Begell House Inc.
Journal of Porous Media
JPM
1091-028X
9
7
2006
Foundry Core Box Vent Positioning as a Problem of Boundary Optimal Control of Convective Gas Flow in Porous Media
609-624
10.1615/JPorMedia.v9.i7.10
Marc
Prat
Institut de Mecanique des Fluides de Toulouse, U.M.R. C.N.R.S. − INP/UPS N°5502, Avenue du professeur Camille Soula, 31400 Toulouse, France
Vincent
Pavan
Institut des Systèmes Thermiques Industriels, 13453 Marseille cedex 13, France
V.
Vidal
Centre Technique des Industries de la Fonderie, 44, av. de la Division Leclerc, 92318 Sévres Cedex, France
This paper discusses the gassing step of the foundry core box-making process for the cold-box/amine system with air as carrier gas. A model of gassing is developed and leads to a good qualitative agreement with experimental visualizations. It is shown that the process efficiency directly depends on the correct opening or closing of the vents positioned at the core box boundary. Then, it is shown how the vents optimal control can be studied as a boundary optimal control of convective gas flow in a porous domain. The problem is to minimize the energy and amount of gas injected in the system while ensuring that the injected gas visits every point of the porous domain in a minimum time. An optimization algorithm based on the adjoint state method is developed. The gas transport is by advection and controlled using Neumann boundary conditions. Application of the optimization procedure leads to results in fair agreement with direct simulations of the process. This opens up interesting perspectives of computer-aided optimal vent positioning.
Mathematical Investigation of Effective Thermal Conductivity in Fractured Porous Media
625-635
10.1615/JPorMedia.v9.i7.20
Shahab
Ayatollahi
Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran
Nima
Saber
School of Chemical and Petroleum Engineering, Shiraz University, P.O. Box 71345-1719, Shiraz, Iran
Mohammad Javad
Amani
School of Chemical and Petroleum Engineering, Shiraz University, P.O. Box 71345-1719, Shiraz, Iran
Ali
Bitaab
School of Chemical and Petroleum Engineering, Shiraz University, P.O. Box 71345-1719, Shiraz, Iran
Calculating the thermal conductivity of the porous material is a challenging task due to the heterogeneity of medium and presence of numerous adjustable parameters. Meanwhile, the prominent role of conductivity in heat transfer through porous media highlights the necessity of a better understanding of effective thermal conductivity. In this study, the unit cell model is used to predict the effective thermal conductivity of the saturated two-phase porous media. The calculated data were proved to be in agreement with those from well-known correlations.
Finally, a unit cell model is further extended to study the heat transfer in fractured porous media and a correlation is developed to predict the effective thermal conductivity of the fractured media based on the size and orientation of the fractures.
Migration versus Diffusion through Porous Media: Time-Dependent Scale Analysis
637-650
10.1615/JPorMedia.v9.i7.30
P.
Begue
Laboratory of Materials and Durability of Constructions, INSA-UPS, Department of Civil Engineering, National Institute of Applied Sciences, 135 Avenue de Rangueil, 31077 Toulouse, France
Sylvie
Lorente
Villanova University
This paper presents a fundamental study of enhanced ionic transport in the case when an electrical field is applied through a saturated porous medium. The study focuses on the very first moments when the potential difference is applied. Scale analysis is used in order to highlight the existence of two different regimes: initially, pure diffusion prevails in spite of the electrical field that will overtake diffusion-driven transport after a transition time. It is shown that the reason for this change in mechanisms is the search for the maximization of the ionic transfer. In the numerical part, the impact of already contaminated porous medium on the amount of ionic species transferred is studied, together with the effect of the concentration and concentration gradients levels. Finally, an optimization method is proposed in order to maximize the transport of species by applying an electrical field increasing in time.
Effect of Anisotropy on Stability of Convection in a Rotating Porous Layer Distant from the Center of Rotation
651-662
10.1615/JPorMedia.v9.i7.40
Saneshan
Govender
School of Mechanical Engineering, University of Kwa-Zulu; School of Mechanical Engineering, University of Natal, Durban, South Africa; Eskom Holdings Ltd, Engineering Department (Gas Division), Eskom Enterprises Park, Simba Road, Sunninghill, Johannesburg
We investigate Rayleigh-Benard convection in an anisotropic porous layer subjected to centrifugal body forces and offset from the center of rotation. The Darcy model (including centrifugal and permeability anisotropy effects) and a modified energy equation (including the effects of thermal anisotropy) are used in the current analysis. The linear stability theory is used to evaluate the critical Rayleigh numbers (one for porous layers placed close to the center of rotation and the other for porous layers placed far away from the center of rotation) for the onset of convection in the presence of thermal and mechanical anisotropy. It is found that the convection is stabilized when the thermal anisotropy ratio (which is a function of the thermal and mechanical anisotropy parameters) is increased in magnitude.
Paradox of the Darcy Free Convection over a Vertical Plate with Prescribed Inverse Linear Surface Heat Flux
663-670
10.1615/JPorMedia.v9.i7.50
E.
Magyari
Chair of Physics of Buildings, Institute of Building Technology, Swiss Federal Institute of Technology (ETH) Zürich, CH-8093 Zürich, Switzerland
It is generally accepted that the Darcy free convection problem for a vertical impermeable plate does not admit similarity solutions when the prescribed surface heat flux is an inverse linear function of the wall coordinate. The present paper explains why this is not unconditionally true, and gives the similarity solution of the problem in an exact analytical form.
Liquid-Vapor Phase-Change and Mixed Convection in a Porous Layer Discretely Heated
671-681
10.1615/JPorMedia.v9.i7.60
Mustapha
Najjari
Laboratoire d'Etudes des Systemes Thermiques et Energetiques, Cite Riadh, Zirig 6072 Gabes, Tunisia
Sassi Ben
Nasrallah
Laboratoire d'Études des Systèmes Thermiques et Énergétiques, Ecole Nationale d'Ingénieurs
de Monastir, Monastir 5019 Tunisie
In this work, we numerically study heat and mass transfers during boiling by mixed convection in a porous layer discretely heated. The porous layer is initially saturated by liquid phase. For the mathematical formulation, an enthalpic method is used. The time-space evolution of thermal and dynamic fields is presented and analyzed in the cases of one and two heat sources. Results concerning the effects of the distance between two heat sources and the length of the heat source are also presented.
Boundary Layer Treatment of Forced Convection over a Wedge with an Attached Porous Substrate
683-694
10.1615/JPorMedia.v9.i7.70
The classical boundary layer analysis of Falkner and Skan (Phil. Mag., vol. 12, pp. 865−896, 1931) for forced convection from a wedge is extended to the case where a thin layer of a porous medium is attached to each surface of the wedge. The analysis involves an expansion in powers of a small parameter ξ = αBJ−1 (Ku∞/vx)1/2 involving the downstream coordinate x, the Beavers−Joseph coefficient αBJ, permeability K, the free stream velocity u∞, and the kinematic viscosity v. The results for the Nusselt number are expressed in terms of a parameter N = αBJDa−1⁄2kr, where kr is the fluid-to-porous-medium conductivity ratio.
Effect of Periodically Oscillating Driving Force on Basic Microflows in Porous Media
695-707
10.1615/JPorMedia.v9.i7.80
O. M.
Haddad
Department of Mechanical Engineering, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan
M. M.
Abuzaid
Mechanical Engineering Department, Jordan University of Science and Technology, Irbid 22110, Jordan
The effect of frequency of fluctuation of the driving force on basic microflows is theoretically investigated in the presence of a porous medium. Four basic flow cases are considered in the slip flow regime: the transient Couette flow, the pulsating Poiseuille flow, the Stokes second problem flow, and the transient natural convection flow. The extended Darcy-Brinkman model is used to model the flow in a porous medium. The study is focused on the effect of frequency on the slip in velocity and jump in temperature at the wall(s). It is found that for all cases considered the increase in frequency or Knudsen number would lead, in general, to an increase in the normalized slip. However, the normalized slip increased as Darcy number decreased, except for Poiseuille flow where the normalized slip increased as Darcy number increased. In addition, the jump in temperature is found to be negligible when the frequency and Knudsen number are relatively small.