The effect of a shock wave on the ignition behavior of aluminum particles in a shock tube

The effect of a shock wave on the ignition behavior of 5 μ m aluminum (Al) particles was studied in a series of experiments by means of a horizontal shock tube with an inner diameter of 70 mm. To isolate the shock effect from other effects, the experiments were conducted in an inert argon (Ar) atmosphere in addition to a few control experiments in air. The use of Ar as driven gas also helps to produce strong shocks. Every aluminum particle is initially covered with a layer of amorphous aluminum oxide (Al2O3). The Al2O3 passivates the particle, thus playing a key role in the ignition and combustion mechanisms of an Al particle. The experiments showed a strong emission of light originating from the particles immediately after the shock wave has passed them. Spectral analysis revealed strong AlO bands even in experiments in which the volatilization temperature of Al2O3 was not exceeded. The emission spectrum of the flame permits the determination of a grey-body temperature. The existence of AlO molecules and the analysis of samples taken after an experiment give a strong evidence of the influence of a shock wave on the ignition and reaction mechanism of Al particle combustion.