Ignition and Combustion of Nickel Coated and Uncoated Aluminum Particles in Hot Post-Flame Environment

It is well known that aluminum particles require a high temperature to achieve ignition, often in excess of 1900 K. Lowering the ignition temperature of Al particles can increase the efficiency of solid propellant-based propulsion systems and thermobaric explosives. Current methods to reduce the ignition temperature include both reducing the particle size to the nano-scale and/or applying coatings that aid ignition. In this work, an experimental study of ignition and combustion of isolated 5 wt% Ni-coated and uncoated aluminum particles was conducted. Two particle sizes (nominally 32 μm and 9 μm in diameter) were examined. The ignition and combustion properties of these aluminum particles were observed in the postflame zone of a multi-diffusion flat-flame burner at atmospheric pressure. The results showed that the applied nickel coating decreased the ignition temperature of the Al particles by 750 K on average for 32 m particles and by 300 K for smaller 9 m particles. Measurements of particle burning times indicated that the application of the nickel coating does not affect the overall burning time of the particles. Thus, the application of nickel coatings on Al particle significantly decreases the ignition temperature while not affecting the overall combustion behavior. I. Introduction luminum is an energetic fuel additive for solid propellants and solid fuels in different propulsion systems, and also has effectiveness for particle damping in suppressing combustion instability in rocket motors. This has led aluminum particles to become widely employed in rocket propulsion applications. The importance of aluminum particle combustion in propulsion systems has led to a large number of combustion studies. Furthermore, aluminum particles have also been used in thermobaric explosive (TBX) applications where Al particles need to be ignited rapidly at low temperatures after the detonation of the high explosive charge. Beckstead 1 performed a comprehensive summary of aluminum particle combustion from a wide range of sources. He found a common ignition temperature of about 2,300 K, which is near the melting temperature of the aluminum oxide shell (2,327 K). Additionally, Beckstead's summary found that the ignition delay of Al particles is dependent on the thickness of the oxide shell. Burning time data of the Al particles was also compiled to determine if the D 2 law was applicable. It was found that the D n law was more applicable for larger particle sizes due to the oxide layer formation with the exponent being 1.5 or 1.8, depending upon the coefficient used in the correlation. Additionally, studies that compared the relative effectiveness of different oxidizer species including oxygen, water vapor, and carbon dioxide were compiled in Beckstead's work 1 . It was concluded that O2 was the most effective oxidizer, H2O was between 50 to 60 percent as effective, and CO2 was about 22 percent as effective. Beckstead's correlation is widely accepted for correlating aluminum combustion behavior for particle sizes above 20 m. Without consideration of the effective oxidizer mole fraction, pressure, and ambient gas temperature, Beckstead’s preliminary correlation indicated a D 1.99 law. When the diameter exponent is close to 2.0, the combustion can be regarded as a diffusion-limited process for relatively large particles beyond 20 m. Bazyn, et al. 2 conducted a study on the combustion of 10 μm-sized aluminum particles in the reflected region of a shock tube and found pressure exponents of −0.9, 0.2, and 0.3, for O2, H2O, and CO2 respectively, (i.e., tb  pi n where pi is the partial pressure of the i th