Thermal Conductivity Of Metal Foams Research Articles

Improvement of isothermal characteristic of isothermal chamber by filling with graded copper foam

Due to the isotropy and high thermal conductivity of metal foam, a novel idea of isothermal chamber filled with graded copper foam (GCF) is developed to improve the isothermal characteristic of isothermal chamber. Firstly, an analytical effective thermal conductivity (ETC) prediction model of metal foam is proposed, with relative root-mean-square error (RRMSE) is 6.90%, which has higher ETC prediction accuracy than the semi-empirical models in the literatures. Secondly, the conventional porous random fiber bundle is replaced by the copper foam as the filler material to improve the isothermal characteristic, and this more accurate ETC prediction model of metal foam is used in the numerical simulation of the discharge process to determine the isothermal characteristic of isothermal chamber filled with homogeneous copper foam (HCF). Experiments are conducted to verify the results of simulation. Thirdly, the graded filling method is adopted to form an isothermal chamber filled with GCF, which enhance the heat transfer efficiency and further improve the isothermal characteristic. The results show that, compared with the conventional isothermal chamber filled with homogeneous porous random fiber bundle, the isothermal characteristic of the isothermal chamber filled with GCF is improved by 32.7%.

Numerical investigation and optimization of melting performance for thermal energy storage system partially filled with metal foam layer: New design configurations

Low thermal performance of storage systems represents a barrier to their industrial/engineering application and commercialization. Among all the proposed methods, combination of phase change material with metal foams appears more promising due to the high thermal conductivity of metal foams. However, the insertion of metal foams reduces the PCM volume; hence, a lower amount of stored energy. The present numerical study thoroughly addresses this issue with a focus on the optimization of melting performance for thermal energy storage system partially filled with metal foam layer. A finite volume method based on the enthalpy–porosity technique has been adopted for the numerical simulations. The metal foam location, porosity, and nanoparticle volume fraction were optimized to explore their effects on the melting performance. The results showed that inserting the foam layer diagonally from the top left to the right bottom leads to the lowest melting time. Compared to pure PCM, the melting time increases by 77.7%, while the stored energy decreases by 6.7%. The optimum porosity was found to be 0.88 as it gives approximately the same amount of stored energy as that of pure PCM with a deviation of 4%. Adding nanoparticles to pure PCM increases the melting rate by approximately 8%, while it decreases the stored energy by almost 3%. It is concluded that hybrid systems, i.e., metal foam at an optimum porosity and nanoparticles is more efficient than using each technique separately.

Characteristics of Cold Start Behavior of PEM Fuel Cell with Metal Foam as Cathode Flow Field under Subfreezing Temperature

ABSTRACT Cold start has been realized as an important issue for PEMFC in its global commercialization. As is well-known, the conventional “channel-rib” type flow field will surely lead to several problems, such as water accumulation under the rib, resulting in slow water removal and serious ice blockage in the pores. In recent years, with the fast development of metal foam materials, metal foam has been recognized as a promising alternative replacement of the conventional “channel-rib” type flow field. In this study, a cold start model of PEMFC is established under sub-zero temperature. The coupled transport processes of heat, mass and charge in PEMFC under various subzero temperatures and startup modes are simulated. The results show that ice formation is slower in the PEMFC with metal foam as cathode flow field in cold start process, due to the superior performance of metal foam in water removal and uniform distribution of gas supply. However, the excellent thermal conductivity of metal foam could also result in faster heat loss from PEMFC. Furthermore, ice formation is highly dependent on the supercooled water transfer behavior, though small amount of supercooled water is maintained in the cell. This work aims to provide a systematic analysis for the cold start behavior of PEMFC with metal foam flow field.

Forced convection in finned metal foams: The effects of porosity and effective thermal conductivity

Metal foams are promising candidates for enhancing the thermal performance of heat exchangers due to their unique features such as high surface-area-to-volume ratio and stochastic orientation. However, limited heat dissipation in high porosity metal foams is found to be a significant constraint on the heat transfer enhancement. To overcome this issue, metal foam structures with fins were proposed. On the other hand, the enhancement of heat transfer obtained by the addition of fins is connected to the porosity and effective thermal conductivity of metal foams. This relationship needs to be considered in the design of the structures with finned metal foams properly. This study, accordingly, investigates the effects of porosity and effective thermal conductivity on flow and heat transfer in a channel with finned metal foams. First, a numerical model was established and validated with the experimental data in the literature. Next, the flow and heat transfer characteristics of metal foams with different porosities and effective thermal conductivities were investigated under different fin configurations. The numerical results show that the addition of fins does not yield a considerable increase in the friction factor, and heat transfer shows a considerable increase up to the addition of a certain number of fins. When the fin number is further increased, the enhancement in heat transfer becomes relatively limited for all metal foams considered in this study. The effective fin number depends on the porosity and effective thermal conductivity. The results also show that the contribution of fins to the heat transfer in the parameter range considered in the study varies between 40-115% and 70–150% at the lowest and highest velocities, respectively.

Preparation and effective thermal conductivity of a Paraffin/ Metal Foam composite

In this work, a paraffin- metal foam composite were prepared by the vacuum impregnation method. Aluminum and nickel foams were used. The aim of this study is to characterize the thermal behavior of these composites with a view to their use for the thermal management of Li-ion batteries. First, thermal conductivities and diffusivities in solid state were evaluated using a Hot Disk Thermal Constants Analyzer (TPS 2500S) and the Transient Guarded Hot Plate Technique (TGHPT) was used to determine thermal conductivity in liquid state. Next, the effective thermal conductivity of the composite was modeled as a function of the structure of the foam, the thermal conductivity of the metal foam and the thermal conductivity of the PCM. The results show a high impact of thermal conductivity of metal foam and a minor impact of pore size on the effective thermal conductivity.

Image-based method for evaluation of effective thermal conductivity of metal foam with hollow ligaments

A feature preserving image-based method was used to estimate the effective thermal conductivity of copper metal foam having hollow ligaments. Hollow portions of the ligament and node were identified from X-ray computed microtomography images, through image processing. The effective thermal conductivity was estimated for foams with hollow ligaments filled with water, air, or vacuum. The fluid filling the pore was considered as water or air. Estimations were also conducted for the case where the metal foam is kept in a vacuum condition, where the heat transfer happens only through the ligaments and nodes. The effective thermal conductivity of metal foam with hollow ligaments was compared with that of metal foams with solid ligaments, maintaining the same ligament cross-sectional area. Two metal foams, having total porosity of 90.8% and 90% and hollowness of 4.45% and 1.65% respectively, were selected for the study. The effective thermal conductivity values obtained were compared with the analytical and empirical models available in the literature. Microtomography-based computational fluid dynamics study was also conducted based on the three-dimensional replica of the metal foam, generated from the two-dimensional tomography images. The thermal conductivity obtained from the computational microtomography based study and the image-based method was found to be in good agreement.

Effective thermal conductivity of high porosity open-cell metal foams

In this paper, a model for effective thermal conductivity of high porosity open-cell metal foams is developed by assuming the foam morphology as 3D tetradecahedron structure. The effective thermal conductivity of metal foams is first studied using the numerical simulation method, which the geometrical model (the ratio of the edge length of node to the radius of ligament) is calibrated with the experimental data. The geometrical model is divided into four distinctive layers along the heat transfer direction. The effective thermal conductivity of four layers is then determined by fitting the numerical results. Consequently, the model for effective thermal conductivity of metal foams is proposed based on the Fourier law. Several other analytical models and the experimental data are used to verify the effectiveness of the present model. The results show that the present model can be satisfactorily accurate to calculate the effective thermal conductivity of metal foam by considering the ratio of the edge length of node to the radius of ligament as a second order polynomial function of the porosity. The effective thermal conductivity of each layer is related to the porosity of each layer and overall porosity of metal foam. Compared with the experimental data, the present model has high accuracy with relative RMS deviation of less than 10%. The finding of this study can help guide the thermal design of porous media.

A theoretical model for the effective thermal conductivity of graphene coated metal foams

Thermal management systems (TMS) are an integral part of electronic devices and ongoing developments using porous structures as TMS have revolutionized this field. Porous composites are extensively used for thermal management due to their light weight and high permeability. To maximize the advantages of porous structures, effective thermal conductivity calculations are critical in designing suitable TMS. Recently, we have developed a graphene coated nickel foam – paraffin composite exhibiting an improvement in thermal conductivity that is 23 times greater than pure paraffin. Current theoretical models, however, cannot predict the thermal conductivity of metal foam with the additional thermally conductive coating layer. Herein we report a theoretical model to determine the effective thermal conductivity of graphene coated metal foam saturated with a filling medium. The model is based on the 2D hexagonal structure of graphene coated metal foams. Samples with various combinations of graphene coated metal foams (nickel and copper foams with different porosities) and filling mediums (paraffin waxes and air) were prepared to validate the model. It is found that the effective thermal conductivities calculated by the model are in good agreement with the experimental results, in which the maximum deviation is less than 2%. The derived theoretical model will be helpful in designing passive TMS using porous structure (graphene coated metal foams) for heat dissipation. Parametric analysis on skeleton and graphene area ratios was also conducted. From the analysis, the node of the metal foam should be minimized. By coating the metal foam with graphene, the thermal conductivity can be increased by 4.4 times from 3.69 W/mK to 19.85 W/mK. This shows that the thin graphene coating is very effective in improving the performance of the graphene coated metal foam saturated with filler for thermal management applications.

Effect of thickness and thermal conductivity of metal foams filled in a vertical channel – a numerical study

PurposeThis paper aims to discuss about the two-dimensional numerical simulations of fluid flow and heat transfer through high thermal conductivity metal foams filled in a vertical channel using the commercial software ANSYS FLUENT.Design/methodology/approachThe Darcy Extended Forchheirmer model is considered for the metal foam region to evaluate the flow characteristics and the local thermal non-equilibrium heat transfer model is considered for the heat transfer analysis; thus the resulting problem becomes conjugate heat transfer.FindingsResults obtained based on the present simulations are validated with the experimental results available in literature and the agreement was found to be good. Parametric studies reveal that the Nusselt number increases in the presence of porous medium with increasing thickness but the effect because of the change in thermal conductivity was found to be insignificant. The results of heat transfer for the metal foams filled in the vertical channel are compared with the clear channel in terms of Colburn j factor and performance factor.Practical implicationsThis paper serves as the current relevance in electronic cooling so as to open up more parametric and optimization studies to develop new class of materials for the enhancement of heat transfer.Originality/valueThe novelty of the present study is to quantify the effect of metal foam thermal conductivity and thickness on the performance of heat transfer and hydrodynamics of the vertical channel for an inlet velocity range of 0.03-3 m/s.

Revised cubic model for theoretical estimation of effective thermal conductivity of metal foams

From the database collected by Ranut (2016), we show that the modified cubic lattice (MCL) is capable to predict without fitting parameters the maximum and minimum of the effective thermal conductivity (ETC) of the open cell foam. Based on these theoretical upper and lower bounds (λeff,max and λeff,min), we propose a simple correlation in order to estimate the ETC. This correlation gives a relative low standard deviation between calculated ETC and experimental data, with the majority of ETC data lying in the ±12% region.

A further discussion on the effective thermal conductivity of metal foam: An improved model

In this study, we explain the causes and effects of the geometrical impossible result encountered in the widely adopted tetrakaidecahedron model (Boomsma and Poulikakos, 2001; Dai et al., 2010) for the effective thermal conductivities (ETCs) of metal foam. The geometrical impossible result is successfully eliminated by accounting for the size variation of the node with porosity. The improved model provides predictions of ETCs that are more precise than available models. For aluminum foams (ks=218Wm-1K-1) using water and air as fluid media, the relative root-mean-square (RMS) deviation of the present predictions from the experimental data is about 5.3%; for the reticulated vitreous carbon (RVC) foams (ks=8.5Wm-1K-1), the relative RMS deviation is about 7.4%.

On the effective thermal conductivity of metal foams

Knowing the effective thermal conductivity is essential in order to design a metal foam heat transfer device. Beside the experimental characterization tests, this quantity can be deduced from empirical correlations and theoretical models. Moreover, CFD (Computational Fluid Dynamics) and numerical modeling in general, at the pore scale, are becoming a promising alternative, especially when coupled with a realistic description of the foam structure, which can be recovered from X-ray computed microtomography (μ-CT). In this work, a review of the most relevant correlations and models published in the literature, usable for the estimation of the effective thermal conductivity of metal foams, will be outlined. In addition, a validation of the models with the experimental values available in the literature will be presented, for both air and water as working fluids. Furthermore, the results of a strategy based on μ-CT – CFD coupling at the pore level will be illustrated.

Analysis of heat transfer in oscillating flow through a channel filled with metal foam using computational fluid dynamics

A computational fluid dynamics analysis of forced convective heat transfer has been conducted numerically on the heat transfer of oscillating flow through a channel filled with metal foam subjected to constant heat flux. The flow field and heat transfer were modeled using the Darcy–Brinkman–Forchheimer-model with corresponding energy equations. The model was validated by comparing the numerical results with available experimental results for a channel filled with aluminum foam. The distribution of surface temperature on the heated plate and local Nusselt number were calculated. The effect of amplitude and frequency of oscillating flow on the heat transfer in porous channel were analyzed. The results of numerical analysis showed significant heat transfer enhancements by inserting metal foam in the channel. Furthermore, local Nusselt number increases with employing high amplitude and frequency of the oscillating air flow. Effects of thermal conductivity of metal foam and Reynolds number were also numerically analyzed. Results showed that an increase in thermal conductivity of the metal foam and Reynolds number can significantly increase the heat transfer. It is revealed that the proposed numerical model can efficiently provide useful information for the design of metal foam filled heat sinks with oscillatory inlet flow.

Correcting and extending the Boomsma–Poulikakos effective thermal conductivity model for three-dimensional, fluid-saturated metal foams

Metal foams have emerged as promising materials for use in heat sink and heat exchanger applications. Boomsma and Poulikakos put forward an important and widely adopted model for the effective thermal conductivity of metal foams; however, the model contains errors in its development and presentation. Whether partially or fully corrected, the model does not provide accurate predictions of the effective thermal conductivity of metal foams. Because the model fails even when corrected, its mechanistic basis is reviewed, and an extension to the approach is put forward to account for ligament orientation in calculating effective thermal conductivity. This modification provides predictions of k eff that are much more accurate than the original Boomsma–Poulikakos model.