Heat and Mass Transfer in Porous Media

Humidity is one of the main causes of decay in buildings, particularly rising damp, caused by the migration of moisture from the ground through the materials of the walls and floors via capillary action. This water comes from groundwater and surface water. The height that moisture will reach through capillary action depends upon factors such as the quantity of water in contact with the particular part of the building, surface evaporation conditions, wall thickness, building orientation and the presence of salts. In historic buildings, rising damp is particularly difficult to treat, due to the thickness and heterogeneity of the walls. Traditional methods of dealing with this problem (chemical or physical barriers, electro-osmosis, etc.) have proved somewhat ineffective. There is therefore a need to study new systems. In recent years, experimental research into the effectiveness of wall base ventilation systems (natural or hygro-regulated) to reduce the level of rising damp, conducted at the Building Physics Laboratory, Faculty of Engineering, University of Oporto, has yielded interesting results. Numerical simulation studies, using the software WUFI-2D, have given similar findings. This paper describes a new system for treating rising damp in historic buildings based upon a hygro-regulated wall base ventilation system, and analyses the results obtained following implementation of the system in churches in Portugal. It was defined criterions to avoid condensation problems inside the system and crystallizations/dissolutions problems at the walls.

Heating and ventilating are fundamental actions for the control of humidity in the indoor environment, but the hygroscopic inertia provided by the materials that contact the inside air can be a complement for that control. The hygroscopic behavior of the walls and ceiling finishing materials, as well as furniture and textiles inside the dwellings, defines their hygroscopic inertia. Reducing the persistence of high relative humidity values inside buildings is essential for the control of mould growth on material surfaces, that can otherwise cause degradation and bring about social and economical problems for the users. As the hygroscopic inertia concept can be very difficult to approach for building designers, a definition of daily hygroscopic inertia classes is presented, based on numerical and laboratory work on this subject. An outline of a simple method, using those classes, that allows for the evaluation of the reduction of mould growth potential associated to a configuration of inside finishes is proposed. The extensive experimental campaign aiming the characterization of the moisture buffering capacity of interior finishing system and the assessment of a room’s hygroscopic inertia is described. The MBV—Moisture Buffer Value is evaluated for different revetments. The assessment of hygroscopic inertia at room level is implemented using a flux chamber designed specifically for this experiment. A daily hygroscopic inertia index, I _h,d , is defined using MBV as a basis for the assessment of materials contribution to the buffering capacity of a room. The correlation between that index and peak dampening is proved using the presented experimental results. Systematic simulation of the set of dynamic experiments of transient moisture transfer in the hygroscopic region is presented; allowing to verifying and correcting the modeling assumptions and the basic data used in simulations, and conclude on the most effective strategies to conduct this type of simulations.

Heat management in high thermal-density systems is confronting rising challenges due to the extremely high heat dissipation capacity requirement in ever more miniaturized and intensified processes. Efforts have been made for heat transfer enhancement. A number of heat transfer technologies including natural convention and radiation, forced convection, pool boiling, micro-channel flow boiling, heat pipes and capillary pumps were introduced. However, the drastically increasing heat transfer requirements are still calling for extensive experimental investigations, especially on the phase change processes in micro-scale. A review of the experimental studies on two-phase flow boiling and heat transfer in small flow passages was presented. Some experimental results published in the year 1993–2010 were overviewed, revealing some controversial conclusions on micro-scale flow boiling heat transfer mechanisms. Besides, several existing heat transfer correlations were assessed. It should be admitted that the existing database is still limited, especially for flow boiling in passages with unique geometries such as high-aspect-ratio cross section. Extensive explorations on two-phase flow boiling and heat transfer in micro-channels are essential for heat transfer predictions and applications in micro-systems.

We present a numerical technique for the simulation of salinity- as well as thermohaline-driven flows in fractured porous media. In this technique, the fractures are represented by low-dimensional manifolds, on which a low-dimensional variant of the PDEs of variable-density flow is formulated. The latter is obtained from the full-dimensional model by the average-along-the-vertical. The discretization of the resulting coupled system of the full- and low-dimensional PDEs is based on a finite-volume method. This requires a special construction of the discretization grid which can be obtained by the algorithm presented in this work. This technique allows to reconstruct in particular the jumps of the solution at the fracture. Its precision is demonstrated in the numerical comparisons with the results obtained in the simulations where the fractures are represented by the full-dimensional subdomains.

Lungs are natural porous structures that are unique, challenging, and high-value media to study. There are multiple drivers to obtain an improved understanding of their architecture and function: to increase high-value information and insights that can be applied in healthcare, to devise control strategies that will limit some hazards effects, and to expand boundaries of what is known that can be applied to produce new (improved) materials. This chapter covers three major topics: shape and structure of lungs, airflow characteristics and the interaction of suspension of particles with the respiratory tract. It is focused on the biological and physical mechanisms involved, in the hope that this will allow an overview of the science related to the respiratory tract.

Convective heat and mass transfer from bodies of classic geometry, a plate and a cylinder, embedded in a porous infiltrated medium and a line source located in a saturated porous medium is studied with the use of the Hele-Shaw analogy. The set of partial differential equations for steady-state, gap-averaged, two-dimensional heat and mass transfer and fluid flow with constant viscosity are identical with those for the Darcy flow regime in porous media and whose permeability K is h ²/12. All theoretical solutions are verified by the experimental data on processes in the Hele-Shaw cell and then compared with available experimental data on convective transfer processes in porous media. The findings indicate that convective transport phenomena in a Hele-Shaw cell share many features of those in porous media even though the phenomena in the cell are constrained to an essentially two-dimensional convection patterns. The validity of Hele-Shaw analogy for convective transport phenomena in porous media is discussed and the limits of applicability are determined where possible.

This chapter provides information related to simultaneous heat and mass transfer and dimension variations in unsaturated porous bodies of arbitrary shape during transient problems. Two mathematical approaches are presented and discussed: lumped and distributed-parameters models. Here, applications have been given to some engineering processes (cooling, heating and drying). A two-dimensional distributed model written in a orthogonal curvilinear coordinate system and which assumes the pure diffusion as the sole mechanism of heat and moisture transport within the solid is applied to ellipsoidal porous bodies (prolate spheroid). A lumped-parameter model written in any coordinate system which includes effects such as shape of the body, heat and mass generation, evaporation and convection is presented, and analytical solution of the governing equations, limitations of the modeling and general theoretical results are discussed.

This chapter presents a practical approach to the identification and interpretation of phenomena in multi-component ceramic granular porous materials (called mould sands) used as the moulds to cast metal alloys at temperatures above 1,300°C. The methodologies chosen for experimental (using artificial and technological heat sources) and numerical studies (using the inverse solution) of these materials and their application in database of simulation systems for virtualization of casting processes are described. The essence of substitute coefficients of mould thermal parameters and the notion of thermal history of heating are explained. The experiment shows the sensitivity of the simulation results to changes in the materials’ coefficients and also that validation is required for the proper use of simulation systems in foundries. Examples of estimating thermal characteristic for the materials chosen with—middle and high thermal instability are presented. Furthermore, the thermo-mechanical parameters of these porous thermolabile materials in original Hot Distortion^® tests are signalled.

This chapter deals with metal foams heat exchangers design. We present effective properties of metal foams such as (a) thermal conductivity and heat transfer coefficient used to model heat transfer, (b) permeability and inertial coefficient used to model flow pattern through foams. All these properties constitute basic inputs for homogeneous equivalent porous media approach widely used to design heat exchanger. We present methods which will allow one to determine thermo-physical and geometrical properties of foams. A numerical approach is used to get better understanding of solid matrix geometry influence on transport properties.