Heat and Mass Transfer in Porous Materials

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Authors: C.M. Rufino Franco, A.G. Barbosa de Lima
Abstract: Hygroscopic materials can suffer variations in water content and temperature according to ambient air conditions. In the case of agricultural products, control the moisture and temperature is of paramount importance for the conservation of their quality and to avoid losses. In this context, the drying is one of the most widely used methods for this purpose. Various drying techniques are reported in the literature,in order to increasingly improve the final product quality and saving energy used. The purpose of this paper is to discuss and analyze the latest advances in intermittent drying research from the standpoint of quality after-drying product and energy efficiency. The most applied modeling techniques are highlighted and considerations for its use are shown.
Authors: M. Vázquez da Silva, J.M.P.Q. Delgado
Abstract: Osmotic dehydration is a water removal process that consists of placing foods, such as pieces of fruits and vegetables, in a hypertonic solution. Both mass fluxes lead to a decrease of the water activity in the product, increasing its shelf life. It can be used as a single dehydration process or as a pre-treatment of other processes such as drying, pasteurization or freezing. Heat and mass transfer gradients associated to the process produce changes in the chemical, physical and structural characteristics of the vegetable tissue. The knowledge and prediction of these changes are very important because they affect the quality of the final product, process modelling and design of process equipment. This work presents some new experimental data during osmotic dehydration of fruits. Some models were used so as to obtain predicted values during osmotic dehydration.
Authors: V.S. Silva, J.M.P.Q. Delgado, W.M. Paiva Barbosa de Lima, A.G. Barbosa de Lima
Abstract: The drying is a thermodynamic process of simultaneous heat transfer and moisture removal of a porous material. Clay products when exposed to drying without control can suffer cracks and deformations, reducing its quality post-drying. Thus, this work aims to study the holed solid drying with arbitrary shape using the global capacitance method. Application has been made for the drying of clay products. The analytical solution of the governing equations was made using the method of separation of variables.Kinetic results for mass loss and heating of the material are presented and analyzed. It was observed that the moisture loss process occurs at a lower speed than the heating of the ceramic material because its thermal diffusivity is greater than the mass diffusivity and that the area/volume relationshipstrongly affects the phenomena of heat and mass transport.
Authors: A. Ahmadi-Sénichault, V. Canseco, N. Sefrioui-Chaibainou, A. Omari, H. Bertin
Abstract: The main objective of this paper is to give more insight on colloids deposition and re-entrainment in presence of a rough surface. Experiments on retention and release of colloids in a porous medium are first presented. The influence of physicochemical and hydrodynamic conditions is investigated. The experimental results cannot be qualitatively interpreted using the DLVO theory and knowledges at pore scale are then needed. A 3D numerical simulation approach at the pore scale is therefore proposed where the motion of colloids is solved in presence of collector surfaces bearing various kinds of asperities and by taking into account physico-chemical interactions calculated at each time step during colloid movement. It is obviously observed that both deposition and mobilization of particles are dependent on solution chemistry and hydrodynamic conditions and are significantly affected by the form and size of the local roughness of the pore surface. Therefore, depending on solution ionic strength and surface topography, colloids may be adsorbed or not and when a particle is retained an increase of flow strength is then needed to remove it and such an increase is specific to the location of occurrence of the adsorption step. In general, simulation results allow us to explain our experimental results that show that by steeply increasing the flow strength, more and more fractions of particles retained inside the porous medium are released until all particles are removed.
Authors: Paweł Popielarski, Zenon Ignaszak
Abstract: The problem described in the paper concerns the thermo-physical properties of the green mould material to which the cast iron is most often poured. The study includes the experiment of pouring the cast iron plate into green bentonite-sand mould. The temperature fields of casting and in different zones of the mould were recorded. The goal of the study was to determine the substitute thermo-physical properties of mould sand containing the over-moisture zone by means of simulation tests (inverse problem). An originality of the related research is an attempt to take into account the effects of the global thermal phenomena occurring in the quartz sand bonded by bentonite-water binder, by application of the substitute thermal coefficients without using the coupled modeling. In the simulation tests in order to achieve the effect of rapid heating of the mould (below temperatures 100 °C) by poured cast iron (T>1300 °C), the function of the latent heat source and the modified values of substitute thermal conductivity and substitute specific heat of the molding sand were used. In order to facilitate the solution, the mould was divided into zones, in which different starting humidity of molding sand was assumed.
Authors: Bernardo Buonomo, Oronzio Manca, Sergio Nardini, D. Ricci
Abstract: Heat transfer enhancement technology has the aim to develop more efficient systems as demanded in many applications in the fields of automotive, aerospace, electronics and process industry. A possible solution to obtain efficient cooling systems is represented by the use of confined impinging jets. Moreover, the introduction of nanoparticles in the working fluids can be considered in order to improve the thermal performances of the base fluids. In this paper a numerical investigation on mixed convection in confined slot jets impinging on a porous media by considering pure water or Al2O3/water based nanofluids is described. A two-dimensional model is developed and different Peclet numbers and Rayleigh numbers were considered. The particle volume concentrations ranged from 0% to 4% and the particle diameter is equal to 30 nm. The target surface is heated by a constant temperature value, calculated according to the value of Rayleigh number. The distance of the target surface is five times greater than the slot jet width. A single-phase model approach has been adopted in order to describe the nanofluid behaviour while the hypothesis of non-local thermal equilibrium is considered in order to simulate the behaviour in the porous media which is featured by a porosity value of 0.87. The aim consists into study the thermal and fluid-dynamic behaviour of the system. Results show increasing values of the convective heat transfer coefficients for increasing values of Peclet number and particle concentration. This behaviour is more evident at low Peclet number values and Rayleigh number ones.
Authors: E. Barreira, J.M.P.Q. Delgado
Abstract: The effect of moisture in buildings is normally related with damage, which may occur due to the presence of moisture itself or due to its evaporation. The drying process plays an important role in the available moisture, both inside the material or at its surface. This paper presents the results of an experimental evaluation of the drying kinetics, considering both surface and bulk moisture transfer. Two different specimens were used: External Thermal Insulation Composite Systems (ETICS) to study surface moisture transfer and Autoclaved Cellular Concrete (ACC) to analyse the bulk moisture transfer. For both samples the drying kinetics was assessed considering different environment conditions (air temperature and humidity). Five different first-order kinetics models, available in the literature, were adjusted to describe the drying process and estimate the equilibrium moisture content of ETICS and ACC.The results point that Logarithmic and Midilli models allow the best fit and that the drying time constant is strongly affected by the moisture transfer phenomenon (at surface or bulk) and by the drying air conditions. It was also estimated the apparent molecular diffusion coefficient for ACC and its variation with temperature.
Authors: J.M.P.Q. Delgado, A.S. Guimarães, V.P. de Freitas
Abstract: The main goal of this work is to present a complete review of rising damp treatment in building heritage using a technique developed in our group to estimate and mitigate the height of the rising damp front and to predict the improvements of some experimental treatment techniques. The paper present a mathematical and a numerical analyse of the problem; an experimental and a practical application of the technique developed. This work is distributed into six main sections, in addition to this general Introduction: This book chapter is divided in several sections. Initially, it is presented the “state of the art” synthesis, where it is studied the techniques traditionally used for the rising damp treatment, showing the existing limitations on its application in monumental heritage and in old or ancient buildings, with specific characteristics. Then, it is performed the idea and the operating principle of the new technique, known as the wall base ventilation system, on the basis of previously conducted studies. An analytical, numerical and “in-field” study it is presented based in a case study described in detail. Finally, it is done a critical analysis of the results obtained, projecting the future work.
Authors: Antonio F. Miguel
Abstract: Living organisms are open dissipative thermodynamic systems that rely on mechano-thermo-electrochemical interactions to survive. Plant physiological processes allow plants to survive by converting solar radiation into chemical energy, and store that energy in form that can be used. Mammals catabolize food to obtain energy that is used to fuel, build and repair the cellular components. The exergy balance is a combined statement of the first and second laws of thermodynamics. It provides insight into the performance of systems. In this paper, exergy balance equations for both mammal’s and green plants are presented and analyzed.

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