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The local heat transfers of a thermally developing region in a rectangular channel filled with porous metal foam are investigated experimentally. A thin metal foil technique and thermal IR imaging are adopted for the measurement of the local temperature distribution. An open-cell metal foam made from copper having a porosity of 0.96 is used. The pore density of the foam is 30 P P I (pores per inch). Fluid flow characteristics like permeability and foam drag coefficient are measured by conducting local pressure drop experiments. Additionally, by utilizing local pressure measurement data, the non-dimensional pressure coefficient is quantified. The non-dimensional pressure coefficient increases in the streamwise direction, irrespective of the Reynolds number, and remains almost constant in the spanwise direction for Reynolds numbers greater than 2000. The effect of the metal foam thickness on local heat transfer and pressure drop is investigated for 13 and 20 mm thick porous metal foam. It is compared with a smooth channel to quantify the heat transfer augmentation in a channel filled with metal foam. Compared to a smooth channel, the channel with 20 mm thick metal foam shows 14 to 25 times augmentation in the Nusselt number. Similarly, the channel with 13 mm foam shows 10 to 15 times augmentation in Nusselt number. The increasing trend of the local Nusselt number is observed in a streamwise direction. The different criteria are studied to understand the thermal performance evaluation. In the Constant Pumping Power Criterion (CPPC), enhancements ranged from 3.14 to 6.72 for a 20 mm thickness and 2.62 and 4.34 for a 13 mm foam thickness. Similarly, in the Performance Evaluation Criterion (PEC), enhancement ranges from 2.23 to 4.83 for a 20 mm foam thickness and 1.89 to 3.16 for a 13 mm foam thickness. A generalized correlation is suggested to describe the average Nusselt number considering the foam material, foam thickness, pore density, and porosity as parameters. [ABSTRACT FROM AUTHOR]

This study intends to address the inquiry of finding the practical ranges of flow conditions during flow boiling of R134a in the presence of copper foam inserts based on analyzing the second law of thermodynamics. The experiments are exercised under mass velocities from 50 kg/m2s to 130 kg/m2s within copper tubes of 3/8 in., while the uniform boundary condition of 5.0 kW/m2 heat flux is fixed for all tests. Metal foam pieces consisting of 10 and 20 PPI pore densities are employed in diverse arrangements. The results reveal that the metal foam inserts systemically affect the dominant entropy generation mechanism during the flow boiling, i.e., heat transfer or friction. The total rate of generated entropy in metal foams is directly affected by vapor quality and mass velocity, especially at high fluid velocities, when frictional pressure drop is the determining mechanism of entropy generation. In addition, at a fixed quality, the number of pores in copper foam inserts affects the rate of total entropy generation, except for the lowest mass velocity. The performance assessment is conducted by computing the entropy generation number. According to this criterion, metal foams show a promising performance under low vapor quality and mass velocity conditions, where the flow patterns are most likely to be slug or stratified–wavy. Eventually, the development of two correlations is undertaken for the estimation of friction and heat transfer contributions to the rate of entropy generation with mean errors of 9.40 % and 5.54 %, respectively. [Display omitted] • The effect of metal foams on flow boiling was studied from the entropy generation point of view. • The heat transfer contribution to entropy generation was dominant for plain tubes. • For metal foams, frictional entropy generation was the primary irreversibility source. • Metal foam PPI increases the frictional and total entropy generation. • Metal foam inserts are recommended for low mass velocity and vapor quality. [ABSTRACT FROM AUTHOR]

Fundamental understanding of turbulent flow and heat transfer in composite porous-fluid systems, which consists of a fluid-saturated stochastic open-cell metal foam and a flow passing over it, is crucial for fostering technological development in numerous applications such as transpiration cooling in aerospace, packed-bed thermal storage and thermal management of electronic devices. In this work, conjugate heat transfer simulations were adopted to explore the turbulent flow and heat transfer features in a composite porous-fluid system at the pore scale. Simulations were performed to account for the influence of the blockage ratios (i.e., BR = 0.5, 0.8 and 1.0) on pressure drop and heat transfer rate by introducing a new concept called penetration cooling length. Furthermore, the effect of Reynolds numbers (i.e., Re = 1800, 3600 and 7200) at different blockage ratios was investigated in terms of pressure drop, fluid and solid temperatures, interstitial heat transfer coefficient, and flow leakage. Results indicate that for a fixed blockage ratio, as the Reynolds number increases by a factor of 3.0, there is a 14.9-fold increase in the pressure drop and a 2.9-fold increase in the interstitial heat transfer coefficient. Additionally, for a fixed Reynolds number, when the blockage ratio increases by a factor of 2.0, there is a 6.8-fold increase in the pressure drop and a 1.8-fold increase in the interstitial heat transfer coefficient. Flow visualization indicated that the penetration cooling length is influenced by flow leakage from the porous-fluid interface. Results show at small blockage ratios and low Reynolds numbers, a significant portion of the flow from the porous region leaves it to the clear region on top of the porous block. While, at high Reynolds numbers and large blockage ratios, the flow leakage is reduced. Additionally, for a low blockage ratio (BR < 0.5), the amount of flow leakage depends on the Reynolds number, while it is independent of the Reynolds number for BR > 0.8. Finally, this study introduces a novel correlation for the interstitial heat transfer coefficient derived from conjugate heat transfer pore-scale simulations. The correlation is established on the basis of Reynolds number, blockage ratio, and development length of the metal foam, offering valuable insights into heat transfer phenomena in porous media. Modelling conjugate heat transfer in a composite porous-fluid system with stochastic open-cell metal foam. Simultaneous effect of blockage ratio (B R) and Reynolds number (R e) on interstitial heat transfer coefficient contour; First row: B R = 0.5; Second row: B R = 0.8; Third row: B R = 1.0. [Display omitted] • Turbulent heat transfer of flow in composite porous-fluid systems is studied. • Pore-scale conjugate heat transfer analysis is performed. • A new concept termed penetration cooling length (PCL) is introduced. • The dependency of PCL on Reynolds number (Re) and blockage ratio (BR) is studied. • For a fixed BR, if Re increases by 3.0 times, the pressure drop rises by 14.9-fold. [ABSTRACT FROM AUTHOR]

In this study, results of a numerical simulation of the 3D laminar mixed convection in an annulus between two confocal elliptic cylinders are presented. The inner cylinder is heated with a constant heat flux, and the outer cylinder is adiabatic. The annulus is filled with isotropic and homogenous metal foam. This system is used to heat a water flow that is axial, uniform and isothermal at the annulus entrance. The slow flow and the heat transfer through the porous annulus are modeled by the continuity equation, the Darcy-Forchheimer-Brinkman flow model, accounting for the significant buoyancy, and the energy equation. The model equations are numerically solved by a validated finite volume numerical method with a second order accurate spatiotemporal discretization. This study details the synergy of the combined effect of the forced flow, the buoyancy and the variable fluid properties on the flow and thermal fields. It is found that the synergy is affected by the geometric aspect ratios, the annulus orientation, the structure and properties of the metal foam and the flow dynamic and thermal controlling parameters. For the same heat input and flowrate, it is found that better mixed convection heat transfer is obtained with the horizontal mounting when the ellipticity of the inner cylinder is relatively high, when the cross-section is rotated such that the major elliptic axis is parallel to gravity and when the annulus thickness is reduced. Moreover, the heat transfer is reduced when the PPI and the porosity of the metal foam are increased. Furthermore, the heat transfer is improved when the Grashof number is increased, the Reynolds number is decreased, or the foam metal is changed from aluminum to copper. • Better heat transfer with horizontal mounting and high ellipticity. • Major elliptic axis parallel to gravity improves mixed convection. • Decreasing the PPI, the porosity and the Reynolds enhances the heat transfer. • The thermal performance is improved when the Grashof number is increased. • The copper metal foam has a better thermal performance than the aluminum one. [ABSTRACT FROM AUTHOR]

The utilization of phase change materials (PCMs) holds tremendous potential of heat storage domain. The PCM's refractory at the latent heat thermal energy storage (LHTES) unit bottom hinders the heat storage efficiency, despite the significant improvement in thermal conductivity achieved through the addition of metal foam. This study employs numerical simulation to examine the impact of applying bottom cross-cut on PCM's spatial distribution in a horizontal LHTES unit. The manuscript analyzes parameters including melting fraction, complete melting time, Rayleigh number, natural convection heat transfer gain, melting phase interface, velocity and temperature distributions, and heat storage. The findings indicate that the proximity to the heating tube results in a reduction of solid volume at the LHTES unit bottom. A 0.6 bottom cross-cut ratio leads to an 18.84 % faster heat storage rate compared to a concentric-circle unit. Furthermore, a bottom cross-cut ratio of 0.5 enhances natural convection heat transfer gain by 3.28 times. [ABSTRACT FROM AUTHOR]

An energy saving way for electronics cooling during peak demand is represented by phase change materials (PCMs), which could maintain the temperature of the electronic equipment at low values by using the latent heat of melting without requiring any power input. Among the organic PCMs, paraffins are widely available in a large range of melting temperatures. This paper explores the possible use of three paraffins with different melting temperatures. Each paraffin is embedded in three different aluminum foams, having approximately the same volumetric porosity (92–93%), but different linear porosity (5, 10 and 40 pores per linear inch). The system made of foam and paraffin is electrically heated from one side by applying three different heat fluxes (10, 15 and 20 kW m−2). Results are given in terms of temperature distribution both of the heated side of the foams and of the paraffin inside the foams, and the effects of the melting temperature of the paraffins and of the linear porosity of the foams are highlighted. In addition, an empirical correlation is proposed to model the paraffin melting phenomenon. • Phase change heat transfer of paraffin waxes is experimentally studied. • Three aluminum foams were used as solid medium to enhance the thermal conductivity of the system. • The results of paraffin melting inside metal foams are compared against a system without foam. • An empirical correlation was proposed to model the paraffin melting phenomenon. [ABSTRACT FROM AUTHOR]

Abstract Previous research has proved that pyramidal core sandwich panel is an excellent kind of passive Thermal Protection Systems (TPSs) for hypersonic aerospace aircrafts suffered from in-service environment. To explore its further insulation performance and load-bearing capability, four kinds of actively cooled pyramidal core sandwich panels are designed and investigated in this paper. Convection flow mechanism for cooling channels is described in details. Influences of the coolant velocity, channel height ratio and flow direction of the two layers on thermal performance are firstly studied and discussed in an appropriate way. Analysis of thermomechanical performance is then performed. The effect of the cover board thickness on thermomechanical performance is also studied. In consequence, a light-weight, structural and thermal integrated thermal protection system (ITPS) has been designed to protect hypersonic aircrafts from intense radiation heat flux in service. Highlights • Four kinds of actively-cooled pyramidal sandwich structures are studied. • Thermal and thermomechanical mechanism of various effects are described. • A structure with good insulation and load-bearing capability is proposed. [ABSTRACT FROM AUTHOR]

Abstract 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. Highlights • Melting of paraffin with volume change in copper foam is simulated at pore scale. • Enthalpy-porosity method, VOF method and Weaire-Phelan foam cell are employed. • Pore-scale details of phase, temperature, heat flux and flow fields are obtained. • Phase change physics of paraffin in metal foam is more thoroughly revealed. • Melting rate is increased to 2.7 times by using high porosity (0.974) copper foam. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

Abstract Highly porous cellular media such as plastic, ceramic and metal foams present specific characteristics that make them interesting for a number of applications related to thermal engineering. In many of these applications, thermal radiation heat transfer can have a significant, ranging from a 20–30% effect for plastic foam insulation to becoming the predominant mode of heat transfer at higher temperatures. To model radiation heat transfer in these media, accurate models of radiation and microscopic structure of the foam are required. Over time, the state of the art has evolved from basic approaches using radiative conductivity to multi-scale models based on equivalent homogeneous media and the Radiative Transfer Equation, with more recent techniques such as MPA and GRTE allowing to model multiple phases, non-Beerian behavior, and history effects. Still, comparisons of homogenized models with reference results are sparse in literature, and results mixed. Furthermore, what more advanced models afford in accuracy they lose in simplicity, leading to cumbersome calculations. In this paper, we combine in a novel way hybrid direct-inverse identification of radiative properties and accounting of history effects, to present modified and improved version of the standard Homogenous Phase Approach (HPA+) and Multi Phase Approach (MPA+), with the intent of increasing accuracy while keeping maximum simplicity. We then compare existing and new models with a reference model based on Monte Carlo Ray Casting. In the comparison, we consider a varied set of porous morphologies and boundary conditions. The improved models consistently outperform existing homogenized models. [ABSTRACT FROM AUTHOR]

Abstract Octet truss lattice structures are a potential candidate for developing multifunctional heat exchangers/sinks owing to their superior structural characteristics compared against stochastic open-cellular metal foams. Structural efficiency is achieved primarily due to increased nodal connectivity in octet-truss lattices. Designed octet trusses and periodic cellular materials, in general, lend themselves well to topology optimization and integration of multiple functions enabling a high degree of tailorability and superior performance benefits. Specific to heat transfer design, octet truss lattices can be tailored to achieve optimal strand cross-sectional area-to-length ratio for improved effective thermal conductivity while meeting surface-area per volume requirements for convective transport. In this work, we explore the heat and fluid flow characteristics using direct numerical simulations of a periodic unit-cell. Effective thermal conductivity, permeability, inertial coefficient, friction factor and Nusselt number for octet truss lattice structures are predicted and compared against stochastic foam experimental data. A range of porosities and pore densities are simulated and heat transfer and fluid flow characteristics are correlated using a single length scale in the form of Brinkman screening length, √K, as against existing literature where different geometric length scales are used. Further, octet truss lattice geometry is compared against existing stochastic metal foam heat and fluid flow empirical data. Normalized permeability of octet lattices were found to be 20–80% higher than stochastic foam for a given porosity. Similarly, for a fixed porosity, inertial coefficients of octet lattices were 50% of stochastic experimental measurements. For the range of Reynolds number considered, octet truss lattices show similar fluid and heat flow characteristics if not better performance than stochastic foams and thus making them ideally suited for tailored multifunctional heat sinks and exchangers. Highlights • For all practical purposes, based on the range of Reynolds number simulated, octet lattice structures can be treated as ordered form of a metal foam geometry as the existing metal foam data matches the octet lattice predictions. • Predicted thermal conductivity for octet-lattice structures compared well against existing thermal conductivity model. • Octet-truss lattice structures show lower or similar friction factor compared to stochastic foam data. • Single geometric length-scale, Brinkman screening length, is able to capture both fluid and heat flow characteristics. • Octet lattice structures are excellent candidates for lightweight compressive load bearing heat sinks. [ABSTRACT FROM AUTHOR]

Abstract The surface wettability determines the condensate mode and the adhesive force of condensate on the metal fiber surface, which can directly affect the heat transfer and pressure drop characteristics of wet air in metal foams. In the present study, the heat transfer and pressure drop characteristics of wet air in metal foams with different surface wettability under dehumidifying conditions were investigated experimentally. The tested samples include the hydrophobic, hydrophilic and uncoated metal foams, and the experimental conditions cover the inlet air temperatures of 27oC-35 °C, the inlet air relative humidity of 30%–90% and the air velocities of 0.5–1.0 m/s. The experimental results show that, the heat transfer coefficients in hydrophobic and hydrophilic metal foams are larger by 4%–33% and 3–21% than that in uncoated metal foams, respectively, and the heat transfer enhancement of the hydrophobic coating is more obvious; compared with the uncoated metal foams, the pressure drop in hydrophobic metal foams is increased by 3%–139%, while the pressure drop in hydrophilic metal foams is reduced by 1%–20%. The heat transfer and pressure drop characteristics of wet air in metal foams were evaluated by the comprehensive performance index (j · f −1/3), and the comprehensive performance of hydrophilic metal foam is better than that of hydrophobic and uncoated metal foams. Highlights • Experiments for wet air in metal foams with different wettability are conducted. • New heat transfer and pressure drop data of wet air were obtained. • Effect of hydrophilic and hydrophobic coatings were compared. • Effect of surface wettability on performance of metal foams was analyzed. [ABSTRACT FROM AUTHOR]

In the present study, fully developed forced convection in tubes partially filled with composite metal foams (CMFs) is numerically investigated. In the CMF region, the Brinkman extended Darcy flow model and the local thermal non-equilibrium model are used to predict fluid and thermal transport. At the foam-foam interface in CMF, fluid temperature and solid energy are assumed to be continuous. At the foam-fluid interface, no-slip coupling conditions are used to couple flow and heat transfer of the foam and free regions. Governing equations are solved numerically with SIMPLE algorithm. Momentum and energy equations are discretized using the SGSD finite volume scheme for convective terms. Velocity distribution, temperature profile, the friction factor and Nusselt number in partially-filled CMF tubes are obtained. The results indicate that, porosity gradient has little effect on the friction factor at a low pore density, but the effect increases with increasing pore density. For the fixed pore density gradient, Nusselt number decreases with increasing porosity, and Nusselt number firstly increases sharply and then increases lineally with increasing Reynolds number. For the fixed material gradient, Nusselt number increases lineally with increasing Reynolds number. [ABSTRACT FROM AUTHOR]

The present study investigates the thermal performance and fluid characteristics of counter flow heat exchangers (CFHEs) computationally. Increasing heat transfer rate in heat exchangers depends on various parameters such as channel geometry, the geometry of obstacles in the channel, frequency and amplitude of corrugated plate, and other factors. In this simulation, the influence of parameters contains the effects of the geometry of the obstacles, the corrugated plate between the channels, the elastic plate between the channels, the frequency and wavelength of the corrugated plate, and its effect on increasing the heat exchanger efficiency have been studied in detail. The numerical method is the finite element method, and a complete set of Navier-Stokes equations are solved unilaterally using the Petrof-Galerkin method. It has been concluded that all methods will improve the performance of the CFHEs. Among the obstacles placed in the channels, the rectangular and edge obstacles show better results. In the case of a corrugated mid-plate, with increasing the wave in mid-plate between channels, heat transfer rate (HTR) increases to a certain amount. It was observed that the elasticity of the mid-plate between two channels and the sinusoidal pulse flow would improve HTR in CFHEs. [ABSTRACT FROM AUTHOR]

Open-cell metal foam has been increasingly applied in heat exchangers to enhance heat transfer. In this work, the coupled radiation-convection in a rectangular channel filled with metal foam under constant wall temperature is simulated numerically. Discrete-scale approach (DSA) and continuum-scale approach (CSA) are respectively developed for the simulation. In DSA, foam domain is constructed through numerous Kelvin structures, and pore scale field is obtained via CFD simulation with re-radiation flux calculated by surface to surface model; while in CSA, local thermal non-equilibrium model (LTNE) is employed with Monte Carlo method to solve radiative heat transfer, and volume-averaged correlations are determined based on Kelvin structure. The flow field and heat transfer at pore-level generated from DSA are analyzed. By means of a representative volume-averaged post-processing method, thermal behaviors from DSA and CSA are compared in terms of pertinent parameters. It shows that spatial fluctuation phenomenon is observed in pore-level temperature field. The overall heat transfer performance characterized by local Nusselt number is enhanced by inclusion of thermal radiation, and the effect is significant for lower pore density and higher porosity case. Increasing the inlet velocity, pore density or decreasing the porosity all can lead to heat transfer enhancement more or less. For all the cases studied, DSA and CSA can predict the temperature field consistently while the discrepancy between them is relatively larger in the local thermal non-equilibrium region. Additionally, it reveals that CSA is not applicable for accurately predicting energy transport in large gradient heat transfer region due to its volume-averaged characteristic. [ABSTRACT FROM AUTHOR]

In this paper, a visualization and measurement investigation is performed on melting phase change of paraffin in high porosity open-cell copper foam at pore scale. With the aid of high definition camera and infrared camera, the phase and temperature fields of paraffin-foam and pure paraffin samples are captured to visualize the detailed melting phenomena. The local temperatures of both paraffin and copper foam matrix are measured by the thermocouples to study the thermal response characteristic and local thermal non-equilibrium effect. The experimental results reveal multiple pore-scale physical processes including the evolution of solid-liquid phase change interface, volume expansion of paraffin, motion of solid paraffin and release of air bubbles from paraffin. Under the combined influences of these physical processes, it shows that copper foam with a high porosity of 0.974 effectively extends the phase change interface and improves the thermal response rate of paraffin. As a result, the melting rate of paraffin has increased by more than two times, while the reduction in the amount of latent heat is only 2.6%. Moreover, significant effect of local thermal non-equilibrium between paraffin and copper foam is found during the melting process of paraffin-copper foam sample. Finally, through generalizing results from present experimental measurements, a correlation for dimensionless melting time of the paraffin-copper foam sample at pore scale is developed. The present investigation can contribute to a better understanding of melting in metal foam and provide guidance for the application and performance analysis of paraffin/metal foam composite. [ABSTRACT FROM AUTHOR]

Metal foams are metallic materials that contain a high percentage of porosity. They can be manufactured as open-cell or closed-cell foams. Open-cell metal foams are ideal for thermal applications due to the high ratio of surface area to volume, which provide more surface contact between the material and the fluid. In this work, a detailed approach to utilize micro-computed tomography scan to create a detailed and accurate 3D metal foam model is discussed. The model is implemented into a finite element analysis software and used in an application of heat sinks. Experiments are performed to record the temperature profile of the heat sink, and then the heat transfer coefficient h for the numerical model is calibrated using the temperature data. The 3D metal foam heat sink model is validated against experiments as well. Finally, the complete validated model is used to study the thermo-mechanical behavior of 1, 2, and 3 fins metal foam heat sinks. The developed model and approach provides more insight into the coupled behavior of metal foam heat sinks by being able to calculate the developed stresses due to the thermal loading of the heat sinks. [ABSTRACT FROM AUTHOR]

Two ideal three-dimensional geometries are generated using cylinder-joint and sphere-subtraction methods, respectively. The former is firstly used to establish simple geometrical relations among average pore diameter, strut diameter, ligament diameter, porosity, and specific surface area. An analytical model of heat transfer in a foamed heat exchanger is then proposed using the transfer matrix method and one-equation model. The heat transfer predictions have been compared with available experimental data and favorable agreements have been obtained. Heat transfer coefficient and pressure drop in both two geometries with different porosities and cell dimensions are investigated using COMSOL Multiphysics. Pressure drop verifications among numerical results, semi-empirical models, and experimental data show good feasibility of Kelvin structure. The proposed Kelvin geometric model is shown to be self-consistency with respect to both its geometrical characteristics and thermal properties, which can be adopted for finer details of pressure drop phenomenon. [ABSTRACT FROM AUTHOR]

The combination of the high conductivity of nanofluids with the huge accessible surface area of open-cell metal foam would indeed be a superb heat exchange technology. While individual ingredients of this technology have been separately investigated, the combination of the two has received only scant attention. Actually, there are only few studies that are focused on nanofluids flow and heat transfer in metal foam. This paper is intended to serve as an orientation for researchers who are interested in both nanofluids heat transfer and metal foam. The paper thus includes discussion of various aspects of nanofluids and their heat-transfer applications, as well as fundamentals of flow and heat transfer of ordinary fluids in metal foam. A section on recent heat transfer studies in metal foam is included, and another section on pore-scale modeling of transport in metal foam is provided. Nanofluids are presented in terms of their definition, brief history, preparation methods, theoretical models, properties, and engineering applications related to heat transfer. The paper then moves to describe metal foam and its morphology, as well as current understanding and theoretical treatment of flow and heat transfer in this class of highly-porous media. Key heat transfer and flow properties of the foam are outlined. The paper also presents the case for nanofluids forced convection heat transfer in open channels to serve as a background for the more complex case that involves heat transfer in porous media in general. The case of forced convection of nanofluids in metal foam in particular is presented next. Studies including complex effects, e.g. magnetism, are briefly described. Based on the above information, some key findings, conclusions and recommendations are made for interested researches. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper presents the analysis of entropy generation, exergy and heat transfer through metal foams partially and completely filled in a vertical channel. The assembly of heater located in between two aluminum plates is located at the core of the channel and for the purpose of heat transfer augmentation aluminum metal foam of pore density 10 with porosity 0.95 are located on either side of the plates. Four different filling rates of 25%, 50%, 75% and 100%, and three metal foam thicknesses of 10 mm, 20 mm and 30 mm are examined in this research for a fluid velocity ranging from 0.1 to 3 m/s. Darcy Extended Forchheimer (DEF) model and local thermal non-equilibrium (LTNE) model is used for forecasting the flow features and heat transfer through the metal foam. The numerical methodology adopted in this research is confirmed by comparing the results with the literature and found fairly good agreement between them. The flow physiognomies in terms of friction factor, heat transfer performance in terms of Nusselt number and performance factor, exergy transfer in terms of mean exergy based Nusselt number, and entropy generation i.e., total irreversibility in terms of Bejan number and entropy generation number are presented and discussed. Results showed that the effect of partially filled metal foams are found to be better in terms of entropy generation, and exergy transfer, and the effect of higher metal foam thickness is found to be better in terms of exergy and heat transfer, but in terms of total entropy generation resulting in terms of both heat transfer and pressure drop smaller metal foam thickness exhibits better performance. • Numerical study is performed for metal foams heated from heart of the vertical channel for heat transfer augmentation. • Integrated Darcy Extended Forchheimer and local thermal non-equilibrium models are used for the prediction of convection parameters. • Optimum filling ratio of the metal foam for enhancing the thermodynamic performance is evaluated. • The thermodynamic performance is assessed based on second law analysis. • The parameters evaluated are exergy transfer, thermal and frictional entropy generation along with total entropy generation. [ABSTRACT FROM AUTHOR]

In this study, the effect of pulsating air flow on heat transfer was investigated experimentally, while constant heat flux was applied with an electric heater to the side surface of the cylindrical duct with metal foam inside. Reynolds numbers in the range of 5932–8592 were studied at three different fan speeds. Pulsating flow experiments were carried out in the range of 12.0–27.9 Womersley numbers. It has been observed that the surface and air temperatures are not affected by the flow frequencies in this range and are independent of time. With the data obtained, Nusselt number values were calculated for the fully developed region, and the Nusselt number ratio was defined in order to compare the steady flow with the pulsating flow. Nusselt correlation was established for the fully developed region depending on the Reynolds number and the Womersley number in the studied range. It has been determined that the effect of frequency on the heat transfer in the case of pulsating flow is weak, but the heat transfer tends to decrease as the frequency increases. • Experimental investigation on heat transfer of pulsating air flow in metal foam. • Fully developed pulsating flow heat transfer coefficient is determined empirically. • Mass flow frequency of pulsating flow affects weakly heat transfer in this range. • Time-averaged Reynolds number is main parameter for heat transfer in this range. [ABSTRACT FROM AUTHOR]

In this paper, the pressure drops and interfacial heat transfer coefficient in metal foam with different pore density which was constructed by tetrakaidecahedron models were numerically studied under high velocity by Computational Fluid Dynamics (CFD). The velocity fields, temperature fields and pressure fields in the computational domain of metal foam were investigated. The equation of pore density and hydraulic diameter of metal foam was obtained. Influence of flow rate on the whole porous zone was also studied and obtaining the fitting formula to predict pressure loss in porous metal which was applied to the pore density range from 10PPI(Pore Per Inch)to 40PPI at high flow rate. Comparing with the previous literatures, the maximum error of pressure drop value was 12.23%. Meanwhile, the relationship between Reynolds number and interfacial heat transfer coefficient was studied, the results shown that the values of heat transfer coefficient increase with the rising of Reynolds number, but the lifting rate decreases gradually. And according to the data points, the numerical formula of interfacial heat transfer coefficient in porous metal was obtained which had a great correctness through contrasting with other data. As a result, the fitting formula could accurately predict the convection heat transfer coefficient of the metal foam interface and provide relevant data for the Thermal Non-equilibrium Model. • The heat transfer and fluid flow characteristics in foam metal under high Reynolds numbers were numerically studied based on tetrakaidecahedron model. • A new correlation for pressure drop in foam metal under high Reynolds numbers was obtained. • A new correlation for interfacial heat transfer coefficient in foam metal was obtained. [ABSTRACT FROM AUTHOR]

In this study, a thermal energy management system that combines passive cooling, heat storage and electrical energy harvest is proposed by using foam/PCM composite and thermoelectric generator (TEG), which are separately fixed upon and under the heat source as the coolers of heat source. Foam/PCM composite is also aimed to enhance the latent-heat energy storage and passive cooling. Results show that the control case of using solitary metal foam harvests the most thermoelectric energy, but has the risk of leading heat source to thermal failure. Using pristine PCM can reduce the wall temperature and prolong the thermal management time during phase change. However, the intrinsic low thermal conductivity of pure PCM still results in high temperature of heat source when the PCM is in solid stage. Attributed to high thermal conductivity and latent heat storage, the foam/PCM composite presents the lowest heat-source temperature, but at the cost of delivering the least thermoelectrical power. [ABSTRACT FROM AUTHOR]

The rapid growth of the internet of things has induced the integration of many microelectronic devices in physical objects, yet the generated heat decreases the performance and stability of microelectronic devices. Therefore, new materials such as gradient metal foams (GMFs) have been recently designed to improve heat transfer. In this paper, an experimental visualization setup was built to investigate the effect of the GMFs gradient layers number and the arrangement order on the pool boiling heat transfer performance. Results show that increasing the number of gradient layers enhances the heat transfer when the copper foam pore density is low. By contrast, at high pore density of 50 PPI, increasing layers hardly changes heat transfer. The bubbles dynamic behavior on the metal foams surface of with different gradient structures is also different. When bubbles detach upward, the temperature of the metal foam is lower, and the temperature gradient is higher. When bubbles detach sideward, the bubble escape is much shorter, and the bubble detachment frequency and size increase. Combined with the theoretical research, the metal foam gas-liquid flow heat transfer model were constructed. The advantages and disadvantages of GMFs with different structures and the applicable scenarios are analyzed. • The effects of gradient layers number and arrangement order of multilayer GMFs on the pool boiling were investigated in detail. • By studying the dynamic behavior of boiling bubbles, two common gas-liquid flow models were obtained. • From the perspective of bubble detachment direction, two foam metal heat transfer models are established. • The enhance heat transfer mechanism of different structures GMFs was analyzed. • The advantages and disadvantages of different gradient structures and the applicable scenarios are analyzed. [ABSTRACT FROM AUTHOR]

While studies concerning heat transfer due to oscillating air (and few other gases) flow in metal foam are available, heat transfer due to oscillating water flow in metal foam has not been offered in the literature. This paper presents characteristics of heat transfer of oscillating water flow in commercial open-cell metal-foam pipe that were obtained experimentally, most likely for the first time. One main difference between gas and liquid flows in porous media is that dispersion is far more significant in the latter. Another difference is the length of the entrance region, which depends strongly on the Prandtl number. The trends in the cycle-averaged wall temperature, length-averaged wall temperature and cycle-averaged Nusselt number were similar to those for oscillating water flow in packed spheres and for oscillating air flow in aluminum, copper and graphite foams in a rectangular channel. For the higher flow frequency of the current study, the cycle-averaged Nusselt number was higher for higher flow displacement amplitude. [ABSTRACT FROM AUTHOR]

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. • Thermal performance of metal foams assessed experimentally and computationally. • Classical approach involving effective-medium models benchmarked against a real digital twin. • Digital twin gives consistent result compared to laser flash experimental data. • Effective-medium models prove to be invalid for metal-foam characterisation. • Metal foams defy classical laser flash analysis. [ABSTRACT FROM AUTHOR]

Nanoparticles have been widely used to enhance the thermal conduction of pure phase change materials (PCMs). However, their effects on the metal foam composite PCMs (MFCPCMs) are not clear. In the present work, we took pore-scale studies on the melting behavior of PCM enhanced by metal foam and Al 2 O 3 nanoparticles simultaneously. By comparing the melting performances of all models, we found that the models without metal foam are more conducive to the development of natural convection, and the nanoparticles have the maximum enhancement on the melting performance up to 7.77%. On the contrary, although the metal foam is helpful to the conductive heat transfer, it will weaken the convective heat transfer. The metal foam with large porosity has more violent natural convection than small porosity. The Al 2 O 3 nanoparticle has a more significant effect (2.55% reduction or −6.70% extension of the complete melting time) on the melting performance in a large porosity metal foam than that in a small porosity metal foam (1.91% ∼ 0.09% reductions of the complete melting time). As a result, the Al 2 O 3 nanoparticle with an appropriate concentration deserves to be added to the pure PCM or the MFCPCM with large porosity. • The melting of metal foam/nano-enhanced PCM is investigated at the pore scale. • The effect of metal foam porosity on the melting is clarified. • The effect of alumina concentration is numerically investigated. • The convective and conductive heat transfers in MACPCM are studied. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The porous metal foam subjected to air jet impinging is set above thin liquid fluid passing through the heating surface to maintain its thickness, in which the heat from heating surface is released in the convection with the thin liquid film to the surroundings, as well as across the thin liquid film and metal foam to the outside surface where the evaporation and convection occur. Local thermal non-equilibrium (LTNE) occurs, so the double energy equations together with Brinkman-Darcy model are employed to describe the heat transfer in metal foam, as well as N–S equations account for the flow of thin liquid film. The effects of the different porosity and particle diameter distributions (e.g. constant, linearly increasing/decreasing and stepwise increasing/decreasing), as well as thickness ratio between the bottom and upper layers with different porosities and particle diameters in stepwise variation on the cooling performances are numerically analyzed. The simulations are validated with the published experiment data. The lower heating surface and larger heat transfer coefficient (HTC) occur in the presented mode with the relatively lower porosity and particle diameter. The 67.47% rise ratio of the peak HTC in the mode with porosity of 0.3 happens than that with porosity of 0.9. The optimized thickness ratio together with the porosity and particle diameter in the bottom and upper layers influence the heat to be transported in the convection with liquid fluid to the downstream side or across the metal foam to its outside surface. All results can be considered into the promotion and application of coupling the air jet impingement and porous metal foam above thin liquid film to enhance heat transfer. [ABSTRACT FROM AUTHOR]

Aiming at solving the heat dissipation problem in a hydrogen unmanned aerial vehicle (UAV), this work uses an open metal foam as the enhanced heat transfer device in the air-cooling channel. The method of improving comprehensive heat transfer capacity and weight reduction by controlling the micropore directions and sizes in the metal skeleton is proposed. The real open cell metal foam structures are scanned and restructured using the X-ray computed tomography imaging system and corresponding computer software. The effects of micropore directions and sizes in the skeleton on pressure drops, conjugate heat transfer, volumetric heat transfer coefficient, comprehensive heat transfer capacity as well as weight reduction are discussed with the inlet velocity ranging from 10 m/s to 80 m/s. Results find that the micropore direction parallel to flow owns a higher comprehensive heat transfer capacity compared with that vertical to flow. There are existing a competitive relationship between heat conduction and heat convection in the metal skeleton. When the micropore size is 0.01, a best enhanced heat dissipation performance can be achieved. The structure with micropore size of 0.03 has a 8.49% weight reduction and 4.72–5.91% improvement of comprehensive heat transfer performance compared with that of the structure without micropores. The above findings can be applied to design a light and high comprehensive heat transfer device in the hydrogen UAV. • Real open-cell metal foam structure is captured by X-ray CT imaging system. • Micropore in skeleton influences on heat and mass transfer in PEMFC are analyzed. • Competitive relationship between heat convection and conduction is identified. • Comprehensive heat transfer improvement and weight reduction is realized. [ABSTRACT FROM AUTHOR]

Phase change material (PCM) are often used to realize thermal energy storage, which is regarded as an effective way to solve the problem of time difference between energy supply and demand. However, conventional phase change materials hinder the energy exchange rate and affect the storage efficiency due to their low thermal conductivity. To solve this problem, this study proposed three types of composite materials "nanoparticle/paraffin", "metal-foam/paraffin" and "nanoparticle/metal-foam/paraffin". The heat transfer properties of proposed composite materials were investigated based on the heat flux method and the heat storage and release test. The effects of metal foams and nanoparticles on the properties of phase change materials with different parameters were obtained, and the effect of the pore density of metal foams on the natural convection process of phase change materials was considered. The results show that the heat transfer performance of the composite phase change material is significantly improved compared with the traditional phase change material. With the increase of metal foam pore density, the heat transfer performance first increases and then decreases which caused by the enhanced obstruction to natural convection, 40 PPI pore density shows better comprehensive performance. In terms of thermal conductivity, nanoparticle/paraffin, metal-foam/paraffin (40 PPI) and nanoparticle-metal foam-paraffin (40 PPI) have been enhanced in turn, which are 123.27%, 740.39%, 847.15% higher than pure paraffin, respectively. The melting time and solidification time are shortened by 6.74%, 62.72%, 72.61% compared with pure paraffin and 26.46%, 49.47%, 38.62%. [ABSTRACT FROM AUTHOR]

In this paper, a new prediction model for the effective thermal conductivity (ETC) of high porosity open-cell metal foams is proposed by introducing the concave tri-prism foam ligament to the tetrakaidecahedron-based foam cell structure as well as considering the effects of ligament orientation and filling medium. The proposed model can avoid using the non-universal empirical parameter that has to be determined experimentally. For validation of the present model, the ETC measurements of copper foams saturated with air, water and paraffin, which are rarely reported in the literature, are also conducted in this work. These ETC data of copper foams, along with the ETC measurements of aluminum and nickel foams from the literature, are used to validate the present model. It shows that the present model, when compared with other ETC models reported in the literature, can better predict the ETC experimental results of high porosity metal foam, with the average prediction deviations for different metal foams (copper, aluminum and nickel foams) saturated with different mediums (air, water and paraffin) within 10%, which can be attributed to the adoption of relatively realistic foam structure and simultaneous consideration of the ligament orientation and filling medium effects in the present model. [ABSTRACT FROM AUTHOR]

As a new-type extending surface, metal foam owns great potential in next generation heat transfer technologies. Convective heat transfer performance in metal foams is numerically investigated based on the local thermal equilibrium (LTE) model and the local thermal non-equilibrium (LTNE) model. The solid–fluid temperature difference and relative deviation are put forward for quantifying LTNE effect. The effects of basic parameters on heat transfer are analysed in depth and the LTNE conditions in metal-foam tube for efficient heat exchangers are summarized. It is indicated that the relative deviation is a more suitable criterion for LTNE effect in metal foam than the solid–fluid temperature difference. The LTNE effect in metal foam is conspicuous for low porosity, large fluid–solid thermal conductivity difference, small duct size, low pore density, and low Reynolds number. Measures lowering proportion of local convective thermal resistance in total thermal resistance, or the ratio of thermal resistance of solid to that of fluid can weaken LTNE effect in metal foam. There is no necessary relationship between thermal performance of metal-foam heat exchangers and corresponding LTNE effect. Clarifying LTNE conditions in porous foams can lay a foundation for the demarcating criterion of LTE/LTNE models. This can also guide quick and accurate thermal design and verification of metal-foam heat exchangers. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation of the solid–liquid phase change process of three natural paraffin waxes, which show slightly different melting temperature: 53 °C, 57 °C, and 59 °C, at three heat fluxes: 6.25, 12.5, and 18.75 kW m −2 . Furthermore, the use of copper foams to improve the phase change process is experimentally studied by employing three different samples with 5, 10, and 40 PPI and constant porosity equal to 0.95. The experimental results clearly show that the presence of the foam matrix improves the heat transfer capabilities of the passive system allowing for lower surface temperature compared to no-foam case, at the same imposed heat flux. A direct video visualization of the process also permitted to show the effects of the porous medium on melting and solidification processes. [ABSTRACT FROM AUTHOR]

The interaction of nanofluids with extended surfaces in the form of porous structures and its effect on the thermal performance of the heat exchanger is not well documented. In this study, the forced convective heat transfer due to flow of Al 2 O 3 /Water nanofluid through a circular tube filled with a metal foam is investigated experimentally. An isothermal boundary condition is created and the pressure drop and the heat transfer rate are measured over a range of flow rates. The results are compared with values for water flowing through a similar tube without the metal foam insert. The experimental data indicate a significant improvement in the heat transfer rate at the cost of a pressure drop increase. Our experimental data also show a direct relationship between the Nusselt number and the volume fraction of Al 2 O 3 . [ABSTRACT FROM AUTHOR]

Metal foams are being widely adopted in a number of applications relevant to heat transfer, in which many include thermal radiation as the predominant heat transfer mode. Radiative characteristics of porous media are determined either by analytical modeling or by experiments. The aim of the present study is to evaluate the feasibility to substitute the real structure, more complex and computationally expensive, with the simpler and lighter ideal foam representation. The radiative heat transfer in foams is investigated using home-made Monte Carlo Ray Tracing codes. The geometry of the real microstructure of a metal foam is obtained by employing X-ray computed tomography (XCT). The actual 3-D structure is imaged, from which the total porosity, surface to volume ratio and size of a representative elementary volume for thermal analysis are determined. Results from the tomographic structures are compared to computer-generated Kelvin and Weaire–Phelan foam structures. Numerical results obtained for the real and ideal geometries are compared. The agreement is good, especially when accounting for the effect of the ligament shape. The good agreement between the results obtained with reference to the Weaire–Phelan convex ligament structures and those for real structures, both in terms of extinction coefficients and scattering phase functions, suggests that such idealized structures can estimate with good accuracy the parameters for equivalent homogeneous media on the basis of the porosity and the specific surface area. [ABSTRACT FROM AUTHOR]

The 3-D numerically simulated steady state results of an industrial size vertical Direct Chill (DC) slab caster fitted with a metallic-foam-plated combo bag melt distributor for AA-1050 aluminum alloy are reported. The turbulence in the melt and the mushy region solidification of the alloy are modeled through the popular low Reynolds number κ – ε model of Launder and Sharma and the enthalpy-porosity scheme, respectively. The transport of melt through the foam is modeled using the Brinkman–Forchheimer extended Darcy equation. The verification of the numerical model is performed by comparing the predicted and measured solidification front for AA-3104 rolling ingot reported in the literature. The inlet melt superheat and the porosity of the metallic foam of the combo-bag were kept fixed at 32 °C and 0.9, respectively. Parametric studies are carried out by varying two important parameters of the process, viz., the casting speed and the heat transfer coefficient (HTC) at the metal-mold contact region. Specifically, the casting speed and the HTC are varied from 40 to 100 mm/min and from 750 to 3000 W/(m 2 -K), respectively. With the increase of the casting speed, the solidification process is delayed while the solid-shell thickness increased with the increase of the HTC. The predicted results are presented for the solid-shell thickness, the sump depth, the mushy thickness, the surface temperature as well as the non-dimensional turbulent viscosity contours. In addition the temperature fields and velocity profiles with streamlines are also provided. [ABSTRACT FROM AUTHOR]

In this paper a long domain, made up of numerous idealized geometrical cells in the flow direction, is used to numerically simulate heat transfer in the foam. The idealized cells mimic the complex morphology of open-cell solid foam. The large number of cells avoided periodicity issues and ensured minimal or no size and entrance effects. The conjugate laminar flow and energy equations are solved directly at the pore level; and the temperature fields are obtained for various approach velocities using a commercial numerical package. The details of the geometrical modeling and simulation are given in this paper. The commercial foam that was simulated had 20 pores per inch and porosity of 91.5%. The simulation showed a thermal development region. To validate the simulation, direct comparisons to analytical local fluid temperatures from the solution of volume-average two-equation model in the thermally fully-developed region were also carried out. Good agreement between the simulation and analytical results were obtained. The results are encouraging and lend confidence to the geometrical modeling and simulation approach. [ABSTRACT FROM AUTHOR]

Heat transfer in metal foams is one of the fields of particular research interest due to the possibility of imparting tailored effective thermo-physical properties for various applications. Their Effective Thermal Conductivity (ETC) varies in a wide range and it is strongly influenced by the morphology of metal foam. This study presents the evaluation of ETC of open cell FeCrAl-alloy metal foams with different porosities and characteristic pore dimensions. The ETC was measured with the transient plane source method at room temperature. Measurements of the ETC for different metal foams were compared with each other and with literature data in order to investigate the influence of morphology of metal foam. Furthermore, measurements were also compared with numerical predictions, obtained using both detailed approach based on 3D thermal lattice-Boltzmann method as well as a simplified modelling approach, and reasonably good agreements were observed. The real foam geometry, obtained from 3D CT-scan images, was used for detailed and simplified numerical simulations. [ABSTRACT FROM AUTHOR]

Rayleigh-Bénard convection significantly affects the melting process of phase change material (PCM). It has been verified to exist in the model with bottom heating. Nevertheless, researchers rarely focus on the Rayleigh-Bénard convection in the metal foam composite PCMs (MFCPCMs). This paper took numerical investigations to study the melting behavior of the MFCPCMs when Rayleigh-Bénard convection was involved, and a visualized experiment was used to verify their accuracy. By analyzing the number of Bénard cells, dimensionless parameters of Rayleigh number and Nusselt number, we found three regimes, including the conductive regime, linear regime, and coarsening regime, forming in the models. The liquid fraction contour captured the evolution of the melting front, and the state of convective heat transfer was characterized by Bénard cells. Results show that the intensity of Rayleigh-Bénard convection in the MFCPCM is obviously weaker than that in the pure PCM. Moreover, the porosity of metal foam has a significant influence on the development of Rayleigh-Bénard convection. With the increase of the porosity, the suppression of liquid PCM flow is weakened, so the convection is more developed. The inhomogeneous evolution of the melting front is more pronounced. Besides, to study the effect of model size on the melting behavior, we built three groups of models with width-height ratios (γ) of 2, 1, and 0.5, respectively. Results show that the model with large γ has more significant inhibition on the development of Rayleigh-Bénard convection. The solid/liquid interface is flat or with a slight fluctuation. On the contrary, the model with small γ has more developed Rayleigh-Bénard convection. The interface of it has a violent fluctuation, which shows as a humped arch. This work interprets the internal heat-transfer characteristics of the MFCPCM model with Rayleigh-Bénard convection. It paves the way for the structural optimization of the metal skeleton to gain efficient heat storage. • The model with bottom heating is investigated in pore-scale. • The Rayleigh-Bénard convection formed in the liquid is clarified. • The effects of the metal foam porosity and model dimension are studied. [ABSTRACT FROM AUTHOR]

Due to the importance of energy-saving and efficiency in the industry, a novel scheme for waste heat recovery was utilized in the cascade refrigeration cycle. This system is composed of a cascade heat exchanger combined with a recuperator intensified with open foam. The exhaust air from the heat exchanger is used for the air handling unit. Also, core flow in an annular channel is proposed as an innovation to enhance heat transfer. An analytical study was performed to evaluate the impact of metal foam properties on the performance of the partially filled annular heat exchanger with asymmetric isothermal boundary conditions. The energy equations were decoupled by normalizing, linear combination, and the variable change method. Then by forming the Bessel differential equations, temperature solutions were achieved in terms of the modified Bessel functions of the first and second kinds. In the energy and momentum equations of the porous region, the local thermal non-equilibrium (LTNE) and the Darcy-Brinkman models were used, respectively. The analytical solutions of velocity, pressure drop, mass flow ratio, temperature, and Nusselt numbers were obtained. A hydraulically critical point exists at the nondimensional interfacial radius of 2.86 for D a = 10 − 4 . It revealed that the Darcy number and the thermal conductivity ratio are two crucial parameters for improving performance. Based on the Performance evaluation criterion (PEC) definition, the optimal porosity value of 0.91 was determined. Also, a design triangle in the range of metal foams was proposed to select the local Nusselt and Biot numbers. At a particular Biot number, the PEC value increases 3.76 times in the design range of metal foams. Results demonstrated that the core flow in an annular heat exchanger leads to high performance, several times greater than non-core flow. [ABSTRACT FROM AUTHOR]

Heat transfer coefficient and pressure drop of the refrigerant R134a flow boiling in metal-foam filled annular tubes are studied. Three test tubes are analyzed. One tube is plain annular and the others are filled with 10 and 20 PPI copper foam pieces. The three test tubes are dimensionally similar with a length of 30 cm, outer and inner diameters of 14.3 mm and 4.7 mm, respectively. The experiments are performed for mass flux ranging from 10 to 80 kgm−2s−1 and vapor quality of 0.13–0.85. Also, a constant and uniform heat flux of 5.7 kWm−2 is applied to the test tube in all of the experiments. It is shown that boiling heat transfer coefficient and pressure drop of the refrigerant profoundly increases using metal foam. Furthermore, they are increased by an increment in pore density, and in particular, heat transfer coefficient is enhanced up to 220% using 20 PPI metal foam. Evidently, both heat transfer and pressure drop are proportionally varying with mass flux. Although these parameters generally increase with an increment in vapor quality, heat transfer coefficient decrement is observed at high vapor quality and low mass flux values. Based on the test data, and due to the lack of existing correlation to span over the testing conditions in this experiment, two new correlations are proposed. New correlations can predict boiling heat transfer coefficient and two-phase pressure drop of refrigerant in the metal-foam filled annular tubes with good accuracies. • Heat transfer and pressure drop of R134a flow boiling in metal-foam filled annular tubes are studied. • Effects of flow parameters and metal foam properties on flow boiling are investigated. • Heat transfer and pressure drop are profoundly increased by inserting metal foam pieces. • The experimental results are compared to the available correlations. • New correlations are proposed to estimate boiling HTC and two-phase pressure drop in metal-foam filled annular tubes. [ABSTRACT FROM AUTHOR]

Pool boiling is specified as boiling stemming from a heated surface that is submerged in a volume of liquid resulting in a phase transition to vapor. The subject of pool boiling heat transfer heat has continuously received attention due to its various applications among researchers. One of the avenues used to improve pool boiling is the employment of open-cell metal foams. In this study, an attempt has been made to study the effect of electrodeposited open-cell metal foam coating on pool boiling heat transfer under atmospheric conditions and deionized water. The foams have three pore densities of 5, 10, and 20 ppi and a constant porosity of 0.95. In each pore density of the open-cell foams with five different thicknesses, copper is deposited on the foams through electrodeposition, and thereafter pool boiling test is performed. Also, to evaluate the number of electrodeposition steps, 4 and 8-step electrodeposition is implemented on the best pore density foam sample from the preceding section. In addition, untreated foams in five different thicknesses and also nickel foams are tested for comparison with electrodeposited copper. The results reveal that 20 ppi 6-step electrodeposited copper open-cell foam with a thickness of 4 mm improves the pool boiling heat transfer in medium and high heat fluxes by up to 160% and 130%, respectively, and thence has the best performance among all the sample foams. However, at a pore density of 5 ppi, the 8-and 6-step electrodeposited open-cell foam with 8 mm thickness achieves the best performance and improves the pool boiling heat transfer at medium and high heat fluxes by approximately 150% and 100%, respectively. In addition, at a pore density of 10 ppi, the 6-step electrodeposited open-cell foam with 4 mm thickness, attained the best results and improved the pool boiling heat transfer at medium and high heat fluxes by 103% and 120%, respectively. SEM, microscopic, and aging images of the foams were carried to compare the structure and strength of the number of electrodeposition steps. [Display omitted] • Electrodeposited open-cell metal foams are experimentally tested for pool boiling. • The thickness effect is studied in three pore densities, 5, 10, and 20 ppi. • The number of electrodeposition steps of open-cell metal foams is investigated. • SEM and microscopic images are taken for the electrodeposited foams' structure. • The durability of the electrodeposition steps is assessed using aging tests. [ABSTRACT FROM AUTHOR]

This study aims to analyze the effect of edge layers on the volumetric radiative properties of open-cell nickel foam based on a three-layer structure model composed of two edge layers and one complete layer. Pore-scale simulation was performed for five nickel foams with typical structural parameters, using their component optical properties and reconstructed digitalized foams. First, the ligament morphology of foam samples was rebuilt and the optical properties of rough ligament surfaces were predicted with the finite difference time domain method. Then, pore-scale radiative transfer models were established which absorption and scattering were computed using Monte Carlo Ray-Tracing (MCRT) method. Afterwards, the size of representative elementary volumes was determined, and it illustrates that at least three foam cells in the thickness of a foam structure are required to characterize volumetric radiative transfer. Moreover, the MCRT results show that accounting for the existence of edge layers, the extinction coefficients were underestimated by 7% for 20PPI nickel foam and 13% for 40PPI nickel foam by Beer's law in the studied wave band. When compared with the results by present simulation, the values of extinction coefficients by the double-thickness model in the reference are quite close within the wave band of 0.83–2.2 μm. While the scattering coefficients was overweighed for about 107 m−1 of sample 2 and 84 m−1 of sample 3 at the wavelength of 0.83 μm. Additionally, the extinction coefficients of five samples changed slightly in the wavelength from 0.75 μm to 12.5 μm. Scattering albedos approach to the reflectivity of the nickel component and scattering phase functions of all nickel foams show an obviously backward predominance. • Establishment of the pore-scale radiative transfer models. • Analysis of the edge-layers effect on the radiative properties. • Determination on representative elementary volumes of open-cell metal foams. • Verification in spectral radiative properties with experimental data. [ABSTRACT FROM AUTHOR]

Abstract: The thermal conductivity of phase change material (PCM) significantly affects the thermal performance of latent heat thermal energy storage (LHTES) system, which is attractive for energy conservation and waste heat utilization. Metal foam can be applied to enhance the low thermal conductivity of pure PCM. In the present study, copper foam and nickel foam with various porosities and pore sizes were impregnated with pure paraffin with vacuum assistance. A steady-state test system which considered the thermal contact resistance (TCR) between the specimen and adjacent surfaces was constructed to measure the effective thermal conductivities of the composite PCMs. The thermal conductivities were also theoretically calculated based on the correlations and models from the literature. The results showed that the thermal conductivities measured with steady-state method showed good agreement with the theoretical predictions, and the thermal conductivities of the composite PCMs were drastically enhanced, e.g., the thermal conductivities of the paraffin/copper foam composite PCMs fabricated by the copper foams with the porosities of 96.95%, 92.31%, 88.89% and pore size of 25 PPI were about thirteen, thirty-one, forty-four times larger than that of pure paraffin, respectively. The ratios of TCR to the total thermal resistances of the composite PCMs with the thickness of about 20.0 mm were in the ranges of 15.0–50.0%. [Copyright &y& Elsevier]

Abstract: The thermo-physical behavior of open-celled metal foams depends on their microscopic structure. An ideal periodic isotropic structure of tetrakaidecahedron shape i.e. Kelvin cell is studied. The geometrical parameters of casted metal foams are obtained using iMorph (in-house code). We have proposed an analytical model in order to obtain geometrical parameters correctly as they have substantial influence on thermal and hydraulic phenomena, where strut geometry is of primary importance. Various relationships between different geometrical parameters and porosities are presented. The analytical results are fully compared with the experimental data in the literature and measured morphological data. The relationship of geometrical parameters with physical properties such as effective thermal conductivity is equally important. The range of solid to fluid phase conductivity ratios (λ s/λ f) studied is from 10 to 30,000 and for different porosities (80–95%). A modified correlation term, F is introduced in order to take account of thermal conductivities of constituent phases using electrical resistor model. An excellent agreement has been observed between the predicted correlation and experimental data. [Copyright &y& Elsevier]

Abstract: A simplified analytical–numerical method to model radiation heat transfer in metallic foams is proposed. It modifies a model taken from the literature and allows to predict the radiative conductivity for high and low porosity foams. A simplified cubic representative elementary volume of the foam is assumed and radiative heat flux is evaluated by computing radiosities and view factors. The analytical approach proposed in this paper slightly modifies some coefficients of the original model. Test ray-tracing and numerical simulations based onto Monte Carlo method are carried out in order to consistently calculate some view factors. The comparison with experimental results shows that predictions of the proposed model are more accurate than those of the original one. [Copyright &y& Elsevier]

In this study, pool boiling heat transfer mechanisms in V-shaped grooved porous metals are studied using our porous metal model. The model eliminates the small Biot number restriction of linear temperature-distribution assumption and thus extends to large Biot number conditions, such as boiling heat transfer of porous metals relating multi-bubble agitation and coalescence. The effects of groove number, groove distance and groove width on pool boiling heat transfer are investigated using lattice Boltzmann method. The experimental study is also conducted to reveal bubble escaping paths. The results show that bubble departure frequency on the porous metal with double V-shaped grooves is higher than those on the porous metal with single V-shaped groove and the uniform porous metal. The experimental result indicates that the venting bubbles in lateral grooves are entrained by the central departure bubbles, which validate the numerical results. At the low heat fluxes, the wall superheat increases with increasing groove distance. The lotus-shaped bubbles form in the central porous metal. The bubble escaping resistance reduction leads to the heat transfer performance enhancement for the porous metal with the wide V-shaped grooves. • Pool boiling heat transfer mechanisms in metal foams with V-shaped grooves are investigated numerically and experimentally. • Groove number, distance and width effects are investigated. • The wall superheat increases with increasing groove distance at low heat fluxes. • 4.Lotus-shaped growing bubbles forms in central area. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]