Numerical and experimental investigation of heat and mass transfer in unsaturated porous media with low convective drying intensity

The heat and mass transfer in an unsaturated wet cylindrical bed packed with quartz particles was investigated theoretically and experimentally for relatively low convective drying rates. The medium was dried by blowing dry air over the top of the porous bed which was insulated by impermeable, adiabatic material on the bottom and sides. Local thermodynamic equilibrium was assumed in the mathematical model describing the multi‐phase flow in the unsaturated porous medium using the energy and mass conservation equations for heat and mass transfer during the drying. The drying model included convection and capillary transport of the moisture, and convection and diffusion of the gas. The wet and dry regions were coupled with a dynamic boundary condition at the evaporation front. The numerical results indicated that the drying process could be divided into three periods: the initial temperature rise period, the constant drying rate period, and the reduced drying rate period. The numerical results agreed well with the experimental data, verifying that the mathematical model can evaluate the drying performance of porous media for low drying rates. ©2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(5): 290–312, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20205     

Heat transfer and phase change of liquid in an inclined enclosure packed with unsaturated porous media

In this paper, we present a mathematical model to describe the simultaneous heat and mass transfer with liquid phase change in unsaturated porous media. Two-dimensional natural convective flow in an inclined rectangular enclosure with porous material unsaturated with fluid is analyzed numerically. The parameter variations are considered for the tilted angle, the aspect ratio and the Darcy–Rayleigh number. Local and global Nusselt numbers are presented as functions of those parameters. Compared with the saturated porous material, the heat transfer characters in the unsaturated case are discussed for the identical aspect ratio and Darcy–Rayleigh number, The discussion is also made for the field synergy of fluid velocity and heat flow in natural convection.     

Heat and mass transfer in saturated porous media with ice inclusions

This paper continues the theoretical exploration of the heat and mass transfer in a biporous medium with the coupled phase transformations. A more complicated system is considered, such as porous medium-ice-aqueous solution. General statements been formulated for a simple medium [4], remain in force for the given system: the heat and mass fluxes each depend linearly on all thermodynamical forces, the transport coefficients satisfy the Onsager reciprocal relations; the coupled phase transformations intensify significantly the cross effects.The presence of contaminants in water imparts the osmotic properties to the biporous medium and decreases the rate of the heat and mass transfer in the system. The dissolved matter takes effect beginning with the concentration of ∼0.001 mol l⁻¹. At the high concentration of the aqueous solution (>0.1 mol l⁻¹) the cross effects become negligible small and the medium loses its unique capacities.     

Simultaneous heat and mass transfer with phase change in a porous slab

Simultaneous heat and mass transfer with phase change in a porous slab has been analytically investigated. Two spatially-steady regimes, corresponding to immobile and mobile condensate, are discovered. Closed-form analytical expressions for the temperature, vapor concentration, condensation rate and liquid-content distributions as well as the location of the condensation region for each of the two regimes is obtained.     

Heat and mass transfer in porous media with ice inclusions near the freezing-point

Heat and mass transfer in biporous medium of regular structure near the phase transition point is studied theoretically. Large pores contain ice. Small pores are filled with pure water. The thermal and filtration problems for a separate cell of the medium are solved by the anisotropic conductivity method. Heat and mass flows depend linearly on the gradients of the temperature and the water pressure. The Onsager reciprocal relations are confirmed for systems with phase transformations. With the advent of the solid phase (ice) in porous media, the straight transport coefficients multiply several times, and the cross coefficients increase more than one order of magnitude.     

Heat and mass transfer in unsaturated porous media: Moisture effects in compost piles self-heating

A mathematical model relating heat and mass transfer in compost pile disposal in field is presented. The unsteady non linear mathematical diffusion model is solved through the finite volume method. The volumetric heat generation caused by the action of aerobic bacteria and by oxidation of cellulose in a porous media is incorporated as a function of the means moisture by a source term. Field studies in compost pile exposed to atmospheric conditions were made trying to reproduce the conditions in which self–heating and combustion may take place. Inside a compost pile made of solid residues obtained after municipal waste water treatment, temperature changes, oxygen contents and moisture were measured in time and depth. The mathematical model and numerical simulations in two dimensions, describe the internal changes of heat, oxygen and moisture observed in fields conditions and with this quantify the effects of the moisture in the temperature and oxygen concentration.     

Heat transfer of phase change materials (PCMs) in porous materials

In this paper, the feasibility of using metal foams to enhance the heat transfer capability of phase change materials (PCMs) in low- and high-temperature thermal energy storage systems was assessed. Heat transfer in solid/liquid phase change of porous materials (metal foams and expanded graphite) at low and high temperatures was investigated. Organic commercial paraffin wax and inorganic calcium chloride hydrate were employed as the low-temperature materials, whereas sodium nitrate was used as the high-temperature material in the experiment. Heat transfer characteristics of these PCMs embedded with open-cell metal foams were studied. Composites of paraffin and expanded graphite with a graphite mass ratio of 3%, 6%, and 9% were developed. The heat transfer performances of these composites were tested and compared with metal foams. The results indicate that metal foams have better heat transfer performance due to their continuous inter-connected structures than expanded graphite. However, porous materials can suppress the effects of natural convection in liquid zone, particularly for PCMs with low viscosities, thereby leading to different heat transfer performances at different regimes (solid, solid/liquid, and liquid regions). This implies that porous materials do not always enhance heat transfer in every regime.     

Numerical investigation on porous media heat transfer in a solar tower receiver

In order to investigate the steady heat transfer characteristics of a porous media solar tower receiver developed in China, this paper applies the steady heat and mass transfer models of the porous media to solar receivers, chooses the preferable volume convection heat transfer coefficient model, solves these equations by using the numerical method, and analyzes the typical influences of the porosity, average particle diameter, air inlet velocity, and thickness on the temperature distribution. The following conclusions have been drawn: in the same position or relative position along the downstream, the bigger the average particle diameter is, the higher the solid matrix dimensionless temperature is, the higher the air dimensionless temperature is. The bigger the porosity is, the lower the solid matrix dimensionless temperature is, the bigger the porosity is, the higher the air dimensionless temperature is. The bigger the thickness is, the lower the solid matrix dimensionless temperature is, the higher the air dimensionless temperature is. In a certain depth, the bigger the air inlet velocity is, the higher the solid matrix dimensionless temperature is. After a certain depth, the bigger the air inlet velocity is, the lower the solid matrix dimensionless temperature is, and the bigger the air inlet velocity is, the higher the air dimensionless temperature is. The paper can provide a reference for this type of receiver design and reconstruction.     

Electrochemical experimental method for natural convective heat and mass transfer in porous media

Natural convection driven by combined thermal and solutal buoyant forces in a fluid-saturated porous enclosure was studied experimentally. An electrochemical method was employed to establish the concentration gradients. The inside temperature profiles and heat and mass transfer coefficients on the vertical walls were determined experimentally. The effects of dimensionless parameter Ra, Le, N on flow, heat, and mass transfer are discussed in detail. © 1999 Scripta Technica, Heat Trans Asian Res, 28(4): 266–277, 1999     

Numerical and experimental investigation for heat transfer in triplex concentric tube with phase change material for thermal energy storage

This paper addresses a numerical and experimental investigation of a thermal energy storage unit involving phase change process dominated by heat conduction. The thermal energy storage unit involves a triplex concentric tube with phase change material (PCM) filling in the middle channel, with hot heat transfer fluid (HHTF) flowing outer channel during charging process and cold heat transfer fluid (CHTF) flowing inner channel during discharging process. A simple numerical method according to conversation of energy, called temperature & thermal resistance iteration method has been developed for the analysis of PCM solidification and melting in the triplex concentric tube. To test the physical validity of the numerical results, an experimental apparatus has been designed and built by which the effect of the inlet temperature and the flow rate of heat transfer fluid (HTF, including HHTF and CHTF) on the thermal energy storage has been studied. Comparison between the numerical predictions and the experimental data shows good agreement. Graphical results including fluid temperature and interface of solid and liquid phase of PCM versus time and axial position, time-wise variation of energy stored/released by the system were presented and discussed.     

Natural convection with coupled mass transfer in porous media

For certain types of packed beds used for thermal energy storage, high discharge rates can be achieved if the effective conductance is increased over that achievable with stagnant gases and liquids. A packed bed containing a fluid mixture with one condensing component and with the fluid mixture simultaneously convecting may achieve the required high conductances.An analogy has been developed with a similar system with no coupled mass transfer. It provides some useful insights into the role of gas/vapour mixture properties on the effective conductivity of such systems, and suggests that very large increases in effective conductivity are achievable.     

Analysis of heat transfer with liquid-vapor phase change in a forced-flow fluid moving through porous media

This study focuses on the experimental analysis of transient-regime heat transfer with liquidvapor phase change in a fluid as it flows through a porous media composed of small bronze spheres. Three distinct zones can be observed: liquid, two-phase and superheated vapor. The boundaries between these zones are determined using temperature and pressure fields. An N-shaped profile is observed for the temperature values along the main flow axis. The first local maximum value on the temperature curve corresponds to the boundary between the liquid zone and the two-phase zone. When a local minimum temperature exists, it corresponds to the boundary between the two-phase and the vapor zones. A finite element numerical simulation is used to predict the saturation field, which is numerically determined from the boundaries of the two-phase zone and of the experimental temperature field. The liquid and vapor pressure fields are then deduced for all three phase zones of the porous medium.     

Heat transfer in a confined rectangular cavity packed with porous media

This paper presents an experimental investigation of convective heat transfer in a confined rectangular cavity packed with porous media, on the opposing vertical walls of which different temperatures are imposed. Measurements are made for each of two kinds of solid particles using three kinds of fluids, i.e. water, transformer oil and ethyl alcohol. The present experiments cover a wide range of Rayleigh number Ra1 between 1 and 10⁵, Prandtl number Pr1 between 1 and 200 and geometrical aspect-ratio $\text{H}\text{W}$ between 5 and 26. The experimental results indicate that Nusselt number Nu1 is correlated by the following relationship: $\text{Nu1 = 0.627 Pr1}{}^{0.130}\text{(}\text{H}\text{W}\text{)}{}^{\mathrm{?0.527}}\text{Ra1}{}^{0.463}$     

Theoretical and experimental study of heat and mass transfer mechanism during convective drying of multi-layered porous packed bed

In this paper, the experimental validation of a combined mass and thermal model for a convective drying of multi-layered packed beds composed of glass beads, water and air is presented. The effects of the drying time, particle size and the layered structure on the overall drying kinetics are clarified in detail. Based on a completed model combining the temperature, total pressure and moisture equations, the results show that the convective drying kinetics strongly depend on the particle size as well as hydrodynamic properties and layered structure, considering the interference between capillary flow and vapor diffusion in the multi-layered porous packed bed. The predicted results are in a good agreement with the experimental results.     

Double porous media model for mass transfer of hemodialyzers

Double porous media model for mass transfer of hollow fiber hemodialyzers is presented. In the model, the hollow fiber bundle is treated as a porous region composed of two interpenetrating porous regions i.e. the blood and dialysate flow regions, and the interface of the two regions is the porous membrane through which mass transfer is performed. Navier-Stokes equations with Darcy source terms are used to describe the flows within the two regions. Modified Kedem-Katchalsky equations as other source terms are added into conservation equations to simulate the permeating flux through the porous membrane. The model is validated with respect to the experimental data in the literature.