Porosidade e infiltração de água do solo sob diferentes sistemas de manejo

Management systems affect soil structure, causing changes in porosity that can influence water infiltration into the soil. This study aimed to evaluate the effect of different management systems on the porosity and water infiltration in Distroferric Red Nitosol with clay texture. The management systems were: conventional with disc plow + two soft harrowing, conventional with heavy disc harrow + two soft harrowing, no tillage and fallow. The following attributes: porosity, pore shape and water infiltration velocity into the soil were determined. Management systems such as no-tillage, conventional with disc plow and heavy disc harrow, and fallow modify the porosity and the shape of pores of a Distroferric Red Nitosol, influencing water infiltration, which was favored in no-tillage for the greater presence of elongated pores.

Stagnant effective thermal conductivity of agro-industrial residues for solid-state fermentation

The stagnant effective thermal conductivities (K0) of sugar cane bagasse (SCB), wheat bran (WB), orange pulp and peel (OPP) and their combination (weight proportion 1:2:2 SCB/OPP/WB) were obtained using the line heat source method. These solid materials were applied to pectinase production via solid-state fermentation. The moisture content ranged from 4 to 80% (w.b.). A conduction mechanism through the porous media was observed, along with conduction through a liquid film and contact thermal resistance between the samples and the probe. K0 was low for intermediate moisture contents and approached the molecular conductivity of water for high moisture contents. © 2013 Copyright Taylor and Francis Group, LLC.

Transferência de calor em leitos fixos com aplicação em reatores de fermentação em estado sólido

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Determinação da difusividade térmica de um solo incorporado com resíduo de E.T.A

Pós-graduação em Física - IGCE

Tank experiments with borehole self-potential data

Despite their importance in the evaluation of petroleum and gas reservoirs, measurements of self-potential data under borehole conditions (well-logging) have found only minor applications in aquifer and waste-site characterization. This can be attributed to lower signals from the diffusion fronts in near-surface environments because measurements are made long after the drilling of the well, when concentration fronts are already disappearing. Proportionally higher signals arise from streaming potentials that prevent using simple interpretation models that assume signals from diffusion only. Our laboratory experiments found that dual-source self-potential signals can be described by a simple linear model, and that contributions (from diffusion and streaming potentials) can be isolated by slightly perturbing the borehole conditions. Perturbations are applied either by changing the concentration of the borehole-filling solution or its column height. Parameters useful for formation evaluation can be estimated from data measured during perturbations, namely, pore water resistivity, pressure drop across the borehole wall, and electrokinetic coupling parameter. These are important parameters to assess, respectively, water quality, aquifer lateral continuity, and interfacial properties of permeable formations.

Mathematical modeling and analysis of the particle interactions in the direct filtration using the colloidal theory

This work used the colloidal theory to describe forces and energy interactions of colloidal complexes in the water and those formed during filtration run in direct filtration. Many interactions of particle energy profiles between colloidal surfaces for three geometries are presented here in: spherical, plate and cylindrical; and four surface interactions arrangements: two cylinders, two spheres, two plates and a sphere and a plate. Two different situations were analyzed, before and after electrostatic destabilization by action of the alum sulfate as coagulant in water studies samples prepared with kaolin. In the case were used mathematical modeling by extended DLVO theory (from the names: Derjarguin-Landau-Verwey-Overbeek) or XDLVO, which include traditional approach of the electric double layer (EDL), surfaces attraction forces or London-van der Waals (LvdW), esteric forces and hydrophobic forces, additionally considering another forces in colloidal system, like molecular repulsion or Born Repulsion and Acid-Base (AB) chemical function forces from Lewis.

Hydrodynamic analysis of particle collection efficiency: comparing downflow and upflow filtration

Models of the filtration phenomenon describe the mass balance in bed filtration in terms of particle removal mechanisms, and allow for the determination of global particle removal efficiencies. These models are defined in terms of the geometry and characteristic elements of granule collectors, particles and fluid, and also the composition of the balance of forces that act in the particle collector system. This work analyzes particles collection efficiency comparing downflow and upflow direct filtration, taking into account the contribution of the gravitational factor of the settling removal efficiency in future proposal of initial collection efficiency models for upflow filtration. A qualitative analysis is also made of the proposal for the collection efficiency models for particle removal in direct downflow and upflow filtration using a Computational Fluid Dynamics (CFD) tool. This analysis showed a strong influence of gravitational factor in initial collection efficiency (t = 0) of particles, as well as the reasons of their values to be smaller for upflow filtration in comparison with the downflow filtration.

Bioremediation of PAHs - Limitations and soultions

Introduction 1.1 Occurrence of polycyclic aromatic hydrocarbons (PAH) in the environment Worldwide industrial and agricultural developments have released a large number of natural and synthetic hazardous compounds into the environment due to careless waste disposal, illegal waste dumping and accidental spills. As a result, there are numerous sites in the world that require cleanup of soils and groundwater. Polycyclic aromatic hydrocarbons (PAHs) are one of the major groups of these contaminants (Da Silva et al., 2003). PAHs constitute a diverse class of organic compounds consisting of two or more aromatic rings with various structural configurations (Prabhu and Phale, 2003). Being a derivative of benzene, PAHs are thermodynamically stable. In addition, these chemicals tend to adhere to particle surfaces, such as soils, because of their low water solubility and strong hydrophobicity, and this results in greater persistence under natural conditions. This persistence coupled with their potential carcinogenicity makes PAHs problematic environmental contaminants (Cerniglia, 1992; Sutherland, 1992). PAHs are widely found in high concentrations at many industrial sites, particularly those associated with petroleum, gas production and wood preserving industries (Wilson and Jones, 1993). 1.2 Remediation technologies Conventional techniques used for the remediation of soil polluted with organic contaminants include excavation of the contaminated soil and disposal to a landfill or capping - containment - of the contaminated areas of a site. These methods have some drawbacks. The first method simply moves the contamination elsewhere and may create significant risks in the excavation, handling and transport of hazardous material. Additionally, it is very difficult and increasingly expensive to find new landfill sites for the final disposal of the material. The cap and containment method is only an interim solution since the contamination remains on site, requiring monitoring and maintenance of the isolation barriers long into the future, with all the associated costs and potential liability. A better approach than these traditional methods is to completely destroy the pollutants, if possible, or transform them into harmless substances. Some technologies that have been used are high-temperature incineration and various types of chemical decomposition (for example, base-catalyzed dechlorination, UV oxidation). However, these methods have significant disadvantages, principally their technological complexity, high cost , and the lack of public acceptance. Bioremediation, on the contrast, is a promising option for the complete removal and destruction of contaminants. 1.3 Bioremediation of PAH contaminated soil & groundwater Bioremediation is the use of living organisms, primarily microorganisms, to degrade or detoxify hazardous wastes into harmless substances such as carbon dioxide, water and cell biomass Most PAHs are biodegradable unter natural conditions (Da Silva et al., 2003; Meysami and Baheri, 2003) and bioremediation for cleanup of PAH wastes has been extensively studied at both laboratory and commercial levels- It has been implemented at a number of contaminated sites, including the cleanup of the Exxon Valdez oil spill in Prince William Sound, Alaska in 1989, the Mega Borg spill off the Texas coast in 1990 and the Burgan Oil Field, Kuwait in 1994 (Purwaningsih, 2002). Different strategies for PAH bioremediation, such as in situ , ex situ or on site bioremediation were developed in recent years. In situ bioremediation is a technique that is applied to soil and groundwater at the site without removing the contaminated soil or groundwater, based on the provision of optimum conditions for microbiological contaminant breakdown.. Ex situ bioremediation of PAHs, on the other hand, is a technique applied to soil and groundwater which has been removed from the site via excavation (soil) or pumping (water). Hazardous contaminants are converted in controlled bioreactors into harmless compounds in an efficient manner. 1.4 Bioavailability of PAH in the subsurface Frequently, PAH contamination in the environment is occurs as contaminants that are sorbed onto soilparticles rather than in phase (NAPL, non aqueous phase liquids). It is known that the biodegradation rate of most PAHs sorbed onto soil is far lower than rates measured in solution cultures of microorganisms with pure solid pollutants (Alexander and Scow, 1989; Hamaker, 1972). It is generally believed that only that fraction of PAHs dissolved in the solution can be metabolized by microorganisms in soil. The amount of contaminant that can be readily taken up and degraded by microorganisms is defined as bioavailability (Bosma et al., 1997; Maier, 2000). Two phenomena have been suggested to cause the low bioavailability of PAHs in soil (Danielsson, 2000). The first one is strong adsorption of the contaminants to the soil constituents which then leads to very slow release rates of contaminants to the aqueous phase. Sorption is often well correlated with soil organic matter content (Means, 1980) and significantly reduces biodegradation (Manilal and Alexander, 1991). The second phenomenon is slow mass transfer of pollutants, such as pore diffusion in the soil aggregates or diffusion in the organic matter in the soil. The complex set of these physical, chemical and biological processes is schematically illustrated in Figure 1. As shown in Figure 1, biodegradation processes are taking place in the soil solution while diffusion processes occur in the narrow pores in and between soil aggregates (Danielsson, 2000). Seemingly contradictory studies can be found in the literature that indicate the rate and final extent of metabolism may be either lower or higher for sorbed PAHs by soil than those for pure PAHs (Van Loosdrecht et al., 1990). These contrasting results demonstrate that the bioavailability of organic contaminants sorbed onto soil is far from being well understood. Besides bioavailability, there are several other factors influencing the rate and extent of biodegradation of PAHs in soil including microbial population characteristics, physical and chemical properties of PAHs and environmental factors (temperature, moisture, pH, degree of contamination). Figure 1: Schematic diagram showing possible rate-limiting processes during bioremediation of hydrophobic organic contaminants in a contaminated soil-water system (not to scale) (Danielsson, 2000). 1.5 Increasing the bioavailability of PAH in soil Attempts to improve the biodegradation of PAHs in soil by increasing their bioavailability include the use of surfactants , solvents or solubility enhancers.. However, introduction of synthetic surfactant may result in the addition of one more pollutant. (Wang and Brusseau, 1993).A study conducted by Mulder et al. showed that the introduction of hydropropyl-ß-cyclodextrin (HPCD), a well-known PAH solubility enhancer, significantly increased the solubilization of PAHs although it did not improve the biodegradation rate of PAHs (Mulder et al., 1998), indicating that further research is required in order to develop a feasible and efficient remediation method. Enhancing the extent of PAHs mass transfer from the soil phase to the liquid might prove an efficient and environmentally low-risk alternative way of addressing the problem of slow PAH biodegradation in soil.

Focused fluid transfer through the mantle above subduction zones

Volcanic arcs above subduction zones are enriched in volatiles and fluid-mobile elements with respect to mid-oceanic ridge basalts. There is general consensus that this particular subduction zone signature is generated by fluid-induced extraction of these elements from subducted oceanic crust and its sedimentary cover. However, how these fluids are transferred through the mantle wedge to the locus of partial melting and what modification the fluids will experience is unresolved. Here we investigate the interaction of slab fluids with the mantle wedge through a series of high-pressure experiments. We explore two end-member processes of focused and porous reactive flow of hydrous slab melts through the mantle. Transfer by porous flow leads to the formation of hydrous minerals that sequester fluid-mobile elements and residual fluids characterized by trace element patterns inconsistent with typical arc lavas. In contrast, no hydrous minerals are formed in the reaction zone of experiments mimicking focused flow, and the typical trace element signature acquired during fluid extraction from the slab is preserved, indicating that this is an efficient process for element transfer through the mantle wedge.

Morphological Functions to Quantify Three-Dimensional Tomograms of Macropore Structure in a Vineyard Soil with Two Different Management Regimes

Soil tomography and morphological functions built over Minkowski functionals were used to describe the impact on pore structure of two soil management practices in a Mediterranean vineyard. Soil structure controls important physical and biological processes in soil–plant–microbial systems. Those processes are dominated by the geometry of soil pore structure, and a correct model of this geometry is critical for understanding them. Soil tomography has been shown to provide rich three-dimensional digital information on soil pore geometry. Recently, mathematical morphological techniques have been proposed as powerful tools to analyze and quantify the geometrical features of porous media. Minkowski functionals and morphological functions built over Minkowski functionals provide computationally efficient means to measure four fundamental geometrical features of three-dimensional geometrical objects, that is, volume, boundary surface, mean boundary surface curvature, and connectivity. We used the threshold and the dilation and erosion of three-dimensional images to generate morphological functions and explore the evolution of Minkowski functionals as the threshold and as the degree of dilation and erosion changes. We analyzed the three-dimensional geometry of soil pore space with X-ray computed tomography (CT) of intact soil columns from a Spanish Mediterranean vineyard by using two different management practices (conventional tillage versus permanent cover crop of resident vegetation). Our results suggested that morphological functions built over Minkowski functionals provide promising tools to characterize soil macropore structure and that the evolution of morphological features with dilation and erosion is more informative as an indicator of structure than moving threshold for both soil managements studied.

Caracterización del hormigón en relación a la difusión de gases y su correlación con el radón

Radon gas (Rn) is a natural radioactive gas present in some soils and able to penetrate buildings through the building envelope in contact with the soil. Radon can accumulate within buildings and consequently be inhaled by their occupants. Because it is a radioactive gas, its disintegration process produces alpha particles that, in contact with the lung epithelia, can produce alterations potentially giving rise to cancer. Many international organizations related to health protection, such as WHO, confirm this causality. One way to avoid the accumulation of radon in buildings is to use the building envelope as a radon barrier. The extent to which concrete provides such a barrier is described by its radon diffusion coefficient (DRn), a parameter closely related to porosity (ɛ) and tortuosity factor (τ). The measurement of the radon diffusion coefficient presents challenges, due to the absence of standard procedures, the requirement to establish adequate airtightness in testing apparatus (referred to here as the diffusion cell), and due to the fact that measurement has to be carried out in an environment certified for use of radon calibrated sources. In addition to this calibrated radon sources are costly. The measurement of the diffusion coefficient for non-radioactive gas is less complex, but nevertheless retains a degree of difficulty due to the need to provide reliably airtight apparatus for all tests. Other parameters that can characterize and describe the process of gas transport through concrete include the permeability coefficient (K) and the electrical resistivity (ρe), both of which can be measured relatively easily with standardized procedure. The use of these parameters would simplify the characterization of concrete behaviour as a radon barrier. Although earlier studies exist, describing correlation among these parameters, there is, as has been observed in the literature, little common ground between the various research efforts. For precisely this reason, prior to any attempt to measure radon diffusion, it was deemed necessary to carry out further research in this area, as a foundation to the current work, to explore potential relationships among the following parameters: porosity-tortuosity, oxygen diffusion coefficient, permeability coefficient and resistivity. Permeability coefficient measurement (m2) presents a more straightforward challenge than diffusion coefficient measurement. Some authors identify a relationship between both coefficients, including Gaber (1988), who proposes: k= a•Dn Equation 1 Where: a=A/(8ΠD020), A = sample cross-section, D020 = diffusion coefficient in air (m2/s). Other studies (Klink et al. 1999, Gaber and Schlattner 1997, Gräf and Grube et al. 1986), experimentally relate both coefficients of different types of concrete confirming that this relationship exists, as represented by the simplified expression: k≈Dn Equation 2 In each particular study a different value for n was established, varying from 1.3 to 2.5, but this requires determination of a value for n in a more general way because these proposed models cannot estimate diffusion coefficient. If diffusion coefficient has to be measured to be able to establish n, these relationships are not interesting. The measurement of electric resistivity is easier than diffusion coefficient measurement. Correlation between the parameters can be established via Einstein´s law that relates movement of electrical charges to media conductivity according to the expression: D_e=k/ρ Equation 3 Where: De = diffusion coefficient (cm2/s), K = constant, ρ = electric resistivity (Ω•cm). The tortuosity factor is used to represent the uneven geometry of concrete pores, which are described as being not straight, but tortuous. This factor was first introduced in the literature to relate global porosity with fluid transport in a porous media, and can be formulated in a number of different ways. For example, it can take the form of equation 4 (Mason y Malinauskas), which combines molecular and Knudsen diffusion using the tortuosity factor: D=ε^τ (3/2r √(πM/8RT+1/D_0 ))^(-1) Equation 4 Where: r = medium radius obtained from MIP (µm), M = gas molecular mass, R = ideal gases constant, T = temperature (K), D0 = coefficient diffusion in the air (m2/s). Few studies provide any insight as to how to obtain the tortuosity factor. The work of Andrade (2012) is exceptional in this sense, as it outlines how the tortuosity factor can be deduced from pore size distribution (from MIP) from the equation: ∅_th=∅_0•ε^(-τ). Equation 5 Where: Øth = threshold diameter (µm), Ø0 = minimum diameter (µm), ɛ = global porosity, τ = tortuosity factor. Alternatively, the following equation may be used to obtain the tortuosity factor: DO2=D0*ɛτ Equation 6 Where: DO2 = oxygen diffusion coefficient obtained experimentally (m2/s), DO20 = oxygen diffusion coefficient in the air (m2/s). This equation has been inferred from Archie´s law ρ_e=〖a•ρ〗_0•ɛ^(-m) and from the Einstein law mentioned above, using the values of oxygen diffusion coefficient obtained experimentally. The principal objective of the current study was to establish correlations between the different parameters that characterize gas transport through concrete. The achievement of this goal will facilitate the assessment of the useful life of concrete, as well as open the door to the pro-active planning for the use of concrete as a radon barrier. Two further objectives were formulated within the current study: 1.- To develop a method for measurement of gas coefficient diffusion in concrete. 2.- To model an analytic estimation of radon diffusion coefficient from parameters related to concrete porosity and tortuosity factor. In order to assess the possible correlations, parameters have been measured using the standardized procedures or purpose-built in the laboratory for the study of equations 1, 2 y 3. To measure the gas diffusion coefficient, a diffusion cell was designed and manufactured, with the design evolving over several cycles of research, leading ultimately to a unit that is reliably air tight. The analytic estimation of the radon diffusion coefficient DRn in concrete is based on concrete global porosity (ɛ), whose values may be experimentally obtained from a mercury intrusion porosimetry test (MIP), and from its tortuosity factor (τ), derived using the relations expressed in equations 5 y 6. The conclusions of the study are: Several models based on regressions, for concrete with a relative humidity of 50%, have been proposed to obtain the diffusion coefficient following the equations K=Dn, K=a*Dn y D=n/ρe. The final of these three relations is the one with the determination coefficient closest to a value of 1: D=(19,997*LNɛ+59,354)/ρe Equation 7 The values of the obtained oxygen diffusion coefficient adjust quite well to those experimentally measured. The proposed method for the measurement of the gas coefficient diffusion is considered to be adequate. The values obtained for the oxygen diffusion coefficient are within the range of those proposed by the literature (10-7 a 10-8 m2/s), and are consistent with the other studied parameters. Tortuosity factors obtained using pore distribution and the expression Ø=Ø0*ɛ-τ are inferior to those from resistivity ρ=ρ0*ɛ-τ. The closest relationship to it is the one with porosity of pore diameter 1 µm (τ=2,07), being 7,21% inferior. Tortuosity factors obtained from the expression DO2=D0*ɛτ are similar to those from resistivity: for global tortuosity τ=2,26 and for the rest of porosities τ=0,7. Estimated radon diffusion coefficients are within the range of those consulted in literature (10-8 a 10-10 m2/s).ABSTRACT El gas radón (Rn) es un gas natural radioactivo presente en algunos terrenos que puede penetrar en los edificios a través de los cerramientos en contacto con el mismo. En los espacios interiores se puede acumular y ser inhalado por las personas. Al ser un gas radioactivo, en su proceso de desintegración emite partículas alfa que, al entrar en contacto con el epitelio pulmonar, pueden producir alteraciones del mismo causando cáncer. Muchos organismos internacionales relacionados con la protección de la salud, como es la OMS, confirman esta causalidad. Una de las formas de evitar que el radón penetre en los edificios es utilizando las propiedades de barrera frente al radón de su propia envolvente en contacto con el terreno. La principal característica del hormigón que confiere la propiedad de barrera frente al radón cuando conforma esta envolvente es su permeabilidad que se puede caracterizar mediante su coeficiente de difusión (DRn). El coeficiente de difusión de un gas en el hormigón es un parámetro que está muy relacionado con su porosidad (ɛ) y su tortuosidad (τ). La medida del coeficiente de difusión del radón resulta bastante complicada debido a que el procedimiento no está normalizado, a que es necesario asegurar una estanquidad a la celda de medida de la difusión y a que la medida tiene que ser realizada en un laboratorio cualificado para el uso de fuentes de radón calibradas, que además son muy caras. La medida del coeficiente de difusión de gases no radioactivos es menos compleja, pero sigue teniendo un alto grado de dificultad puesto que tampoco está normalizada, y se sigue teniendo el problema de lograr una estanqueidad adecuada de la celda de difusión. Otros parámetros que pueden caracterizar el proceso son el coeficiente de permeabilidad (K) y la resistividad eléctrica (ρe), que son más fáciles de determinar mediante ensayos que sí están normalizados. El uso de estos parámetros facilitaría la caracterización del hormigón como barrera frente al radón, pero aunque existen algunos estudios que proponen correlaciones entre estos parámetros, en general existe divergencias entre los investigadores, como se ha podido comprobar en la revisión bibliográfica realizada. Por ello, antes de tratar de medir la difusión del radón se ha considerado necesario realizar más estudios que puedan clarificar las posibles relaciones entre los parámetros: porosidad-tortuosidad, coeficiente de difusión del oxígeno, coeficiente de permeabilidad y resistividad. La medida del coeficiente de permeabilidad (m2) es más sencilla que el de difusión. Hay autores que relacionan el coeficiente de permeabilidad con el de difusión. Gaber (1988) propone la siguiente relación: k= a•Dn Ecuación 1 En donde: a=A/(8ΠD020), A = sección de la muestra, D020 = coeficiente de difusión en el aire (m2/s). Otros estudios (Klink et al. 1999, Gaber y Schlattner 1997, Gräf y Grube et al. 1986) relacionan de forma experimental los coeficientes de difusión de radón y de permeabilidad de distintos hormigones confirmando que existe una relación entre ambos parámetros, utilizando la expresión simplificada: k≈Dn Ecuación 2 En cada estudio concreto se han encontrado distintos valores para n que van desde 1,3 a 2,5 lo que lleva a la necesidad de determinar n porque no hay métodos que eviten la determinación del coeficiente de difusión. Si se mide la difusión ya deja de ser de interés la medida indirecta a través de la permeabilidad. La medida de la resistividad eléctrica es muchísimo más sencilla que la de la difusión. La relación entre ambos parámetros se puede establecer a través de una de las leyes de Einstein que relaciona el movimiento de cargas eléctricas con la conductividad del medio según la siguiente expresión: D_e=k/ρ_e Ecuación 3 En donde: De = coeficiente de difusión (cm2/s), K = constante, ρe = resistividad eléctrica (Ω•cm). El factor de tortuosidad es un factor de forma que representa la irregular geometría de los poros del hormigón, al no ser rectos sino tener una forma tortuosa. Este factor se introduce en la literatura para relacionar la porosidad total con el transporte de un fluido en un medio poroso y se puede formular de distintas formas. Por ejemplo se destaca la ecuación 4 (Mason y Malinauskas) que combina la difusión molecular y la de Knudsen utilizando el factor de tortuosidad: D=ε^τ (3/2r √(πM/8RT+1/D_0 ))^(-1) Ecuación 4 En donde: r = radio medio obtenido del MIP (µm), M = peso molecular del gas, R = constante de los gases ideales, T = temperatura (K), D0 = coeficiente de difusión de un gas en el aire (m2/s). No hay muchos estudios que proporcionen una forma de obtener este factor de tortuosidad. Destaca el estudio de Andrade (2012) en el que deduce el factor de tortuosidad de la distribución del tamaño de poros (curva de porosidad por intrusión de mercurio) a partir de la ecuación: ∅_th=∅_0•ε^(-τ) Ecuación 5 En donde: Øth = diámetro umbral (µm), Ø0 = diámetro mínimo (µm), ɛ = porosidad global, τ = factor de tortuosidad. Por otro lado, se podría utilizar también para obtener el factor de tortuosidad la relación: DO2=D0*-τ Ecuación 6 En donde: DO2 = coeficiente de difusión del oxígeno experimental (m2/s), DO20 = coeficiente de difusión del oxígeno en el aire (m2/s). Esta ecuación está inferida de la ley de Archie ρ_e=〖a•ρ〗_0•ɛ^(-m) y la de Einstein mencionada anteriormente, utilizando valores del coeficiente de difusión del oxígeno DO2 obtenidos experimentalmente. El objetivo fundamental de la tesis es encontrar correlaciones entre los distintos parámetros que caracterizan el transporte de gases a través del hormigón. La consecución de este objetivo facilitará la evaluación de la vida útil del hormigón así como otras posibilidades, como la evaluación del hormigón como elemento que pueda ser utilizado en la construcción de nuevos edificios como barrera frente al gas radón presente en el terreno. Se plantean también los siguientes objetivos parciales en la tesis: 1.- Elaborar una metodología para la medida del coeficiente de difusión de los gases en el hormigón. 2.- Plantear una estimación analítica del coeficiente de difusión del radón a partir de parámetros relacionados con su porosidad y su factor de tortuosidad. Para el estudio de las correlaciones posibles, se han medido los parámetros con los procedimientos normalizados o puestos a punto en el propio Instituto, y se han estudiado las reflejadas en las ecuaciones 1, 2 y 3. Para la medida del coeficiente de difusión de gases se ha fabricado una celda que ha exigido una gran variedad de detalles experimentales con el fin de hacerla estanca. Para la estimación analítica del coeficiente de difusión del radón DRn en el hormigón se ha partido de su porosidad global (ɛ), que se obtiene experimentalmente del ensayo de porosimetría por intrusión de mercurio (MIP), y de su factor de tortuosidad (τ), que se ha obtenido a partir de las relaciones reflejadas en las ecuaciones 5 y 6. Las principales conclusiones obtenidas son las siguientes: Se proponen modelos basados en regresiones, para un acondicionamiento con humedad relativa de 50%, para obtener el coeficiente de difusión del oxígeno según las relaciones: K=Dn, K=a*Dn y D=n/ρe. La propuesta para esta última relación es la que tiene un mejor ajuste con R2=0,999: D=(19,997*LNɛ+59,354)/ρe Ecuación 7 Los valores del coeficiente de difusión del oxígeno así estimados se ajustan a los obtenidos experimentalmente. Se considera adecuado el método propuesto de medida del coeficiente de difusión para gases. Los resultados obtenidos para el coeficiente de difusión del oxígeno se encuentran dentro del rango de los consultados en la literatura (10-7 a 10-8 m2/s) y son coherentes con el resto de parámetros estudiados. Los resultados de los factores de tortuosidad obtenidos de la relación Ø=Ø0*ɛ-τ son inferiores a la de la resistividad (ρ=ρ0*ɛ-τ). La relación que más se ajusta a ésta, siendo un 7,21% inferior, es la de la porosidad correspondiente al diámetro 1 µm con τ=2,07. Los resultados de los factores de tortuosidad obtenidos de la relación DO2=D0*ɛτ son similares a la de la resistividad: para la porosidad global τ=2,26 y para el resto de porosidades τ=0,7. Los coeficientes de difusión de radón estimados mediante estos factores de tortuosidad están dentro del rango de los consultados en la literatura (10-8 a 10-10 m2/s).

Método multigrid algébrico: reutilização das estruturas multigrid no transporte de contaminantes

A necessidade de obter solução de grandes sistemas lineares resultantes de processos de discretização de equações diferenciais parciais provenientes da modelagem de diferentes fenômenos físicos conduz à busca de técnicas numéricas escaláveis. Métodos multigrid são classificados como algoritmos escaláveis.Um estimador de erros deve estar associado à solução numérica do problema discreto de modo a propiciar a adequada avaliação da solução obtida pelo processo de aproximação. Nesse contexto, a presente tese caracteriza-se pela proposta de reutilização das estruturas matriciais hierárquicas de operadores de transferência e restrição dos métodos multigrid algébricos para acelerar o tempo de solução dos sistemas lineares associados à equação do transporte de contaminantes em meio poroso saturado. Adicionalmente, caracteriza-se pela implementação das estimativas residuais para os problemas que envolvem dados constantes ou não constantes, os regimes de pequena ou grande advecção e pela proposta de utilização das estimativas residuais associadas ao termo de fonte e à condição inicial para construir procedimentos adaptativos para os dados do problema. O desenvolvimento dos códigos do método de elementos finitos, do estimador residual e dos procedimentos adaptativos foram baseados no projeto FEniCS, utilizando a linguagem de programação PYTHONR e desenvolvidos na plataforma Eclipse. A implementação dos métodos multigrid algébricos com reutilização considera a biblioteca PyAMG. Baseado na reutilização das estruturas hierárquicas, os métodos multigrid com reutilização com parâmetro fixo e automática são propostos, e esses conceitos são estendidos para os métodos iterativos não-estacionários tais como GMRES e BICGSTAB. Os resultados numéricos mostraram que o estimador residual captura o comportamento do erro real da solução numérica, e fornece algoritmos adaptativos para os dados cuja malha retornada produz uma solução numérica similar à uma malha uniforme com mais elementos. Adicionalmente, os métodos com reutilização são mais rápidos que os métodos que não empregam o processo de reutilização de estruturas. Além disso, a eficiência dos métodos com reutilização também pode ser observada na solução do problema auxiliar, o qual é necessário para obtenção das estimativas residuais para o regime de grande advecção. Esses resultados englobam tanto os métodos multigrid algébricos do tipo SA quanto os métodos pré-condicionados por métodos multigrid algébrico SA, e envolvem o transporte de contaminantes em regime de pequena e grande advecção, malhas estruturadas e não estruturadas, problemas bidimensionais, problemas tridimensionais e domínios com diferentes escalas.

Surface growth for molten silicon infiltration into carbon millimeter-sized channels: Lattice–Boltzmann simulations, experiments and models

The process of liquid silicon infiltration is investigated for channels with radii from 0.25 to 0.75 [mm] drilled in compact carbon preforms. The advantage of this setup is that the study of the phenomenon results to be simplified. For comparison purposes, attempts are made in order to work out a framework for evaluating the accuracy of simulations. The approach relies on dimensionless numbers involving the properties of the surface reaction. It turns out that complex hydrodynamic behavior derived from second Newton law can be made consistent with Lattice-Boltzmann simulations. The experiments give clear evidence that the growth of silicon carbide proceeds in two different stages and basic mechanisms are highlighted. Lattice-Boltzmann simulations prove to be an effective tool for the description of the growing phase. Namely, essential experimental constraints can be implemented. As a result, the existing models are useful to gain more insight on the process of reactive infiltration into porous media in the first stage of penetration, i.e. up to pore closure because of surface growth. A way allowing to implement the resistance from chemical reaction in Darcy law is also proposed.

New method to determine PSD using supercritical adsorption: Applied to methane adsorption in activated carbon

Adsorption of supercritical fluids is increasingly carried out to determine the micropore size distribution. This is largely motivated by the advances in the use of supercritical adsorption in high energy applications, such as hydrogen and methane storage in porous media. Experimental data are reported as mass excess versus pressure, and when these data are matched against the theoretical mass excess, significant errors could occur if the void volume used in the calculation of the experimental mass excess is incorrectly determined [Malbrunot, P.; Vidal, D.; Vermesse, J.; Chahine, R.; Bose, T. K. Langmuir 1997, 13, 539]. 1 The incorrect value for the void volume leads to a wrong description of the maximum in the plot of mass excess versus pressure as well as the part of the isotherm over the pressure region where the isotherm is decreasing. Because of this uncertainty in the maximum and the decreasing part of the isotherm, we propose a new method in which the problems associated with this are completely avoided. Our method involves only the relationship between the amount that is introduced into the adsorption cell and the equilibrium pressure. This information of direct experimental data has two distinct advantages. The first is that the data is the raw data without any manipulation (i.e., involving further calculations), and the second one is that this relationship always monotonically increases with pressure. We will illustrate this new method with the adsorption data of methane in a commercial sample of activated carbon.

MRI pressure and stress measurement in novel homogeneous soft solids

This paper presents MRI measurements of a novel semi solid MR contrast agent to pressure. The agent is comprised of potassium chloride cross linked carageenan gum at a concentration of 2% w/v, with micron size lipid coated bubbles of air at a concentration of 3% v/v. The choice for an optimum suspending medium, the methods of production and the preliminary MRI results are presented herein. The carageenan gum is shown to be ideally elastic for compressions relating to volume changes less than 15%, in contrast to the inelastic gellan gum also tested. Although slightly lower than that of gellan gum, carageenan has a water diffusion coefficient of 1.72×10-9 m2.s-1 indicating its suitability to this purpose. RARE imaging is performed whilst simultaneously compressing test and control samples and a maximum sensitivity of 1.6% MR signal change per % volume change is found which is shown to be independent of proton density variations due to the presence of microbubbles and compression. This contrast agent could prove useful for numerous applications, and particularly in chemical engineering. More generally the method allows the user to non-invasively image with MRI any process that causes, within the solid, local changes either in bubble size or bubble shape. © 2008 American Institute of Physics.