Comparison on the ignition and combustion characteristics of single Al-Li alloy and Al fuel microparticles in air

Abstract

To address the resistance effect of protective oxide shell on the ignition of micron-sized aluminium (Al) and the particle agglomeration and residue deposition of Al-based fuels, aluminium-lithium (Al–Li) alloy by introducing tiny elemental Li could have a promising potential in substituting for Al. The paper presents a comparative study on the surface structure and the ignition and combustion characteristics between single Al-Li alloy and pure Al fuel microparticles in air. The Al-Li alloy with the content of 1.0 wt.% Li was prepared by gas-atomized method and characterized by multiple methods. The characterization of the surface morphology, elemental distribution, and residues of incomplete combustion show the dendritic structure of eutectic compounds like veins at the surface and interior of the Al-Li alloy microparticles. Compared to the dense oxide shell of Al, the oxide shell of Al-Li alloy provides many more pathways like cracks/openings for the Al and O2 contact as inductively heated by a highly-focused laser beam, resulting in a reduction of ignition delay. The difference of ignition delay between the two demonstrated that the addition of Li element shortened the ignition delay, especially, over 80% for the particle size of > 45 µm. The ignition delay can be reduced by minimizing the particle size or enhancing the ignition power density. Owing to the variations in reactivity, melting, and boiling points compared to pure Al, the Al-Li alloy particles exhibit distinct gas-phase flame and micro-explosion behaviour. These differences result in considerably higher flame temperatures and more intense radiation. Finally, a sequence of reaction mechanisms that potentially control the ignition and combustion of single Al-Li alloy particles (as distinct from pure Al) is proposed, the physical and chemical properties of highly reactive Li are crucial to improve the combustion performance of Al.