Open-cell Metal Foam Research Articles

Experimental Study on Capillary Microflows in High Porosity Open-Cell Metal Foams.

Metal foams have been widely used in heat pipes as wicking materials. The main issue with metal foams is the surface property capillary limit. In this paper, a chemical blackening process for creating a superhydrophilic surface on copper foams is studied with seven different NaOH and NaClO2 solution concentrations (1.5~4.5 mol/L), in which the microscopic morphology of the treated copper foam surface is analyzed by scanning electron microscopy. The capillary experiments are carried out to quantify the wicking characteristics of the treated copper foams and the results are compared with theoretical models. A the microscope is used to detect the flow stratification characteristics of the capillary rise process. The results show that the best wicking ability is obtained for the oxidation of copper foam using 3.5 mol/L of NaOH and NaClO2 solution. Gravity plays a major role in defining the permeability and effective pore radius, while the effect of evaporation can be ignored. The formation of a fluid stratified interface between the unsaturated and saturated zone results in capillary performance degradation. The current study is important for understanding the flow transport in porous materials.

Numerical Modeling of Pure and Metal-Foam Loaded PCMs

Despite many desirable properties, most phase change materials (PCMs) undergo timing issues during the phase change process due to a low thermal conductivity, which limits their application in heat storages. Thus, many techniques have been pointed out to overcome these disadvantages and improve heat transfer, such as coupling PCMs with metal inserts, like high porosity open-cell metal foams. Indeed, the presence of a metal foam increases the effective thermal conductivity of the composite medium and speeds up the charging and discharging phases. In the present paper, a numerical model developed in COMSOL ambient has been calibrated by comparison with experimental results on the melting of pure and metal-foam loaded PCMs, placed in a small case and heated from the top by an electric resistance. The numerical model considers the metal foam as a static solid, filled with a phase changing fluid and employs a literature correlation to evaluate the effective thermal conductivity of the composite medium. The performance of two different paraffinic PCMs (RT35 and RT35HC byRubitherm GmbH,D), loaded either with a copper foam (20 PPI, 95% porosity, byPorometal,China) or with an aluminum one (10 PPI, 96% porosity, byRecemat,NL) has been investigated in terms of speed rate of the phase change front, time required to complete the melting process, temperature distribution and effect of foam porosity. The obtained results clearly evidence the significant heat transfer improvement yielded by metal foams, whose presence increases the effective thermal conductivity of the composite medium (from 0.2 to 7.03 W/mK for copper foam and to 3.52 W/mK for aluminum one), leading to a significant decrease of the charging time and to a lower temperature gradient within the PCM (from about 16 to 3 K).

Prediction of high-temperature infrared radiative properties of nickel foam ligaments

High-temperature radiative properties of open-cell foam ligaments are rather sparse due to measurement limitation. This study aims to numerically predict the infrared radiative properties of nickel foam ligaments at high temperatures. In this work, rough surfaces of ligaments are modeled with SEM scanning pictures, and optical properties of nickel components are simulated using ab initio molecular dynamics method at 298 K, 673 K, 873 K, 1073 K and 1273 K. The reflectivity of smooth surface is determined with empirical formulas at high temperatures. Moreover, the spectral radiative properties of nickel foam ligaments are predicted with finite difference time domain method at the waveband of 0.8–8.5 μm based on the modeled surfaces and high-temperature optical properties of components. It is found high temperature can marginally strengthen radiation scattering of the ligament surfaces, while having little effect on the distribution of radiation scattering. For instance, the directional-hemispherical reflectivity increases approximately linearly with temperature. As the incident wavelength grows, the directional-hemispherical reflectivity and proportion of specular reflection both show a rapid increase in the studied waveband, while the value of reflection main peak of directional-hemispherical reflectivity declines. With the growth of incident angle, the center position of main reflection peak is moving to the direction of increasing angles. Polynomial fit is applied to anticipate the relationship between proportion of specular reflection and incident wavelength with a RSME less than 0.02. This work demonstrates a practical method for obtaining the infrared radiative properties of open-cell metal foam ligaments at high temperatures.

Elastic Properties of Open Cell Metallic Foams-Modeling of Pore Size Variation Effect.

Elastic properties of open-cell metallic foams are investigated in correlation with relative density and pore size variation. A variety of foam architectures, with open porosity above 70% (relative density below 0.30) and various pore size distributions, were modeled using Laguerre–Voronoi tessellations (LVT). The coefficient of pore volume variation, CV(V), was introduced to quantify the uniformity of designed structures and ranged between 0.5 to 2.1. Elastic behavior of the modeled foams to uniaxial compression along three orthogonal directions was analyzed using the finite element (FE) method. It is shown that Young’s modulus and Poisson’s ratio of open-cell metals is not solely a function of relative density (porosity) but the pore size variation as well. For similar porosity (approx. 74–98%), Young’s modulus and Poisson’s ratio may be reduced by approx. 25–30% and 10–25%, respectively, when CV(V) increases from 0.5 to 2.1. Furthermore, the incorporation of a relationship between Young’s modulus and the coefficient of pore volume variation to the Gibson–Ashby model is proposed.

Forced convection heat transfer: A comparison between open-cell metal foams and additive manufactured kelvin cells

The Kelvin cells are normally applied to represent the open-cell metal foams for simulation due to the similar pore structure. However, the experimental comparison between Kelvin cells and open-cell metal foams is rarely reported. This study compares the forced convective heat transfer properties of four Ni70Cr30 metal foams and one GH4169 Kevin cells manufactured from additive manufacturing. The results indicate that the pressure drop and overall heat transfer coefficient (HTC) increase with the increase of PPI, while decreasing with the increase of porosity. Moreover, the comparisons between present data and data from the literature prove the reasonability of the experiments. The pressure drop and overall HTC of Kelvin cells are 64.4% and 12.3% higher than open-cell metal foams, while showing 26.2% lower overall heat transfer performance at 6 m/s. The pore structure, thicker skeleton, higher porosity, and surface roughness of Kelvin cells contribute to the higher pressure drop than open-cell metal foams. The comparison among experimental correlations and present data indicates that the correlations for predicting the behavior of open-cell metal foams could not be used directly to predict Kelvin cells.

Facile coating of Co3O4 on open-cell metallic foam for N2O catalytic decomposition

Co3O4 coated on open-cell metallic foams, which can merge the catalytic activity of Co3O4 and advantages derived from the geometry of the support, are promising for environmental applications such as the decomposition of N2O (deN2O). This study demonstrates how it is possible to easily and reproducibly prepare a well-adhered layer of Co3O4 with a similar specific surface area as the materials prepared with the precipitation method (∼ 23 m2 g−1coating) and controlled thickness on FeCrAlloy open-cell foams of different porosity and dimension by the electrosynthesis of Co(OH)2 followed by thermal treatment (600 °C). The obtained structured catalysts show stable performance after three cycles of catalytic decomposition of N2O between 250 and 550 °C, with a similar T50 of approximately 460 °C, which is comparable to the combustion or precipitated catalysts but provides a lower pressure drop. Under the operating conditions with a sequential test, the presence of NO and/or O2 in the feed gas inhibits partially the catalytic activity of the structured catalysts while the presence of H2O shows a slight enhancement in the N2O conversion at a temperature above 450 °C.

Influence of bubble departure control on nucleate pool boiling heat transfer of electrodeposited copper foam: Experiments and correlation

Pool boiling is highly suitable for cooling in various industries due to its high heat transfer capability. Among pool boiling enhancement methods, embedding open-cell metal foams to the heater surface are particularly beneficial as they increase the effective heat transfer area. In this study, by analyzing 30 samples, the effect of pore density, thickness, and channels with different diameters on copper metal foams were investigated. In addition, the electrodeposition method was used to deposit a porous layer on the edges of copper metal foam to increase boiling efficiency in atmospheric conditions with deionized water as a working fluid. As an innovation, holes with different diameters were created inside the specimens using machining to construct a suitable exit path for the bubbles. According to the results, creating a channel with a diameter of 3 mm inside foams with thicknesses of 8 mm and 10 mm, both in the untreated and electrodeposited modes, improved the heat transfer coefficient (HTC) compared to foam without the channel. On the other hand, a 5 mm diameter channel decreased boiling efficiency due to a substantial reduction in effective surface area compared to a 3 mm channel. At low heat fluxes, the electrodeposited sample with a pore density of 20 ppi, a thickness of 10 mm, and a channel diameter of 5 mm achieved an HTC of 13.56 W/cm2. K that was approximately 3.5 times higher than the plain copper sample. A high-speed camera was utilized to observe the bubble departure from the channels during changing the heat flux. Finally, by considering the dimensionless parameters of the problem, an experimental correlation was derived to predict the HTC values.

Significance of SWCNTs and MWCNTs on the dynamics of hybrid nanofluid flow over a stretching surface

In this article, the hybrid nanofluid flow across the hot substrate of a heated sink coated with an open-cell metallic foam is explored to increase the rate of heat transfer. Two sorts of nanoparticles are considered to be specific MWCNT and SWCNT taking water as the base liquid. Tiny particles are examined because of their remarkable properties, such as exceptional thermal conductance, which is important in modern nano-technology, cooling systems, material sciences, and electronic parts. The primary goal of this extensive study is to improve heat transportation. The mathematical equations for the considered flow are the Navier–Stokes equations in the form of the Darcy–Brinkman–Forchheirmer model. Using similarity variables, a system of PDEs is converted into a system of dimensionless ODEs. The subsequent ODEs are settled by utilizing the numerical technique bvp4c in MATLAB. The significant results are presented with the guide of figures and tables. From this analysis, it is observed that the Xue model has a larger heat transport rate than the Yamada–Ota and Tiwari–Das models. Furthermore, the Xue model achieves a higher skin friction coefficient and Nusselt number values than the Yamada–Ota and Tiwari–Das models.

Free Vibration Analysis of a Graphene-Platelet-Reinforced, Porous, Two-Cylindrical-Panel System

In this study, a novel, dynamic model of a graphene-platelet-reinforced, porous (GPLRP) double-cylindrical-panel system is proposed. The material properties of a graphene-platelet-reinforced, porous, double-cylindrical-panel system were determined by the Halpin–Tsai micromechanics model and the typical mechanical properties of open-cell metal foams. Different types of porosity distribution and graphene platelet (GPL) distribution patterns were considered. Love’s shell theory was utilized to derive the theoretical formulation, and the Rayleigh–Ritz method was used to calculate the natural frequencies of the system. The proposed model was validated by several comparison studies with the natural frequencies in the existing literature. Finally, the effects of stiffness of Winkler springs, boundary condition, porosity coefficient, porosity distribution, GPL distribution pattern, and GPL weight fraction on the free vibration characteristics of the system were evaluated.

Radial heat transport in a fixed-bed reactor made of metallic foam pellets: Experiment and particle-resolved computational fluid dynamics

Open-cell metallic foams in the shape of pelletized catalysts are regarded as potential catalyst substrates for the application in fixed-bed reactors. This work reports an experimental study and a detailed Computational Fluid Dynamics (CFD) model to investigate the heat transfer performance of a fixed-bed reactor made of open-cell metallic foam pellets. The steady-state heat transfer behavior of a hot gas flowing through such a packed bed under wall-cooled conditions was examined. The proposed CFD model is in line with the particle-resolved CFD (PRCFD) approach that accounts explicitly for the random packing structure, so that the transport processes in the interstitial regions are fully resolved. The momentum and energy transports inside the highly porous foam particles are considered by closure equations based on the porous-media model. The Rigid Body Dynamics (RBD) method is adopted to create the packing geometry. The axial temperature and the pressure drop obtained by the CFD simulations show good agreement with experimental data. To evaluate the thermal performance, effective heat transport parameters in a packed bed such as the effective radial thermal conductivity and the apparent wall-fluid heat transfer coefficient are determined, with the aid of a 2D pseudo-homogenous plug-flow reactor model. The correlations for heat transport characteristics as a function of particle Reynolds number for the metallic foam packed bed are also presented.

Experimental Set-Up of the Production Process and Mechanical Characterization of Metal Foams Manufactured by Lost-PLA Technique with Different Cell Morphology

A flexible and versatile method for manufacturing open-cell metal foams, called lost-PLA, is presented in this work. With a double extruder 3D printer (FDM, Ultimaker S3, Utrecht, The Netherlands), it is possible to make polymer-based samples of the lost model. Through CAD modeling, different geometries were replicated so as to get black PLA samples. This method combines the advantages of rapid prototyping with the possibility of manufacturing Al-alloy specimens with low time to market. The production process is articulated in many steps: PLA foams are inserted into an ultra-resistant plaster mix, after which the polymer is thermally degraded. The next step consists of the gravity casting of the EN-6082 alloy in the plaster form, obtaining metal foams that are interesting from a technological point of view as well as with respect to their mechanical properties. These foam prototypes can find application in the automotive, civil and aeronautical fields due to their high surface/weight ratio, making them optimal for heat exchange and for the ability to absorb energy during compression. The main aspects on which we focus are the set-up of the process parameters and the characterization of the mechanical properties of the manufactured samples. The main production steps are examined at first. After that, the results obtained for mechanical performance during static compression tests with different geometry porosities are compared and discussed. The foam with truncated octahedron cells was found to show the highest absorbed energy/relative density ratio.

Pore-scale direct numerical simulation of fluid dynamics, conduction and convection heat transfer in open-cell Voronoi porous foams

The pore-scale analysis includes the pore network simulation and direct numerical simulation, which the latter is more accurate and provides more details about the problem. This study presents a pore-scale direct numerical simulation of flow and convection-conduction heat transfer in open-cell metal foams with a complex structure. First, using the Laguerre–Voronoi tessellations (LVT) algorithm, three foam samples with various porosities (76.6%, 84.4%, and 93.8%) and a constant pore density of 20 pores per inch (PPI) are generated. Next, the open-source library of OpenFOAM is employed to perform heat transfer and flow analysis in Darcy and non-Darcy regimes. Results were verified through comparison with available experimental and numerical data. The effects of foams' porosity, solid material, and Reynolds number on flow characteristics and heat transfer are investigated. Results are presented in terms of the permeability, Forchheimer coefficient, the volumetric and average heat transfer coefficient, and temperature and velocity distribution. By decreasing porosity, the flow complexity increases with subsequent increments in heat transfer and pressure drop. Also, the conductive heat transfer through the foam plays a vital role in the total heat transfer rate. These pore-scale outcomes could be used in macro-scale thermo-fluid models to perform more accurate simulations with less computational costs.

Experimental investigation of the pressure-drop characteristics of open-cell metal foam with air-gaps under non-Darcy flow regimes

Multi-layer metal foam with discrete air gaps between neighbored layers can have wide engineering applications, and its pressure drop characteristic is the crucial performance that needs to be predicted precisely. Experimental studies were carried out to investigate the influences of the air gaps and their layouts on the pressure drop characteristics of the multi-layer open-cell metal foam within a wide flow velocity range to develop efficient models for pressure drop predictions. The accumulated metal foam thickness and air gap distance were kept unchanged in the tests, and four sets of test sections with different air gap layouts were tested. The varying airflow speed across the metal foam was realized by adjusting the inlet pressure level of the test section to reveal the relations of the pressure drop versus the mass flow rate. Results indicate that in the multi-layer and multi-gap configurations, the Forchheimer model in the transition regime and bilinear model in the turbulent regime is still applicable for predicting the pressure drop; however, the insertion of air gaps between the metal foam slices leads to an increased pressure drop coefficient. Meanwhile, the first air gap leads to a higher pressure drop compared to the subsequent air gaps and shifts the flow regime transition point towards an even more minor flow rate operating point, while the subsequent air gaps present almost the same influences on the metal foam pressure drop characteristics and transition point. With the experimental study, two models for predicting the pressure drop characteristics of open-cell metal foam with and without air gaps in the transition and turbulent flow regimes with a broadened range are presented.

A hybrid shape memory polymer filled metallic foam composite: shape restoring, strain sensing, Joule heating, strengthening, and toughening

In this paper, an open-cell metallic foam was filled in by a tough shape memory polymer (SMP), to form a hybrid metal/polymer composite with multifunctionalities and enhanced mechanical properties. This work aims to study the positive composite actions between the metallic skeleton and the SMP filler. Mechanical, thermal, and conductive properties of the resulting hybrid composite were evaluated and compared to the individual components. Uniaxial compression tests and shape memory effect tests were conducted. Results demonstrated an improvement in the compressive strength and toughness. The hybrid composite also exhibited excellent shape recovery and high recovery stress of 1.76 MPa. Infrared thermography has been used to verify the free shape recovery by Joule heating. Sandwich structures with the hybrid composite as the core were studied through low velocity impact test and three-point bending test. The sandwich structures with the composite foam core showed significant performance improvement in both tests. Electrical resistivity study during the three-point bending test validates the possible application of this multifunctional polymer-aluminum open cell foam composite as strain sensor. This type of hybrid composites can be beneficial in many industrial sectors that search for an ideal combination of high strength, high toughness, low weight, damage sensing, and excellent energy absorption capabilities.

Experimental investigations on the acoustical performance of open-celled metal foams under grazing flow

Open-celled metal foams offer a possible solution for implementing acoustic liners in non-traditional nacelle locations, such as directly over the turbofan rotor. Previously, we have shown that the normal incidence absorption performance of metal foams can be further improved by compressing them to reduce their effective pore size. In this paper, we focus on the acoustical performance of compressed metal foams under grazing flow conditions. Experimental tests were conducted using a grazing flow impedance tube set up within Spirit AeroSystems' acoustics lab. Metal foams with four different compression ratios were tested under various flow rates and incident sound pressure levels. The sound pressure measurements were then utilized to educe their effective impedance under grazing flow conditions. Our results show that a uniform through-thickness compression can improve the acoustic performance of open-celled metal foams under grazing flow conditions and can aid the further development of acoustic liners for over the rotor applications.

Surrogate modeling of open-celled metal foam sound absorbers with stepwise property gradients

In this paper, we develop a Kriging-based surrogate model to predict the sound absorption performance of open-celled metal foams with user-defined stepwise relative density gradients. An experimentally validated transfer matrix model is used to generate training data capturing the absorption profiles of metal foams with various geometrical parameters. We deploy a spatial analysis method to create influence surfaces around each sample data point and depict its effect on the observed output by using Kriging's basis functions. The surrogate model has an in-built machine learning framework that uses a genetic algorithm and a maximum likelihood function to tune the weights that define these influence surfaces. Using this approach, we successfully developed a data-fed, supervised prediction model that estimates absorption coefficient of a virtual metal foam sample. Post-validation predicted values for unknown sample configurations show satisfactory agreement with their actual absorption coefficients. Finally, we implement the developed metamodel as an iOS application as a potential low-cost tool to reduce testing costs and enable rapid design iterations for acoustic design problems.

On the thermal buckling and postbuckling responses of temperature-dependent graphene platelets reinforced porous nanocomposite beams

In this work, thermal buckling and postbuckling behaviors of graphene platelet (GPL) reinforced porous nanocomposite beams are studied with inclusion of temperature-dependent material properties for the first time. The Halpin-Tsai micromechanics model and extended rule of mixture as well as open-cell metal foam model are applied to determine the effective properties of nanocomposites. A high order shear deformation theory with the aid of von Kármán nonlinearity is implemented to yield governing equations corresponding to thermal buckling and postbuckling problems. The numerically stable admissible functions constructed through Gram-Schmidt procedure are developed to describe end restraints of beams. An eigenvalue equation without geometric nonlinearity is solved to obtain critical buckling temperatures, while an iterative methodology is implemented to find the temperature-dependent thermal postbuckling paths of beams. An extensive numerical study is performed to check the influences of internal pores and GPL reinforcements on the thermal buckling and postbuckling responses of GPL-reinforced metal foam beams. It is found that the temperature-dependency of material properties plays important roles on thermal buckling/postbuckling behaviors of GPL-reinforced porous beams, and dispersing more GPLs on the upper and lower surfaces and fabricating more internal pores near central portion of beams can improve thermal instability-resistance capability of composite structures.

PIV measurement of turbulent flow characteristics inside an open-cell metal foam replica

Metal foam structures have been of interest because their geometrical characteristics are theoretically desirable to many applications such as heat exchanger, chemical reactor, filter, etc. Better understanding of flow characteristics inside metal foam is essential to utilize metal foam in thermo-fluid systems. A test section with a 3-D printed 8 times scaled-up transparent metal foam replica has been prepared. Time-resolved 2-D particle image velocimetry was used to measure velocity fields in the center planes at upstream, inside, and downstream of the replica model. The investigation was conducted for 3 different Reynolds numbers, 1000, 5,000 and 10,000 based on the channel hydraulic diameter. The turbulent flow characteristics inside an open-cell metal foam was compared between laminar and turbulent inlet flow conditions. Upstream turbulence intensity was 10% in case of turbulent inlet condition. For the ensemble averaged fields, clear similarities are found in mean velocity distributions and turbulent stresses inside the metal foam regardless of Reynolds number. The velocity magnitude inside metal foam increased up to 1.6 times of the channel bulk velocity for all Reynolds number. The temporal fluctuation of streamwise velocity component is increased up to 1.4 times to channel bulk velocity inside the metal foam replica. This increase was similar to that of laminar inlet condition even there is no fluctuations in the upstream. Normalized vorticity and turbulent kinetic energy also showed the similarity in three different Reynolds numbers, but the magnitude is higher at lower Reynolds number. Temporal analysis was also conducted at several points inside metal foam. The integral length scale is similar to 0.2 size of ligament in the case of Re = 10,000. Energy spectrum shows that the spectral slope is -5/3 in inertial subrange which confirm that flow inside metal foam is turbulent even the inlet flow is laminar.

A theoretical model for effective thermal conductivity of open‐cell‐coated metal foams saturated with fluid/phase change material

The current research presents geometrical effective thermal conductivity (ETC) models for externally coated open-cell metal foams (MFs) saturated with fluid/phase change material (PCM) by considering three-dimensional configuration adopted from the tetrakaidecahedron structure. The three-dimension (3D) models consider different geometries, such as hexagonal and square, involving different shapes of ligaments (cylindrical, concave triprism) and nodes (cubic, pyramidal) for the analysis. The ETC of MF-composite increases with the coating thickness, the thermal conductivity of coating material, and infiltrating medium. The percentage enhancement in ETC is found to be significant with the increase in coating thickness compared with the increase in thermal conductivity of coating material and infiltrating medium. In addition, the ETC value increases with the increase in the porosity value, which can be explored at the manufacturing process during the design of thermal management systems (TMS). The present model involving hexagonal geometry with concave triprism ligaments and pyramidal nodes is found to be in excellent agreement with test data for all the infiltration cases with an average deviation of less than 3%. The present study reports the mathematical expressions for ETC as a function of various modeling parameters, which can be useful during the design of the TMS.

Metal foams as a gas diffusion layer in direct borohydride fuel cells

Renewable energy sources have become an important issue due to global warming and the diminishing of fossil fuel resources that have become a major problem for the world. Fuel cells are one of the most important renewable energy systems. The use of metal foams within the scope of commercialization and cost reduction of fuel cells attracts attention today. Metal foams are a material that provides excellent performance in many engineering applications, especially in fuel cells, thanks to the high permeability provided by the porous structure, narrow flow channels, large specific surface area, capillary, and diffusive forces. Among the metallic foams, the focus is on the use of aluminum foam, due to its low cost, superior strength, and thermal conductivity performance properties as well as ease of production. As an alternative to gas diffusion layers, the use of different metallic foam materials in fuel cell systems and performance improvements were analyzed. The use of aluminum foams as flow distributors and electrodes in the anode diffusion layer, which is one of the DBHYP components, provides significant increases in cell performance, cell weight, and volume. It has been found that open-cell metallic foams used in fuel cell systems cause an increase in performance due to their positive effects on the amount of catalyst coating, flow characteristics, and electrical properties.