S. Ben Nasrallah

Study of two-dimensional heat and mass transfer during desorption in a metal-hydrogen reactor

A study of two-dimensional dynamic heat and mass transfer in a metal-hydrogen reactor during desorption is presented in this paper. A mathematical model has been established and solved numerically by the method of finite domains. The numerical simulation is used to present the time-space evolution of the temperature, the pressure and the hydride density in the reactor and to determine the sensitivity to some parameters (reactor geometry, outlet pressure, temperature of heating fluid and heat conductivity). This simulation allows us to study the effect of neglecting the term related to heat transport by convection in the model.

Heat and mass transfer models in metal-hydrogen reactor

Abstract To analyse heat and mass transfer in a metal-hydrogen reactor, the following hypotheses are typically used: (i) solid and gas are at the same temperature, (ii) the effect of pressure variation in the reactor is negligible, (iii) the effect of hydrogen concentration on the equilibrium pressure variation is negligible. In this paper, we test the validity of these hypotheses by comparing numerical results obtained with and without these hypotheses. For a LaNi5-hydrogen reactor, the results show that these hypotheses are valid under almost all conditions.

Control volume finite element numerical simulation of mixed convection in an air-cooled cavity

The knowledge of flow structure and heat transfer by mixed convection in an open cavity is of interest in relation to a number of physical and technological applications such as ventilation, heat or pollution agent clearance, and electronic cooling. This paper presents a numerical study of transient mixed convection of laminar flows in an air-cooled cavity in which a fluid is injected at a temperature lower than the initial temperature of the cavity. A control volume finite element method (CVFEM) using triangular elements is employed to discretize the governing equations. The performance of the numerical algorithm is first tested. In order to investigate the quality of the ventilation and cooling, the dynamic and thermal fields are numerically determined for a large range of Reynolds and Richardson numbers and for different inlet-outlet locations. Cooling efficiency is examined at transient and at steady regimes.

MIXED CONVECTION IN A PLANE CHANNEL WITH A BUILT-IN TRIANGULAR PRISM

The superposition of Von Karman street and convective cells in a horizontal plane channel containing a triangular prism and heated from below constitute the principal subject of this numerical investigation. The numerical scheme is based on the control volume finite element method (CVFEM) adapted to the standard staggered grid with the SIM PLER algorithm for pressure-velocity coupling and an Alternating Direction Implicit (ADI) scheme for the time integration. Many standard test flows are simulated successfully. Results are obtained for a Grashof number ranging from 0 to 1.5 10 4 at Pr 0.71 and Re 100 with constant physical properties. At the outlet of the computational domain a convective boundary condition (CBC) is used. Results are presented to show the effect of development of convective cells on the flow pattern and on the Strouhal number. Regarding the heat transfer rate, we focus on the effect of the presence of the triangular prism on the heat flux transferred from the hot wall to the flow.The superposition of Von Karman street and convective cells in a horizontal plane channel containing a triangular prism and heated from below constitute the principal subject of this numerical investigation. The numerical scheme is based on the control volume finite element method (CVFEM) adapted to the standard staggered grid with the SIM PLER algorithm for pressure-velocity coupling and an Alternating Direction Implicit (ADI) scheme for the time integration. Many standard test flows are simulated successfully. Results are obtained for a Grashof number ranging from 0 to 1.5 10 4 at Pr 0.71 and Re 100 with constant physical properties. At the outlet of the computational domain a convective boundary condition (CBC) is used. Results are presented to show the effect of development of convective cells on the flow pattern and on the Strouhal number. Regarding the heat transfer rate, we focus on the effect of the presence of the triangular prism on the heat flux transferred from the hot wall to the flow.

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.

Evaporation of water by free or mixed convection into humid air and superheated steam

Abstract This paper concerns a numerical analysis of the evaporation of water in pure air, humid air and superheated steam in an externally insulated channel. Results were obtained for mixed and free convection driven by combined thermal and mass buoyancy forces. For natural convection case, the analysis is restricted to situation in which combined buoyancy forces act in the downward direction. The results show that below a certain temperature of the free stream, water evaporation rate decreases as the humidity of air increases and above it this relation reverses. This temperature “inversion point temperature” was treated in previous experimental and numerical studies in the case of forced convection. In this work, particular attention is paid to study the effect of the ambient conditions on evaporation rate of water and the inversion temperature phenomenon in the condition of free and mixed convection.

Evaporation of water by free convection in a vertical channel including effects of wall radiative properties

Abstract The present study consists of a numerical investigation of coupled heat and mass transfers by natural convection during water evaporation in a vertical channel heated symmetrically by a uniform flux density by taking into account radiative heat transfer between the plates. The governing equations are solved numerically by a finite difference method. The spatial profiles of velocity, temperature and moisture are presented. The effect of ambient conditions, channel width and walls radiation are also analysed in this study.

Heat and mass transfer during drying of granular products — simulation with convective and conductive boundary conditions

Abstract Numerical simulation of grain drying in a vertical cylindrical bed has been carried out with specific boundary conditions: evaporation, which occurs with forced air flowing through the bed, is intensified with a conductive heat flux from the wall. We consider two mathematical models of heat and mass transfer through granular medium: two temperature model (No Local Temperature Equilibrium Model: NLTEModel) and one temperature model (Local Temperature Equilibrium Model: LTEModel). The air–grain mass transfer is expressed with a drying rate equation outcome from experiments. The resolution of the equations set is carried out, using a finite volume method. Different effects, such as the effect of the porosity variation on energy and mass transport, are discussed. Sensitivity to external conditions is determined. Finally, validity of local thermal assumption is examined.

Unstructured Control-Volume Finite-Element Method for Radiative Heat Transfer in a Complex 2-D Geometry

Abstract This article describes a new approach based on the control-volume finite-element method (CVFEM) for computing radiative heat transfer in a complex two-dimensional geometry using a general unstructured grid. To examine its accuracy and computational efficiency, five test cases are investigated, and the results obtained agree very well with other published works. In addition, the study presented in this article shows that not only this method is flexible in treating radiative heat transfer in complex geometry, but also that the computer procedure based on this numerical method needs an accurate computer process unit (CPU) time and can be combined easily with developed codes for fluid dynamics.

HEAT AND MASS TRANSFER DURING DRYING IN CYLINDRICAL PACKED BEDS

In this paper, we present a numerical simulation of forced convection drying of granular products. The granular product is in a vertical cylinder opened at both ends and exposed to a constant wall heat flux. We have established a general formulation of heat and mass transfer in the medium. The solution of the equation set is carried out using a finite volume method with axisymmetrical coordinates. Here we study the time-space evolution of temperatures and water content fields, drying kinetics, and heat flux exchange with the exterior. W e have also reported, in the course of this study, the effects of considering intermittent flow (the fluid is periodically blown by the bottom, then by the top of the cylinder).