Evaluation of iron flame discreteness in monoatomic gases based on single particle combustion behaviour

The combustion behaviour of single iron particles is experimentally investigated at a fixed oxygen concentration of 40%, diluted in 60% He, Ar, and Xe, using an electrostatic levitator with laser ignition. While the three gas mixtures have identical adiabatic flame temperature predictions using thermodynamic equilibrium calculations, the experiments reveal a noticeable difference in particle temperatures due to differences in Lewis number. This temperature variation is evident in particle luminosity and in nano-oxide formation, observed through the attenuation of white LED light. In the helium-diluted mixture, insignificant nano-oxide formation is observed due to lower particle temperatures, which subsequently reduces Fe evaporation and vapour-phase combustion. In contrast, xenon dilution enhances vapour-phase combustion, yielding pronounced nano-oxide formation, while argon shows intermediate behaviour. Despite large differences in oxygen mass diffusivity, measured combustion times, defined from melting onset to peak temperature, are comparable for helium- and argon-diluted mixtures, and only slightly longer for the xenon-diluted mixture. Using these combustion times, the discreteness parameter, χ , of iron flames is qualitatively compared: it is 1.71 times higher in 40% O 2 –60% Ar than in 40% O 2 –60% Xe, and 5.28 times higher in 40% O 2 –60% He than in 40% O 2 –60% Xe. The results indicate that discrete flame propagation is likely in argon-diluted mixtures but unlikely in helium-diluted mixtures. The present study highlights how the Lewis number influences the interplay of oxygen mass diffusivity and vapour-phase combustion in evaluating particle combustion time and flame discreteness, providing essential insights for designing future discrete flame microgravity experiments in different gas mixtures and advancing iron combustion models. • The Lewis number of the gas mixture influences the particle temperature. • Nano-oxide formation is denser in xenon dilution than in helium dilution. • Combustion time is not significantly affected by the monoatomic diluents used. • Discrete flame propagation can likely occur in a 40%O2–60%Ar gas mixture.