An exact equation for the free surface of a fluid in a porous medium

We study the problem of the evolution of the free surface of a fluid in a saturated porous medium, bounded from below by a. at impermeable bottom, and described by the Laplace equation with moving-boundary conditions. By making use of a convenient conformal transformation, we show that the solution to this problem is equivalent to the solution of the Laplace equation on a fixed domain, with new variable coefficients, the boundary conditions. We use a kernel of the Laplace equation which allows us to write the Dirichlet-to-Neumann operator, and in this way we are able to find an exact differential-integral equation for the evolution of the free surface in one space dimension. Although not amenable to direct analytical solutions, this equation turns out to allow an easy numerical implementation. We give an explicit illustrative case at the end of the article.

Transient non-Darcy forced convective heat transfer from a flat plate embedded in a fluid-saturated porous medium

Transient non-Darcy forced convection on a flat plate embedded in a porous medium is investigated using the Forchheimer-extended Darcy law. A sudden uniform pressure gradient is applied along the flat plate, and at the same time, its wall temperature is suddenly raised to a high temperature. Both the momentum and energy equations are solved by retaining the unsteady terms. An exact velocity solution is obtained and substituted into the energy equation, which then is solved by means of a quasi-similarity transformation. The temperature field can be divided into the one-dimensional transient (downstream) region and the quasi-steady-state (upstream) region. Thus the transient local heat transfer coefficient can be described by connecting the quasi-steady-state solution and the one-dimensional transient solution. The non-Darcy porous inertia works to decrease the velocity level and the time required for reaching the steady-state velocity level. The porous-medium inertia delays covering of the plate by the steady-state thermal boundary layer. © 1990.

Approximate solution for non-Darcy transient film condensation in a porous medium

The problem of non-darcian transient film condensation adjacent to a vertical flat plate embedded in a porous medium has been considered. The governing equation for the boundary layer thickness was obtained by an integral method and solved approximately by the method of integral relations. It is shown that the results are in good agreement with those obtained exactly by the method of characteristics.

Approximate solution for non-darcy transient film condensation in a porous medium

The problem of non-darcian transient film condensation adjacent to a vertical flat plate embedded in a porous medium has been considered. The governing equation for the boundary layer thickness was obtained by an integral method and solved approximately by the method of integral relations. It is shown that the results are in good agreement with those obtained exactly by the method of characteristics.

Some Physical and Mechanical Properties of Medium-Density Fiberboard Made from Giant Bamboo

The objective of this study was to evaluate the density, density profile, water swelling and absorption, modulus of elasticity and rupture from static bending, and tensile strength of experimental medium-density fiberboards manufactured using Dendrocalamus giganteus (Munro bamboo). The fiber production was carried out through the chemo-thermo-mechanical pulping process with four different conditions. The panels were made with 10% urea formaldehyde resin based on dry weight of the fibers, 2.5% of a catalyzer (ammonium sulfate) and 2% paraffin. The results indicate that treatments with the highest alkali (NaOH) percentage, time and splinter heating temperature improved the physical properties of the panels. The root-fiber interface was evaluated through scanning electron microscopy in fracture zones, which revealed fibers with thick, inflexible walls. The panels' mechanical properties were affected due to the fiber wall characteristics and interaction with resin. Giant bamboo fiber has potential for MDF production, but other studies should be carried out.

Painéis de Medium Density Fiberboard fabricados com bagaço de cana-de-açúcar e madeira de reflorestamento

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Medium density particleboard reinforced with bamboo laminas

The objective of this work was to evaluate the effect of the addition of bamboo laminas of the species Dendrocalamus giganteus to three-layer medium density particleboard (MDP). These laminas were glued onto both the top and the bottom of each panel. With the manufactured panels laminated with bamboo, mechanical tests based on the Brazilian Standard ABNT NBR 14810 were carried out to determine the modulus of rupture (MOR) in static bending and the tensile strength parallel-to-surface. These mechanical tests were realized in particleboards of eucalyptus and in reinforced particleboard, both produced in the laboratory. The modulus of rupture and tensile strength parallel-to-surface of the laminated MDP had values close to those that have been reported. The reinforcements increased the values of these studied properties. Nevertheless, this fact indicated the possibility to produce a stronger MDP using bamboo lamina as surface layers. These results show the possibility of using coatedbamboo MDP for utilization in large spans, for example, in flooring for mezzanines with finish on both sides, and for robust furniture as bookshelves, beds, tables, etc.

Characterization of medium density particleborads using agricultural residues

In recent years the production of products derived from wood and bamboo are increasing, due to the search for a more rational exploitation of these raw materials. Amongst these products, the particleboards production combine sustainability and rationality in the use of these materials. In this context, this work has the objective to study the application of alternative raw materials in the manufacture of Medium Density Particleboards (MDP), using residues from industrial processimg of coffee and bamboo. MDP had been produced with particles of giganteus bamboo of the Dendrocalamus species and particle of coffee rind in the intermediate layer of the particleboard, bonded with polyurethane resin based on castor oil. The physical and mechanical characterization was carried out accordingly to NBR 14810-3 (2006). The physical properties evaluated were: of water absorption for 2h and 24h; thickness swallowing for 2h and 24h; density, humidity content. The mechanical properties evaluated were: Tensile strength, static bending (MOR and MOE). The results were compared with NBR 14810-2 (2006) and also with the ANSI A208-1 (1993). The physical performance of these particleboards was below the values recommend by the Brazilian norm. Also the mechanical characteristics are not improve, demonstrating that the inclusion of coffee rind did not benefit the physical characteristics and nor the mechanical ones. However it can be used as construction materials for partitions and ceiling panels.

Characterization of medium density particleboards using agricultural residues

In recent years the production of products derived from wood and bamboo are increasing, due to the search for a more rational exploitation of these raw materials. Amongst these products, the particleboards production combine sustainability and rationality in the use of these materials. In this context, this work has the objective to study the application of alternative raw materials in the manufacture of Medium Density Particleboards (MDP), using residues from industrial processimg of coffee and bamboo. MDP had been produced with particles of giganteus bamboo of the Dendrocalamus species and particle of coffee rind in the intermediate layer of the particleboard, bonded with polyurethane resin based on castor oil. The physical and mechanical characterization was carried out accordingly to NBR 14810-3 (2006). The physical properties evaluated were: of water absorption for 2h and 24h; thickness swallowing for 2h and 24h; density, humidity content. The mechanical properties evaluated were: Tensile strength, static bending (MOR and MOE). The results were compared with NBR 14810-2 (2006) and also with the ANSI A208-1 (1993). The physical performance of these particleboards was below the values recommend by the Brazilian norm. Also the mechanical characteristics are not improve, demonstrating that the inclusion of coffee rind did not benefit the physical characteristics and nor the mechanical ones. However it can be used as construction materials for partitions and ceiling panels.

Propriedades físico-mecânicas de painéis de mdp (medium density particleboard) constituído de bambu e fibra de coco

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Analysis of the accumulated energy by thermal stratification of liquid inside storage tanks with porous layers

A mathematical model was developed in order to study the behavior of thermal stratification of liquid in a typical storage tank with porous medium. The model employs a transient stream function-vorticity formulation to predict the development of stream function and temperature fields in a charging process. Parameters analyzed include Biot, Darcy, Reynolds and Richardson numbers, position, and the thickness of the porous medium. The results show the influence of these physical parameters that should be considered for a good design of storage tanks with thermal stratification.

População de Metarhizium anisopliae em diferentes tipos e graus de compactação do solo

Este trabalho objetivou investigar a influência do tipo e compactação do solo na sobrevivência do fungo Metarhizium anisopliae. A sobrevivência do fungo foi determinada em quatro tipos de solos: Latossolo Vermelho textura argilosa, Latossolo Vermelho textura média, Argissolo Vermelho Amarelo textura arenosa média e Argissolo Vermelho Amarelo textura areno-argilosa, com maior teor de matéria orgânica. Para determinar o efeito da compactação na sobrevivência do fungo usaram-se os três primeiros tipos de solos nas densidades de 1,12, 1,32, 1,50g cm-3; 1,22, 1,44, 1,65g cm-3; 1,30, 1,50, 1,70g cm-3, respectivamente. Por meio da contagem de unidades formadoras de colônias (UFC) em placas de Petri, fizeram-se avaliações da sobrevivência do fungo, após zero, 20, 40, 60, 80, 100 e 120 dias de incubação a 27 ± 1ºC. Houve influência significativa do tipo de solo e do grau de compactação na sobrevivência do fungo, obtendo-se maior quantidade de UFC no solo textura areno-argilosa. Entre os demais solos, a maior sobrevivência ocorreu no solo textura arenosa e a menor no solo textura argilosa. O efeito da compactação foi significativo para o tipo de solo, exceto no solo textura arenosa. Independentemente do tipo de solo, a maior sobrevivência foi observada nos valores médios de densidade. A compactação teve maior impacto no solo textura média, onde ocorreu queda mais acentuada na quantidade de UFC em todas as densidades.

Fluid dynamics of air in a packed bed: velocity profiles and the continuum model assumption

Air flow through packed beds was analyzed experimentally under conditions ranging from those that reinforce the effect of the wall on the void fraction to those that minimize it. The packing was spherical particles, with a tube-to-particle diameter ratio (D/dp) between 3 and 60. Air flow rates were maintained between 1.3 and 4.44 m3/min, and gas velocity was measured with a Pitot tube positioned above the bed exit. Measurements were made at various radial and angular coordinate values, allowing the distribution of air flow across the bed to be described in detail. Comparison of the experimentally observed radial profiles with those derived from published equations revealed that at high D/dp ratios the measured and calculated velocity profiles behaved similarly. At low ratios, oscillations in the velocity profiles agreed with those in the voidage profiles, signifying that treating the porous medium as a continuum medium is questionable in these cases.

Simulação numérica de trocadores de calor multitubulares equipados com fitas helicoidais externamente aos tubos

Classical shell-and-tube heat exchangers are usually equipped with segmental baffles. These baffles serve two basic functions: (a) they provide tube supports, thereby preventing or reducing mechanical problems, such as sagging or vibration; (b) they direct the fluid flow over the tubes so as to introduce a cross-flow component, thereby increasing the heat transfer. Segmented baffles have several sources of performance loss, some due to various leakage flows and others caused by stagnation zones. A new concept of longitudinal flow heat exchanger - based on placing twisted tapes along the tube bundle subchannels - was developed to mitigate drawbacks of other types of tubular heat exchangers. In this paper, a numerical model has been implemented in order to simulate the thermal-hydraulic feature of tubular heat exchangers equipped either with segmental baffles or with subchannel twisted tapes. The tube bundle has been described by means of an equivalent porous medium type model, allowing a macroscopic description of the shell-side flow. The basic equations - continuity, momentum and energy - have been solved by using the finite volume method. Typical numerical results have been compared with experimental data, reaching a very good agreement. A comparative analysis of different types of heat exchangers has been carried out, revealing the satisfactory thermal-hydraulic efficiency level of the twisted tapes heat exchangers.

Avaliação da distribuição da densidade em MDF a partir da técnica da onda de ultra-som

The goal of this research was to determine the density distribution in medium density fiberboard (MDF), manufactured with polyurethane derived from castor oil using, ultrasonic wave technique. The equipment used in this test is Steinkamp BP7 with plan and exponential transducers, both with 45 kHz frequencies, located in several zones on the plate in order to determine wave ultrasonic velocity. The Pinus caribaea and Eucalyptus grandis fiberboard were manufactured in the quality control and products development laboratory of Duratex with 500 mm long, 500 mm large, 8 and 15 mm of thickness. Three MDF for each fiber specimen and thickness were fabricated, totalizing twelve plates tested. The MDF were produced with 5% polyurethane addition, in temperature of 160°C, tension press of 53 bars and addition of moisture content of 12%. For determination of fiberboard density, samples were extracted from the same zones where the wave ultrasonic velocity was determined. In this case, DAX-Ray equipment was used. Statistical analysis shows good agreement with wave ultrasonic velocity and the density profile, validating the application of non-destructive technique in order to determine the density profile of MDF's.