Combustion of porous titanium particles in air and in composite propellants

Combustion of a carbon particle in air is considered on the basis of a diffusion-kinetic model. Two regimes of combustion are shown to exist: high- and low-rate regimes. In the high-rate combustion regime, oxygen is totally consumed in a homogeneous reaction running above the particle surface and does not reach the particle, whereas carbon is consumed in a reaction with carbon dioxide. In the low-rate combustion regime, oxygen reaches the particle surface, and the reaction of carbon with oxygen producing carbon dioxide runs within the porous particle.

Combustion of levitated titanium particles in air

Combustion of single biomass particles in air and in oxy-fuel conditions

Inhalable particles and pulmonary host defense: In vivo and in vitro effects of ambient air and combustion particles

Oxidizer coarse-to-fine ratio effect on microscale flame structure in a bimodal composite propellant

For the diffusion-kinetic model of combustion of a porous carbon particle in air, the structure of the reaction zone of carbon with reactive gases inside the porous particle is studied. It is shown that, for the given kinetics of reaction of carbon with oxygen, the dependence of combustion rate of the porous particle on its internal surface area is ambiguous, which is related to the strong dependence of the reaction rate of carbon with oxygen on temperature. To obtain an unambiguous dependence, it is necessary to use the kinetic equation for the reaction rate of carbon with oxygen with an activation energy lower than the experimentally determined value.

Detection of allergens adsorbed to ambient air particles collected in four European cities

The burning of titanium particles in air has recently attracted the serious attention of researchers [1, 2]. This work continues the present authors' studies of large (up to 700 um) titanium particles burning in the free fall in air [3-5]. The particles in [3] were monodisperse agglomerates formed of a large number of small irregular shape particles. In [4,5], a modified approach, which makes it possible to obtain initially monolithic burning titanium particles with a diameter up to 700 yum, was used.

The combustion spectra of loose magnesium particles in air and in carbon dioxide at atmospheric pressure are investigated. Experimental graphs of the combustion time as a function of the initial particle size are plotted. It is shown that the luminescence time of the metal and oxide vapors are always shorter than the total particle luminescence time. It is observed for the first time that the magnesium oxide vapor disappears before the vapor of the metal itself when combustion takes place in carbon dioxide gas. This is attributable to the fact that the rate of the gas-phase reaction of magnesium with carbon dioxide decreases as the production of carbon monoxide increases during the combustion stage.

The nanosized powders have gained attention to produce materials exhibiting novel properties and for developing advanced technologies as well. Nanosized materials exhibit substantially favourable qualities such as improved catalytic activity, augmentation in reactivity, and reduction in melting temperature. Several researchers have pointed out the influence of ultrafine aluminium (∼100 nm) and nanoaluminium (<100 nm) on burning rates of the composite solid propellants comprising AP as the oxidizer. The inclusion of ultrafine aluminium augments the burning rate of the composite propellants by means of aluminium particle’s ignition through the leading edge flames (LEFs) anchoring above the interfaces of coarse AP/binder and the binder/fine AP matrix flames as well. The sandwiches containing 15% of nanoaluminium solid loading in the binder lamina exhibit the burning rate increment of about 20–30%. It was noticed that the burning rate increment with nanoaluminium is around 1.6–2 times with respect to the propellant compositions without aluminium for various pressure ranges and also for different micron-sized aluminium particles in the composition. The addition of nano-Al in the composite propellants washes out the plateaus in burning rate trends that are perceived from non-Al and microaluminized propellants; however, the burning rates of nanoaluminized propellants demonstrate low-pressure exponents at the higher pressure level. The contribution of catalysts towards the burning rate in the nanoaluminized propellants is reduced and is apparent only with nanosized catalysts. The near-surface nanoaluminium ignition and diffusion-limited nano-Al particle combustion contribute heat to the propellant-regressing surface that dominates the burning rate. Quench-collected nanoaluminized propellant residues display notable agglomeration, although a minor percentage of the agglomerates are in the 1–3 µm range; however, these are within 5 µm in size. Percentage of elongation and initial modulus of the propellant are decreased when the coarse AP particles are replaced by aluminium in the propellant composition.

Aluminum (Al) particles are commonly used in ammonium perchlorate (AP) composite propellants of solid rockets for increasing performance. When propellants including Al particles burn, Al particles easily agglomerate on the burning surface of the propellant. The diameters of agglomerated Al particles are greater than those of mixed particles. The combustion efficiency of the propellant decreases with increasing burning time of the agglomerated Al particles. Therefore, it is important to observe how the agglomerated Al particles burn on the burning surface of AP composite propellant. A lot of researchers have studied Al agglomerate characteristics. Previous studies clarified the relation between the agglomerated Al particle diameter and luminous flame diameter around Al particles near the burning surface. The shapes of luminous flames around agglomerated Al particles are spherical or elliptical. This study evaluates the shapes of the luminous flame around agglomerated Al particles at a constant diameter or a different diameter. When the proportion of the luminous flame diameter (Df) to the diameter of agglomerated Al particles (D0) is 1.54–1.71 at a constant D0, the luminous flames are almost perfectly spherical. Otherwise, the luminous flames are elliptical at a constant D0. Furthermore, when Df/D0 is close to the mean value, the luminous flame is more spherical than elliptical at different D0. The evaporation rate and the burning rate of Al vapor are inversely proportional to D0. The oxidation gas temperatures were changed and the activation energy of Al vapor was obtained as 39.2 kJ mol−1.

Three-dimensional carrier-phase direct numerical simulations (CP-DNS) of reacting iron particle dust clouds in a turbulent mixing layer are conducted. The simulation approach considers the Eulerian transport equations for the reacting gas phase and resolves all scales of turbulence, whereas the particle boundary layers are modelled employing the Lagrangian point-particle framework for the dispersed phase. The CP-DNS employs an existing sub-model for iron particle combustion that considers the oxidation of iron to FeO and that accounts for both diffusion- and kinetically-limited combustion. At first, the particle sub-model is validated against experimental results for single iron particle combustion considering various particle diameters and ambient oxygen concentrations. Subsequently, the CP-DNS approach is employed to predict iron particle cloud ignition and combustion in a turbulent mixing layer. The upper stream of the mixing layer is initialised with cold particles in air, while the lower stream consists of hot air flowing in the opposite direction. Simulation results show that turbulent mixing induces heating, ignition and combustion of the iron particles. Significant increases in gas temperature and oxygen consumption occur mainly in regions where clusters of iron particles are formed. Over the course of the oxidation, the particles are subjected to different rate-limiting processes. While initially particle oxidation is kinetically-limited it becomes diffusion-limited for higher particle temperatures and peak particle temperatures are observed near the fully-oxidised particle state. Comparing the present non-volatile iron dust flames to general trends in volatile-containing solid fuel flames, non-vanishing particles at late simulation times and a stronger limiting effect of the local oxygen concentration on particle conversion is found for the present iron dust flames in shear-driven turbulence.

On modeling the diffusion to kinetically controlled burning limits of micron-sized aluminum particles

Aluminum particle burn rates are known to be a strong function of particle size as the mode of burning transitions from diffusion to kinetically controlled. To better understand the rate dependent diffusion and kinetic processes, a fully compressible, one-dimensional, spherically symmetric particle burn model is developed. Several cases are studied to explore the burning of aluminum particles in air, carbon-dioxide and steam environments. Predictions of burn rates versus particle size reveal significant deviations from a diffusion controlled burning limit highlighting the importance of accounting for finite-rate chemistry in modeling the burning of sub-micron aluminum particles. While overall agreement to data is satisfactory, the detailed model cannot be directly used in system level tools due to computational cost. A reduced modeling strategies are therefore explored to account for finite-rate chemistry effects in simpler models for use in system level CFD analysis. An augmented D − law where the finite-rate chemistry is treated as a perturbation to flame sheet approximation via augmented burn rate “constants”. Predictions using this approach of deflagration speeds in dusty aluminum-air gases agree well with experiments and show evidence of a maximum flame speed for a given mass loading.

The mechanism of convective drying of single coarse lignite particles has important theoretical and practical significance for the development of the lignite drying technologies. The drying kinetics of single lignite particles in hot air were investigated experimentally with particles diameters of 20 and 30 mm. The experiments were performed at hot air velocities of 0.7 and 1.5 m/s and hot air temperatures of 100 and 140 °C. The results from these experiments show that the lignite particle size, the hot air temperature, and the velocity all significantly influence the drying. For the range of operating conditions investigated in this study, the drying time was reduced by increasing the temperature and velocity of the hot air flow and decreasing the particle size. The constant drying rate period was not obvious with the lignite characterized by a long decreasing drying rate. Both types of lignite had a good linear relationship between the drying rate and the dry-basis moisture content. A mathematical model for the drying of single particles was developed assuming local thermodynamic equilibrium to describe the multiphase flow in the porous particle with the energy and mass conservation equations to describe the heat and mass transfer during the drying. The drying model included convection of the free water, diffusion of the bound water, and convection and diffusion of the gas mixture in the lignite particle. The numerical results agree well with the experimental data verifying that the mathematical model can evaluate the drying performance of porous lignite particles. The effects of the drying conditions such as the particle size, the temperature, the absolute humidity, and the velocity of the drying air on the drying process were evaluated using the numerical model.