Combustion Of Aluminum Droplets Research Articles

Динамічні процеси у твердопаливних ракетних двигунах та їх взаємодія з вібраціями конструкції ракети: стан питання та актуальні проблеми

The most critical operating conditions of solid rocket motors (SRMs) are often due to the development of dynamic processes characterized by excess values of operating parameters. Pressure surges and a sharp increase in the combustion product temperature may impair the strength of the combustion chamber structure, cause its failure, and lead to critical conditions of the motor operation, up to extinguishing the propellant combustion in the motor. It is shown that both in steady and in unsteady operating conditions of an SRM, dynamic processes in its combustion chamber feature a complex interrelation of a large number of processes in the gas-dynamic space of the combustion chamber: physical, chemical, and thermodynamic (heat and mass exchange) processes. It is found that current studies of SRM operation instability are aimed at identifying mechanisms of combustion chamber pressure oscillations, which are usually due to combustion product vortex formation in the chamber space and acoustic feedback resulting from collisions of vortices with the SRM’s combustion chamber components or nozzle. Other lines of investigation are the analysis of SRM resonant damping and the establishment of a relationship between aluminum droplet combustion and SRM internal instability. It is noted that accelerations and vibrations of mixed-propellant combustion surfaces may greatly affect the combustion rate and the agglomeration, on-surface confinement, and burn-up of metal additives, which, in its turn, governs the combustion chamber acoustics. It is pointed out that the interaction of SRM combustion chamber pressure oscillations and the response of the SRM structure observed in flight tests of some rockets should be taken into account in predicting the stability of SRM dynamic processes. This interaction may call into question the sufficiency of SRM static tests and subsequent conclusions on the magnitude of its dynamic effect on the rocket structure.

Sharp-interface calculations of the vaporization rate of reacting aluminum droplets in shocked flows

The vaporization rate of reacting aluminum droplets in high-speed flows plays a crucial role in determining the energy released during the combustion of aluminized solid propellants and explosives. This work develops a model for the vaporization rate of burning aluminum droplets in shocked flows using data from high-resolution simulations. A levelset-based sharp-interface method is used to resolve the dynamics at the droplet-air interface. Several reactive calculations of shock-droplet interaction are performed in the parameter space of Mach number (1.1 - 3.5) and Reynolds number (100 – 1000) to understand the influence of local flow parameters on the flame structure around the reacting aluminum droplet. The results show a transition from diffusion limited to kinetically limited combustion of the aluminum droplets when the Mach number is increased and the droplet size or the Reynolds number is decreased. The transition from diffusion-limited combustion to kinetically limited combustion is found to significantly affect the flame dynamics and vaporization rate of the reacting droplets. Therefore, empirical models for the vaporization rate of droplets, developed for diffusion limited combustion regimes, do not represent the kinetically limited combustion in high Mach number conditions. A new model for the Sherwood number spanning diffusion and kinetically limited regimes of the reacting aluminum droplets is developed using an economical multi-fidelity surrogate modeling technique. The current model for the Sherwood number for reacting aluminum droplets will be useful in predicting the energy released from the combustion of aluminized energetic materials in shocked flows. Although the current method is developed in the context of aluminum droplet combustion, it can be extended to develop surrogate models of Sherwood number for general liquid fuel droplets in compressible flows.

Model for Acoustic Induced Aluminum Combustion Fluctuations in Solid Rocket Motors

Combustion of aluminum droplets released by the solid propellant is widely used to increase the thrust in solid rocket motors. The combustion dynamics of the released droplet cloud in the unsteady flow is however susceptible to trigger thermoacoustic instabilities. A theoretical analysis is conducted to determine the heat release rate fluctuations produced by the burning droplet cloud for small acoustic disturbances. Two contributions to heat release fluctuations are identified. The first originates from fluctuations of the evaporation rate due to the oscillating flow around the droplets. This leads to local fluctuations of the volumetric rate of heat release within the droplet cloud. The second one originates from the motion of the boundary of the burning droplet cloud at the end of the combustion process. This motion is due to droplet lifetime oscillations leading to an additional source of heat release disturbances. Both contributions to heat release disturbances take place within the acoustic boundary layer along the solid propellant surface. Quasi-steady models for the response of the droplet diameter fluctuations and droplet velocity fluctuations are derived. Combined with a model for the gas velocity fluctuations within the acoustic boundary layer they lead to expressions for the resulting heat release rate disturbances within the droplet cloud and at its boundary. Results are compared to a previous low order model and to numerical flow simulations. It is shown that the new model leads to close agreement with simulations over the entire flow. The derived expressions yield a better understanding on heat release disturbances and can be used to predict the linear stability of a solid rocket motor at reduced computational costs.

Experimental investigation on the combustion characteristics of aluminum in air

With the aim of revealing the detailed process of aluminum combustion in air, this paper reports an experimental study on the combustion of aluminum droplets. In this work, the aluminum wires were exposed and heated by a CO2 laser to produce aluminum droplets, and then these droplets were ignited and burnt in air. The changing processes of aluminum wires, droplets and flames were directly recorded by a high-speed camera, which was equipped with a high magnification zoom lens. Meanwhile, the spectrum distribution of the flame was also registered by an optical spectrometer. Besides, burning residuals were collected and analyzed by the methods of Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS). Experimental results show that, during combustion, the aluminum droplet is covered by a spherical vapor-phase flame, and the diameter of this flame is about 1.4 times of the droplet diameter, statistically. In the later stages of combustion, the molten aluminum and condensed oxide products can react to generate gaseous Al and Al2O spontaneously. Little holes are found on the surface of residuals, which are the transport channels of gaseous products, namely the gaseous Al and Al2O. The combustion residuals are consisted by lots of aluminum oxide particles with diameters less than 1μm.

Mechanical Erosion of Graphite Nozzle in Solid-Propellant Rocket Motor

A detailed theoretical/numerical framework is established to study the mechanical erosion of graphite-nozzle materials in solid rocket motors with aluminized ammonium perchlorate/hydroxyl-terminated polybutadiene composite propellants. The analysis is based on a combined Eulerian–Lagrangian approach for treating multiphase motor flowfields. The multicomponent reacting gas-phase dynamics is formulated using the conservation equations of mass, momentum, and energy in the Eulerian framework. Turbulence closure is achieved using the standard two-equation model. The dispersed phase, consisting of aluminum and alumina droplets, is treated in the Lagrangian framework. Combustion of aluminum droplets to aluminum-oxide smoke is considered. Two empirical correlations are first calibrated and then employed to predict the mechanical-erosion rate of the nozzle surface. The estimated erosion rates fall within the range of the available experimental data. Mechanical erosion is prevalent in the convergent section of the rocket nozzle due to the particle impingement on the nozzle surface. No such erosion, however, is observed at the nozzle throat or downstream because the droplet trajectories move away from the nozzle surface in those regions.

Dissolution kinetics of carbon in aluminum droplet combustion: Implications for aluminized solid propellants

An analytical model describing the kinetics of carbon dissolution in burning aluminum droplets has been developed in order to simulate its effects under solid rocket motor conditions. A carbon dissolution rate ( k) was introduced in different droplet regression laws and depending on the heterogeneous kinetics between the Al surface and the surrounding gases. The model was validated using previous experiments performed by the authors on millimeter-sized Al droplets burning in several CO 2-containing atmospheres at atmospheric pressure ( P = 1 atm ). It has been shown that the carbon dissolution is affected by the presence of hydrogen due to competition between CO and H 2 chemisorption. The model was then applied to aluminized propellants (AP/HTPB) at high pressures ( P = 60 atm ) and high temperatures ( T = 3000 and 3500 K), as well as at various burning rates and adsorption conditions. Though the accuracy of the extrapolation results needs further improvement, it has been shown that the carbon dissolution process should not be neglected in order to achieve global understanding of the combustion of Al particles, particularly agglomerates.

Size and morphology of the nanooxide aerosol generated by combustion of an aluminum droplet

The increasing recent interest in characteristics ofsubmicron oxide smoke generated by combustion ofaluminum droplets stems from environmental problemsassociated with application and utilization of alumi-nized propellant rocket motors and from the develop-ment of technologies of production of metal nanoox-ides in aerodisperse flames [1, 2]. Oxide particles rang-ing from nanometer to submicrometer size form andgrow in the flame zone surrounding a burning particle,this process being the starting point for their furtherevolution. Rational design of particle combustion pro-cesses in technical devices is based on an understand-ing of physicochemical processes that occur duringcombustion of a particle and on knowledge of how theircharacteristics depend on combustion conditions. Oneof the challenges in studying the aluminum particlecombustion mechanism is to study the formation andproperties of oxide nanoparticles. In this work, we stud-ied how the size distribution and morphology of theoxide aerosol formed by combustion of aluminum par-ticles in atmospheric air depends on the size of a burn-ing droplet. We intend to extend the pressure range inthe future.Burning droplets were produced by combustion of asmall sample (from 2 × 2 × 20 to 2 × 4 × 40 mm in size)of aluminized solid propellant, which was burnt in a20-L container in air filtered from aerosols. Duringcombustion, the sample expelled burning aluminumdroplets (agglomerates), which fell freely under grav-ity. The combustion lasted a few seconds. During thistime, the container was filled with “oxide smoke,” a fineaerosol. After completion of combustion, aerosol parti-cles coagulated, sedimented, and were partially depos-ited on the walls of the container. To study the evolutionof particles after burning, the resulting aerosol wasperiodically sampled. The aerosol sample was fedeither into a thermophoretic precipitator, to collect par-ticles for subsequent dispersion and morphologicalanalysis based on their electron microscopic images, orinto a Millikan cell, to observe aerosol particles andrecord their motion by a video camera with a micro-scope lens and a laser illuminator. The cell constructionimplies that a homogeneous electric field can beapplied. This, in combination with video recording ofparticle motion, makes it possible to determine the elec-tric charge of the particles. The sampling, video record-ing, and image processing techniques were describedelsewhere [1, 3].We carried out four series of experiments with pro-pellants differing in the size of generated burning drop-lets.In series 1, a model solid propellant containing25 wt % ammonium perchlorate (AP), 35 wt %cyclotetramethylenetetranitramine (HMX), 20 wt %binder, and 20 wt % aluminum was used. Inasmuch asaluminum droplets are agglomerated as the propellantburns, the distribution function of burning droplets gen-erated by the propellant was estimated in the followingmanner. The size distribution of agglomerates, as wellas the fraction of the metal involved in agglomeration(≈0.6), was determined using the technique in [4]. If weassume that the metal not involved in agglomerationpasses into the gas phase in the same form as it has inthe propellant, the set of particles generated by combus-tion consists of agglomerates and particles of initialaluminum. The distribution function of initial alumi-num was preliminarily determined on a Malvern 3600Eparticle size analyzer. Table 1 summarizes the charac-teristics of the set of polydisperse particles calculatedtaking into account the weight fractions. Hereinafter,D

Aluminum combustion in wet and dry CO 2: Consequences for surface reactions

The combustion of aluminum droplets in wet (3 mol% H 2O) and dry CO 2 is studied in order to identify the influence of the two atmospheres on the surface processes. Millimeter-sized samples are maintained contactless in an aerodynamic levitation system and are heated continuously during burning by a laser. Ignition and combustion of the aluminum droplet are observed with a high-speed camera, the Al surface temperature is measured by an optical pyrometer, and unburnt residues are analyzed by X-ray diffraction. The determination of the burning rates and of the droplet temperatures reveals no differences between wet and dry CO 2 ( β = 1.28 ± 0.05 mm 2 / s , T = 2600 ± 50 K ), which shows that the gas-phase combustion regime is not affected by the presence of water vapor. However, the oxide cap, initially formed by the oxide coating breakdown at ignition, is progressively removed in wet CO 2, whereas it is unvarying in dry CO 2. Comparison between Al burning in CO 2/H 2 and in CO 2/(Ar or He) demonstrates that the oxide cap regression in a wet atmosphere is related to a chemical effect of hydrogen produced in the flame, and then diffusing and reacting at the droplet surface. It is suggested that the adsorption mechanism of H 2 on the Al surface may slow down the contribution of adsorbed oxygen-containing species (CO) to the oxide cap, which would consequently promote its decomposition (removal). Furthermore, the carbon dissolution process is observed in wet and dry CO 2. When the carbon concentration reaches the saturation limit in the burning Al droplet ( x C = 0.23 at T = 2600 K ), the excess of carbon is ejected at the surface and forms a solid coating. In the absence of the oxide cap (wet CO 2), the refractory carbon coating prevents strong surface oxidation, and the combustion definitely stops. In the presence of the oxide cap (dry CO 2), the carbon coating reacts and produces an oxycarbide phase which is melted by the laser heating; a new burning regime occurs mainly controlled by direct surface reactions, and leading to the slow oxidation of the droplet and the expulsion of dissolved carbon into CO. Finally, a qualitative model of the combustion of aluminum in CO 2 atmospheres is proposed.

ON THE ROLE OF CARBON DIOXIDE IN THE COMBUSTION OF ALUMINUM DROPLETS

ABSTRACT In this paper, the influence of carbon dioxide on the combustion of aluminum droplets is investigated. Millimeter-sized droplets were heated and ignited by a laser in an aerodynamic levitation system in several CO2 containing atmospheres (H2O/CO2, H2O/CO2/N2) with a large range of compositions (wet – xH2O < 3%, 80/20, 50/50, 12.5/87.5, 50/25/25, 20/40/40). The combustion processes were observed with a high-speed CCD camera, and the droplet radiation was recorded by two optical pyrometers. The ignition occurs with the oxide coating breakdown, which liquefies to form an initial cap moving on the droplet surface. Aluminum vaporizes and burns with the oxidizers as a detached diffusion flame. The droplet regression rate, i.e., the burning rate, strongly depends on the oxidizing atmosphere, from β = 0.58 mm2/s in wet 50% CO2/50% N2 to β = 2.45 mm2/s in 80% H2O/20% CO2. It is shown that CO2 is the worst oxidizer with a smaller “oxidizer efficiency” compared to H2O and O2 . The burning droplet temperature in wet CO2 and in the H2O/CO2 mixtures is around T ≈ 2600 K, and is smaller in H2O/CO2/N2 (T ≈ 2450 K). The non-correlation between the burning rates and the droplet temperatures confirms that the combustion processes are limited by molecular diffusion, and highlights the influence of H2 in the gas-phase transport. An estimation of the exponent n of the “d n law” shows that n decreases with the increase of CO2 from n = 1.7 to n = 0.6. The stagnant burning rates are evaluated as being 7 to 9 times smaller than the measured ones with convective effects. Furthermore, during the droplet regression, the oxide cap dimensions also regress, and it is generally completely removed. Oxide cap regression rates are estimated and are slower than their respective burning rates. Nevertheless, there is a good correlation between the oxide cap regression rate and the droplet temperature that shows that the oxide cap regression results from the chemical decomposition of Al2O3 by the liquid Al droplet producing gaseous AlxOy. For CO2 concentration higher than 40%, a solid phase suddenly appears on the liquid Al surface which entirely covers the droplet leading to the end of the gas-phase combustion. This phenomenon is the consequence of the massive dissolution of carbon in the droplet during burning. Analyses of the unburnt residues showed amounts of dissolved carbon up to 20–23% molar, which is near the saturation concentration limit. Thus, the solid coating corresponds to the ejection of carbon from the droplet because of its continuing regression and is expected to be also present for smaller particles. Therefore, carbon dioxide plays a double role. First, it participates in the gas-phase combustion, but it is the worse oxidizer with smaller burning rates. Second, CO2 causes the carbon dissolution in the Al droplet and finally stops the gas-phase burning. A further implication could be that carbon dioxide may promote the appearance of a combustion regime with surface reactions only.

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 CO 2 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 (H 2O/O 2, H 2O/Ar, H 2O/N 2, and air) has shown that they depend only on the nature and concentration of the oxidizers (O 2 and H 2O); a comparison of experiments in nitrogen and in argon containing mixtures demonstrated that N 2 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 x O y species. Therefore, nitrogen effects are significant at the droplet surface rather than in the gas phase, and it is suggested that N 2 is probably one of the main species causing the manifestation of unsteady processes during aluminum droplet burning.

Formation of charged aggregates of Al 2O 3 nanoparticles by combustion of aluminum droplets in air

The mechanism of Al 2O 3 nanoparticles aggregation in air was studied under atmospheric pressure conditions. Alumina nanoparticles were generated by combustion of a small sample of solid rocket propellant. Size and morphology of nanoparticles were studied by transmission electron microscopy. It was determined that alumina nanoparticles form aggregates of size about 1 μm composed of primary particles with a diameter of few tens of nm. The aggregates' coagulation was observed directly by a video system. In addition, the aggregate movement in the electric field was recorded by the video system. These observations showed that the majority of aggregates are charged. The typical aggregate charge is about 10 elementary units. Some aggregates are dipoles as indicated by the aggregate rotation when the electric field polarity is changed. The mechanism of aggregate formation has been proposed. The initial stage of alumina nanoparticle synthesis includes the formation of liquid oxide particles in the reaction zone located around the surface of burning Al droplet. When the particles pass to the relatively low-temperature region, solidification of particles occurs followed by clustering of solid spherical Al 2O 3 particles. Later, an aggregation of clusters formed by different burning Al droplets occurs resulting in formation of final aggregates. Estimations of thermal emission of electrons by the burning Al droplets were carried out via solution of the Poisson–Boltzmann equation. These estimates showed that the negative volume charge in the reaction zone around the Al droplet is 10 8–10 9 elementary charges per cm 3.

Aluminum droplet combustion in fluorine and mixed oxygen/fluorine containing environments

Experimental and numerical results are reported on the combustion of single aluminum (Al) droplets, 187 μm diameter, that were laser ignited in quiescent environments of SF 6 and SF 6 /O 2 or Ar mixtures. Combustion measurements consisted of luminosity versus time traces, photographic observations, and electron probe microanalysis (EPMA) and electron microscope inspection of condensed-phase products of combustion surrounding burn-intercepted droplets. Planar laser-induced fluorescence (PLIF) was employed to image the gaseous intermediated reaction species ALO during droplet combustion in SF 6 /O 2 mixtures. Aluminum droplets burning in pure SF 6 burned without condensed-phase products in the surrounding envelope flames. Accordingly, no condensed-phase products were found on surfaces of droplets captured by quenching into an inert environment. In SF 6 , classical d 2 law burning behavior was observed. Droplets burned in SF 6 /Ar mixtures exhibited similar behavior, with diminished burning rates. Disruptive burning was not observed for pure SF 6 or any of the SF 6 /Ar mixtures. Condensed-phase deposits on quench plates intercepting burning droplets showed the surrouunding gaseous flame structure to consist of two zones for burning in SF 6 and three zones in SF 6 /O 2 mixtures. For each system, near the surface where the gas phase was aluminum rich, significant amounts of sulfur were found in the condensed products. PLIF showed the species profile of AlO to shift closer to the surface with increasing fluorine content in SF 6 /O 2 mixtures. Experimental results were compared with a local equilibrium combustion model. The model predicted burn rates within 20% and qualitatively predicted radial profile speciation. As the O/F ratio was increased, the final equilibrium products were predicted to shift from AlF 3 to AlF 2 O to Al 2 O 3 . The formation of AlF 2 O reduced condensed-phase products at the reaction temperature for a wide range of O/F ratios.

Trajectories of sparks from arcing aluminum power cables

Can sparks produced by arcing overhead aluminum power distribution lines ignite a brush fire? A model of aluminum droplet combustion is proposed and used to calculate combustion rates, lifetimes, and trajectories of such sparks for various wind conditions. Although there are some uncertainties in the modeling, the results show that sparks of about 1 mm initial diameter can reach the ground before burning up. Large sparks (1.5 mm diameter) can travel more than 200 m before impact. Since the spark flame temperature is about 3,800 K, subsequent ignition of dry brush is quite possible.

Differences between the Combustion of Aluminum Droplets in Air and in an Oxygen‐Argon Mixture

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