Abdel Hassan Sweidan

This research work presents an experimental and numerical study of the coupled thermo-hydro-mechanical (THM) processes that occur during soil freezing. With focusing on the artificial ground freezing (AGF) technology, a new testing device is built, which considers a variety of AGF-related boundary conditions and different freezing directions. In the conducted experiments, a distinction is made between two thermal states: (1) The thermal transient state, which is associated with ice penetration, small deformations, and insignificant water suction. (2) The thermal (quasi-) steady state, which has a much longer duration and is associated with significant ice lens formation due to water suction. In the numerical modeling, a special focus is laid on the processes that occur during the thermal transient state. Besides, a demonstration of the micro-cryo-suction mechanism and its realization in the continuum model through a phenomenological retention-curve-like formulation is presented. This allows modeling the ice lens formation and the stiffness degradation observed in the experiments. Assuming a fully saturated soil as a biphasic porous material, a phase-change THM approach is applied in the numerical modeling. The governing equations are based on the continuum mechanical theory of porous media (TPM) extended by the phase-field modeling (PFM) approach. The model proceeds from a small-strain assumption, whereas the pore fluid can be found in liquid water or solid ice state with a unified kinematics treatment of both states. Comparisons with the experimental data demonstrate the ability and usefulness of the considered model in describing the freezing of saturated soils.

Latent heat storage systems using phase change materials (PCMs) are considered an attractive technique for storing the available thermal energy due to their high energy storage capacity, compactness and ability to store heat at an almost constant temperature. However, the PCMs suffer from their low thermal conductivity that affects the heat transfer rates and leads to slow charging and discharging processes. Therefore, many approaches have been adopted for improving the low thermal conductivity of PCMs. In the underlying research work, a latent heat storage medium using a PCM‐saturated highly conductive porous metal foam is being studied. High porosity cellular metal foams are proved to be promising materials for enhancing the heat transfer performance of the PCMs due to their high surface area to volume ratio, ultra light weight and relatively high thermal conductivity. A macroscopic description of the PCM‐saturated porous material with intrinsically coupled and incompressible solid and fluid constituents is presented, based on the Theory of Porous Media (TPM). The heat transfer process between the metal foam and the PCM is described using the local thermal non‐equilibrium model, where the phase‐field method (PFM) is employed to account for the phase change process. The PFM relies on specification of the free energy density function and employs a phase‐field variable that defines the state of the material (solid or liquid). Finally, numerical examples using the finite element method are presented to show the ability of the phase‐field‐porous media approach in simulating the phase change problems on the macro scale.