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The article describes a new model of water–ice phase transition in pores of a saturated porous medium. The model takes into account the difference in specific volume between ice and water which causes structural changes in the porous medium. Describing details of heat, phase, and structure dynamics, the model contributes to a deeper understanding of phenomena in upper soil layers subjected to either seasonal conditions or climate changes. Governing multi-physics system of equations includes the conservation of mass, momentum and energy at the pore level and includes the anisotropic Allen–Cahn equation for tracking the position of ice during nucleation and growth inside the pores. The model provides space–time behavior of key quantities and describes the interaction of growing ice with pore geometry and surrounding grains. The governing system of equations is solved by the finite-element method to provide several qualitative computational studies of the ice growth inside a porous structure. • Soil freezing and thawing described by conservation laws. • Detailed phase interface tracking provided by the phase-field method. • Dynamics of phase transition in porous structure. • Capturing structural changes produced by the phase change. • Finite-element numerical solution. [ABSTRACT FROM AUTHOR]

The article studies a time dependent mathematical model of two-dimensional two-phase system describing a cross-section of saturated soil sample in terms of its temperature. The model is used to control the structural conditions in the medium by coupling it with the Navier equations. The computational studies of this coupling are presented at the end of the article. The thermal model setting and its variational formulation are presented as well as a basic analysis of mathematical properties of the partial problem solution. [ABSTRACT FROM AUTHOR]

As part of our long-term aim at developing a complex CFD simulation tool for bubbling fluidized bed boilers operating in oxyfuel regime, we have performed specialized fluidization experiments to validate our numerical algorithm for multiphase flow simulation. The algorithm combines and extends the function of several components of OpenFOAM, using the multiphase particle-in-cell method implemented into a custom solver derived from coalChemistryFoam. In the first part of this contribution, the mathematical model and the important improvements in the implementation of the numerical algorithm are discussed. The second part focuses on the design of the experiments and the methodology of the validation. The experimental device used is a model of the combustion chamber, the distributor, and the windbox. Its transparent walls are made of polymethyl methacrylate. In the experiments, three different materials (sand and two types of LECA suitable for use in fluidized bed combustion) were subjected to fluidization at different flow rates, using air at room temperature as the fluidization medium. The flow rates and the pressure drop at the fluidized bed were measured. Video recordings were taken and automatic image postprocessing algorithms allowed to identify the surface of the fluidized bed, its height and its shape as viewed from the front. Based on the sieve analysis, accurate particle size distributions were calculated and implemented in the model. Afterward, series of simulations were performed in order to find the correct settings of the particle stress model and the gas–solid drag model. In particular, to avoid excess complexity when treating non-spherical particles, the modified Ergun–Wen–Yu drag model was used with added amplification factors depending on the local regime of the multiphase flow (dense vs. dilute). The best obtained combination of parameter settings is material-dependent and allows to achieve satisfactory agreement with the experiments in terms of the time-averaged pressure drop and bed height. Qualitative assessment of fluidization in terms of the properties inherent to materials in different Geldart groups was also performed and found to be strongly influenced by the choice of the particle stress model. Finally, the results of the simulations were analyzed to evaluate the minimum fluidization velocity and the average bubble size. The obtained values were compared to semi-empirical correlations available in literature. [Display omitted] • Multiphase particle-in-cell method simulates the fluidization of sand and LECA. • Rosin–Rammler particle size distribution is fitted to granulometry data. • Validation of the model using a specifically tailored experiment is performed. • Particle stress with kinetic-collisional pressure allows qualitative agreement. • Ergun–Wen–Yu drag modified for irregular particles allows quantitative agreement. [ABSTRACT FROM AUTHOR]