Search

This study presents 2-D simulations of a flow-through a sudden expansion/contraction micro-channel with the existence of obstacles. The bottom wall is maintained at constant flux, while the other walls are adiabatic. Rectangular adiabatic obstacles are mounted before the expansion region on the upper and lower wall of the channel used. The finite element method was used to discretize the equations that govern the physical model. Results indicate the apparition of a separate vortex, situated in the corner after the sudden expansion of the micro-channel for low Reynolds numbers. For higher values and expansion ratios, the vortex separation length increases. The obtained results show that the obstacles have a considerable effect on the dynamics of the flow and enhancement of heat transfer. [ABSTRACT FROM AUTHOR]

Summary: This article presents a numerical investigation of two discharging procedures of a metal hydride‐hydrogen storage tank (MH‐HST). The former is categorized as "simultaneous procedure" as the heating of the MH‐HST and the discharging of hydrogen from the same start simultaneously. The latter is categorized as "separated procedure" as the heating of the MH‐HST was carried out initially and the discharging of hydrogen from the same began only when the desired desorption temperature is attained. To the best of authors' knowledge, the separated procedure is simulated and compared for the first time with the simultaneous case. For the two procedures, the discharging process was carried out in two ways, that is, at constant and variable applied pressure. A computational model governing heat and mass transfer within the MH‐HST was developed and successfully validated with experimental data. The simulation results ascertain that the simultaneous procedure is faster than the separated procedure. For example, with a desorption pressure of 0.1 bar and a heating fluid temperature of 353 K, the "simultaneous procedure" provides a reduction of the discharging time by 57% in comparison with the "separated procedure." [ABSTRACT FROM AUTHOR]

Summary: This study presents the numerical investigation of the performance improvement of Metal Hydride (MH) bed equipped with nano‐enhanced phase change material (NePCM) jacket for heat reaction recovery via U‐tube heat exchanger using Nanofluid. In this study, Mg2NiH6 is used as an MH bed, sodium nitrate (NaNO₃) as heat storage medium (PCM), and sodium‐potassium nitrate (60% NaNO₃‐40% KNO₃) as a heat transfer fluid (HTF). A computational model for this system is developed and validated. Furthermore, the hydriding of the MH bed, the melting of NePCM, and the overall heat transfer characteristics are simulated. With this model, the effect of a few types of nanoparticle additives on the hydriding process is investigated. The nanoparticles of iron oxide (Fe2O3) and copper oxide (CuO) are dispersed at various concentrations (to a range of 0‐1.5 wt.%) in the HTF. Further, CuO and graphene nanoparticles are added to the PCM to a range of 0 to 5 wt.%. The simulation shows that a reduction of hydriding process time by 33.5% is possible by using this MH‐Nanofluid‐NePCM system in comparison with a conventional MH bed system. The use of nanoparticles in the HTF, at an optimum flow rate, did not show a significant effect on the enhancement of the hydriding process. Also, the results show that CuO‐PCM composite has a greater impact in decreasing the time of complete hydriding relative to graphene‐PCM composite. [ABSTRACT FROM AUTHOR]

Latent heat thermal energy storage (LHTES) systems and their applications have been very substantive for the developments in energy science and engineering. The efficiency of LHTES systems largely depends on the thermal conductivity of the phase change materials (PCMs) and the heat transfer mechanisms in them. This review focuses on the methods employed to enhance heat transfer in LHTES systems which accordingly improve their storage performance. This includes the possible geometrical configurations of LHTES systems and the effects of their design parameters. The various methods adopted for enhancing LHTES systems' performance through either applying nanomaterials additives, using cascaded or encapsulated PCMs, or employing extended surfaces, such as fins, are discussed in detail. Additionally, various designs of PCM based heat exchangers with active and passive heat transfer techniques were highlighted. These systems are critically analyzed to help selecting reliable techniques and compatible materials for effective designs in order to achieve more efficient LHTES systems. This review would be helpful for the researchers to further develop heat transfer intensification mechanisms in LHTES systems. [ABSTRACT FROM AUTHOR]

Summary: The efficacy of integrating organic coconut oil (co‐oil) phase‐change material (PCM) into an unconditioned building with a lightweight envelope is explored experimentally and numerically for heat gain reduction. In what we think is the first test of its kind for co‐oil PCM, twin side‐by‐side single‐room buildings (with and without PCM) are constructed and studied experimentally under ambient weather conditions. The effects of the co‐oil on the thermal performance of the buildings are investigated with respect to the window orientation (east, west, north and south). Furthermore, numerical simulation of the buildings is carried out to evaluate the contribution of the co‐oil to their thermal performance and to determine the effect of the co‐oil layer thickness on the heat storage capacity. Moreover, by employing a simplified heat transfer analysis, an approximate relation for the optimal thickness of co‐oil PCM layer is developed. Experimental and numerical results show that co‐oil PCM can be a promising solution to improve the indoor thermal environment. It is found that with a south‐facing window equipped with co‐oil PCM, the indoor temperature is lowered by 23.8% compared to the case without PCM, when an optimal PCM layer of ~4 cm in thickness is embedded in the wall. [ABSTRACT FROM AUTHOR]

I n the present work, a high temperature Metal Hydride Water Pumping System (MHWPS) equipped with a latent heat exchanger was investigated numerically. The operating concept of the pump was presented and the mathematical model of heat and mass transfer within the pump was established. We simulated the pump under different operating conditions using the alloy Mg 2 Ni as a metal hydride and the KNO 3 as a Phase Change Material (PCM). The obtained results have shown that i) the developed numerical model is flexible and accurate in predicting the dynamic behavior of the pump ii) the numerical model without radiative heat transfer gives results with errors that can reach 38%, particularly for pumping time iii) the integration of the PCM provides a reduction in the pumping time of about 90% and an increase of the efficiency of the pump of about 7.6 times compared to the case without PCM which represents an improvement of 86% and iv) the Mg 2 Ni alloy requires high temperature and a mass of PCM about 9 times larger than the case of LaNi 5 alloy to pump a volume 6.5 times greater than that pumped with LaNi 5. • A 2D mathematical model is developed for a MHWPS. • Effect of radiative heat transfer on the performance of the MHWPS is evaluated. • Effect of integrating PCM on the performance of the MHWPS is studied. • A comparison between MHWPSs (with and without PCM) is carried out. • A comparison between Mg 2 Ni and LaNi 5 pumps is carried out. [ABSTRACT FROM AUTHOR]

The purpose of the present paper is testing an in‐house efficiency algorithm based on lattice Boltzmann method (LBM) and using it to resolve the obtained coupled nondimensional governing equations to analyze two‐dimensional free convection inside a cold outer cavity subjected to a heated cylindrical diamond array. Steady state or oscillatory results are obtained using the Bhatnagar‐Gross‐Krook collision model associated to the thermal LBM. Both the velocity and temperature fields are solved using the D2Q9 models. With different Rayleigh numbers (Ra), the tested free convection can either achieve to steady state or oscillatory. We extended our in house Fortran 90 code using curved boundary conditions and implemented them into a cavity with a diamond array. The numerical simulations were done using distinct Ra (106 and 10 7) and distances 0.3≤ε≤0.7 between the four neighboring circular cylinders aligned in a diamond array. The effects of several physical parameters, including Ra and position of the hot body array on flow and heat transfer characteristics are investigated. The obtained results are highlighted in the form of streamlines, isotherms, and velocities plots. We show in this paper the stability and the efficiency of the LBM to deal with a complex geometry and its ability to reach suitable convergence criteria for high Ra (10 6 and 10 7). The numerical results indicate that LBM can simulate numerical problems with a high Ra reaching a steady state where we can depict the change of the flow pattern and enhancement of the heat transfer in the presence of heated diamond array. [ABSTRACT FROM AUTHOR]

In this paper, coupled heat and mass transfer during the desorption process of a metal-hydrogen reactor (LaNi5-H2) is numerically investigated. To predict the dynamic behavior of this reactor, a new algorithm based on the lattice Boltzmann method (LBM) is proposed as a potential solver. Based on this algorithm, a computer code is developed using fortran 90. This algorithm is validated successfully by comparison with experimental data reported in the literature and results obtained by finite volume method (FVM). Using the developed code, the time-space evolutions of the temperature and the hydride density within the reactor are presented. In addition, the effect of some parameters (applied pressure, heating temperature, and overall heat transfer coefficient) on the dynamic behavior of the reactor is evaluated. Compared to the FVM, the proposed algorithm presents simple implementation on a computer and with reduced CPU time. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a numerical study of coupled heat and mass transfer during the desorption process of metal–hydrogen reactor (Mg2Ni–H2), is presented. Analytical expressions describing, the reaction kinetic and the equilibrium pressure of the Mg2Ni-H2 system have been determined and integrated into a theoretical model that describes the dynamic behavior of the reactor. This model, which takes into account radiative heat transfer, is solved by the control volume finite element method (CVFEM). The numerical simulation is used to present the time–space evolutions of the temperature and the hydride density within the reactor and to evaluate the effect of radiative heat transfer and the governing operating parameters (outlet pressure, temperature of heating fluid, heat exchange coefficient) on the dynamic behavior of the reactor. In addition, a new geometric configuration of the reactor is proposed and simulated. [Copyright &y& Elsevier]

A new algorithm based on the lattice Boltzmann method and the control volume finite element method is proposed as an hybrid solver for two dimensional transient conduction and radiation heat transfer problems in an optically emitting, absorbing and scattering medium. The lattice Boltzman method was used to solve the energy equation and the control volume finite element method was used to compute the radiative information. The advantages of the proposed methodology is to avoid problems that confronted when previous techniques are used to predict radiative heat transfer, essentially, in complex geometries and when there is scattering and or non black boundaries surfaces. This method combination, which is applied for the first time to solve this unsteady combined mode of heat transfer, has been found to accurately predict the effects of various thermophysical parameters such as the scattering albedo, the conduction radiation parameter and the extinction coefficient on temperature distribution. The results of this method combination were found to be in excellent agreement with the lattice Boltzmann collapsed dimension method this proposed numerical approach include, among others, simple implementation on a computer, accurate CPU time, and capability of stable simulation. [ABSTRACT FROM AUTHOR]

A three-dimensional inverse radiation problem of parameter estimation in absorbing, emitting and scattering medium is solved, given temperature profile. The radiative transfer equation is solved by the control volume finite element method (CVFEM) and the Levenberg Marquardt method (LMM) is used to minimize an objective function, expressed by the sum of square residuals between calculated and experimental data. Also, the effects of initial guesses, measurement error and number of measurement points are examined. It is shown that the present technique is robust and can be used to make reliable predictions of the radiative parameters. [ABSTRACT FROM AUTHOR]

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 Darcy''s 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. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

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 LaNi5 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). [Copyright &y& Elsevier]

In this paper, a new hybrid numerical algorithm is developed to solve coupled convection-radiation heat transfer in a two-dimensional cavity containing an absorbing, emitting, and scattering medium. The radiative information is obtained by solving the radiative transfer equation (RTE) using the control volume finite element method (CVFEM), and the density, velocity, and temperature fields are calculated using the two double population lattice Boltzmann equation (LBE). To the knowledge of the authors, this hybrid numerical method is applied at the first time to simulate combined transient convective radiative heat transfer in 2D participating media. In order to test the efficiency of the developed method, two configurations are examined: (i) free convection with radiation in a square cavity bounded by two horizontal insulating sides and two vertical isothermal walls and (ii) Rayleigh-Benard convection with and without radiative heat transfer. The obtained results are validated against available works in literature, and the proposed method is found to be efficient, accurate, and numerically stable. [ABSTRACT FROM AUTHOR]

The integration of a phase change material (PCM) with a metal hydride (MH) reactor has received considerable attention recently. In such a system, the exothermic and endothermic processes of the MH reactor can be utilized effectively by enhancing the thermal exchange between the MH reactor and the PCM bed. In this study, a novel design that integrates the MH reactor with cascaded PCM beds is proposed. Magnesium nickel (Mg 2 Ni) alloy is used as the hydride reactor. Two different types of PCMs with different melting temperatures and enthalpies are arranged in series. A parametric study is carried out to identify the optimum distribution of the different PCMs. The results indicate that the proposed cascaded MH-PCM sandwich design improves the heat transfer rate which consequently shortens the time duration of the hydrogenation and dehydrogenation processes by 26% and 51%, respectively, compared to an MH-PCM sandwich design that includes only a single PCM. Image 1 • A new design of a metal hydride reactor equipped with cascaded PCMs was proposed. • Cyclic behaviors of the new design were investigated. • The effect of the PCMs' storage capacity distribution in the new design was studied. • The hydrogenation/dehydrogenation times were reduced by 26% and 51%, respectively. [ABSTRACT FROM AUTHOR]

Phase change material (PCM) is integrated with a metal hydride (MH) storage reactor to enhance its performance. The main function of the PCM is to store the heat released by the MH reactor during the hydrogenation process and reuse it later during the dehydrogenation process. In such a system, the exothermic/endothermic reactions are highly dependent on the heat transfer rate between the MH reactor and the PCM bed. To increase the heat transfer rate, a new configuration of the MH-PCM storage system is proposed. The new configuration consists of an MH reactor encircled by a cylindrical sandwich bed packed with PCM. The results indicate that the proposed configuration of the MH-PCM system improved the heat transfer rate which accordingly reduced the time duration of the hydrogenation and dehydrogenation processes by 81.5% and 73%, respectively, compared to a conventional MH-PCM system that includes only a single PCM bed. Image 1 • A new configuration of a metal hydride reactor equipped with PCM was proposed. • The new configuration consists of an MH reactor encircled by PCM sandwich bed. • The effect of the PCM volume distribution in the new configuration was investigated. • The hydrogenation/dehydrogenation times were reduced by 81.5% and 73%, respectively. [ABSTRACT FROM AUTHOR]

It is known that the hydrogen has a very high mass energy density, in fact, that it is a lightest gas; therefore, its storage is a great problem. The aim of the hydrogen storage technologies is thus to reduce the volume that hydrogen occupies in its thermodynamically stable state under conditions close to ambient salt. Recent work on hydrogen storage is mainly based on the use of metal hydrides. These metal hydrides have a high capacity for the hydrogen storage in the operating conditions. The effecting parameters on the performance of such a metal-hydrogen reactor are its design and configuration. In this case, there are a number of problems that need to be considered in designing a reactor. Among these parameters are the reactor configuration, the thermal and the mechanical strength, the kinetics of hydrogen storage and the security. Our study is concentrated on the problem of the thermal and the mechanical strength while focusing on the nature of the metal makes the reactor. In this work, the experimental studies of the hydrogen absorption phenomenon in different reactors, based on metal hydrides, were evaluated. The characteristics of the reaction kinetics in three different reactors using the same measurement conditions were compared. A numerical model describing the reaction kinetic of the H 2 absorption by LaNi 5 alloy validates the results were obtained. Of these results, it is found that the rate constant varies from one reactor to another. Moreover, the activation energy of the absorption kinetics were identified. [ABSTRACT FROM AUTHOR]

The objective of this work is to study experimentally the behavior of a metal–hydrogen reactor (MHR) during hydrogen desorption by the LaNi 5 hydride. The reactor is surrounded by a small coil traversed by an alternative sinusoidal current. The effect of the voltage applied to the coil is studied. Two kinds of tests are done. In the first one, the reactor is heated to the desired temperature and then it was put in contact with a tank at weak pressure (desorption), and in the second one, desorption and heating start simultaneously. Desorbed mass and temperature evolution within the hydride bed and in the reactor wall, as a function of time, are plotted. A comparison of results obtained by the electromagnetic induction heater and by heat exchanger provisioned by a hot fluid shows an improvement of the efficiency of the reactor when compared to the traditional hot system (fluid system). [ABSTRACT FROM AUTHOR]