Abstract:
An experimental study is conducted to investigate the effects of particle size and packing density on the combustion of nickel-aluminum pellets. The particle sizes of Al and Ni are varied in the range of 1–25 µm, and the packing density is varied between 63% and 76% of the theoretical maximum density (TMD). The pellets are burned in a quartz tube, and the combustion process is recorded using a high-speed video camera. Temperature evolution during combustion is recorded, and the composition of burned pellets is analyzed. Computer simulations of the pellet combustion process are also conducted using a multiscale model, in which the diffusion process in the particles is coupled to energy transport in the pellet. Two distinct propagation modes are observed – (i) a quasi-steady mode characterized by high combustion velocities and continuous front propagation and (ii) a relay-race mode characterized by low combustion velocities and discrete jumps of the combustion front. The combustion velocity increased with decreasing Ni particle size due to a reduction in the length scale of the atomic diffusion process. Increasing the Al particle size generally increased the combustion velocity due to increased permeability of the pellet and reduced oxide content in the Al powder. The combustion velocity decreased by a factor of 3–4 when the packing density increased from 63% to 76% TMD. Super-adiabatic flame temperatures are not observed and the product analysis revealed the presence of intermediates (such as Ni2Al3) and layered structures in the burned pellets for the relay-race propagation mode. The study suggests that the combustion rate of Ni-Al pellets is strongly dependent on the rate of atomic diffusion processes, the oxide content in the powders, and the ability of the Al melt to wet the surface of Ni particles and preheat the unburned regions of the pellet.