Magnetic behavior and physicochemical properties of Ni and NiO nano-particles

A series of nano-crystalline Ni/NiO particles was synthesized by a combustion route depending upon the glycine-nitrate process. The as prepared samples were characterized by X-ray diffraction (XRD), scanning electron micrograph (SEM), transmission electron micrograph (TEM), nitrogen adsorption isothems at 77 K and vibrating sample magnetometer (VSM) techniques.The XRD results revealed that the Ni powder crystallizes was formed with the cubic phase when the molar ratio of glycine to nitrate is 1.5. Above or below that molar ratio, NiO phase coexists as an impurity along with the Ni phase. The SEM and TEM measurements of the as synthesized powders showed that the particles are irregular in shapes and have porous morphology. Increasing the ratio between glycine and Ni-nitrate resulted in slightly agglomeration and grain growth of nano-particles with subsequent decrease in the value of surface area depending upon high combustion heat. The magnetization value of Ni measured at room temperature is very close to the value observed for commercial Ni powder.     

Influence of nitrogen in aluminum droplet combustion

The influence of nitrogen on the aluminum droplet combustion under forced convection conditions has been studied. An aerodynamic levitation technique of millimetric size liquid droplets heated with a CO₂ laser has been adopted to characterize the combustion of aluminum droplets and, in particular, to observe the surface phenomena. The determination of the burning rate and of the droplet temperature in several atmospheres (H₂O/O₂, H₂O/Ar, H₂O/N₂, and air) has shown that they depend only on the nature and concentration of the oxidizers (O₂ and H₂O); a comparison of experiments in nitrogen and in argon containing mixtures demonstrated that N₂ did not influence the gas phase combustion. However, for nitrogen containing atmospheres we observed the formation of solid aluminum nitride (AlN) at the droplet surface after a latency time depending on the nitrogen pressure. AlN first interacts with the oxide cap producing an aluminum oxynitride, then completely covers the droplet, and finally prevents combustion. The existence of a latency time varying with the nitrogen pressure suggests that the AlN formation is controlled by heterogeneous kinetics. The phenomenon of oxide cap regression during combustion was also observed in all gases, and it is attributed to a chemical decomposition process of alumina by aluminum forming gaseous Al_xO_y species. Therefore, nitrogen effects are significant at the droplet surface rather than in the gas phase, and it is suggested that N₂ is probably one of the main species causing the manifestation of unsteady processes during aluminum droplet burning.     

Effect of hydrogen content in titanium on the structure of the front and the specific features of the combustion of a Ti + 0.5C granular mixture in a cocurrent nitrogen flow

Specific features of the combustion of a titanium–carbon black granular mixture in a quartz tube blown with nitrogen is studied. In contrast to the previous studies, titanium with increased hydrogen content is used. The gas flow (cocurrent filtration) is produced by applying a fixed pressure differential less than 1 atm across the inlet and outlet of the tube. The characteristics of the combustion of a Ti + 0.5C granular mixture in a nitrogen flow are investigated. It is shown that an increased content of hydrogen in titanium leads to the formation of a two-front combustion wave. It is demonstrated that the first and second fronts of the two-front structure arise due to the interaction of the charge with nitrogen. The velocities of propagation of the fronts increase with the nitrogen pressure differential.     

A study of the characteristics of the combustion of Ti + <Emphasis Type="Italic">x</Emphasis>C (<Emphasis Type="Italic">x</Emphasis> &gt; 0.5) powder and granular compositions in a gas coflow

The characteristics of the combustion of Ti + 0.5C, Ti + 0.75C, and Ti + C powder and granular mixtures in a flow of inert (argon) and reactive (nitrogen) gases at various pressure differences are studied. It is shown that the influence of the pressure difference on the burning velocity of the powder mixture decreases with increasing fraction of carbon in it, but a pressure difference of 1 atm producing practically no effect on the burning rate of the Ti + C mixture. The data obtained are indicative of a nonequilibrium mechanism of the combustion of Ti + xC granular mixtures in a nitrogen coflow, in which case the sequence of chemical reactions in the combustion wave is determined by the kinetic characteristics of the interaction of titanium with nitrogen and carbon. It is concluded that the reactive gas flow ignites the surface of the granules and thereby leads the propagation of the combustion wave. It is established that, for all the mixtures studied, the mechanism of the combustion of a granular charge in a nitrogen flow is fundamentally different from the combustion of a powder charge under the same condition.     

Scaling-Up fire

The role of combustion research in fire safety is revisited through the process of Scaling-Up fire. Scaling-Up fire requires the adequate definition of all the building blocks and couplings associated with the construction of a fire model. The model then has to deliver predictions of the evolution of a fire and its environment with the precision, completeness and robustness relevant to fire safety. Areas of combustion research relevant to the development of fire models emerge from an assessment of methodology, complexity, incompatibility and uncertainty associated to the Scaling-Up process.     

Behaviour of a confined fire located in an unventilated zone

The behaviour of a fire in an enclosure is studied for a configuration where the fuel source is located in the upper hot unventilated zone trapped by a soffit. The experimental study, undertaken in a laboratory-scale compartment with a fuel source above the level of a soffit, included the determination of the parameters (ventilation factor, rate of fuel supply) controlling the combustion or leading to extinction. Measurements (PIV, thermocouples, gas sampling and analysis) were performed to propose a hypothesis on the structure of the flame (flame stabilisation mechanisms, premixed or diffusion types). Video photography is used to determine the area covered by the flames. This information is used as a criterion to identify the combustion regimes. The results show that the gaseous fuel is diluted in the combustion products (CP) in the upper layer and that a recirculatory motion is formed, driven by buoyancy forces, which enhances the mixing of fuel and CP. These then travel horizontally towards the vent along the interface between the lower fresh air and upper zones, and are premixed with the convected air in the enclosure, before entering the reaction zone and being burnt. The flame stabilises at the interface between the upper hot and lower ventilated layers in the compartment. The observed “ghosting flame” is stabilised by a triple flame if the flame speed of the premixed flame is higher than the natural convection velocity induced in the compartment. The flame stability is quantified by a criterion based on the area of the horizontal flame. It has been observed that the combustion is controlled by the available mass fuel flux at the reaction zone if the ventilation is sufficient. This information is essential for the modelling of the phenomena involved in fires with such an underventilated fuel source.     

Validation of flow simulation and gas combustion sub-models for the CFD-based prediction of NOx formation in biomass grate furnaces

While reasonably accurate in simulating gas phase combustion in biomass grate furnaces, CFD tools based on simple turbulence–chemistry interaction models and global reaction mechanisms have been shown to lack in reliability regarding the prediction of NO_x formation. Coupling detailed NO_x reaction kinetics with advanced turbulence–chemistry interaction models is a promising alternative, yet computationally inefficient for engineering purposes. In the present work, a model is proposed to overcome these difficulties. The model is based on the Realizable k–? model for turbulence, Eddy Dissipation Concept for turbulence–chemistry interaction and the HK97 reaction mechanism. The assessment of the sub-models in terms of accuracy and computational effort was carried out on three laboratory-scale turbulent jet flames in comparison with the experimental data. Without taking NO_x formation into account, the accuracy of turbulence modelling and turbulence–chemistry interaction modelling was systematically examined on Sandia Flame D and Sandia CO/H₂/N₂ Flame B to support the choice of the associated models. As revealed by the Large Eddy Simulations of the former flame, the shortcomings of turbulence modelling by the Reynolds averaged Navier–Stokes (RANS) approach considerably influence the prediction of the mixing-dominated combustion process. This reduced the sensitivity of the RANS results to the variations of turbulence–chemistry interaction models and combustion kinetics. Issues related to the NO_x formation with a focus on fuel bound nitrogen sources were investigated on a NH₃-doped syngas flame. The experimentally observed trend in NO_x yield from NH₃ was correctly reproduced by HK97, whereas the replacement of its combustion subset by that of a detailed reaction scheme led to a more accurate agreement, but at increased computational costs. Moreover, based on results of simulations with HK97, the main features of the local course of the NO_x formation processes were identified by a detailed analysis of the interactions between the nitrogen chemistry and the underlying flow field.     

Inclined fire whirls

Air entrainment, leading to strong fire whirls, is commonly thought to be caused by the buoyant rise of the hot combustion products under the influence of gravity. We have, however, created in the laboratory steady, axisymmetric strong fire whirls with axes inclined 30° from the vertical orientation, whirls which model an inclined fire whirl, about 30 m tall, observed in California wildland near the Cleveland National Forest. The results contradict the common notion of buoyancy being significant for the structure of the whirl, implying that strong fire whirls instead are dominated by rotation only, even if their axis is vertical. The new concept of rotation-controlled fire whirls is explained by a Rossby number displacing the Froude number (or the Richardson number) in describing the phenomenon.     

烟火药和压缩氮气声源水下声辐射特征对比研究

为研究烟火药水下燃烧声辐射机理,采用烟火药和压缩氮气喷射声源对比的方法,利用水声测试系统,通过实验研究不同体积流量下两种声源装置的声辐射规律。结果表明,烟火药水下燃烧声源与压缩氮气声源的声辐射特征相似,辐射频率主要集中在0～1000 Hz内,峰值频率均位于100 Hz附近,总声压级、峰值声压级均随着气体流量增加而增强。当气体流量从60 ml/s增加到84 ml/s时,烟火药峰值声压级由155 d B增加到163 d B,0～1000 Hz内总声压级由159 d B增加到165 d B;当喷气流量从70 ml/s增加到141 ml/s时,压缩氮气源峰值声压级由136 d B增加到139 d B,0～1000 Hz内总声压级由144 d B增加到147 d B。当气体流量相近时,烟火药相比压缩氮气声压级相差显著,其声压级均高于同频率下压缩氮气源,两者的峰值声压级分别为157 d B、139 d B,0～1000 Hz内总声压级分别为160 d B、147 d B。     

Modeling of HMX combustion

A mechanism of HMX combustion was proposed and the corresponding model was developed under the assumption that the combustion wave consists of two zones, with consideration given to the reaction of decomposition and vaporization of the initial energetic material in the condensed phase and the subsequent decomposition of its vapor in the gas phase. An analysis of the results showed that, at low pressures, the burning rate is largely determined by the exothermic decomposition of the material in the condensed phase, but at pressure above ∼20 atm, the processes in the gas phase begin to play an increasingly important role, where the limiting process is the bimolecular activation reaction with the subsequent dissociation of HMX accompanied by the secondary reactions between the products. A comparison of the calculation results with experimental data showed that the model adequately describes a number of characteristics of the combustion wave and ballistic properties, such as the burning rate and its sensitivity to pressure and initial temperature.     

Effects of the penetration height of ethylene transverse jets on flame stabilization behavior in a Mach 2 supersonic crossflow

Effects of fuel jet penetration height on supersonic combustion behaviors were investigated experimentally in a supersonic combustion ramjet model combustor at a Mach speed of 2 and at a stagnation temperature of 1900 K. The jet-to-crossflow momentum flux ratio was varied to control the fuel-jet penetration height, using several injectors with different orifice diameters: 2, 3, and 4 mm. First, transverse nitrogen jets were observed to identify a relationship between the fuel jet penetration height and the momentum flux ratio by focusing Schlieren photography. Then, supersonic combustion behaviors of ethylene were investigated through combustion pressure measurements. Simultaneously, time-resolved images of CH* chemiluminescence and shadowgraphs were recorded with high-speed video cameras. Furthermore, a morphology of supersonic combustion modes was investigated for various equivalence ratios and fuel penetration heights in a two-dimensional latent space trained by the shared Gaussian process latent variable models (SGPLVM), considering CH* chemiluminescence images and the shock parameters. The results indicated that the penetration height of nitrogen jets was a function of the jet momentum flux ratio; this function was expressed by a fitting curve. Five typical combustion modes were identified based on time-resolved CH* chemiluminescence images, shadowgraphs, and pressure profiles. Even for a given equivalence ratio, different combustion modes were observed depending on the fuel penetration height. For an injection diameter of 3 and 4 mm, cavity shear-layer and jet-wake stabilized combustions were observed as the scram modes. On the other hand, although the cavity shear-layer and lifted-shear-layer stabilized combustions were observed, no jet-wake stabilized combustion was observed for an orifice diameter of 2 mm. Fuel penetration heights above the cavity aft wall were expected to affect the combustion behavior. Finally, a morphology of the supersonic combustion modes was clearly shown in the two-dimensional latent space of the SGPVLM.     

Nitrogen determination by SEM‐EDS and elemental analysis

This paper describes a methodology for the analysis of nitrogen by scanning electron microscope with an energy dispersive X‐ray spectrometer (SEM‐EDS). The methodology was developed to have a rapid and accurate alternative method to the elemental analysis by combustion and thermoconductivity detection that does not imply the decomposition of the sample. Two methods by SEM‐EDS were established: a quantitative method trying to construct a calibration curve with reference materials and another using the standardless method provided with the instrument software, and the results were compared with those obtained by elemental analysis using two instruments that work at different temperature. An important matrix effect was found when trying to construct a calibration curve for SEM‐EDS for any kind of material, being corrected when using the standardless method because this method corrects the matrix effect. The quantification of nitrogen by SEM‐EDS is a good alternative to elemental analysis by combustion and thermoconductivity detection in those cases where the sample has a very high decomposition temperature. Copyright © 2013 John Wiley & Sons, Ltd.     

Specifics of the combustion of aluminum-water mixtures

Experimental studies of the combustion of mixtures of micron-sized flaky aluminum powder with unthickened water in different conditions at atmospheric and high pressure in nitrogen and argon are performed. The density and composition of the mixture are varied. The regularities of the combustion have been established. A filtration wave of hot hydrogen ahead of the combustion front in samples with high porosity has been revealed. For the combustion under a nitrogen atmosphere, the pressure exponent in the burning rate law is close to 0.47 in a wide range of pressures. For the combustion under an argon atmosphere at pressures above 50 atm, the pressure exponent becomes zero or negative. Aluminum powder is demonstrated to be able to burn under conditions of a separated charge, where the fuel (aluminum) and oxidizer (water) are separated by a thin partition or brought in direct contact. The fast convective burning of aluminum-water mixtures in a semiclosed volume is discovered.     

Modelling biodiesel–diesel spray combustion using multicomponent vaporization coupled with detailed fuel chemistry and soot models

A multicomponent vaporization model is integrated with detailed fuel chemistry and soot models for simulating biodiesel–diesel spray combustion. Biodiesel, a fuel mixture comprised of fatty-acid methyl esters, is an attractive alternative to diesel fuel for use in compression-ignition engines. Accurately modelling of the spray, vaporization, and combustion of the fuel mixture is critical to predicting engine performance using biodiesel. In this study, a discrete-component vaporization model was developed to simulate the vaporization of biodiesel drops. The model can predict differences in the vaporization rates of different fuel components. The model was validated by use of experimental data of the measured biodiesel drop size history and spray penetration data obtained from a constant-volume chamber. Gas phase chemical reactions were simulated using a detailed reaction mechanism that also includes PAH reactions leading to the production of soot precursors. A phenomenological multi-step soot model was utilized to predict soot emissions from biodiesel–diesel combustion. The soot model considered various steps of soot formation and destruction, such as soot inception, surface growth, coagulation, and PAH condensation, as well as oxidation by oxygen and hydroxyl-containing molecules. The overall numerical model was validated with experimental data on flame structure and soot distributions obtained from a constant-volume chamber. The model was also applied to predict combustion, soot and NOx emissions from a diesel engine using different biodiesel–diesel blends. The engine simulation results were further analysed to determine the soot emissions characteristics by use of biodiesel–diesel fuels.     

Numerical modelling of the work of a pulsed aerosol system for fire fighting at the ignitions of liquid hydrocarbon fuels

The work of a pulsed aerosol system for fire fighting is modelled, which is designed for fire fighting at oil storages and at the spills of oil products, whose vapors were modelled by gaseous methane. The system represents a device for separate installation, which consists of a charge of solid propellant (the gas generator) and a container with fine-dispersed powder of the flame-damper substance. The methane combustion was described by a one-stage gross-reaction, the influence of the concentration of vapors of the flame-damper substance on the combustion process was taken into account by reducing the pre-exponent factor in the Arrhenius law and was described by an empirical dependence. The computational experiment showed that the application of the pulsed aerosol system for fire fighting ensures an efficient transport of fine-dispersed aerosol particles of the flame-damping substance and its forming vapors to the combustion zone; the concentration of particles ensures the damping of the heat source. The work was financially supported by the President of the Russian Federation (NSh-9886.2006.9), Presidium of RAS (Project No. 4.1), and the Program of Interdisciplinary Integration Basic Research of SB RAS No. 26.     

Effet de la température sur la taille des gouttes d'un jet diesel

The fuel spray from a one-hole injector is characterized by experimental conditions close to those of the Diesel engine. Droplet sizes and velocity histories were measured using a phase Doppler analyzer in a transparent constant volume combustion chamber. This bomb was filled with nitrogen at a pressure of 2 MPa, in a temperature range of 20 to 325 °C. The combined effects of temperature coalescence and aerodynamic drag lead to an increase of the Sauter mean diameter with the penetration of the spray. At room temperature, the coalescence process predominates. At higher temperatures, this growth is mainly due to the evaporation of the smallest drops.     

Autowave synthesis of cast aluminum oxynitrides with a high nitrogen content

Aluminum oxynitrides Al₂₂O₃₀N₂ and Al₂₃O₂₇N₅ and a number of Al-O-N solid solutions with a nitrogen content of 2.3 to 6.5 wt % were prepared from (CrO₃-Al-Al₂O₃-AlN) thermite-type mixtures with a high combustion temperature (2950–3200 K) by using the autowave synthesis method. The synthesis was conducted in a hermetically sealed reactor at an initial nitrogen pressure of 4.0 to 8.0 MPa. The aluminum nitride content was demonstrated to influence the nitrogen content in cast aluminum oxynitrides and on their microstructure and phase composition.     

Surface combustion on a ceramics-coated foamed-metal matrix

The results of a comparative study of the characteristics of surface combustion in the infrared mode on a flat and volumetric foamed-metal matrix with a ceramic (alumina) coating are reported. The coating of thickness ~200 μm was applied by using the detonation method. It was shown that the covering of the matrix with a material having a lower emissivity and thermal diffusivity causes the flame front to immerse into the matrix and increases the surface layer temperature. For combustion on a flat matrix at a firing rate of ~75 W/cm², the concentrations of nitrogen oxides and carbon monoxide were up to two times lower. For combustion in a volumetric matrix at a firing rate of ~30 W/cm², the reduction in the concentration of nitrogen oxides was two to three times lower.     

Simulating fire whirls

A numerical investigation of swirling fire plumes is pursued to understand how swirl alters the plume dynamics and combustion. One example is the ‘fire whirl’ which is known to arise naturally during forest fires. This buoyancy-driven fire plume entrains ambient fluid as heated gases rise. Vorticity associated with a mechanism such as wind shear can be concentrated by the fire, creating a vortex core along the axis of the plume. The result is a whirling fire. The current approach considers the relationship between buoyancy and swirl using a configuration based on fixing the heat release rate of the fire and imposing circulation. Large-eddy methodologies are used in the numerical analyses. Results indicate that the structure of the fire plume is significantly altered when angular momentum is imparted to the ambient fluid. The vertical acceleration induced by buoyancy generates strain fields which stretch out the flames as they wrap around the nominal plume centreline. The whirling fire constricts radially and stretches the plume vertically.(Some figures in this article are in colour only in the electronic version; see www.iop.org)