Filtern
Dokumenttyp
- Vortrag (3)
- Zeitschriftenartikel (2)
- Beitrag zu einem Sammelband (2)
Sprache
- Englisch (7)
Schlagworte
- Porous media (7) (entfernen)
Organisationseinheit der BAM
Eingeladener Vortrag
- nein (3)
A three-phase transport model for high-temperature concrete simulations validated with X-ray CT data
(2021)
Concrete exposure to high temperatures induces thermo-hygral phenomena, causing water phase changes, buildup of pore pressure and vulnerability to spalling. In order to predict these phenomena under various conditions, a three-phase transport model is proposed. The model is validated on X-ray CT data up to 320 ◦C, showing good agreement of the temperature profiles and moisture changes. A dehydration description, traditionally derived from thermogravimetric analysis, was replaced by a formulation based on data from neutron radiography. In addition, treating porosity and dehydration evolution as independent processes, previous approaches do not fulfil the solid mass balance. As a consequence, a new formulation is proposed that introduces the porosity as an independent variable, ensuring the latter condition.
Spalling of concrete structures is a serious issue for their safety. A better understanding of the pore water distribution and state during a fire is a prerequisite for numerical approaches to such problems.
Temperature-driven water transport in concrete consists of multiple phenomena, such as convection, diffusion, adsorption and dehydration. Distinguishing the different influences experimentally is difficult because typically they cannot be disentangled.
A common experimental setup approximates a one-dimensional flow, and places temperature and pressure gauges along the propagation direction. For direct information about the water content inside a sample, methods such as NMR or neuron radiography are necessary.
A multiphase model for the flow in porous media is presented, with dehydration and changes in the pore size distribution taken into consideration. NMR measurements for temperature-driven flow have been performed. The numerical and experimental results are compared for water transport at temperatures below the critical point. Since both the finite-element model and the experiment allow the distinction between adsorbed, capillary and bulk water, a more fine-grained view of the pore water state is obtained.
Porous media flow is an important aspect of geomechanics and material behaviour of concrete under heating and drying. We will present a model for multiphase nonisothermal flow and its solution using the finite element method. In contrast to single phase models (e.g.), which have to consider vaporization or condensation as a temperate dependency on the thermal capacity, the phase changes will be explicitly considered. More complex models have been proposed (e.g.), which also capture the multiphase nature of the flow field. These models, however, also include the coupling to a mechanics field, with many of the constitutive relations of the flow field dependent on the specific damage formulation employed. The resulting complexity of the constitutive models leads to a high number of experiments and difficult calibration procedures to determine their parameters.
The simulation of the moisture distribution under fast and slow heating is presented. The constitutive relations employed here do not assume any particular damage-mechanical model and will therefore allow free choice when mechanical coupling is desired. The resulting pore pressure field is an important prerequisite for the modeling of concrete spalling. Suitable numerical methods to achieve optimal convergence will be discussed.
By using direct coupling between the parts of a multiphysics problem, the models for each part can be simplified, making calibration and sensitivity calculations easier. This is also important for more complicated problems, where the need for simpler material laws that handle coupled phenomena becomes more evident. In particular, this contribution is an essential step towards the modeling of concrete spalling.
Fire simulation is mostly understood as simulation of room fires, i.e. where the free volume is
large compared to the volume occupied by the combustible matter. In such a case, the fire
propagation is predominantly a fluid dynamics problem. Fire propagation through bulk
materials, insulation layers or wooden construction elements is, however, determined by
conduction and diffusion. For these cases modified modelling principles and different
software solutions have to be applied. Computational results are presented for temperature
evolution and charring progress in a timber beam exposed to ISO standard temperature curve,
for self-ignition and fire propagation in a waste deposit with consideration to
conduction/diffusion and convection inside the porous medium and for a fire in an
underground coal seam where conduction/diffusion is the prevailing mechanism.
An evolutionary optimization technique is used to reconstruct digitized material models of 3003 nm3 size for mesoporous two-phase systems. The models are adapted to the two-point probability (TPP) and to a volume-based pore-size distribution (PSD) which were derived from SANS and adsorption experiments and which carry statistical information about morphology and topology of the pore system. To avoid extreme update-costs, the bulk of mutations are assessed by means of a suitable approximation of the PSD; it is demonstrated that a sporadic insertion of the PSD suffices to drive the algorithm towards satisfactory models in acceptable time. Our approach is knowledge-based in the sense that (i) the mutations are restricted to expedient exchanges of phase-voxels by a heuristic rule, and (ii) the sporadic calculation of the PSD from the current state of the model, in essence, provides an efficient self-control for the evolutionary process. We applied the method to reconstruct periodic models of the xerogel Gelsil 200. Such reconstructs of real mesoporous solids could be utilized, for instance, to verify theories of adsorption and capillary condensation.