Iron as a fuel differs significantly from other solid fuels such as coal or biomass and thus, previous results or modelling approaches for these carbonaceous fuels cannot be transferred directly. The combustion of iron particles is governed by heterogeneous reactions, such that typically no fuel volatiles or oxidation products are released. Instead, the iron oxides form as a result of combustion continue to be solid, resulting in an increasing mass with advancing oxidation. A porous layer of various iron oxides (FeO, Fe2O3, Fe3O4) is formed as an outer shell, which represents a further transport resistance for the diffusion of oxygen to the inner reactive surface. Due to the repeated process of adding and removing the oxidation layer, the size and shape of single iron particles is modified along with the porosity of the outer layer. The impact of these modifications on the transport processes across the particle boundary layer is insufficiently understood so far.
This subproject aims at unravelling the relevance of volume and shape changes and the presence of porous layers on the transport process within the boundary layer of individual iron particles. To tackle this issue the iron particle itself and the gas phase in its direct vicinity will be studied numerically. In this framework the particle is fully-resolved by means of an immersed boundary method. The laminar flow around single particles of varying size, shape and porosity is simulated in order to deduce the impact of these variations on the momentum, heat and mass transfer correlations that are included in the full-scale simulations of laminar and turbulent iron dust flames.
The project is complementary to that led by A. Scholtissek (reaction-transport coupling of single, iron-based microparticles). It strongly collaborates with the projects led by C. Hasse (numerical investigation of iron dust/air combustion) and O. Stein (particle ignition and combustion in turbulent flow). It further collaborates with the experimental project led by H. Nirschl (structure-property relationships of iron particles and its oxides).
Scientific challenges:
1. Implementation of an Immersed Boundary Methods in OpenFOAM that allows to study and extract transport properties across the particle boundary layers
2. Evaluation of the impact of particle volume changes on the boundary layer transport properties of reacting iron particles
3. Evaluation of the impact of particle porosity on the boundary layer transport properties
