STD 6001 Test 2) is modified to provide a bench-scale test environment that simulates the low velocity buoyant or ventilation flow generated by or around a burning surface in a spacecraft or extraterrestrial gravity level. The equivalent low stretch apparatus (ELSA) uses an inverted cone geometry with the sample burning in a ceiling fire (stagnation flow) configuration. For a fixed radiant flux, ignition delay times for characterization material PMMA are shown to decrease by a factor of 3 at low stretch, demonstrating that ignition delay times determined from normal cone tests significantly underestimate the risk in microgravity. The critical heat flux for ignition is found to be lowered at low stretch as the convective cooling is reduced. At the limit of no stretch, any heat flux that exceeds the surface radiative loss at the surface ignition temperature is sufficient for ignition. Regression rates for PMMA increase with heat flux and stretch rate, but regression rates are much more sensitive to heat flux at the low stretch rates, where a modest increase in heat flux of 25 kW/m² increases the burning rates by an order of magnitude. The global equivalence ratio of these flames is very fuel rich, and the quantity of CO produced in this configuration is significantly higher than standard cone tests. These results demonstrate that the ELSA apparatus allows us to conduct normal gravity experiments that accurately and quantifiably evaluate a material’s flammability characteristics in the real-use environment of spacecraft or extraterrestrial gravitational acceleration. These results also demonstrate that current NASA STD 6001 Test 2 (standard cone) is not conservative since it evaluates a material’s flammability with a much higher inherent buoyant convective flow.     

Physical aspects of polymer combustion and the inhibition mechanism

The characteristics of polymer combustion were studied. The contributions from the conductive, convective, and radiation components of the total heat flow from the flame to the polymer surface were determined. The influence of inhibitors on the rate of chemical reactions in the preignition zone and on the rate of heat and mass exchange between the flame and the burning surface was estimated. It was shown that a change in the heat balance and heat and mass exchange accompanied by changing the optical properties of the flame and burning surface has a pronounced effect on the combustion rate at the flame edge. The formation of a protective coke layer reduces heat flow to the surface of a nonreacted polymer, leading to a decrease in the rate of evolution of the volatile combustible polymer-destruction products into the gas phase. As a result, the flame temperature decreases and it is extinguished.     

Using level sets for creating virtual random packs of non-spherical convex shapes

Random packs of spheres have been used to model heterogeneous and porous material morphologies during simulations of physical processes such as burning of coal char, convective burning in porous explosives, and regression of solid rocket propellant. Sphere packs have also been used to predict thermo-mechanical properties, permeability, packing density, and dissolution characteristics of various materials. In this work, we have extended the Lubachevsky–Stillinger (LS) sphere packing algorithm to create polydisperse packs of non-spherical shapes for modeling heterogeneity in complex energetic materials such as HMX and pressed gun propellants. In the method, we represent the various particle shapes using level sets. The LS framework requires estimates of inter-particle collision times, and we predict these times by numerically solving a minimization problem. We have obtained results for dense random packs of various convex shapes such as cylinders, spherocylinders, and polyhedra, and we show results with these various particles packed together in a single pack to high packing fraction.     

Single star evolution I. Massive stars and early evolution of low and intermediate mass stars

We survey the results of those model calculations of the early evolution of single stars that have been obtained over approximately the last decade, and compare some of these results with the observations, concentrating particularly on the comparison between theoretical predictions regarding surface abundances of red giants and Cepheids and abundance estimates obtained by an analysis of spectral data.For massive stars, we discuss the ramifications of the fact that the time scale for mass loss (via stellar winds) during main sequence and red supergiant evolution can be comparable to the nuclear burning timescale, noting in particular the unusual distribution of stars in the Hertzsprung-Russell diagram, and the probability that significant mass loss is responsible for the chemically highly evolved spectra of Wolf-Rayet stars. finally, we sketch (1) recent progresses in following the evolution of massive stars to the presupernova stage, which is described by a configuration consisting of a core of near-Chandrasekhar mass made up of iron peak elements and a series of “onion”-skin layers of less highly thermonuclearly processed matter and (2) recent progress in understanding the nature of the type II supernova phenomenon.For low and intermediate mass stars, we discuss postulated “extramixing” (beyond convective) processes, which may occur on the main sequence and on the first red giant branch, and continue on to a discussion of asymptotic giant branch evolution, placing considerable emphasis on the character of the thermal pulses that occur in such stars and, in particular, on the nucleosynthesis that occurs in the helium burning convective shells during these pulses and on the dredge-up phenomenon that brings fresh carbon and neutron-rich isotopes to the surface following pulse peak.     

Theory of the transition from conductive to convective burning

The experimentally known phenomenon of an abrupt transition from slow conductive to fast convective (penetrative) burning in a confined gas-permeable explosive is discussed. A simple model, involving only the most essential physical ingredients, is formulated and analyzed. A good qualitative agreement between theoretical and experimental dependencies is obtained. The transition is triggered by a localized autoignition in the extended resistance-induced preheat zone formed ahead of the advancing deflagration, provided the pressure difference between hot gas products and gases deep inside the pores of the unburned solid exceeds a certain critical level. In line with observations the critical overpressure increases with diminishing permeability.     

Three stage cool flame droplet burning behavior of n-alkane droplets at elevated pressure conditions: Hot,warm and cool flame

Transient, isolated n-alkane droplet combustion is simulated at elevated pressure for helium-diluent substituted-air mixtures. We report the presence of unique quasi-steady, three-stage burning behavior of large sphero-symmetric n-alkane droplets at these elevated pressures and helium substituted ambient fractions. Upon initiation of reaction, hot-flame diffusive burning of large droplets is initiated that radiatively extinguishes to establish cool flame burning conditions in nitrogen/oxygen “air” at atmospheric and elevated pressures. However, at elevated pressure and moderate helium substitution for nitrogen (X_He?>?20%), the initiated cool flame burning proceeds through two distinct, quasi-steady-state, cool flame burning conditions. The classical “Hot flame” (～1500?K) radiatively extinguishes into a “Warm flame” burning mode at a moderate maximum reaction zone temperature (～ 970?K), followed by a transition to a lower temperature (～765?K), quasi-steady “Cool flame” burning condition. The reaction zone (“flame”) temperatures are associated with distinctly different yields in intermediate reaction products within the reaction zones and surrounding near-field, and the flame-standoff ratios characterizing each burning mode progressively decrease. The presence of all three stages first appears with helium substitution near 20%, and the duration of each stage is observed to be strongly dependent on helium substitutions level between 20–60%. For helium substitution greater than 60%, the hot flame extinction is followed by only the lower temperature cool flame burning mode. In addition to the strong coupling between the diffusive loss of both energy and species and the slowly evolving degenerate branching in the low and negative temperature coefficient (NTC) kinetic regimes, the competition between the low-temperature chain branching and intermediate-temperature chain termination reactions control the “Warm” and “Cool” flame quasi-steady conditions and transitioning dynamics. Experiments onboard the International Space Station with n-dodecane droplets confirm the existence of these combustion characteristics and predictions agree favorably with these observations.     

Direct observation of the phenomenology of a solid thermal explosion using time-resolved proton radiography

We present a new phenomenology for burn propagation inside a thermal explosion based on dynamic radiography. Radiographic images were obtained of an aluminum cased solid cylindrical sample of a plastic bonded formulation of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine. The phenomenology observed is ignition followed by cracking in the solid accompanied by the propagation of a radially symmetric front of increasing proton transmission. This is followed by a further increase in transmission through the sample, ending after approximately 100 micros. We show that these processes are consistent with the propagation of a convective burn front followed by consumption of the remaining solid by conductive particle burning.     

Investigation of interaction between methanol fed tandem porous spheres burning in a mixed convective environment

Flame interaction during the burning of two porous spheres in tandem arrangement fed with methanol and subjected to a mixed convective environment, has been studied experimentally and numerically. Porous sphere technique is employed for experimentally simulating the burning characteristics of methanol transpired spheres of different sizes, separated by fixed distances. The mass burning rates from both the spheres and visible flame stand-off distances from the sphere surfaces have been measured in the experiments. In the numerical simulations, transient, axisymmetric, mass, momentum, species and energy conservation equations are solved using a finite volume technique based on non-orthogonal semi-collocated grids. Features of the numerical model include finite rate chemistry and temperature and mixture composition dependent thermo-physical properties. Burning of tandem porous spheres in an air stream flowing vertically upwards, at atmospheric pressure has been simulated for different sphere sizes, separation distances and free stream velocities. The numerical predictions have been compared with experimental results. Results reveal that when two spheres burn one over the other, the transition from envelope to wake flame is delayed when compared with that of an isolated sphere. For two spheres of same diameter burning one over the other, depending on the separation distance, flame blows-off after the occurrence of transition from envelope to wake flame in the bottom sphere. For the case of larger sphere at the top, either the flame stabilises in the recirculation zone formed in between the spheres or the flame from the smaller sphere lifts off and stabilises near the front portion of the larger sphere, depending on the separation distance. At higher separation distances, around four times the diameter of the sphere, both the spheres burn independently. The burning rate undergoes complex variations with air stream velocity depending on the sphere sizes and separation distances.     

Microgravity droplet combustion: effect of tethering fiber on burning rate and flame structure

Droplets tethering on fibers has become a well established technique for conducting droplet combustion experiments in microgravity conditions. The effects of these supporting fibers are frequently assumed to be negligible and are not considered in the experimental analysis or in numerical simulations. In this work, the effect of supporting fibers on the characteristics of microgravity droplet combustion has been investigated numerically; a priori predictions have then been compared with published experimental data. The simulations were conducted using a transient one-dimensional spherosymmetric droplet combustion model, where the effect of the supporting fiber was implicitly taken into account. The model applied staggered convective flux finite volume method combined with high-order implicit time integration. Thermal radiation was evaluated using a statistical narrow band radiation model. Chemical kinetics and thermophysical properties were represented in rigorous detail. Tether fiber diameter, droplet diameter, ambient pressure and oxygen concentration were varied over a range for n-decane droplets in the simulations. The results of the simulations were compared to previously published experiments conducted in the Japan Microgravity Center (JAMIC) 10 second drop tower and the NASA Glenn Research Center (GRC) 5.2 second drop tower. The model reproduces closely nearly all aspects of tethered n-decane droplet burning phenomena, which included droplet burning history, transient and average burning rate, and flame standoff ratio. The predictions show that the presence of the tethering fiber significantly influences the observed burning rate, standoff ratio, and extinction.     

High power single-mode cw dye laser with Michelson mode selector

The application of a Michelson mode selector for high power single-mode operation of a cw dye laser is reported. By employing the selector for compensation of spatial hole burning effects in a standing-wave cavity, a tunable single-mode output power of 1 W has been obtained at 590 nm. A double Michelson selector has also been applied with the spatial hole burning taken into account.     

Transient convective burning of a periodic fuel-droplet array

The transient convective burning of fuel-droplets interacting within 3-D infinite periodic arrays in a hot gas stream is numerically studied for the first time, with considerations of droplet regression, deceleration due to the drag of the droplets, internal liquid motion, variable properties, non-uniform liquid temperature, surface tension, and n-octane one-step oxidation kinetics. Depending upon the initial conditions and other constraints, a flame is established early as either a wake flame or an envelope flame. An initial envelope flame remains an envelope flame, and an initial wake flame has a tendency to develop from a wake flame to an envelope flame. The flame shows no strong tendency to modify significantly the standoff distance during the lifetime of the droplet. For an initial wake flame, the moment of wake-to-envelope transition is advanced as the initial droplet spacing (intermediate) is decreased, but tends to be postponed as the initial droplet spacing is further reduced. The burning rate at smaller initial droplet spacing or smaller initial Reynolds number might be greater for some period during the lifetime because of an earlier wake-to-envelope transition which elevates the average surface temperature. Lower ambient temperature yields a later wake-to-envelope transition time and smaller mass burning rate. At the lower ambient pressure with the same initial relative stream velocity, the average surface temperature is reduced, the wake-to-envelope transition is later, and the mass burning rate is smaller. Validation of our analysis is made by comparing with the results of an isolated droplet Wu and Sirignano [11].