The role of recirculation zones on flow and transport phenomena in periodic porous media. A multi-scale study.

In recent years, the injection of nanoscopic particles of zero-valent iron (nZVI) proved to be an effective intervention to remediate contaminated aquifer systems. Thus, the fate and transport of nZVI, as well as those of contaminants and harmful particles, needs to be completely characterised in order to design successful interventions. The theoretical framework in which this research field is enclosed is the study of flow and transport phenomena in porous media. Under laminar flow regime, computational fluid-dynamic (CFD) simulations of flow field and transport of both reacting and non-reacting solutes and colloidal particles were performed. The aim was to explore the correlations that link the spatial arrangement of the porous medium grains to the morphology of the flow field and to the onset of anomalous transport. In particular, the following dissertation is devoted to the study of three different periodic pore structures, namely face-centered cubic (FCC), body-centered cubic (BCC) and sphere-in-cube (SIC) arrangements, as they excellently balance the complexity of real porous media systems with the ease of implementation and low computational costs. The nature of the present thesis is essentially multi-scale: performing pore-scale simulations it was possible to describe the intimate characteristics of periodic porous media, and couple them with the analysis of macro-scale effective parameters (dispersion above all) that are of interest in practical applications. The emphasis was put on the presence of recirculation zones, portions of the fluid region where the streaklines bend and curl. Recirculation zones are strongly related to the structure of the porous medium and their presence overwhelmingly emerges also at low Reynolds numbers. The role of recirculation zones is highlighted in the various aspects of transport and deposition phenomena, starting from their link to the onset of anomalous transport, to their influence on filtration performances of the investigated periodic packing structures. At the macro-scale level, for periodic porous media, the choice of a proper transport model was thus influenced by the presence (or not) of recirculation zones. Both the classical advection-dispersion model and the dual porosity model were considered, the latter to take into account recirculation zones and their effects. The onset of anomalous transport was considered also through the assumption of the model parameters as functions of both the pore-scale Péclet number and the space variable, thus allowing a proper description of the transition from anomalous to the Fickian regime. Moving within the framework of colloid filtration theory (CFT), deposition phenomena were investigated for both FCC and BCC arrangements, thus highlighting some interesting potentiality of the use of periodic porous media in filtration processes. The macro-scale analyses were supported by the application of the method of moments, that under the Fickianity assumption is a rather good alternative to derive information about model parameters.