Experimental studies on the ignition of single nickel/aluminum, iron/aluminum, and titanium particles
Abstract
The work presented here focuses on the ignition of single Ni- and Fe-coated Al particles in pure CO2 and Ar atmospheres and Ti particles in O2/N2 and O2/Ar environments. The importance of studying Ni- and Fe-coated Al particles is related to their potential use in propulsion and materials synthesis applications. Meanwhile, Ti combustion may be utilized to eliminate hazardous chemical and biological air pollutants. Ni- and Fe-coated Al particles. The use of Ni- or Fe-coated Al particles (10–100 μm) as a fuel component in propellants is expected to improve the performance characteristics of solid rocket motors by reducing Al agglomeration. They can also be used for combustion synthesis of nickel and iron aluminides. The ignition mechanism of such particles, however, is not well understood. In this work, to provide sufficient spatial and time resolutions, single ∼2.5-mm Ni- and Fe-coated Al particles were laser-heated in argon and carbon dioxide atmospheres. The ignition process was investigated using a high-speed digital video camera, thermocouple measurements, and analysis of particles quenched at different pre-ignition stages. It was shown that the Ni-coated Al particle ignition temperature is ∼1325°C and does not depend on atmosphere (Ar or CO2) or Ni content. The established ignition mechanism includes exothermic intermetallic reactions and phase transformations of different Ni-Al compounds. Specifically, the melting of NiAl3 at 854°C was determined to play a critical role in particle ignition. Despite the lower exothermicity of Fe-Al reactions, iron-coated Al particles were investigated because, unlike Ni, Fe is not toxic. Further, a prior study showed that Fe coatings may exhibit a more favorable effect on Al agglomeration than Ni coatings. It was shown that significant differences exist between the ignition of the Fe-Al and Ni-Al particles. In contrast to Ni coatings, the ignition of Fe-coated Al particles is influenced by atmosphere (Ar or CO2) and the post-ignition product phases are not well-mixed. Again, the ignition mechanism relied on exothermic intermetallic reactions and phase transformations. For Fe-coated Al particles, the intermetallic reactions contributed significantly to heating rate upon Al melting at 660°C. The above differences between the Ni- and Fe-coated Al particle systems may be related to the manner by which buoyancy influences phase formation in each system. Thus, microgravity (10-2–10-3 g) experiments were conducted to reduce convective mixing, allowing its effect on ignition to be examined. The results indicated that Nicoated Al particle ignition is not influenced by microgravity. Meanwhile, for Fe-coated Al particles, microgravity lowered ignition temperature by ∼100°C, indicating a significant influence of buoyancy. In normal gravity, similar effects are expected to occur with decreasing the initial particle size from 2.5 mm to 250–550 μm. Ti particles. Combustion of Ti-rich pyrotechnic mixtures in air may potentially be used for the in-situ synthesis of nano-scale TiO 2 particles, which can photocatalytically degrade chemical and biological air pollutants. Knowledge of Ti particle reactions in O2-containing atmospheres is required to develop this method. In the present work, large (∼3 mm) single Ti particles were heated by a laser in O2/N 2 and O2/Ar environments. High-speed digital video recording, thermocouple measurements and quenching at different stages of the process were used for diagnostics. Analysis of the obtained temperature-time curves and quenched particles does not show a significant influence of nitrogen on oxidation of solid Ti. In all experiments, noticeable surface oxidation started at temperatures between ∼850–950°C, leading to a sharp temperature rise at ∼1400°C. During prolonged heating at the Ti melting point (1670°C), a liquid TiO2 bead formed and, after an induction period, ejected fragments. It was shown that this phenomenon may result from an excess of oxygen in the liquid bead. Fragment ejection was more intense in O2/N2 atmospheres than in O 2/Ar, indicating that N2 accelerates the oxidation of liquid Ti.
Degree
Ph.D.
Advisors
Shafirovich, Purdue University.
Subject Area
Chemical engineering
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