Begell House
Journal of Porous Media
Journal of Porous Media
1091-028X
13
2
2010
DIFFUSION OF HYDROGEN IN POROUS SILICON-BASED SENSORS
Porous silicon (PS) layers of 65% porosity on n-type (111) Si substrates were prepared by anodic etching. Au/PS/Si structures have been fabricated by evaporation of thin Au film onto the PS surface. Current-voltage characteristics and the open-circuit voltage (Voc) generation in Au/PS/Si structures were examined at different ambient humidities (water vapor) in the temperature range 295-365 K. Generation of a voltage from 10 to 400 mV with the increase of the relative humidity from 50% relative humidity (RH) to 95% RH was observed in Au/PS/Si sensors. The response and recovery time of open-circuit voltage generated in Au/PS/Si Schottky-type sensors under humid atmosphere depends on ambient temperature. The effective diffusion coefficient of hydrogen estimated from response (or recovery) Voc − t curves on placing (or removal) of Au/PS/Si sensors in (or out) of humid ambient (90% RH) in the temperature range 313−363 K increases from 3.1 × 10−8 to 1.7 × 10−7 cm2/s and is described as D = 9.2 × 10−3 exp(−0.34 eV/kT). The possible mechanism of the hydrogen diffusion in porous silicon layers of Au/PS/Si sensors was considered.
Tayyar
Dzhafarov
Yildiz Technical University
Sureyya Aydin
Yuksel
Department of Physics, Yildiz Technical University, 34210 Esenler/Istanbul
97-102
EFFECT OF HALL CURRENTS ON INTERACTION OF PERISTALTIC FLOW WITH PULSATILE MAGNETOFLUID THROUGH A POROUS MEDIUM
The interaction of purely periodic mean flow with a peristaltic induced flow is investigated within the framework of a two-dimensional analogue. The mathematical model considers a viscous incompressible fluid under the effect of a transverse magnetic field, taking into account the effect of Hall currents through a porous medium between infinite parallel walls on which a sinusoidal traveling wave is imposed. A perturbation solution to the complete set of Navier-Stokes equations is found for the case in which the frequency of the traveling wave and that of the imposed pressure gradient are equal. The ratio of the traveling wave amplitude to channel width is assumed to be small. For this case, a first-order steady flow is found to exist, as contrasted to a second-order effect in the absence of the imposed periodic pressure gradient, and the effect of Hall parameter, permeability parameter, Hartmann number, and the various parameters included in the problem are discussed numerically.
N. S.
Gad
Department of Mathematics, Faculty of Education, Roxy, Ain Shams University, Cairo
103-110
POROUS STRUCTURE AND HYDRIC PROPERTIES OF COB
The aim of this work is to determine porous structure and hydric properties of cob, a local traditional material of the Rennes basin in Brittany (France) that is now used in low-energy buildings for its hygrothermal properties: this material shows low thermal conductivity (0.4 W m−1 K−1), high thermal capacity (815 J kg−1 K−1), and is said to bean efficient hydric regulator. This material, made of clay and reinforced with animal or vegetal fibers, is traditionally pressed in situ. Nowadays, manually compacted blocks of cob are prefabricated and are stabilized with a low quantity of cement (3% to 5% of clay mass). In this work, porous structure and hydric properties are investigated experimentally. The pore size distribution is measured by mercury porosimetry. Water vapor sorption isotherm (adsorption and desorption) and water vapor permeability are investigated by measuring weight differences according to time at 23° C. Then, porous structure and hydric properties are coupled in a theoretical study. The pore size distribution is computed from water vapor sorption. The specific surface area is then computed from mercury porosimetry and from water vapor sorption isotherm. From water vapor sorption and permeability measures, the water vapor diffusivity is evaluated.
Florence
Collet
Laboratoire de Génie Civil et Génie Mécanique, Equipe Matériaux Thermo-Rhéologie, Universite Européenne de Bretagne, 35704 Rennes
Marjorie
Bart
Laboratoire de Génie Civil et Génie Mécanique, Equipe Matériaux Thermo-Rhéologie, Universite Européenne de Bretagne, 35704 Rennes
Laurent
Serres
Laboratoire de Génie Civil et Génie Mécanique, Equipe Matériaux Thermo-Rhéologie, Universite Européenne de Bretagne, 35704 Rennes
Jacques
Miriel
Laboratoire de Génie Mécanique et de Génie Civil (LGMGC) de l'IUT Saint Malo et de l'INSA de Rennes 1
111-124
MICROSCOPIC MODELING OF THE TWO-TEMPERATURE MODEL FOR CONDUCTION IN HETEROGENEOUS MEDIA
We consider conduction in a two-phase composite solid or, equivalently, a stagnant porous medium saturated with a single fluid. In particular, we derive and calculate values for the interphase heat transfer coefficient, h, which multiplies the source/sink terms in the two-energy model for conduction in a porous medium. On allowing a uniform heat generation to take place within one of the phases, it is possible to determine h from the difference in the average temperatures of the two phases after the decay of transients. An exact analytical expression is obtained for periodic striped media, which suggests that a new nondimensional parameter might usefully be defined. Exact numerical solutions are obtained for randomly striped media. Precise expressions are also found for the two-dimensional checkerboard pattern and its three-dimensional analogue. We also consider other types of two-dimensional periodic media, and finally, randomly constituted media are analyzed.
D. Andrew S.
Rees
Department of Mechanical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
125-143
THE STABILITY OF EVAPORATING FRONTS IN POROUS MEDIA
Evaporating fronts in porous media occur during drying processes, underground coal gasification, geothermal energy production from hot, dry rock, and around nuclear waste repositories. The stability of such fronts is pertinent to understanding the heat and mass transfer at the front. This article reports the results of linear stability analyses of evaporating fronts in infinite and semi-infinite domains. For the case of an infinite domain, subcooled or saturated liquid flows toward hot matrix rock above the saturation temperature, resulting in a moving evaporation front. For the semi-infinite domain, subcooled or saturated liquid flows toward a surface maintained at a temperature above the saturation temperature. For the infinite domain and an evaporating front normal to the gravitational vector, the front can be destabilized by gravity when a dense liquid overlies a less dense vapor. Whether the flow is vertical or horizontal, the front can be destabilized by mobility. It is demonstrated that evaporation, as characterized by the vapor phase Jakob number, has a destabilizing effect on mobility, however, this is countered by a stabilizing effect due to heat transfer at the interface. The stabilizing effect of heat transfer can dominate destabilizing effects of mobility and gravity at large wave numbers. For the semi-infinite domain, a stationary front can be shown to exist under certain conditions, and in this case, the stabilizing effect of heat transfer is always greater than the destabilizing effect of mobility for a fluid having the properties of water over a wide range of pressures.
O. A.
Plumb
Department of Mechanical Engineering, University of Wyoming, Laramie, WY 82072
Alexander
Klimenko
University of Queensland, BRISBANE 4072
145-155
VISCOUS DISSIPATION AND OHMIC HEATING EFFECTS ON MAGNETOHYDRODYNAMIC MIXED CONVECTION ALONG A VERTICAL MOVING SURFACE EMBEDDED IN A FLUID-SATURATED POROUS MEDIUM
The effects of both first- and second-order resistance, due to the solid matrix of non-Darcy porous media, viscous dissipation, Ohmic heating, and temperature-dependent viscosity on laminar mixed convection boundary layer flow and heat transfer on a continuously moving vertical surface, have been studied. The fluid viscosity is assumed to vary as an inverse linear function of temperature under the action of a transverse magnetic field. Local similarity solutions are obtained for the boundary layer equations governing the flow along a moving vertical isothermal surface with uniform velocity. The effects of various parameters, such as the first- and second-order solid matrix resistance, magnetic field, Eckert number, buoyancy force, and viscosity/temperature parameter, on the velocity and temperature profiles as well as the skin friction coefficient and wall heat transfer are presented graphically and in tabular form.
A.M.
Rashad
Department of Mathematics, Aswan University, Faculty of Science, Aswan, 81528, Egypt
Ahmed Y.
Bakier
Department of Mathematics, Assiut University, Assiut, Egypt
Rama Subba Reddy
Gorla
Department of Mechanical Engineering, Cleveland State University, Cleveland, OH, 44115 USA; Department of Mechanical Engineering, University of Akron, Akron, Ohio 44325, USA; Department of Mechanical & Civil Engineering, Purdue University Northwest, Westville, IN 46391, USA
159-170
FLOW THROUGH ANISOTROPIC POROUS MEDIUM WITH MULTISCALE LOG-NORMAL CONDUCTIVITY
In this paper we study effective coefficients in steady filtration problems for anisotropic mediums. The conductivity is represented mathematically with Kolmogorovâ€™s multiplicative cascade in three-dimensional space. The dimension of solution domain of the problem is considered to be large compared to the sizes of heterogeneities of the medium. The subgrid modeling approach, associated with problems of subsurface hydrodynamics, is presented. Theoretical results are compared to the results of direct 3D numerical modeling and results of conventional perturbation theory.
O. N.
Soboleva
The Institute of Computational Mathematics and Mathematical Geophysics, SB RAS, 630090 Novosibirsk
E. P.
Kurochkina
The Institute of Thermophysics, SB RAS, 630090 Novosibirsk
171-182
COMPARISON BETWEEN DIFFERENT POROUS BED (POROUS FILTER AND RODS BUNDLE) IN OPEN CHANNELS
In this study the characteristics of turbulent flow in an open channel with porous bed are studied experimentally with the use of hot-film anemometry. Experiments were carried out with two types of porous bed with the same porosity, ε = 0.85: (a) porous filter and (b) rods bundle. The main emphasis is given to the effects of relative porous thickness s/h (s = porous thickness, h = total flow depth) on the flow characteristics over the porous bed. The relative thickness s/h varies between 0.524 and 0.647. Measurements of mean velocity and turbulence characteristics (Reynolds stresses) indicate the effects of the bed material used on the flow characteristics.
Evangelos
Keramaris
University of Thessaly
183-193