The mechanism of aluminium oxidation is quantified and a simplified ignition model is developed. The model describes ignition of an aluminium particle inserted in a hot oxygenated gas environment: a scenario similar to the particle ignition in a reflected shock in a shock tube experiment. The model treats heterogeneous oxidation as an exothermic process leading to ignition. The ignition is assumed to occur when the particle's temperature exceeds the alumina melting point. The model analyses processes of simultaneous growth and phase transformations in the oxide scale. Kinetic parameters for both direct oxidative growth and phase transformations are determined from thermal analysis. Additional assumptions about oxidation rates are made to account for discontinuities produced in the oxide scale as a result of increase in its density caused by the polymorphic phase changes. The model predicts that particles of different sizes ignite at different environment temperatures. Generally, finer particles ignite at ...

Effect of polymorphic phase transformations in alumina layer on ignition of aluminium particles

Characteristics of aluminum combustion are summarized in an overview of the subject, focusing on the burning time of individual particles. Combustion data from over ten different sources with almost 400 datum points have been cataloged and correlated. Available models have also been used to evaluate combustion trends with key environmental parameters. The fundamental concepts that control aluminum combustion are discussed, starting from a discussion of the D

 n
 law. The exponent in the D

 n
 law is shown to be lower than two, with nominal values of ≈1.5 to 1.8 being typical. The effect of the ambient medium on the burning time is considered, oxygen as an oxidizer being twice as effective as water and about five times more effective than carbon dioxide. The effect of pressure and initial temperature is minor.

Correlating Aluminum Burning Times

The combustion characteristics of aluminum combustion are summarised in an overview of the subject, focusing on the burning time of individual particles. The fundamental concepts that control aluminum combustion are discussed starting with a discussion of the “D n ” law. Combustion data from over ten different sources with almost 400 datum points have been cataloged and correlated. Available models have also been used to evaluate combustion trends with key environmental parameters. The exponent is shown to be less than two, with nominal values of ~1.5 to 1.8 being typical. The effect of oxidizer is pronounced with oxygen being twice as effective as water and about five times more effective than carbon dioxide. The observed effect of pressure and initial temperature is minimal. In the second part of the paper a two-dimensional unsteady state kinetic-diffusion-vaporization controlled numerical model for aluminum particle combustion is presented. The model solves the conservation equations, while accounting for the species generation and destruction with a 15 reaction kinetic mechanism. Two of the major phenomena that differentiate aluminum combustion from hydrocarbon droplet combustion, namely the condensation of the aluminum oxide product and the subsequent deposition of part of the condensed oxide, are accounted for in detail with a sub-model for each phenomenon. The effect of the oxide cap in the distortion of the profiles around the particle has been included in the model. The results obtained from the model, which include two-dimensional species and temperature profiles, are analyzed and compared with experimental data. The combustion process is found to approach a diffusion controlled process for the oxidizers and conditions treated. The flame zone location and thickness is found to vary with oxidizer. The result shows that the exponent of the particle diameter dependence of burning time is not a constant and changes from about 1.2 for larger diameter particles to 1.9 for smaller diameter particles. Due to the deposition of the aluminum oxide on the particle surface, particle velocity oscillates. The effect of pressure is analyzed for a few oxidizers.

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A Summary of Aluminum Combustion

Ignition and combustion of aluminum/magnesium alloy particles in O2 at high pressures

https://www.mne.psu.edu/sfnm/Publications1_files/Purdue/1-s2.0-S0010218014002508-main.pdf

Exploring mechanisms for agglomerate reduction in composite solid propellants with polyethylene inclusion modified aluminum

Ignition of gas suspensions of metallic powders in reflected shock waves

Results are offered from an experimental study of propagation of a low-speed detonation wave through a chain of liquid hydrocarbon drops in a pure oxygen atmosphere. Detonation was initiated by a planar shock wave at Mach numbers M=2.0–3.5. The detonation wave structure is reflected by the x, t-diagrams obtained. The propagation of the flame front was found to be pulsating in character, and reasons were established for this phenomenon. It is shown that the microspray in the drop wake does not self-ignite, but is ignited by combustion products from the preceding drop.

A. N. Papyrin

Papers

Ignition of gas suspensions of metallic powders in reflected shock waves

Mechanism of low-speed detonation wave propagation in gas-drop mixtures