Theresa Schollenberger war bis 30.04.2025 Mitarbeiterin am Lehrstuhl für Hydromechanik und Hydrosystemmodellierung (LH2) und im SFB 1313. Sie ist Mitglied der SimTech Graduate School (GS SimTech) und wird am Freitag, den 13. Juli 2026 ihre Doktorarbeit mit dem Titel "Numerical investigation of evaporation-driven processes in brine-saturated porous media - density instabilities and salt precipitation" verteidigen.
Datum: Freitag, den 13. Juli 2026
Uhrzeit: 09:30 Uhr
Ort: MML, U. 1.003, Pfaffenwaldring 61, 70569 Stuttgart
Abstract
The evaporation from a brine-saturated porous medium is an important process in many technical and environmental systems. It can introduce the precipitation of salt and the development of density-driven instabilities. The precipitation of salt in the porous medium can lead to challenges in various contexts. The associated stresses can for example damage building material. The accumulation and precipitation in soil, called soil salinization, proposes great challenges for agriculture, due to the resulting degradation of soil and the intolerance of most plants to high salt concentrations. Soil salinization is used as exemplary application in this work.
During evaporation, brine is transported towards the evaporation front at the top. There, the water evaporates, whereas the salt accumulates. This results in an increase of salt concentration at the evaporation front. If the solubility limit of the salt is exceeded, the salt precipitates as solid salt. This leads to alterations of the pore space and the formation of salt crusts on the porous medium. The increase in salt concentration in the brine is accompanied by an increase in fluid density. This results in an unstable layering, which can lead to density-driven instabilities. The instabilities have the potential to transport the accumulated salt downwards and so can prevent salt precipitation.
The thesis aims to develop numerical models, which are able to represent the relevant processes induced by the evaporation of brine from a porous medium, in particular the development of density-driven instabilities and the precipitation of salt. The models are subsequently used to develop a detailed understanding of these processes and to investigate the dependence on different parameters.
Therefore, numerical models on different scales are developed in the framework of DuMuX. The development of density-driven instabilities is investigated on the macro scale using a REV-model, where the porous medium is represented with volume-averaged quantities. A linear stability analysis is used to validate the model, by comparison of the characteristic onset time of the instabilities.
The model enables the investigation of the initial development of density-driven instabilities and the conditions under which the system gets unstable. The initial development of the instabilities can be divided in three phases based on the dominant transport processes.
The understanding of the underlying processes enables the analysis under which conditions the system gets unstable and fingers form. For lower permeabilities, the system gets more stable as later onset times are observed for the instabilities as well as for the fingers. For the lowest considered permeability, salt starts to precipitate and no fingers form. This is based on the higher resistance of the porous medium for advective transport associated with the lower permeability. Thus, diffusive fluxes get more dominant, which stabilize the system. Therefore, the REV-model enables to determine criteria whether instabilities develop or salt precipitates.
For the investigation of salt precipitation, a model on the micro-scale is developed using a pore-network model. The pore-network model represents the pore space with a network of pore-bodies and pore-throats and considers pore-space alterations due to the precipitation. The amount of precipitation in the pore bodies is determined by a chemical reaction term in the mole balance and alters the volume of the pore body. In the throats, no mole balance and reaction is considered. The volume of the throats, however, determines the flow resistance of the pore network. Thus, four different concepts to approximate the amount of precipitation in the pore throats are presented and compared for one-phase flow. The concepts use different information from the adjacent pore bodies to determine the amount of precipitation in the pore throats and lead to a different development of the pore-network permeability. Based on the altered throat volume, a new transmissibility of the throats is determined by two different concepts. These two concepts do not show great influence on the permeability of the system.
A two-phase pore-network model is further used to investigate micro-scale processes during salt precipitation. The pore-network model is able to represent the influence of pore-body and pore-throat volume alteration, the influence of the interface location and the influence of micro-scale heterogeneities on the precipitation distribution in the pore-network, including processes like capillary pumping.
The developed pore-network model is a strong tool to consider micro-scale processes, also in larger domains. The model can be used to determine upscaled relations for the macro scale. It further can be deployed to represent the upper soil layers in a coupled model. As a middle layer coupled to a free-flow model above and macro-scale model below, micro-scale processes are so taken into account in the relevant area.
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