Research Project CX3

The fundamental understanding of salt precipitation in porous media due to evaporation processes is important in different environmental and technical applications. For example, salt precipitation in building materials generate stresses, which can lead to weathering and damage of constructions and cultural monuments. The salinization of soil in arid and semi-arid zones is a further challenge. This is often induced by use of saline water for irrigation, due to the limited availability of irrigation water of high quality. In combination with high evaporation rates and poor drainage this can lead to salt precipitation in the root zone or at the soil surface and a resulting reduction of crop yield.
During this processes water evaporates at the evaporation front at the brine-air interface. Due to the loss of water the salt concentration in the brine increases at the evaporation front and salt precipitates when the solubility limit is exceeded. The evaporation process is influenced by the fluid properties like the salt concentration, by the surrounding atmospheric conditions like temperature, humidity of the air or wind speed and the porous medium properties like permeability.

During the evaporation of a saline solution, the salt concentration increases at the evaporation front and solid precipitates in the porous medium when the solubility limit is exceeded. At the evaporation front a layer of higher densitiy develops due to the accumulation of salt in the liquid phase. This can lead to density-driven instabilities which prevent a further increase of the salt concentration and the precipitation of salt. Many process-controlling properties of salt precipitation depend on interface-driven processes on the pore-scale like the spatially varying salt concentration. To investigate this interface-driven processes experiments and simulations are done on the pore scale, where the single pores and interfaces can be resolved. The formation of density-driven instabilities is considered on the REV-scale, where the porous medium is averaged over a representive elementary volume (REV).

In this project a so-called pore-network model is used, that efficiently resolves pore-scale phenomena using a network of pore bodies and pore throats, representing the larger pore spaces and the connections in-between. The aim is to developed a reactive transport model for the pore-network model including the precipitation reaction and the resulting change in pore-space geometry. To investigate the influence of the free-flow on the evaporation processes in detail the pore network must be coupled to a free-flow domain.

Further, experiments are conducted at the microCT-facility of the Oregon State University to improve and validate the model. A column filled with a porous medium and salt solution which is open to the atmosphere is scanned after several periods of evaporation using X-ray tomography. From these scans three-dimensional information about the distribution of the liquid and gaseous phase can be gained as well as knowledge about the position and amount of precipitated salt. From this experiments an extensive dataset is obtained which is of great value for the further development and improvement of the numerical model.