Characterisation of multi-stage self-ignition of iron particle clouds in sheared turbulence

In this paper, a three-dimensional carrier-phase direct numerical simulation (CP-DNS) approach is employed to analyse the self-ignition conditions of reacting iron particle clouds in sheared turbulence, i.e., a reacting mixing layer. The gas phase is described by the Eulerian transport equations for mass, momentum, sensible enthalpy and chemical species, while the solid phase follows the Lagrangian equations for particle mass, momentum and temperature. The interaction between the two phases is realised through interphase coupling terms. Various operating conditions are tested by adapting the key parameters for the ignition and combustion process, namely gas temperature, oxygen mass fraction, equivalence ratio and particle diameter. Moreover, the effect of radiation is explored, as well as the impact of initialising the lower stream of the mixing layer with fully-oxidised particles. The particle self-ignition behaviour is characterised in terms of the spatially-averaged gas temperature, the total heat released by the particles and the gradient of the latter. Starting from a reference case, one key parameter is changed at a time to isolate its individual effects. The results reveal that two distinct ignition stages with different times occur under most conditions. Under specific conditions, the first and second stages merge, resulting in a single global cloud ignition time. Significant impacts on the self-ignition behaviour are observed for the initial gas phase temperature and particle diameter, while the effects of the oxygen concentration, particle equivalence ratio and the inclusion of radiation are smaller. The addition of fully-oxidised particles to the lower stream does not show any substantial effect on the global ignition behaviour of the iron particle combustion system. Despite being derived for well-controlled, synthetic conditions and using simplified sub-models, the trends identified in this study may serve as a first indicator for controlling iron particle cloud combustion and determining optimised operating conditions for stable self-ignition in future iron particle combustors. • Fundamental carrier-phase DNS to analyse and quantify iron particle self-ignition stages under various ambient conditions. • Characterising particle ignition-stages in shear-driven turbulence. • Identifying key parameters to control the iron self-ignition behaviour. • Fundamental trends found here can help to indicate optimised self-ignition conditions in future iron particle combustors.