Modeling the melt dispersion mechanism for nanoparticle combustion
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Thermite particles have long been known to increase in reactivity as they decrease in size. However, during fast heating (106 - 108 K/s) of Al nanothermites, the diffusion mechanism that explains micron size thermite reactions cannot explain the extremely fast ignition times and much higher flame propagation velocities. A new mechanism known as the melt dispersion mechanism has recently been introduced to explain the fast oxidation of these Al nanothermites. A model has been created dependant upon key parameters to predict the reactivity of Al nanothermites. In this study, flame propagation velocities are statistically evaluated in terms of an integral that employs a probability density function (pdf) for key parameters and a flame velocity equation dependent on relative particle size (Al core radius divided by oxide shell thickness), oxide shell formation temperature, and oxide shell strength. It is shown that flame propagation velocity depends sensitively on relative particle size, relative particle size distribution, oxide shell formation temperature, and shell strength. It is also dependant upon particle size, and oxide shell thickness but not as sensitively. Both single and bimodal particle sizes were studied. Combining smaller nanoparticles with larger nanoparticles in a bimodal mixture significantly increases the flame propagation velocity as compared to a composite consisting of only the larger particles. The results presented here suggest that better reproducibility of the flame velocity may be achieved experimentally by selecting a material with a narrow relative particle size distribution. A combination of increased oxide shell formation temperature and increased oxide shell strength could be used to maximize the flame velocity in particles with increased relative particle size.