Experimental formula for estimating porosity in a metal hydride packed bed

An experimental formula for estimating porosity in a metal hydride packed bed is presented. The formula was developed by direct observation of the volume changes of a metal hydride packed bed under free expansion in a vessel. The experimental results showed that the cycles of expansion and contraction were repeated at large porosities above 60% after a rapid state change caused by early particle breakup. The formula for porosity was expressed as a function of the reacted fraction and as a function of the cycle number. The function formula of the reacted fraction can be used to compute different values of porosity for expansion by absorption and for contraction by desorption. The coefficients assuming 100% hydrogen storage based on the experiments with LaNi₅ were an expansion ratio of 16.7% and a contraction ratio of 8.4%, on average. This experimental porosity formula is useful for effective thermal conductivity calculations and for numerical simulations of metal hydride packed bed behavior.     

Experimental study of porosity and effective thermal conductivity in packed bed of nano-structured FeTi for usage in hydrogen storage tanks

The porosity and effective thermal conductivity, which were measured simultaneously, changed owing to expansion during hydrogen absorption and contraction during hydrogen desorption. The metal hydride used in the experiment was a newly developed nano-structured FeTi (n-FeTi) by mechanical alloying. The porosity was measured by directly observing the changes in the volume of a metal hydride packed bed under free expansion. In the authors' previous study of LaNi₅, the volume of the packed bed gradually increased, whereas the total volume of the n-FeTi packed bed decreased with increase in the cycles of expansion and contraction. The effective thermal conductivity measurements of the metal hydride packed bed were carried out using a cylindrical device with heating from a central heater. The results showed that the effective thermal conductivity of n-FeTi changes from 0.4 to 1.1 W/mK with rise in reacted fraction and decrease in porosity.     

相变储能堆积床传热特性的研究

基于多孔介质传热理论,建立了储能堆积床的传热模型.在此基础上分析了换热流体流量、温度、单元尺寸、相变材料导热系数和孔隙率等参数对堆积床传热特性的影响.研究表明,提高相变材料的导热系数,增大换热流体温差或减小单元尺寸对堆积床传热速率有明显提高,而提高换热流体流量,增大对流换热系数对传热速率的影响不明显.因而强化相变材料侧的传热过程是提高堆积床传热速率的有效途径.     

Simulation of Effective Thermal Conductivity of Metal Hydride Packed Beds

Several mathematical models are available for estimation of effective thermal conductivity of nonreactive packed beds. Keeping in view the salient differences between metal hydride beds in which chemisorption of hydrogen takes place and conventional nonreactive packed beds, modified models are proposed here to predict the effective thermal conductivity. Variation in properties such as solid thermal conductivity and porosity during hydrogen absorption and desorption processes are incorporated. These extended models have been applied to simulate the effective thermal conductivity of the MmNi_4.5Al_0.5 hydride bed and are compared with the experimental results. Applicability of the extended models for estimation of the effective thermal conductivity at different operating conditions such as pressure, temperature, and hydrogen concentration is discussed.     

Heat transfer characteristics of the metal hydride vessel based on the plate-fin type heat exchanger

Heat transfer characteristics of the metal hydride vessel based on the plate-fin type heat exchanger were investigated. Metal hydride beds were filled with AB₂ type hydrogen-storage alloy’s particles, Ti_0.42Zr_0.58Cr_0.78Fe_0.57Ni_0.2Mn_0.39Cu_0.03, with a storage capacity of 0.92 wt.%. Heat transfer model in the metal hydride bed based on the heat transfer mechanism for packed bed proposed by Kunii and co-workers is presented. The time-dependent hydrogen absorption/desorption rate and pressure in the metal hydride vessel calculated by the model were compared with the experimental results. During the hydriding, calculated hydrogen absorption rates agreed with measured ones. Calculated thermal equilibrium hydrogen pressures were slightly lower than the measured hydrogen pressures at the inlet of metal hydride vessel. Taking account of the pressure gradient between the inlet of metal hydride vessel and the metal hydride bed, it is considered that this discrepancy is reasonable. During the dehydriding, there were big differences between the calculated hydrogen desorption rates and measured ones. As calculated hydrogen desorption rates were lower than measured ones, there were big differences between the calculated thermal equilibrium hydrogen pressures and the measured hydrogen pressures at the inlet of metal hydride vessel. It is considered that those differences are due to the differences of the heat transfer characteristics such as thermal conductivity of metal hydride particles and porosity between the assumed and actual ones. It is important to obtain the heat transfer characteristics such as thermal conductivity of metal hydride particles and porosity both during the hydriding and dehydriding to design a metal hydride vessel.     

Determination of the thermal properties of ceramic sponges

The knowledge of thermal properties of technical components or internals in chemical reactors is often a key characteristic for planning and designing chemical engineering processes. As an alternative to packed beds or packings, sponges turned out to be used in new application fields in chemical and process engineering. Therefore an experimental study was performed to investigate the two-phase thermal conductivity of solid ceramic sponges made of alumina, mullite and oxidic-bonded silicon carbide (OBSiC) at moderate temperatures. A two-dimensional model is used for analysing the measured temperature profiles and for calculating the thermal conductivity. It can be observed, that the thermal conductivity increases with decreasing porosity and is nearly constant when the pore size (ppi number) is varied. The thermal conductivity data are modelled by an approach similar to the well known Krischer model. Compared to a packed bed of spherical particles, the values of the thermal conductivity of sponges turn out to be about five times higher.     

含湿非饱和多孔介质的完全分形传热研究

为了探究在含湿情况下多孔介质有效导热率的变化,基于分形理论,考虑多孔介质在含湿时加热过程中相变的影响,结合加热过程中的热量守恒方程和傅里叶导热定律推导出计算有效导热率的新公式。将该模型相关数据代入进行计算,分析了孔隙率、含湿率、面积分形维数和迂曲分形维数对有效导热率的影响。研究发现,孔隙率与有效导热率呈负相关,含湿率与有效导热率呈正相关,分形维数与有效导热率呈负相关。该研究能够反映多孔介质内的传热进程,对于探究微孔结构物质的传热具有一定的指导意义。     

Kinetic modeling and simulation of catalyst pellet in the high temperature sulfuric acid decomposition section of Iodine-Sulfur process

In the iodine-sulfur (I-S) thermochemical process for hydrogen production, the most endothermic high temperature step is the sulfuric acid decomposition which is catalytic process. Here, we introduce a model-based framework to address the effect of model uncertainty on catalyst identification, designing and optimization. To investigate the kinetic parameters, the experiments are performed in the packed bed reactor under the conditions of diffusion free regime. Further, the kinetic model is implemented in a two-dimensional heterogeneous model of the catalytic packed bed reactor developed in Multiphysics software in order to investigate the effects of catalyst composition, support material, catalyst pellet shapes, sizes (i.e., diffusional length), pellet intrinsic properties (i.e., porosity and thermal conductivity) and temperature on the catalyst effectiveness factor. The 7-hole cylindrical shapes with 0.75 and 1.5 mm principle dimension at 1073 K has been found to have negligible diffusional limitation and temperature gradients across their cross sections.     

Effects of porosity, pumping power, and L/D ratio on the thermal characteristics of an N2O catalytic igniter with packed bed geometry

In this study thermal characteristics of an N₂O catalytic igniter with packed bed geometry which is recently introduced as a hybrid ignition system for small satellites are theoretically considered. For the purpose a so-called porous medium approach has been opted for modeling the N₂O catalytic igniter with packed bed geometry. Using the Brinkman-extended Darcy equation with Ergun term for fluid flow and the one-equation model for heat transfer, both velocity and temperature distributions are presented. To ensure the validity of the approach, the calculated wall temperature solutions have been compared with previous experimental data. Based on the results, parameters of engineering importance such as the porosity, the pumping power and the ratio of length to diameter of the catalytic igniter are identified. Their effects on thermal performance of the catalytic igniter are systematically presented. Finally an effective volume flow rate has been defined to determine the optimum values of the parameters capable of delivering maximum catalyst performance.     

Controlled degradation of highly compacted sodium alanate pellets

Compaction of sodium alanate doped with 3 mol% titanium chloride (TiCl₃) into rigid cylindrical pellets improves thermal conductivity, density and volumetric hydrogen capacity of a traditionally poorly conductive material. However, hydrogen cycling of alanate pellets results in significant expansion which counteracts the advantages of compaction. Restricting the area in which pellets can expand into minimizes these losses with no adverse effect to cycling capacity. Confined pellets had a 50% less decrease in density over 30 cycles, 5 times greater thermal conductivity within 10 cycles and maintain structural integrity through 50 cycles compared to free pellets. In addition, pellets within mechanical confinement fused into a rigid stack within the first few hydrogen cycles thereby reducing surface contact resistance between pellets by 3.5 times. Improved thermal conductivity and heat transfer through a pellet bed of materials such as complex metal hydrides, is a key aspect for on-board storage applications.     

Thermal management of metal hydride hydrogen storage reservoir using phase change materials

Metal hydride hydrogen storage reservoir should be carefully designed to achieve acceptable performance due to significant thermal effect on the system during hydriding/dehydriding. Phase change materials can be applied to metal hydride hydrogen storage system in order to improve the system performance. A transient two-dimensional axisymmetric numerical model for the metal hydride reservoir packed with LaNi₅ has been developed on Comsol platform, which was validated by comparing the simulation results with the experiment data from other work. Then, the performances of metal hydride hydrogen storage reservoir using phase change materials were predicted. The effects of some parameters, such as the thermal conductivity, the mass and the latent heat of fusion of the phase change materials, on the metal hydride hydrogen storage reservoir were discussed. The results shown that it was good way to improve the efficiency of the system by increasing the thermal conductivity of phase change materials and selecting a relatively larger latent heat of fusion. Due to the relatively lower thermal conductivity of phase change materials, different metal foams were composited with the phase change materials in order to improve the heat transfer from the metal hydride bed to the phase change materials and the hydrogen storage efficiency. The effect of aluminium foam on the metal hydride reservoir was studied and validated. The phase change materials composited with copper foam shown better performance than that composited with aluminium foam.     

Effect of geometrical and thermophysical characteristics of bed materials on the enhancement of thermal performance of packed bed solar air heaters

In this experimental investigation, a packed bed solar air heater has been designed, fabricated and tested under the local weather conditions of Roorkee, India. Data were obtained from May to June 1992. Tests were conducted to cover a wide range of influencing parameters,including the geometrical and thermophysical characteristics of absorber matrices, mass flow rates and input solar energy fluxes under actual outdoor conditions. The effects of these parameters on the thermal performance have been investigated, and the results have been compared with those of flat plate (plane) collectors. Based on thermal performance, the woven screen of different geometry (having the lowest values of bed thickness to element size ratio, bed porosity and extinction coefficient) and the copper woven screen (having highest value of thermal conductivity) have been found to be the best absorber matrices for packed bed solar air heaters. It is observed that the performance of the collector improves appreciably as a result of packing its duct with blackened absorber matrices, and this improvement is a strong function of the bed and operating parameters.     

Heat and mass transfer studies on metal-hydrogen reactor filled with MmNi4.6Fe0.4

A transient, three-dimensional computational investigation of coupled heat and mass transfer in an annular cylindrical hydrogen storage tank, equipped with fins and filled with MmNi_4.6Fe_0.4, is presented. The effects of different parameters such as length, thickness and thermal conductivity of fins and overall heat transfer coefficient on the hydrogen storage performance of the tank are studied. The predicted hydrogen storage capacity at different supply pressures showed good agreement with the experimental data reported in the literature. In addition, it is observed that the use of fins enhances heat transfer within the hydride bed and consequently 40% improvement of the time required for 90% storage can be achieved over the case without fins.     

Numerical heat transfer analysis of the packed bed latent heat storage system based on an effective packed bed model

Due to the complexity of the fluid flow and heat transfer in packed bed latent thermal energy storage (LTES) systems, many hypotheses were introduced into the previous packed bed models, which consequently influenced the accuracy and authenticity of the numerical calculation. An effective packed bed model was therefore developed, which could investigate the flow field as the fluid flows through the voids of the phase change material (PCM), and at the same time could account for the thermal gradients inside the PCM spheres. The proposed packed bed model was validated experimentally and found to accurately describe the thermo-fluidic phenomena during heat storage and retrieval. The proposed model was then used to do a parametric study on the influence of the arrangement of the PCM spheres and encapsulation of PCM on the heat transfer performance of LTES bed, which was difficult to perform with the previous packed bed models. The results indicated that random packing is more favorable for heat storage and retrieval as compared to special packing; both the material and the thickness of the encapsulation have the apparent effects on the heat transfer performance of the LTES bed.     

低体积分数水基SiO2纳米流体沸腾换热关键物性参数研究

在不添加任何分散剂和改变pH值的情况下,通过两步法将比表面积为150 m~2/g的气相SiO_2纳米颗粒制备成均匀稳定、透明度高、分散性能好的纳米流体。并对该功能性纳米流体进行了导热系数、黏度、表面张力和壁面接触角的测量。低体积分数下,功能性纳米流体较基液的导热系数几乎没有变化,但黏度却有较大改变。传统固液两相混合物黏度模型不再适用功能性纳米流体的计算,其主要原因是传统公式低估了分子间作用力对纳米流体黏度的影响。因此,建立了功能性纳米流体的黏度经验公式。由于纳米颗粒的存在提高了沸腾表面的粗糙度,从而使纳米流体的壁面湿润性能大大提高。实验结果表明,纳米流体的黏性和壁面接触角是沸腾换热发生骤变的关键。     

Effective thermal conductivity and thermal contact resistance of gas diffusion layers in proton exchange membrane fuel cells. Part 1: Effect of compressive load

Heat transfer through the gas diffusion layer (GDL) is a key process in the design and operation of a PEM fuel cell. The analysis of this process requires determination of the effective thermal conductivity as well as the thermal contact resistance associated with the interface between the GDL and adjacent surfaces/layers.In the present study, a custom-made test bed that allows the separation of effective thermal conductivity and thermal contact resistance in GDLs under vacuum and ambient conditions is described. Measurements under varying compressive loads are performed using Toray carbon paper samples with a porosity of 78% for a range of thicknesses. The measurements are complemented by compact analytical models that achieve good agreement with experimental data. A key finding is that thermal contact resistance is the dominant component of the total thermal resistance; neglecting this phenomenon may result in significant errors in evaluating heat transfer rates and temperature distributions.     

Thermal Modeling of Mg2Ni-Based Solid-State Hydrogen Storage Reactor

In this paper, a numerical study of coupled heat and hydrogen transfer characteristics in an annular cylindrical hydrogen storage reactor filled with Mg₂Ni is presented. An unsteady, two-dimensional (2-D) mathematical model of a metal hydride reaction bed of cylindrical configuration is developed for predicting the hydrogen storage capacity. The effect of volumetric radiation is accounted in the thermal model. Effects of hydride bed thickness, initial absorption temperature, hydride bed thermal conductivity, and hydrogen supply pressure on the hydrogen storage capacity are studied. A thinner hydride bed is found to enhance the hydriding rate, accomplishing a rapid reaction. At an operating condition of 20 bar supply pressure and 573 K initial absorption temperature, Mg₂Ni stores about 36.7 g hydrogen per kg alloy. For a given bed thickness and an overall heat transfer coefficient, there exists an optimum value of hydride bed thermal conductivity. The present numerical results are compared with the experimental data reported in the literature, and good agreement was observed.     

多孔介质中高温气体非稳态渗流传热数值计算

针对水平导管中填充颗粒物料层内的高温气体参流传热现象，考虑渗流与传热的相互作用并采用局部非平衡假设建立多孔介质中的瞬态渗流传热物理数学模型。研究不同情况下填充物料中的渗流速度和气固温度分布。计算结果表明，高温热气体对水平导管中移动颗粒料层的热渗透主要发生在渗流入口端区域，随着渗流时间延长，热渗透深度沿导管推进。增大入口渗流速度以及减小出料速度，将导致物料温度沿导管慢速下降，热渗透深度扩大，热渗透作用区域内的物料温度水平提高。在热渗透作用区域，孔隙率对流场和温度场有很大的影响。研究对于高温反应器的颗粒输运和给料器的设计与运行有一定的参考作用。     

An application of a homogenization method to the estimation of effective thermal conductivity of a hydrogen storage alloy bed considering variation of contact conditions between alloy particles

A homogenization method is applied to the estimation of effective thermal conductivity of a hydrogen storage alloy bed. By including the contact conditions between the alloy particles, the effect of contact conditions on the thermal conductivity is investigated.     

Numerical simulations of HI decomposition in packed bed membrane reactors

Numerical simulations to develop an understanding of transport processes inside PBMRs (packed bed membrane reactors) and to evaluate effectiveness of PBMRs in increasing the conversion of HI (hydrogen iodide) decomposition reaction of IS (iodine–sulfur) thermochemical cycle are reported. The computational approach used in the simulations has been validated using the data reported for HI decomposition in a packed bed reactor (PBR). The validated computational approach has been used for parametric studies. Effects of different parameters (temperature, pressure, space velocity, membrane permeability, permselectivity, packed bed porosity and reactor diameter) on HI conversion are reported. The parameters having the maximum impact on the conversion are identified. The findings show that using a PBMR instead of a PBR leads to significant enhancement in conversion and the parameters having high impact on conversion are wall temperature, feed temperature, reactor diameter and packed bed porosity. Based on the findings of parametric studies, ranges of the parameters having maximum impact on conversion are suggested, e.g. the reactor wall temperature is recommended to be in the range of 690–700 K, the bed porosity is recommended to be in the range of 0.2–0.4.