Open-cell Metal Foam Research Articles

The effect of PPI on thermal parameters in compact heat exchangers with aluminum foam

A numerical analysis on a compact heat exchanger in aluminum foam is accomplished. The governing equations in two-dimensional steady state regime are written in local thermal non-equilibrium (LTNE). The physical domain under investigation is made up of a plate in aluminum foam with inside a single array of five circular tubes. The presence of the open-celled metal foam is modelled as a porous media by means of the Darcy-Brinkman-Forchheimer law. The metal foam is characterized by a porosity of 0.93 and different pores per inch (PPI), equal to 5, 10, 20 and 40 are analized. The LTNE assumption is considered to simulate the heat transfer between the fluid phase and the solid matrix of the foam. The compact heat exchanger at different air flow rates is studied with an assigned surface tube temperature. The results are shown in terms of heat and mechanical power ratio. Global parameters such as effectiveness and NTU, typical in compact heat exchangers, are showed. Furthermore, local air temperature and velocity profiles in the smaller cross section, between two consecutive tubes, as a function of Reynolds number are presented.

Measurement model for near-infrared radiative properties of open-cell metallic foams based on transmittance spectra

Considering the existence of edge effect caused by the cut surface of struts or sintering points will affect the actual radiative transfer process in metallic foams, a three-layer structure model is constructed to obtain their spectral radiative properties. Using the double-thickness model for reference, the real spectral radiative properties of metallic foams are calculated based on transmittance spectra. To verify the feasibility of this method, transmittance spectra of nickel foams with different cell sizes and thicknesses are measured by FTIR spectrometer system in the near infrared band. Through comparing extinction coefficients obtained by traditional single-layer structure model and the three-layer structure model proposed in this paper, it is found that extinction coefficient will be underestimated by the latter method, while the deviation can reach 13% for 20ppi nickel foam and 8% for 40ppi nickel foam. Then absorption coefficient and scattering coefficient are calculated by a prediction model with parameters corrected. Based on this method, temperature influence on extinction coefficient of nickel foam is studied from 293 K to 773 K and it is found that extinction coefficient decreases with temperature in the researched band while the maximum changing rate is 2.1% in the researched band.

Gas Diffusion Layers in Fuel Cells and Electrolysers: A Novel Semi-Empirical Model to Predict Electrical Conductivity of Sintered Metal Fibres

This paper introduces novel empirical as well as modified models to predict the electrical conductivity of sintered metal fibres and closed-cell foams. These models provide a significant improvement over the existing models and reduce the maximum relative error from as high as just over 30% down to about 10%. Also, it is shown that these models provide a noticeable improvement for closed-cell metal foams. However, the estimation of electrical conductivity of open-cell metal foams was improved marginally over previous models. Sintered porous metals are widely used in electrochemical devices such as water electrolysers, unitised regenerative fuel cells (URFCs) as gas diffusion layers (GDLs), and batteries. Having a more accurate prediction of electrical conductivity based on variation by porosity helps in better modelling of such devices and hence achieving improved designs. The models presented in this paper are fitted to the experimental results in order to highlight the difference between the conductivity of sintered metal fibres and metal foams. It is shown that the critical porosity (maximum achievable porosity) can play an important role in sintered metal fibres to predict the electrical conductivity whereas its effect is not significant in open-cell metal foams. Based on the models, the electrical conductivity reaches zero value at 95% porosity rather than 100% for sintered metal fibres.

Electrodeposition of CeO2 and Pd-CeO2 on small pore size metallic foams: Selection of deposition parameters

The electrodeposition, in particular the electro-base generation method, is an alternative to conventional washcoating to coat open-cell metallic foams, especially small pore size ones. In this work, the method was applied for the in-situ synthesis of cerium-based coatings, CeO2 and Pd-CeO2, on 100 pores per inch (ppi) FeCrAl foams. The range of parameters suitable for the electrodeposition of CeO2 films of different thickness and morphology was firstly investigated, i.e. Ce(NO3)3 concentration, potential applied, and deposition time. Then the most challenging one step Pd-CeO2 electrodeposition was studied; the Pd content and distribution were optimized considering also the electrochemistry and chemistry of Pd2+ species, i.e. by selection of a suitable Pd2+ complex precursor (Pd(NH3)4(NO3)2 or PdCl2 in HCl). Coated foams were calcined at 550 °C to obtain the structured catalysts. The CO oxidation was used as a model reaction to test the activity of Pd-CeO2 catalysts.The electrodeposition of cubic fluorite CeO2 coatings on the surface of the foam ranging from few to 18 μm and made by compact and/or platelet particles was easily achieved and with a high reproducibility. The Pd-CeO2 samples prepared from the ammine-containing electrolyte, though generated a well-adhered coating, resulted in a lower Pd content than the nominal value of the electrolyte. A high electrolyte concentration containing PdCl2 combined with a short time allowed to deposit a rather thick Pd-containing CeO2 coating avoiding the massive Pd° deposition. CO oxidation tests, especially at high flow rates, confirmed the key role of the coating and Pd distribution on the activity of the structured catalysts. A comparison of the conversion in mass transfer regime and estimates with literature correlations was presented, showing that, despite the very complex geometry of the support, a remarkably high quantity of the available surface is effectively exploited, paving the way for compact catalytic converters.

Role of porous metal foam on the heat transfer enhancement for a thermal energy storage tube

Thermal energy storage technology has attracted extensive attentions due to its remarkable energy-saving benefits. However, the low thermal conductivity of phase change materials seriously limits the energy storage efficiency, which put forward more stringent requirements for heat transfer enhancement. In this study, a two-dimensional axisymmetric simulation model with natural convection was established for the shell-and-tube thermal energy storage unit. Open-cell metal foam with a porosity of 0.94 and pore density of 15 pore per inch was employed to be arranged in either heat transfer fluid or phase change materials domains. The effects of the metal foam location and the metal foam porosity on the heat storage performance were studied. The numerical method was verified by experimental measurement, achieving good agreement. Results demonstrated that metal foam can significantly enhance heat transfer due mainly to the reduction of thermal resistance in heat transfer fluid. The case that both domains for heat transfer fluid and phase change materials were embedded in porous media can provide the best heat transfer enhancement. Compared with smooth tube without metal foam, the full melting time for this case was reduced by 88.548%; meanwhile, temperature response rate, heat flux and j-factor was increased by 834.27%, 774.90%, 5186.91% respectively. Besides, embedding metal foam into phase change materials can improve the temperature uniformity of phase change materials.

Impingement jet hydrogen, air and Cu[sbnd]H2O nanofluid cooling of a hot surface covered by porous media with non-uniform input jet velocity

In this paper, a numerical investigation is performed on the flow and thermal performance of a heat sink, covered with an open cell metal foam, under the influences of uniform and non-uniform velocity impinging jets. Hydrogen, air and Cu-water nanofluid are considered as the cooling fluids. Navier-Stokes PDEs including the porous drag induced terms – represented by Darcy-Brinkman-Forchheimer relation – in line with the energy equation, are transformed to a system of ordinary differential equations (ODE) through definition of non-dimensional parameter and similarity variables. The system of non-linear ODEs has been solved numerically and results are validated by comparison with those of a commercial software. Afterward, the influences of hydrodynamic variables, porous medium properties and nanofluid volume fraction on flow and heat transfer performance of the heat sink, have been scrutinized. Results presented in terms of non-dimensional velocity and temperature profiles, as well as the stream function, velocity and temperature contours. Results indicate that increasing the volume fraction of nanofluid have increased the heat transfer rate. In addition, under the constant heat sink inlet mass flow rate, the use of non-uniform impingement jet with decreasing velocity distribution improves the thermal performance of the heat sink.

Effect of inclination on the thermal response of composite phase change materials for thermal energy storage

Using paraffin as phase change material, the melting behaviors of pure phase change material and phase change material embedded in open-cell metal foams (composite phase change material) at different inclination angles are studied. Experimental system enabling continuous rotating is designed and built to explore the influence of inclination angles on phase interface evolution and temperature responses inside pure and composite phase change materials. Constant wall temperature is applied on one surface while others are thermally insulated. Experiments are carried out at angles of 0°, 30°, 60°, and 90° respectively. The effects of inclination on the melting behavior of phase change materials are analyzed by visualizing the solid-liquid interface and analyzing the temperature responses. Results demonstrate that the inclination angle has a great influence on the formation and development of natural convection during melting of pure phase change material, affecting the solid-liquid interface propagation and heat transfer rate. Compared with the case at 90°, the full melting time is reduced by 12.28%, 22.81% and 34.21% at 0°, 30° and 60°, respectively. However, when phase change material is melted in open-cell metal foam, heat conduction dominates, and inclination angle has little influence. Melting fractions at different inclination angles are the same and temperature curves at given points overlaps with each other.

Pore-scale simulation of melting process of paraffin with volume change in high porosity open-cell metal foam

In this paper, a pore-scale model for melting phase change of paraffin with volume change in high porosity open-cell metal foam is first developed by comprehensively adopting the enthalpy-porosity method, the volume-of-fluid (VOF) method and the Weaire-Phelan foam structure. This model is validated with the pore-scale experimental result from literature, which can more reasonably consider the effects of paraffin volume variation and foam structure when compared with previously reported models. Then the validated model is utilized to directly simulate the paraffin melting in foam pore region and obtain the detailed phase change information including the phase, temperature, heat flux and flow fields, which helps exploring the phase change transport physics in metal foam/paraffin composite (MFPC). The numerical results show that during the melting process, the solid-liquid phase change interface evolves from a multiply-connected domain to many isolated curved surfaces and is remarkably extended due to the prominent volume expansion of paraffin and high specific surface area of metal foam, which is beneficial to reducing the characteristic length for phase change. Local thermal non-equilibrium between metal foam and paraffin is prominent during most of the phase change process. The metal foam can effectively weaken the heavily thermal insulating effect of the phase change interface and avoid the seriously thermal stratification phenomenon existing in the pure paraffin unit. Besides, the paraffin flow driven by the volume expansion pressure difference is strengthened (which has not been revealed before) while the natural convection flow is depressed by the metal foam. As compared to the pure paraffin, the MFPC has much higher phase change rate, volume expansion rate, temperature uniformity and heat storage rate. This work can contribute to new viewpoints for better understanding the phase change transport process in MFPC as compared to previous macroscopic-scale and pore-scale investigations.

Enhancing the efficiency of Photovoltaic panel using open-cell copper metal foam fins

In this experimental study, a passive cooling technique by open-cell copper metal foam fins was performed for a photovoltaic (PV) panel to enhance its performance by reducing the operating temperature of the PV panel. The experiment was carried out in Baghdad-Iraq climatic conditions during February, March, and April 2019. Three polycrystalline PV panels were used, two panels were equipped with the proposed cooling technique and the other without modification for the purposed of comparing. The open-cell copper metal foam fins mounted on the backside of the PV panel by thermal grease. Four longitudinal fins arrangements (4, 6, 8, and 10 fins) were investigated. The porosity of the metal foam helps the penetration of air through the fins to extract more heat from the PV panel. It was found that the adding of ten longitudinal fins can reduce the average PV panel temperature by about 8.4% and improve the power output by an average of 4.9%.

Free Vibration of Rectangular Plates with Porosity Distributions under Complex Boundary Constraints

Free vibration of rectangular plates with three kinds of porosity distributions and different boundary constraints has been performed by means of a semianalytical method. The distribution of porous varies along the thickness of the plate, in which the mechanical properties are defined by open‐cell metal foam. Regardless of boundary conditions, displacement admissible functions are represented by combination of standard cosine Fourier series and auxiliary sine series. The kinetic energy and potential energy of plates are also expressed on the basis of first‐order shear deformation theory (FSDT) and displacement admissible functions. Finally, the coefficients in the Fourier series which determine natural frequencies and modal shape are derived by means of the Rayleigh–Ritz method. Convergence and dependability of the current method are verified by comparing with the results of FEM and related literatures. In addition, some new results considering geometry parameters under classical and elastic boundary constraints are listed. The effects of geometry parameters, material parameters, and boundary constraints have been discussed in detail.

Thermal conduction in open-cell metal foams: Anisotropy and Representative Volume Element

It is well known that the manufacturing process of open-cell foams slightly elongates their struts in one direction, thus making them anisotropic; in turn, anisotropy affects foam characteristics. Furthermore, actual foams can be characterized with reference to a Representative Volume Element (RVE), defined as the cubic sub-volume having the same characteristics as those of the whole foam. An appropriately chosen RVE is very helpful to pass simulation data from a micro-to a macro-scale. The important role played by RVE in characterizing foams performance suggests further research in its determination, in order to reduce computational power and to allow to apply the volume-averaging technique to open-cell foams. In this paper, anisotropy and RVE for the effective thermal conductivity of open-cell metal foams are numerically analyzed. After scanning with Computed Tomography (CT) and postprocessing four open-cell aluminum foams with different porosities and the same Pores Per Inch (PPI) value, their morphologies are investigated in order to evaluate the effects of porosity on cells anisotropy. Foam cell elongation is quantified by an anisotropy ratio. Simulations are performed on CT data with a finite-element method to compute the effective thermal conductivities along three orthogonal directions, and results are compared with data published in the literature. A new correlation between effective thermal conductivity, porosity and direction is presented. The dependence of effective thermal conductivity on cells elongation is also pointed out. RVE sizes for different porosities, directions and inaccuracy thresholds are then investigated. Finally, an existing analytical model is suitably adapted to reliably predict the RVE size with a new correlations which accounts for the directionally-averaged effective thermal conductivity.

Modelling the pressure drop of two-phase flow through solid porous media

This contribution discusses a physically meaningful model for the description of the pressure drop of two-phase flow through bi-continuous solid porous media like metal sponges (open-cell metal foams) at high mass fluxes (>25 kg m-2 s-1). Such conditions typically occur during flow boiling in tubes filled with solid sponges as a means to enhance heat transfer. We propose combining the homogeneous model for two-phase pressure drop in empty tubes and the Forchheimer equation describing the pressure drop of single-phase flow in porous media. For known geometrical characteristics of the sponge (i.e. the porosity, the specific surface area, and the window diameter), different prediction methodologies for the Forchheimer coefficient based on models developed for the single-phase pressure drop are compared. The validity range of the model is determined on the basis of all available literature data. If the geometric properties of the sponge are known, the model can predict 80% of all experimental data with a deviation of less than 30%. For experimentally determined single-phase Forchheimer coefficients, 96% of all data are predicted within 30% uncertainty.

Multiscale characterisation and simulation of open cell metal foams

AbstractThe complex microstructure of open cell metal foams results in beneficial global material properties like a good weight to stiffness ratio, which makes this special group of cellular materials interesting for several applications. Open cell metal foams are suitable for lightweight applications as well as for energy absorbing systems. The dependency of the entire samples' mechanical behaviour on their microstructure induces the demand of experiments and simulations on different scales. Hence, a couple of experiments are necessary to specify the material properties of open cell aluminium foams. On the macroscopic level, several tests with different load cases such as pure tension, pure compression or pure torsion and the superposition of these loading conditions are needed to obtain the yield surface for the different types of foams. The material parameters on the microscopic level can be identified by microtensile tests and inverse calculations using a 3D model of the strut. The deviation of the simulation results from the experimental data is minimised by an optimisation. Thus, the used material parameters in the simulations are changed until the numerical results match the experiments. This contribution focuses on the characterisation of open cell metal foams on different hierarchical levels. It introduces an approach to describe the macroscopic material behaviour with a homogeneous material model based on micromechanical experiments and material parameters.

Charging nanoparticle enhanced bio-based PCM in open cell metallic foams: An experimental investigation

In this paper, an experimental investigation is carried out to examine the melting process of nanoparticle enhanced phase change material (i.e., nano-PCM) inside a metal foam enclosure under constant heat flux boundary condition. Visualization experiments were carried out using a bio-based nano-PCM (i.e., copper oxide (CuO) nanoparticles dispersed in the bio-based coconut oil PCM) inside an open-cell metal foam. Rectangular blocks, made from aluminium metal foam of pore density of 5 PPI and porosities of 88%, 92%, and 96%, were considered. An experimental setup was constructed to track the evolution of the melting process and observe the transient variation in temperature. Temperatures were measured at selected locations inside the nano-PCM filled metal foam. Melt fraction was calculated by means of image analysis. Experimentally obtained images corresponding to the melting process of PCM, nano-PCM, PCM in metal foam, and nano-PCM in metal foam are presented for selected times and applied wall heat fluxes. Corresponding melt fractions and energy storage rates are calculated and presented as well. The results showed that utilizing both nanoparticles and metal foam increase the melting and energy storage rates. The results further show uniform melting for low porosity (88%) porous medium as heat is transferred primarily by conduction. For high porosity (96%) porous medium, non-uniform melting, e.g., more melting at the upper part compared to the lower part is observed as heat is transferred by convection at the upper part and by conduction at the lower part. Outcome of the current research can potentially be applied to latent heat thermal energy storage systems, hybrid-electric vehicles’ rechargeable prismatic-battery thermal management systems, and electronic cooling systems in remote locations. The melting process is enhanced by 1.2% when nanoparticles were added to the PCM; however, higher enhancement was observed, i. e. 41.2%, when the metal foam was embedded in the pure PCM at 1814 W/m2 and 2160 s. The energy stored rate accelerated by utilizing the metal foam in comparison with the pure PCM and the nano-PCM, i. e. 2.61% by adding nanoparticles and 28.81% by utilizing the metal foam at 2835 W/m2 and 2280 s.

The Influence of the Washcoat Deposition Process on High Pore Density Open Cell Foams Activation for CO Catalytic Combustion

Spin coating was evaluated as alternative deposition technique to the commonly used dip coating procedure for washcoat deposition on high-porosity metallic substrates. By using spin coating, the washcoating of metallic open cell foams with very high pore density (i.e., 580 μm in cell diameter) was finely controlled. Catalytic performances of samples prepared with conventional dip coating and spin coating were evaluated in CO catalytic combustion in air, using palladium as active phase and cerium oxide as carrier. The incipient wetness method was used to prepare catalytic powder, which was dispersed by means of an acid-free dispersing medium. After washcoating, deposited layers were evaluated by optical microscopy and adhesion test. In comparison to dip-coated samples, the use of spin coating demonstrated better performances from both catalytic and coating quality points of view, highlighting the possibility of the industrial adoption of these supports for process intensification in several catalytic applications.

An experimental study on the thermal and hydraulic characteristics of open-cell nickel and copper foams for compact heat exchangers

High-porosity open-cell metal foams made of copper and nickel were inserted into a rectangular channel containing R245fa refrigerant with mass flux ranging from 76 to 391 kg/m2 s. The single-phase heat transfer from the surrounding channels is driven by hot water in a closed cycle and ranges from 0.53 to 1.37 kW. The heat transfer and pressure drop across the embedded metal-foam samples were measured while maintaining constant conditions of the refrigerant inlet pressure and hot water supply. The heat transfer coefficient was calculated based on a modified Wilson-plot method. The results were compared with published correlations, and new correlations are proposed to take into account the characteristics of the foam geometry and material. In addition, three parameters are introduced to compare different types of metal foam samples. The results of the experiment show that the metal-foam-filled channel increases the Colburn j-factor by up to 6.3 times compared to an empty channel. As a result, the metal foams with higher pore density and higher thermal conductivity perform better than other samples in single-phase flow.

Experimental study on confined metal foam flow passage as compact heat exchanger surface

This work focuses on forced convection through asymmetrically heated rectangular confined porous flow passage of aspect ratio of 50 (width/height = 150/3). Heat transfer characteristics of the porous flow passage were investigated with four different open-cell aluminium metal foams (MF) as porous media. Two sets (four samples) of aluminium foams of 20 ppi and 40 ppi pore densities with each set had two different relative densities (RD) of 9–11% and 12–16% (as per supplier's specification). One set of MFs was uncompressed (9–11% relative density) while another set was obtained by unidirectional compressing the MFs of 6–8% RD and 6 mm height flow passages. The study was systemically conducted with constant heat flux at the bottom surface of the flow passage, and on the basis of compact heat transfer surface analogy. Steady state condition was achieved before acquiring data. Temperature distribution along flow direction was investigated to study thermal development of air flow through it. It was found that thermally developed flow exists in such confined flow passages. Moreover, as a case study of a potential application, a systematic method was followed to obtain performance curves for all the flow passages as compact heat transfer surfaces for use in a Polymer Electrolyte Membrane Fuel Cell(PEMFC). The performance curves were utilised to obtain the best compact heat transfer surface for its possible use in a PEMFC.

Experimental investigation on the melting behavior of phase change materials in open-cell metal foams in an inclined rectangular enclosure

To explore the heat and mass transfer characteristics during phase change, a well-designed test rig with melting front visualization was designed and fabricated. The results show that the different inclination angles have a great influence on the melting process of pure paraffin. Compared with the 60-degree inclination, the full melting time of pure paraffin at 30-degree obliquity decreased by 25.30%. The visualization demonstrated an inclined melting front, which indicated the contribution of local natural convection in the fluid phase. However, there seems to be little influence of inclination angles for the melting process of paraffin embedded in metal foam. Local thermal non-equilibrium between metallic ligaments and the saturating PCM (paraffin) was experimentally observed. This revealed the feasibility of two-equation model for modelling the phase change process in open-cell metal foam.

The Introduction of Thin Open-Cell Metal Foams and their Wider Engineering Applications

Metal foams, having a great of specific surface areas and three-dimensional open cell structures, can be used for electromagnetic shielding and thermal management in electronics, battery electrodes in new energy, catalyst carriers in chemical engineering and lightweight structure in aerospace. The key performance indicators of these open cell metal foams are how to effectively control their pore structures such as pore diameter distribution and porosity and how to make it thinner that can meet different requirements for the component preparation. Summarized about three manufacturing processes are usually used to build metal foams. The first is well-known as physical process. The second is chemical or electrochemical method. The third is a combined process between physical and chemical processes. No matter what kind of process is selected to manufacture open cell metal foam, the specific surface area directly related to the microstructure of the foam is an important parameter in a material selection and design for its application.This paper will introduce a special powder composite plus manufacture process that is developed by Jiangsu Green Materials Hi-Tech. Co. Ltd. The leading manufacture process is a new one that is combined by powder metallurgy, casting, deformation and physical-chemical synthesis. It is environmentally friendly and recyclable from raw material selection, manufacturing process to thermal-mechanical treatment. This paper will also focus on the introduction of the microstructure characterization of open-cell metal foams such as copper, nickel, iron, silver and their alloying foams that we manufactured and give some examples to demonstrate their potential applications in the field of new energy, such as being an electrode for lithium-ion battery, membranes for fuel cell and super-capacitor, in the field of electronic engineering such as thermal management and electromagnetic shielding, in the field of chemical engineering such as separation and catalysts. These examples show their lead roles of these open-cell metal foams and different applications by our developed process.

Effect of porous media on the heat transfer enhancement for a thermal energy storage unit

Thermal energy storage (TES) can effectively recover thermal energy from low-temperature waste heat and it has now been received increasing attentions in practical engineering applications. Nevertheless, the relatively low thermal conductivity of engineering available phase change materials (PCMs) greatly limits the energy efficiency of TES applications. To enhance the phase change process, open-cell metal foam with a porosity of 0.94 and pore density of 15 PPI (pore per inch) was employed to be inserted either in heat transfer fluid (HTF) or in phase change material (PCM). A two-dimensional axis-symmetric problem was numerically solved and was validated through comparing temperature history at selected points. Results demonstrated that the involvement of open-cell metal foam can effectively enhance the phase change heat transfer, greatly reducing the full melting time. By comparing the four cases (without metal foam, inserting metal foam into HTF, PCM and both domains), the case that both HTF and PCM domains were embedded with porous media can provide the best heat transfer enhancement, from which practical applications with thermal engineering may benefit.