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Experimental research was conducted to study the combustion characteristics of clouds composed of micrometer-sized aluminum particles with an oxidizer blend of oxygen and superheated steam. Both ignition and combustion of an aluminum particle are difficult because it is enveloped by an oxide layer that has a high melting point (above 2300 K). To overcome the problem, we induced a particular shape of the combustor that can keep a stable flame of aluminum dust clouds at a lower feeding-rate. The combustion characteristics were examined using a system applied combustor involving a variety of parameters affecting the burning of the dust clouds. Based on these attempts, the aluminum dust clouds had an upper flammability limit above the equivalence ratio (Ф) 1.9 for oxygen diluted by air. Varying the lower limit of flammability, we found, crucially, that the feeding-rate and velocity of the particles are more effective than the equivalence ratio or combustor length. Furthermore, we could confirm the changing of the lower limit from 0.3 to 0.26 because the steam was additionally fed in the combustion of aluminum-oxygen.

In this study, the effects of gaseous methane/oxygen injection velocity ratio on the shear coaxial jet flame structure are analyzed using high-speed imaging along with OH* and CH* chemiluminescence. The images show that, as the velocity ratio is increased, the visual flame length increases and wrinkles of the flame front are developed further downstream. The region near the equivalence ratio 1 condition in the flame could be identified by the maximum OH* position, and this region is located further downstream as the velocity ratio is increased. The dominant CH* chemiluminescence is found in the near-injector region. As the velocity ratio is decreased, the signal intensity is higher at the same downstream distance in each flame. From the results, as the velocity ratio is decreased, there is increased entrainment of the external jet, the mixing of the two jets is enhanced, the region near the stoichiometric mixture condition is located further upstream, and consequently, the flame length decreases.

Aluminum particles 15–25 μm in size are widely used in fuel propellants and underwater propulsion systems in national defense research. However, these particles are covered with an aluminum oxide film, which has a high melting point, so ignition is difficult. To improve the ignitability of high-energy aluminum powder and to understand the reaction phenomenon as a function of particle size(15–25 μm, 74–105 μm, and 2.38 mm) and oxidizer(air, CO2, and argon), the natural oxide films are chemically removed. The particles are then coated with nickel using an electro-less method. The degree of nickel deposition is confirmed qualitatively and quantitatively through surface analysis using scanning electron microscopy/energy dispersive spectroscopy. To characterize the nickel coatings, elemental analysis is also conducted by using X-ray diffraction. Thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) enable comparisons between the uncoated and coated aluminum, and the reaction process are investigated through fine structural analysis of the particle surfaces and cross sections. There are little difference in reactivity as a function of oxidant type. However, a strong exothermic reaction in the smaller nickel-coated aluminum particles near the melting point of aluminum accelerates the reaction of the smaller particles. Explanation of the reactivity of the nickel-coated aluminum depending on the particle sizes is attempted.