Sassi Ben Nasrallah

Experimental and theoretical study of ametal–hydrogen reactor

Abstract An experimental and theoretical study of a metal–hydrogen reactor (LaNi5–H2) is presented. The first goal of this study is to experimentally determine the effectivethermal conductivity, the conductance between the hydride bed and the fluid around the reactor,the equilibrium pressure and the expression of the reaction kinetics, taking into account the initialcondition, the temperature and the applied hydrogen pressure temporal evolution. The secondgoal is to test the validity of the theoretical model by comparison between theoretical andexperimental results.

Study of two-dimensional and dynamic heat and mass transfer in a metal–hydrogen reactor

Abstract To analyse heat and mass transfer in a metal–hydrogen reactor, the hypothesis that disregards the radiative heat transfer in the reactor, is typically used. In this paper, we take into account the radiative heat transfer and we test the validity of this hypothesis in the case of the LaNi 5 and in the case of the magnesium. A theoretical model is conducted for the two-dimensional system where conduction, convection radiation and chemical reaction take place simultaneously. This model is solved by the finite volume method. The numerical simulation is used to present the time–space evolutions of the temperature and the hydride density in the reactor and to determinate the sensitivity to some parameters (absorption coefficient, scattering coefficient, reactor wall emissivity).

Two-dimensional laminar fluid flow and heat transfer in a channel with a built-in heated square cylinder

A numerical investigation was conducted to analyze the unsteady flow field and heat transfer characteristics in a horizontal channel with a built-in heated square cylinder. Hydrodynamic behavior and heat transfer results are obtained by the solution of the complete Navier–Stokes and energy equations using a control volume finite element method (CVFEM) adapted to the staggered grid. The Computation was made for two channel blockage ratios (β=1/4 and 1/8), different Reynolds and Richardson numbers ranging from 62 to 200 and from 0 to 0.1 respectively at Pr=0.71. The flow is found to be unstable when the Richardson number crosses the critical value of 0.13. The results are presented to show the effects of the blockage ratio, the Reynolds and the Richardson numbers on the flow pattern and the heat transfer from the square cylinder. Heat transfer correlation are obtained through forced and mixed convection.

Numerical investigation of forced convection in a plane channel with a built-in triangular prism

Abstract Structure of laminar flow and heat transfer, in a two-dimensional horizontal channel differentially heated, with a built-in triangular prism is investigated from the numerical solutions of complete Navier–Stokes and energy equations. The numerical scheme is based on Control Volume Finite-Element Method with the SIMPLER algorithm for pressure-velocity coupling. Many standard test flows are successfully simulated. Results are obtained for Reynolds number ranging from 20 to 250 at Pr= 0.71 with constant physical properties. The flow is especially studied in details for two Reynolds numbers, Re= 30 as a sample of the symmetric flow, and Re= 100 as a sample for the periodic flow. Results are presented to show how the presence of such bluff body affects the flow pattern and the heat transfer from the hot bottom plate to the flow in both cases, symmetric and periodic flows.

Prediction of transient heat and mass transfer in a closed metal–hydrogen reactor

Abstract The metal–hydrogen reactor is usually composed of a porous medium (hydride bed) and an expansion volume (gaseous phase). During the sorption process, the hydrogen flow and the heat transfer in the expansion part are badly known and can have some effects on the sorption phenomena in the hydride medium. At our knowledge, the hypothesis that neglects those effects is typically used. In this paper, a 2D study of heat and mass transfer has been carried out to investigate the transient transport processes of hydrogen in the two domains of a closed cylindrical reactor. A theoretical model is conducted and solved numerically by the control-volume-based finite element method (CVFEM). The result on temperature and hydride density distribution are presented and discussed. Moreover, this paper discusses in detail the effects of some governing operating conditions, such as dimensions of the expansion volume, height to the radius reactor ratio, and the initial hydrogen to metal atomic ratio, on the evolution of the pressure, fluid flow, temperature and the hydrogen mass desorbed.

Dynamic behavior of metal–hydrogen reactor during hydriding process

Abstract The present study involves numerical simulation of transient transport of hydrogen and heat within a metal–hydrogen reactor connected to a hydrogen tank during the hydriding process. This problem is of particular interest in the design of many installations in the field of energy technology (compressor, heat pumps, thermal or hydrogen storage systems). The reactor presents an expansion volume for hydrogen. The hydrogen flow is described by general momentum conservation equations instead of Darcys law. The evolutions of the temperature, of the hydrogen concentration and of the hydrogen flow velocity are presented. The effects of the reactor dimensions, the inlet diameter, the volume of the expansion part, the tank volume, the initial pressure and the amount of hydrogen in the tank, on the heat and hydrogen transfer are determined.

Thermodynamic analysis of the Stirling heat engine with regenerative losses and internal irreversibilities

Abstract A model of an irreversible cycle of the Stirling heat engine, using air as the working substance, has been established. Several irreversibilities due to the non-adiabatic regenerator dead volumes of hot space, cold space, and regenerator that the regenerator effective temperature is an arithmetic mean of the heater and cooler temperature. In addition, the general irreversibilities of the non-quasistatic cycle are taken into account. Numerical simulation is performed and the effects of the regenerator effectiveness and dead volumes are studied. Some fundamental optimal relations and general performance characteristic curves of the cycle are obtained. The results from this study indicate that the engine net work is affected by the cycle irreversibilities, while the heat input and engine efficiency are affected by both the regenerator effectiveness and dead volumes. It is concluded that the system operating temperature and the overall system efficiency depend on the internal irreversibility of the heat engine.

Mass and heat transfer during water evaporation in laminar flow inside a rectangular channel — validity of heat and mass transfer analogy

Abstract A detailed numerical analysis concerning the mechanism of heat and mass transfer during water evaporation in a two-dimensional steady laminar flow of dry air or air–vapor mixture in a horizontal channel is studied. The gas is considered as absorbing, emitting and nonscattering medium with variable thermophysical properties. The results show the effect of different state variables on the coefficients of heat and mass transfer and the domain where the analogy between the heat and mass transfer is valid. They also show the effect of the thermal radiation on the ratio between Sherwood and Nusselt numbers. The comparison between the present results and those obtained in previous published studies [32–34] features to a satisfactory agreement.

Numerical Analysis of Natural Buoyancy-Induced Regimes in Isosceles Triangular Cavities

This article deals with a numerical simulation of natural-convection flows using the control-volume finite-element method in isosceles triangular cavities, submitted to a uniform heat flux from below when inclined sides are maintained isothermal, without symmetry assumptions for the flow structure. The aim of the study is to examine a pitchfork bifurcation occurrence. The study provides useful information on the thermal exchange sensitivity to two governing parameters, the Rayleigh number and the tilt angle in a basin still receiving a uniform heat flux. Results show that the heated wall is not isothermal and the flow structure is sensitive to the cover tilt angle. Three regimes with two vortices or more, symmetrical or asymmetrical, can be observed. Results can be constructive for design enhancements in energy systems such as solar water distillers and air conditioning processes.

Heat Transfer Between a Porous Layer and a Forced Flow: Influence of Layer Thickness

The heat transfer between a forced flow of a pure fluid phase and a porous medium is theoretically studied. A unified enthalpy model is used to investigate the transient temperature field in the pure fluid phase and in the porous medium. Possible transition from saturated to nonsaturated porous medium is considered. From the temperature field obtained, the time variation of the local temperature and convective heat transfer coefficient at the interface porous-pure fluid regions are studied. Numerical results provide also the parametric information concerning effects of the thickness of the porous layer on theses parameters at the transient and the steady regimes.