Open-cell Foams Research Articles

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.

Open-cell bio-polyurethane foams based on bio-polyols from used cooking oil

Used cooking oil is a widely available and inexpensive waste with a high application potential as a feedstock for the bio-based polyurethane production. Usually, bio-polyols from vegetable oils have higher viscosity and lower hydroxyl values compared to commercial petrochemical polyols, which limits their usefulness. This article reports on the development of open-cell polyurethane foam systems wherein 100% of the polyol components were bio-polyols obtained from used cooking oil. What is particularly considered is the effect of bio-polyol properties (molecular weight, viscosity and hydroxyl value) on the properties of the final open-cell polyurethane systems - apparent density, thermal conductivity coefficient, content of closed cells, mechanical strength, brittleness and short-term water absorption. It was found that the key step in the synthesis of bio-polyols designed for open-cell polyurethane foams is the epoxidation reaction. The epoxy value has a significant effect on the occurrence of side reactions (mainly oligomerization) during the oxirane ring-opening process determining the properties of bio-polyols. The resulting open-cell foams were characterized by apparent densities from 12.4 to 13.3 kg/m3, thermal conductivity coefficients from 36.6 to 38.2 mW/m∙K, and closed cell contents below 10%, which makes them comparable to commercial products. The results demonstrate that used cooking oil-based polyols can provide an alternative starting material for open-cell polyurethane foam production.

Development of open-porosity magnesium foam produced by investment casting

High-porosity, open-cell AZ91 magnesium alloy foams of two pore sizes were fabricated by means of investment casting technology, using PUR foam patterns. Foam casting variables such as pressure, mould temperature and metal pouring temperature were thoroughly investigated to define the most optimal casting conditions. The mechanical properties of the fabricated foams were measured in compression tests. A potential application for the foams considered is temporary bioresorbable bone implants, therefore the mechanical properties of the foams were compared with those of cancellous bone tissue. Foams with smaller pore size and lower porosity (20 PPI and 80%–87%) exhibited mechanical properties in the lower regions of the cancellous bone property range (Young's modulus 36.5–77.5 MPa), while foams with higher pore size and porosity (10 PPI and ∼90%) were found to have insufficient compression strength (Young's modulus 11.65–23.8), but thickening their walls and lowering their porosity below 90% yielded foams with Young's modulus between 36.5 and 77.5 MPa. Foam fractures were also investigated to determine their collapse mechanism. A series of corrosion tests in stimulated body fluid was carried out to determine their applicability as a biomaterial. The Plasma Electrolytic Oxidation (PEO) process was used in a feasibility study to examine the microstructure and chemical composition of foams with protective coating.

A novel approach for volumetric solar receiver performance assessments

A novel design and an efficient operation of solar volumetric receivers remain critical for successfully integrating such receivers into solar energy harvesting devices. The performance of the solar volumetric receiver is highly dependent on the geometric connectivity of the actual pore-sites in the SiC open-cell foams. Although the performance of SiC foam receivers has been investigated previously considering the homogeneous foam structure, further studies are needed to explore the influence of the actual path of the pores and their connectivity on the flow and thermal performance of the receiver. Hence, the present study considers the novel design of a solar volumetric absorber incorporating the actual SiC foam receiver geometric features. Accordingly, the thermal and flow behavior of the working fluid within the receiver is examined considering the 3D connectivity of the pore sites. Moreover, the performance of the solar volumetric receiver is evaluated for different Reynolds numbers, solar concentrations, and foam porosities. A 3D-dimensional numerical investigation is carried out incorporating the interactions between convection, conduction, and radiation heat transfer for which the discrete ordinate radiation model is used. Porous SiC foam is placed in a channel having an optically transparent wall at the top and air is used as the working fluid extracting heat from foam. The overall heat transfer coefficient, normalized Nusselt number, and thermal effectiveness are evaluated for various operating conditions. Findings revealed that the receiver thermal performance improves at high Reynolds numbers. The convection heat transfer is found to be the apparent heat transfer mode within the porous SiC foam. In addition, the pumping power to overcome the flow resistance in the channel is considerably less than that of the rate of heat transfer gain.

Ultralight and hydrophobic PVDF/PMMA open-cell foams with outstanding heat-insulation and oil-adsorption performances fabricated by CO2 molten foaming

Heat-insulation and oil-water separation are critical issues in the area of energy conservation and environmental protection. Developing high-expansion open-cell polymer foams with excellent heat insulation and oil-absorption properties is an essential way to solve the above problems. An innovative method for fabricating lightweight PVDF open-cell foam via blending with PMMA followed by a supercritical carbon dioxide molten foaming process was proposed, and ultralight and hydrophobic polymer open-cell foams with outstanding heat insulation and oil-absorption properties were successfully prepared. PMMA not only delays cooling crystallization behavior of PVDF but also improves the melt viscoelasticity. As a result, the open-cell polymer foam with an expansion ratio of up to 43.2, open porosity of 98.6 %, and a density of 0.0361 g/cm3 can be obtained. Benefiting from high expansion ratio and high open porosity, the blend foam displays a thermal conductivity of 31.07 mW∙m−1∙K−1, an adsorption capacity for various oils as high as 8.1–23.1 g/g, and favorable cycle durability. This work provides a green and sustainable method for mass production of better-behaved foam materials for heat insulation and oil-water separation.

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.

Digital twin of a laser flash experiment helps to assess the thermal performance of metal foams

Metal foams are a class of materials with immense potential. The complex topology in these lightweight systems is responsible for their outstanding mechanical and thermal properties. Furthermore, this is the same reason why thermal assessment of metal foams is challenging. While conventional analytical techniques provide useful data, modern-age technology allows to explore thermal transfer in metal foams in great detail by building a digital twin of the thermal characterisation experiment. In this work, high-accuracy measurements with a laser flash analysis instrument, processed with two effective-medium heat conduction models, are compared to a digital twin built with computed tomography and finite element analysis. This combination helps resolving complexity in analysing AlSi7Mg open-cell foams. Evidence of non-Fourier macroscopic heat conduction is found, which makes standard laser-flash analysis non-scalable.

Experimental study of the oxidative steam reforming of fuel grade bioethanol over Pt–Ni metallic foam structured catalysts

In this work, Pt–Ni/CeO2 catalysts have been used for the oxidative steam reforming of fuel grade bioethanol. To transfer the above formulation on structured carriers, due to the requirement of a washcoat (wc) deposition, it was mandatory to carry out a preliminary study on the catalysts in the form of powder. An initial experimental campaign was performed to optimise the ceria loading (between 25 and 45 wt%) as well as the Pt content (between 2 and 5 wt%): stability tests were carried out for 24 h at 500 °C, WHSV = 12.3 h−1, H2O/C2H5OH ratio of 4, O2/C2H5OH of 0.5. The highest activity, selectivity and durability was recorded over the 3Pt–10Ni/35CeO2/wc, which assured an ethanol conversion of almost 98% at the end of the test with a corresponding H2 yield of 50%. This interesting formulation was transferred on a Ni–Fe substrate, made of an open cell foam, which was tested under the same operative conditions described above: the structured catalyst, due to the very good heat management within the catalytic bed and the improved mass transport, displayed a more stable behaviour compared to the corresponding powder, even in the presence of the typical bioethanol impurities; moreover, the formation of unwanted by-products was negligible throughout the whole investigated interval.

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.

Determination of the effect of the open cell foam material geometry on the value of energy efficiency

THE PURPOSE. Improving the energy efficiency of open cell foam materials with different geometries (SC, BCC, FCC, DEM) and with different medium porosities (ε=0.7; ε=0.75; ε=0.8; ε=0.85; ε=0.9; ε=0.95) by numerical simulation. To determine the influence of the geometry and porosity of an open cell foam material on the values of pressure drop, heat flux and energy efficiency factor.METHODS .Numerical simulation was carried out using the ANSYS Fluent 19.2 software package. Geometric models of porous structures are sets of intersecting spheres with different packing structures: periodic Simple Cubic packing (SC), Face Centered Cubic packing (FCC), Body Centered Cubic packing (BCC), and random structure generated by the discrete element method (DEM).The calculations were carried out at the following air flow velocities: 0.01; 0.05; 0.25; 0.5; 0.75; 1; 1.25 m/s.RESULTS. Atair flow velocities of 0.01 m/s and 0.05 m/s, all the studied structures show approximately the same heat flux. With porosity values ε=0.75; ε=0.8; ε=0.85 the highest values of heat flow were shown by the FCC structure, with porosity ε=0.7; ε=0.9; ε=0.95 the BCC structure had the highest heat flux. This is explained by the fact that, at the corresponding porosity values, the FCC or BCC structure had the largest surface area, which provided the largest heat flux. With the porosities of media ε=0.7 and ε=0.75, the BCC and FCC cell packages show a high pressure drop. With the porosities of media ε=0.8 and ε=0.85, the highest pressure drop corresponds to FCC cell packing, and for porosities ε=0.9 and ε=0.95, to BCC cell packing.CONCLUSION. With equal high porosity, the BCC cell packing provides a higher value of heat flux than the FCC structure. The SC package has the lowest heat flux values for all studied porosities. The SC package also has the lowest pressure drop values and therefore the highest energy efficiency values.

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.

Developments on auxetic closed cell foam pressure vessel fabrications

Auxetic foam can have higher indentation resistance, better protection under impact and higher vibration damping than conventional foam. Unlike auxetic open cell foam, with established, commercially viable options for manufacturing, methods for making auxetic closed cell foam are not established. We revisited pressure-vessel methods, proposed in 1996, for making auxetic closed cell foam. We processed low-density polyethylene foam for 6 h at 400–700 kPa and 100 °C, causing foams to shrink by a factor of two to five. The volumetric compression kinked cell walls, producing negative Poisson’s ratios as low as −0.2 and Young’s moduli from 0.2 to 1.2 MPa. Trends between applied volumetric compression and Poisson’s ratio agree with those for open cell foam—initially decreasing to negative values as volume reduced by a factor of two after processing, then plateauing or slightly increasing as volume decreased by a factor of two to five. Foams of different sizes and shapes (15–75 mm sides) processed in the same conditions (700 kPa, 6 h, 100 °C) shrank evenly in all three axes and had similar final volume ratios. We noticed a long settling period, of up to three months, where foams slowly shrank. Placing foam in a vacuum after processing reduced the settling period to within 24 h.

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.

An open-source workflow for open-cell foams modelling: Geometry generation and CFD simulations for momentum and mass transport

• We developed an open-source workflow to characterize open-cell solid foams. • The geometry generation is performed by using the computer graphics code Blender. • Fluid flow and mass transfer are simulated using open-source CFD code OpenFOAM. • From the CFD results permeability and filtration rate are calculated. • The workflow (available online) is used to investigate a large number of different foams. Open-cell foams are porous materials characterized by high porosity and large specific surface, industrially employed as catalyst supports or particulate filters. These materials can be modelled using periodic lattices, or with more complex approaches based on Voronoi tessellations. This work introduces, tests, and makes available an open-source workflow ( Agostini, 2021 ) based on Blender that can reproduce great varieties of geometries with a limited cost for their modeling. An example of the exploration capabilities of this workflow is presented in the form of CFD simulations for flow field and mass transfer (performed with OpenFOAM) on the created geometries, eventually obtaining an effective particle deposition/filtration coefficient K d . The results are interpreted using constitutive equations and other functional forms depending on geometric parameters which prove to be insufficient in explaining the variations in filtration performance, highlighting the need for more detailed exploration, the objective for which this workflow was developed.

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.

Studies on Microstructure, Mechanical Properties, and Corrosion Behavior, of Partially Open-Cell Magnesium Foam through Powder Metallurgy Route

Studies on Microstructure, Mechanical Properties, and Corrosion Behavior, of Partially Open-Cell Magnesium Foam through Powder Metallurgy Route

RhNi/CeO2 catalytic activation of alumina open cell foams by dip-spin coating for the CO2 methanation of biogas

Production of CH4 by methanation of biogas provides a promising route towards sustainable energy production for the future. Utilization of biogas from biomass as CO2 source combines the advantage of reduction of greenhouse gas emissions producing a fuel with high energy density. In this study we apply a dip-spin coating method for washcoating Ni(7.5 wt%)Rh(0.5 wt%)/CeO2 catalyst on alumina open cell foams with different cell densities (20, 30 and 40 PPI).The catalytic activity of the structured samples was assessed using a gas mixture that simulates a typical biogas composition (CH4 = 60%vol; CO2 = 40%vol). The effects of the reaction temperature (300–600 °C) and H2/CO2 molar ratio in the feed (3–5) on the CO2 conversion and CH4 selectivity were evaluated under a gas hourly space velocity (GHSV) of 5100 h−1.All samples showed appreciable CO2 conversion ranging between 62.5 and 66.7% at reaction temperature of 500–600 °C, and a CH4 productivity of 3.4 molCH4·gMe−1·h−1 was obtained with the catalyst supported on open cell foam with 30 and 20 PPI under a reaction temperature of 400 °C.

Study on enhanced heat transfer performance of open-cell metal foams based on a hexahedron model

Due to its high heat transfer performance, the open-cell metal foam has a great potential of applications in heat exchanger industries. Based on the cell structure of the perforated metal foam, a simplified hexahedral structure model was proposed herein. The fluid flow in metal foams was simulated by computational fluid dynamics software. The accuracy of the model was verified by experiments. Based on the numerical simulation, heat transfer performance and the mechanism of enhanced heat transfer were investigated and discussed. The results show that the hexahedron model proposed in this article is feasible and accurate. The heat transfer and pressure drop properties of metal foams were numerically calculated and analyzed at the inlet air velocity of 1–5 m/s. At a certain PPI (pores per inch) and porosity, with the increase of Reynolds number Re, both the heat transfer coefficient h and pressure drop ΔP/L increase gradually. The comprehensive performance of convective heat transfer of metal foams with a lower PPI is relatively better. Increasing PPI or decreasing porosity is conducive to the destruction of the fluid boundary layer, enhancing the fluid disturbance, thus forming a vortex at the back of the skeleton and strengthening the heat transfer.

Influence of metal foam thickness on the conduction and convective heat transfer for a flat plate with metal foam impinged by a rectangular slot jet

The local heat transfer distribution is measured for the case of a flat plate with a porous metal foam impinged by a rectangular slot air jet. The main objective is to separate the convection and conduction heat transfer effects in a flat plate with metal foam. For this purpose, flat plate with attached and detached metal foam are studied. The effect of the thickness of the metal foam on the convection and conduction heat transfer is also investigated. An open-cell aluminium metal foam having a porosity of 92% and pore density of 20 PPI (pores per inch) is studied. The thickness of the metal foam studied is 4 mm, 8 mm and 12 mm. Reynolds number ranges from 5200 to 12,000 and impinging distance varies from 2 to 10 times the width of the rectangular slot. The local Nusselt number of a flat plate with attached and detached metal foam is independent of the impinging distance irrespective of the metal foam thickness. The impinging jet on the flat plate with metal foam experiences extra hydraulic resistance depending on the thickness of the metal foam. The 12 mm thick metal foam offers the highest extra hydraulic resistance. However, the 4 mm thick metal foam offers negligible extra hydraulic resistance. The flat plate with detached metal foam experiences attenuation in the convective heat transfer due to extra hydraulic resistance. However, the flat plate with attached metal foam experiences conduction heat transfer due to metallic foam along with attenuation in the convective heat transfer due to extra hydraulic resistance. The attenuation in the convection heat transfer and conduction heat transfer due to the presence of the metal foam is quantified by the attenuation factor and enhancement factor respectively. The average attenuation factor is 1.12, 0.96 and 0.84 for 4 mm, 8 mm and 12 mm respectively. The enhancement factor for the metal foam having 4 mm, 8 mm and 12 mm are 1.91, 2.20 and 2.90 respectively. Hence, the effect of the enhancement due to conduction heat transfer dominates over the effect of the attenuation in the convective heat transfer due to extra hydraulic resistance. Under slot jet impingement, a flat plate with 4 mm, 8 mm and 12 mm thick metal foam shows 2.02, 2.12 and 2.41 times overall enhancement in comparison with the smooth flat plate respectively. Region-wise correlations for Nusselt number are developed using multiple regression analysis.