Ignition and combustion of a dense powder jet of micron-sized aluminum particles in hot gas

Aiming at the potential implementation of aluminum as a primary fuel in powder-fueled ramjets or engines, this work seeks to investigate the ignition and combustion characteristics of a dense gas-suspended jet of micron-sized aluminum particles in a hot flow with controlled temperature and compositions. Aluminum particles with a mean diameter of 40 µm are aerosolized using a custom-made feeder and carried into the burner by a nitrogen stream. The powder jet with a particle density of up to 1–3 kg/m³ can be ignited and burned violently at a surrounding gas temperature as low as 1500 K. The lowered ignition temperature of the powder jet can be attributed to a cooperative mechanism resulting in fast reactions. Meanwhile, the ignition delay time decreases from ∼25 to ∼5 ms when the surrounding temperature increases from 1500 to 2200 K. The burning powder jet generates strong luminance and AlO emission signals detected by a spectrometer. Particle image velocimetry (PIV) and camera pyrometry are used to derive the two-dimensional velocity and average projected temperature distribution, respectively. Furthermore, a high-speed camera with a microscopic lens captures the transition from dispersed combustion to group combustion that forms a large-scale flame column wrapping the entire powder jet. The aluminum oxide produced in the columnar flame forms a large number of nanosized smoke particles in the condensation region. Finally, a numerical model considering the collective effect of the powder jet is developed to predict the particle temperature history during the ignition stage, which shows good agreement with the temperature profiles derived from camera pyrometry and PIV techniques.