A. A. Mohamad

Heat transfer enhancements in heat exchangers fitted with porous media Part I: constant wall temperature

This work investigates heat transfer enhancement for a flow in a pipe or a channel fully or partially filled with porous medium. The porous layer inserted at the core of the conduit. Forced, laminar flow is assumed and the effects of porous layer thickness on the rate of heat transfer and pressure drop were investigated. The Darcy number (permeability) is varied in the range of 10−6 to 10.0. Developing and fully developed flow conditions are considered in the analysis. It is found that the plug flow assumption is not valid for Da>10−3. The effect of varying the inertia term (Forchheimer term) is also investigated and it is found that the inertia term is not that important for Da<10−4 for the range of the parameters investigated. Partially filling the conduit with porous medium has two advantages: it enhances the rate of heat transfer, and the pressure drop is much less than that for a conduit fully filled with a porous medium.

Natural convection from a discrete heat source on the bottom of a horizontal enclosure

Abstract Steady, natural convection from a discrete flush-mounted rectangular heat source on the bottom of a horizontal enclosure is studied numerically. Three-dimensional form of Navier–Stokes equations are solved by using multigrid technique. Rayleigh number based on the enclosure height is varied from 103 until unstable flow is predicted for a fixed Prandtl number of 0.71. Aspect ratio of the source is varied until it fully covered the entire width of the bottom plate. The enclosure is cooled from above and insulated from the bottom. Effect of vertical boundary conditions on the rate of heat transfer from the heat source is studied. It is found that the rate of heat transfer is not so sensitive to the vertical wall boundary conditions. The limit of the maximum Rayleigh number to obtain a convergent solution decreases as the aspect ratio of the source is increased. The variation of Nusselt number as a function of Rayleigh number and aspect ratio of the source is reported.

Double diffusive convection in a cubic enclosure with opposing temperature and concentration gradients

A three-dimensional numerical study has been performed to investigate double-diffusive, natural convection in a cubic enclosure subject to opposing and horizontal gradients of heat and solute. The flow is driven by buoyancy forces due to temperature and solutal gradients. Constant temperature and concentration are imposed along the two vertical side walls of the cubic enclosure, while the remaining walls are impermeable and adiabatic. The numerical simulations presented here span a wide range of thermal Rayleigh number (10.0<Ra<2×105), buoyancy ratio (−2.0<N<0) and Lewis number (0.1<Le<150) to identify the different flow patterns and bifurcations. Nusselt and Sherwood numbers are presented as functions of the governing parameters. Most of the published results approximate the problem as two dimensional. In the present results we indicate that the double diffusive flow in enclosures with opposing buoyancy forces is strictly three dimensional for a certain range of parameters.

Combustion in Porous Media

Abstract The combustion research in porous medium burners has been reviewed from the early development in 1912 up to date. A collection of documented works by different authors was classified and their specific interests were addressed. These interests included the evolution and earlier work on the excess enthalpy flame in solid porous medium burners, the analytical treatment, and experimental study of flame stability in such burners. The characterization of the porous burner performance with respect to the relevant parameters was also recorded for both numerical and experimental works. The features of innovative geometries and turbulent flow conditions were demonstrated. The extension of the porous burner operation to incorporate liquid fuel combustion was also included in the classification. Relevant comments and recommendations for future work were attached at the end of each section.

Simulation of Conjugate Heat Transfer Using the Lattice Boltzmann Method

The Lattice Boltzmann Method (LBM) is utilized to investigate conjugate heat transfer. Hot and cold streams enter the computational domain, and heat transfer takes place between the two streams through a finite thickness and finite thermal conductivity wall. The main objective of the work is to demonstrate that LBM can solve conjugate heat transfer by using one energy equation for solid and fluid phases. The flux continuity insures automatically. Furthermore, the effects of extended surfaces were investigated on the rate of heat transfer and pressure drop.

A TRANSIENT STUDY OF COUPLED NATURAL CONVECTION AND RADIATION IN A POROUS VERTICAL CHANNEL USING THE FINITE-VOLUME METHOD

The present article deals with a numerical study of coupled fluid flow and heat transfer by transient natural convection and thermal radiation in a vertical channel opened at both ends and filled with a fluid-saturated porous medium. The bounding walls of the channel are isothermal and gray. In the present study we suppose the validity of the Darcy law and of the local thermal equilibrium assumption. The radiative transfer equation (RTE) is solved by the finite-volume method (FVM). The net radiative heat flux as well as its divergence is also calculated using the same method. The sensitivity of the fluid flow and the heat transfer to different controlling parameters, namely the conduction-radiation parameter or Planck number or Stark number, N, the optical thickness, τ D , and the wall emissivity, ϵ, are addressed. The results indicate that the controlling parameters of the problems, namely, N , τ D , and ϵ, have significant effects on the flow and thermal fields and on the transient process of heating or cooling of the medium. It has also been shown that the volumetric flow rate, q v , and the convected heat flux at the channels exit increase when N is decreased and/or τ D and ϵ are increased.

Three-dimensional double-diffusive convection in a porous cubic enclosure due to opposing gradients of temperature and concentration

A three-dimensional mathematical model based on the Brinkman extended Darcy equation has been used to study double-diusive natural convection in a fluidsaturated porous cubic enclosure subject to opposing and horizontal gradients of temperature and concentration. The flow is driven by conditions of constant temperature and concentration imposed along the two vertical sidewalls of the cubic enclosure, while the remaining walls are impermeable and adiabatic. The numerical simulations presented here span a wide range of porous thermal Rayleigh number, buoyancy ratio and Lewis number to identify the dierent steady-state flow patterns and bifurcations. The eect of the governing parameters on the domain of existence of the three-dimensional flow patterns is studied for opposing flows ( N< 0). Comprehensive Nusselt and Sherwood number data are presented as functions of the governing parameters. The present results indicate that the double-diusive flow in enclosures with opposing buoyancy forces is strictly three-dimensional for a certain range of parameters. At high Lewis numbers multiple dipole vortices form in the transverse planes near the horizontal top and bottom surfaces, which the two-dimensional models fail to detect. The dipolar vortex structures obtained are similar to those created in laboratory experiments by the injection of fluid into a stratied medium. Natural convection flow resulting from the combined action of temperature and concentration, which is also called double-diusive convection, has recently been the subject of intense research activity in view of its importance in various engineering and geophysical problems. Among these are the migration of moisture through the air contained in brous insulations and grain storage installations, solute exchange in sediments in coastal environments, the transport of chemical contaminants through water-saturated soil and disposal of nuclear wastes in underground sites. As reviewed by Song & Viskanta (1994) the mushy zone existing during the solidication of alloys consists of a ne mesh of dendritic crystals growing into the melt, owing to the solubility dierence between the solid and liquid phases. The composition of the resulting solid is generally dierent from that of the melt when a melt of two or more components solidies. Therefore, heat and mass transfer occur simultaneously in the mushy zone, which can be modelled as double-diusive convection in a porous medium. Heat and solute diuse at dierent rates, as a result of which complex flow structures may form which have no counterpart in buoyant flows driven by a single component. Research in this eld is mainly based on two congurations. The rst

Three-Dimensional Simulation of Laminar Rectangular Impinging Jets, Flow Structure, and Heat Transfer

The flow and heat transfer characteristics of impinging laminar jets issuing from rectangular slots of different aspect ratios have been investigated numerically through the solution of three-dimensional Navier-Stokes and energy equations in steady state. The three-dimensional simulation reveals the existence of pronounced streamwise velocity off-center peaks near the impingement plate. Furthermore, the effect of these off-center velocity peaks on the Nusselt number distribution is also investigated. Interesting three-dimensional flow structures are detected which cannot be predicted by two-dimensional simulations. Impinging jets are commonly used in many industrial applications to enhance heat or mass transfer from the impingement surface. Typical applications include tempering of glass, annealing of materials, cooling of turbine blades and drying of paper.

Double diffusion natural convection in a rectangular enclosure filled with binary fluid saturated porous media: The effect of lateral aspect ratio

Three-dimensional, double diffusion, natural convection in a rectangular enclosure filled with binary fluid saturating porous media is investigated numerically. The effect of lateral aspect ratio on the heat, mass, and momentum transfer is systematically studied. For certain range of parameters, it is interesting to find that the flow patterns may duplicate themselves as the lateral aspect ratio increases by integer factors, which is similar to longitudinal roll formation in a Rayleigh–Benard problem. For the mentioned range of parameters the change in the lateral aspect ratio has no influence on the rates of heat and mass transfer. However, for other ranges of parameters, the flow exhibits completely different patterns and the rates of heat and mass transfer are influenced drastically compared with that of cubic cavity. In general, the flow of three- and two-dimensional results are difficult to justify, especially if interest is on the flow structure.

Suppressing free convection from a flat plate with poor conductor ribs

An experimental and numerical analysis is performed to investigate the effect of attaching poor conducting ribs on a vertical heated flat plate. Accordingly, a test cell is constructed and a mathematical model is developed. The ribs are made of a poor conducting material (plexiglass). The present work is intended to study the suppression of free convection from a vertical plate with ribs attached on the surface. The effects of rib height to span ratio and Rayleigh number are investigated experimentally and numerically for air as the working fluid. It has been found that adding ribs on the surface can reduce the rate of free convection heat transfer by as much as 75% compared with a bare plate. Rather complex recirculatory flow patterns are predicted in the space between the ribs at high Rayleigh numbers.