ANALYSES OF FREE CONVECTION FLOW OF AN INCLINED PLATE EMBEDDED IN A DOUBLE LAYER POROUS MEDIUM

Transport Processes in a Rapidly Changing World.- Modelling of Transport Phenomena in Porous Media.- Fundamentals of Mechanics of Saturated Porous Media: Basic Equations and Waves.- The Stability of Convective Flows in Porous Media.- Free Convection Heat and Mass Transfer in a Porous Medium.- Natural Convection in a Vertical Porous Annulus.- Non-Darcy Natural Convection in Saturated Porous Media.- Mixed Convection in Saturated Porous Media.- Forced Convective Flow and Heat Transfer Through a Porous Medium Exposed to a Flat Plate or a Channel.- Forced Convection Heat Transfer in a Porous Medium.- Radiation Transport in Porous or Fibrous Media.- Fundamentals of Drying of Capillary-Porous Bodies.- Heat Transfer During Unsaturated Flow in Porous Media.- Buoyancy-Induced Flow and Heat Transfer in Saturated Fissured Media.- Effect of Randomness on Heat and Mass Transfer in Porous Media.- Analytical Solutions to Transient Convective Mass Transfer Within Porous Media.- Natural Convection in Porous Media with Variable Porosity and Thermal Dispersion Effects.- Convective Flow Interaction and Heat Transfer Between Fluid and Porous Layers.- Temperature Distribution in a Porous Slab with Random Thermophysical Characteristic.- Forced Convection in Packed Tubes and Channels with Variable Porosity and Thermal Dispersion Effects.- Transient Double Diffusive Convection in a Horizontal Fluid Layer Situated on top of a Porous Substrate.- Drying of Wood Residues in a Fixed Bed.- Heat and Mass Transfer in Adsorbent Beds.- Solidification of a Binary Mixture Saturating a Bed of Glass Spheres.- Melting in the Presence of Natural Convection in a Saturated Porous Medium.- Air-Water Two-Phase Flow Pressure Drop in Large Scale Porous Media.- Boiling and Dryout in Unconsolidated Porous Media.- Heat Transfer from a Surface Covered with Hair.- Measurements of Thermal Conductivity in Porous Media.- Determination of Velocity Vectors in Porous Media with Fluorescent Particle Image Velocimetry (FPIV).- Non Invasive Measurement Techniques in Porous Media.- Flash Method of Measuring Thermal Diffusivity and Conductivity.- Mechanics, Heat and Mass Transfer in Saturated Porous Media. Application to Petroleum Technology.- Drying Complex Porous Materials-Modelling and Experiments.- Some Geophysical Problems Involving Convection in Porous Media.- Porous Surface Boiling and Its Application to Cooling Microelectronic Chips.- Heat and Mass Transfer in Spouted Beds.- Liquid Seeping into Porous Ground.- Future Research Needs in Convective Heat and Mass Transport in Porous Media.

A numerical study has been carried out for free convection in a vertical cylindrical annulus filled with a porous medium and whose inner wall is isothermally heated and the outer wall is isothermally cooled, the horizontal walls being insulated. The porous medium is assumed to be both hydrodynamically and thermally anisotropic. Numerical results are reported for 0.1 ? K*? 10, 0.1 ? ? ? 10,1 ? A ? 20,2 ? Rr ? 20, and Ra*? 10000. Anisotropy of the porous medium is found to affect fluid flow, temperature distribution and heat transfer significantly. Higher permeability in the vertical direction enhances convective flow intensity and heat transfer inside the annulus. Average Nusselt number on the inner hot wall increases with increase in Rayleigh number or radius ratio, while it decreases with increase in aspect ratio or permeability ratio. The influence of thermal anisotropy is not so significant as that of hydrodynamic anisotropy. The numerically predicted temperature distribution at various locations inside the annulus shows reasonable agreement with experimental results available for isotropic porous medium. Based on a parametric study, correlation for heat transfer is presented in terms of Rayleigh number, aspect ratio, radius ratio, and permeability ratio.

In the chain of steel production, the blast furnace converts iron ore into liquid iron. The hearth of the blast furnace, where the liquid metal is collected and tapped off, is filled with relatively large coke particles (D ~ 20 – 100 mm). The meandering flow of hot liquid metal in the coarse-grained porous medium in the hearth causes erosion of the refractory walls containing the hearth through the formation of hot spots. This has a severe impact on the lifetime of blast furnaces. Therefore, it is crucial to understand the liquid metal flow and heat transfer through the packed bed of relatively large coke particles. With the hot metal flowing in from the top and the refractory walls being cooled, the flow of the liquid metal in the hearth is a natural and mixed convection flow characterized by the dimensionless Rayleigh and Reynolds numbers, and their ratio, viz. the Richardson number. Since the pores between the large coke particles are not small compared to the flow and thermal length scales, there is a strong interaction between the flow and the pore geometry. Therefore, it is important to capture the details of fluid flow and temperature distribution at the pore level and to resolve the strong interaction between the flow and the solid grains.In order to gain a fundamental understanding of the above types of flow, this dissertation reports on experimental investigations of natural and mixed convection in cubical cavities filled with coarse-grained porous media consisting of packed beds of relatively large solid spheres. Bottom-heated natural convection, side-heat natural convection, and vented mixed convection configurations have been considered. Accurate global heat transfer measurements have been performed for various sphere packings, sphere sizes, and sphere conductivities in a wide range of Rayleigh numbers (and Reynolds numbers in the mixed convection case). Refractive index matching between water and hydrogel spheres enabled the use of optical measurement techniques, i.e. Particle Image Velocimetry and Liquid Crystal Thermography, to obtain highly-resolved pore-scale velocity and temperature fields.In bottom-heated natural convection, it was observed that at lower Rayleigh numbers, the Nusselt numbers for the porous medium-filled cavity are reduced compared to the pure-fluid cavity (Rayleigh-Benard convection) and the Nusselt number reduction strongly depends on packing type, size, and conductivity of spheres. However, at high Rayleigh numbers, when the flow and thermal length scales become sufficiently smaller than the pore length scale, the flow penetrates into the pores with much higher velocities and is not obstructed by the presence of coarse-grained porous media. This leads to an asymptotic regime in which the convective heat transfer for all sphere conductivities, sizes and packing types converge into a single curve which is very close to the pure Rayleigh-Benard convection curve. The results indicate that the ratio between the thermal length scale and the pore length scale is a determining factor in the effect of porous media on flow and heat transfer.In side-heated natural convection, the presence of the porous medium decreases the heat transfer compared to the corresponding pure-fluid cavity. This is due to the fact that the layers of the spheres touching the isothermal side walls hinder the boundary layers along these walls and divert a portion of the boundary layer fluid away from the walls. This subsequently alters the temperature distribution and reduces the mean temperature gradient at the walls. The heat transfer measurements demonstrate a transition from Darcy to non-Darcy regime by increasing the Rayleigh number and the size of spheres. A new Nusselt number correlation for coarse-grained porous is presented which takes into account the strong effect of the porous medium conductivity. In vented mixed convection, three flow and heat transfer regimes were observed depending on the Richardson number. For Ri l 10, the porous medium directs a portion of the strong forced inflow downward towards the hot wall, and therefore the flow structure and the Nusselt number scaling are similar to pure forced convection and are independent of Rayleigh number. For Ri g 40, the strong upward directed natural convection flow dominates and the Nusselt number becomes less sensitive to the Reynolds number. For 10 l Ri l 40, the upward directed natural convection flow competes with the downward directed forced flow at the hot wall, leading to a minimum effective Nusselt number. A Nusselt number correlation is presented which covers all three regimes. This dissertation concludes by discussing the contribution of this work in improving the knowledge on the physics of natural and mixed convection in coarse-grained porous media and its relevance for understanding and modelling of the fluid flow and heat transfer processes in the blast furnace hearth, as well as in other application fields such as in air ventilation and in the food industry.

Keywords: Natural convection, anisotropic porous medium, inversion of density.Abstract. The natural convection in the porous medium has attracted considerable attention with its applications in various industrial sectors, such as the agro-alimentary, the pharmaceutical and oil processing industries. The present work is about the study of the phenomenon of natural convection at 4°C on the vertical plate lining in an anisotropic porous medium in permeability. The wall of the vertical surface is subjected to a constant temperature with defined hydrodynamic conditions and thermal limits. Generalized Darcy’s law was used to establish the governing equations of the system. The control parameters governing the system are respectively the permeability ratio K*, the angle ϴ of the major axis and the inversion parameter R. The general basic equations were solved numerically using Runge-Kutta and shooting method. There was validated against previous work. The effect of these parameters on the heat transfer was highlighted. From the analysis result, it comes out the following conclusions: The convective flow is significantly affected by the anisotropic parameter; heat transfer along the vertical surface is maximum (minimum) when the main shaft having the high permeability is oriented parallel (perpendicular) to the gravitational field; the rate of heat transfer is depending on the inversion parameter R. The convective transfer rate illustrated by the local Nusselt number is symmetric with respect to R=0.45, where it reaches its smallest value. It is inversely proportional to the distribution of the thickness of boundary layer. A high convective transfer rate corresponds to a low boundary layer thickness and this inversely.

Natural convection heat transfer in a horizontal enclosure filled with anisotropic porous media, being isothermally heated at bettom and cooled at top while the vertical walls being adiabatic, is numerically studied by applying the Brinkman model- a modified form of Darcy model giving consideration to the viscous effect. The results show that: (1) a larger permeability ratio (K*) causes a lower flow intensity in the enclosure and a smaller Nusselt number, all Nusselt numbers approach unity in the limit of K* → ∞; a larger thermal conductivity ratio (λ*) causes a stranger distortion of isotherms in the enclosure and a higher flow velocity near the walls, all the Nusselt numbers approach unity in the limit of λ* → 0; the permeability and thermal conductivity ratios generally have opposing effects on the Nusselt number. (2) an increasing Darcy number decreases the flow intensity and heat transfer rates, which is more significant at a lower permeability ratio. In particular, with K* ≤ 0.25, the Nusselt number for Da=10−3 would differ from that of Darcy flow up to an amount of 30%, an analysis neglecting the non-Darican effect will inevitably be of considerable error.

Chapter 8 - Nanoparticles-enhanced energy storage materials in solar thermal desalination

Analyses of free convection flow of variable spaced plates embedded in a porous medium

Combined Heat and Mass Transfer in a Horizontal Channel with an Open Cavity

Chapter 3 - Fundamental Relationships of Heat and Mass Transfer in Solar Seawater Desalination Systems

Cooling Performance of Bearing Ring in High-Pressure Gas-Quenching Furnace

This paper examines the problem of mass transfer on MHD unsteady free convective flow of a polar fluid through a porous medium of variable permeability bounded by an infinite horizontal porous plate in slip flow regime.The permeability of the porous medium decreases exponentially with time about a constant mean.Using perturbation technique the expressions for the velocity distribution, mean angular velocity of rotation of particles, concentration distribution and skin friction are obtained.The effects of permeability parameter K 0 , Magnetic parameter M, Prandtl number P r , Schimdt number S c ,and Grashof number G r ,Modified Grashof number G m entering into the problem on velocity, temperature distribution and concentration distribution are shown graphically and discussed numerically.It can be observed that this velocity decreases with the increase in M, P r, S c and increases with the increase in K 0 , G r , G m .Temperature and Concentration decreases with the increase in the value of P r and S c respectively.

An experimental study of thermal convection in a horizontal porous layer bounded by isothermal planes has been performed with and without a mean flow of the saturating fluid phase. The temperature distribution and heat transfer resulting from convection have been determined. The theoretical criterion for the onset of convection (Rayleigh number NRa > 4p2) has been verified. For low values of NRa (< 260) a regular pattern of convective cells has been observed which may be motionless or moving depending on the experimental conditions. For NRa values higher than 260, another convective state has been found that is mainly unstable. Numerical computations have been worked out that confirm the experimental results on the heat transfer and size of convective cells. Introduction Thermal convective currents may exist in a porous medium when the vertical component of the temperature gradient runs in the same direction as the gravity vector. Such a configuration is possible in some cases during a real in-situ combustion test4 or during hot fluid injection. A general understanding of convection is also of interest for analyzing the abnormal temperature gradients sometimes observed in oil and gas reservoirs.22 From the phenomenological standpoint a distinction must be made between the natural or free convection occurring in a closed volume and the mixed or combined free and forced convection arising when there is a mean flow of the fluid phase. Natural convection in porous media has been the subject of numerous experimental attempts to determine a convection criterion, e.g., by using a linear theory,1,2,9 and to measure the mean heat transfer.3,7 Numerical studies8,10,15,19,20 have also been made, and their results are not always in good agreement with experimental observations. However, no thorough study of mixed convection has been made except for a theoretical analysis of the convection onset criterion5 and some qualitative experimental observations.4 This paper presents some findings concerning both the natural and mixed convection of an incompressible fluid in a homogeneous horizontal porous layer bounded by two parallel impervious isothermal surfaces. The temperature for the cooler upper surface is T1 and that of the lower surface is T2=T1+?T.

Desalination processes are energy consuming and it is required to apply clean energy sources for supplying them to prevent environmental issues. Solar energy is one of the attractive clean energy sources for desalination. In solar thermal desalination systems, different thermal components could be used for heat transfer purpose. In solar desalination technologies, heat pipe as efficient heat transfer mediums could be employed to transfer absorbed and/or stored thermal energy. The objective of this study is to review applications of heat pipes in solar energy desalination systems. Regarding the performance dependency of these thermal systems on the variety of factors, scholars have investigated these systems by consideration of the effect of different influential factors. Based on the results, it is concluded that use of heat pipes could lead to proper performance of solar desalination systems. Aside from direct transfer of absorbed heat from solar radiation, heat pipes can be applied in the storage units of solar desalination systems to keep the systems active in night-hours or low solar irradiation conditions. The overall performance of the solar desalinations systems with heat pipes can be influenced by some factors such as filling ratio and operating fluid that affect the performance of heat pipes.

In order to analyze the influence of condensation behavior on the heat transfer process of large heat exchangers under wet conditions, a three-dimensional numerical model of condensation heat transfer of twelve-row wavy fin-and-tube heat exchangers under wet conditions was established. The effects of inlet velocity and relative humidity on heat and mass transfer characteristics were investigated. Through the analysis of the field synergy principle, it is found that the wavy fin can effectively improve the synergy between the velocity field with the temperature field and the concentration field. The wavy fin not only increased the disturbance of fluid but also continuously disrupted the developments of thermal and mass concentration boundary layers, thereby greatly enhancing the heat and mass transfer. The condensation mainly occurs on the first six pipe rows, and the increase of inlet velocity makes the condensation area obviously larger. The inlet relative humidity has little effect on the temperature distribution and sensible heat transfer under wet conditions but has a significant effect on the latent heat transfer and total heat transfer. Finally, the relationship for convective heat transfer coefficients under wet and dry conditions is obtained by multivariate fitting.

A review on evaporation improvement of solar still desalination using porous material