High Porosity Open-cell Metal Foams Research Articles

The Experimental Investigation of a 98% Hydrogen Peroxide Monopropellant Thruster Comprising the Metal-Foam-Supported Manganese Oxide Catalyst

The article presents alternative metal-supported catalysts for decomposition of the highest-class hydrogen peroxide: 98%+ (Type 98 HP, according to MIL-PRF-16005F). The aim of this study was the experimental investigation of an alternative solution for decomposition of 98%+ hydrogen peroxide, strictly for chemical propulsion. High-porosity open cell metal foams have been identified as structures with great potential. Low density, good mechanical and thermal properties, availability of various materials and alloys as well as new technologies of manufacturing, make metal foam a potential solution for many different propellants, not only hydrogen peroxide. Open cell NiCrAl foam has been processed to prepare several catalysts, with different content and dispersion of the active phase. Cleaning and drying were performed to prepare carriers for further processing: wet impregnation, slow drying, and calcination. Simple drop tests with 98% hydrogen peroxide have been conducted to estimate activity level of catalysts, in a simplified scale: low, medium, high. Then, real-environment tests have been performed in a catalyst bed. Temperature, pressure along the bed and propellant mass flow rate were measured while testing. The analysis of the test results provided a general conclusion that metal foam supported manganese oxide catalyst is a promising solution for hydrogen peroxide propulsion.

Pore-scale fluid flow and conjugate heat transfer study in high porosity Voronoi metal foams using multi-relaxation-time regularized lattice Boltzmann (MRT-RLB) method

This study aims to investigate pore-scale flow and conjugate heat transfer in high-porosity open-cell metal foams. Five representative elementary volume (REV) samples with different values of porosity (0.80, 0.85, and 0.90) and pore density (10, 20, and 30 PPI) are generated using the Laguerre-Voronoi tessellation (LVT) algorithm. To perform direct numerical simulations, a decoupled lattice Boltzmann solver is developed utilizing multi-relaxation-time (MRT) model for flow and regularized lattice Boltzmann (RLB) model for advection-diffusion. As a novelty, this solver is able to simulate advection-diffusion in the non-Darcy regime. To consider thermal conduction in REV samples more prominently, a constant high temperature is assigned to the solid ligaments on the inlet plane. The flow results show that the transition from the Darcy to the non-Darcy regime occurs at lower Reynolds numbers in foams with lower porosity, whereas the pore density does not affect the transition point significantly. According to the heat transfer results, metal foams cool down rapidly at high flow velocities while the temperature rise in the fluid is minimal. Also, increasing pore density from 10 PPI to 30 PPI and decreasing porosity from 0.90 to 0.80 increase the h-ReK slope in the non-Darcy regime by 190% and 17.5%, respectively.

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

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

Numerical Modeling of Pure and Metal-Foam Loaded PCMs

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

The evaluation of the effective thermal conductivity of metal-foam loaded phase change materials

Phase Change Materials (PCMs) are suitable materials to be included in Latent Thermal Energy Storage Systems (LTESS) to enhance the storage capacity per unit of volume. In order to increase their typical low thermal conductivity, PCMs are often loaded with high-porosity open-cell metal foams. Several correlations have been proposed in the literature to evaluate the effective thermal properties of the composite medium made of PCM and metal foam. However, the values of the effective thermal conductivity (keff) predicted by the different relationships can be very different from each other, with a consequent strong impact on the results of numerical simulations. In this work, a critical overview of the accuracy of the most used literature correlations for the evaluation of the effective thermal conductivity of open-cell metal-foam loaded PCMs is made: the temperature distribution obtained through a numerical model using different correlations is compared with the experimental values measured by testing different commercial paraffins loaded with copper or aluminum foams, subjected to complete melting. Since no correlation proves to yield accurate results for all the composite PCMs tested in this work, a new method for the calculation of the effective thermal conductivity of the PCM-metal foam medium is suggested.

Pore-scale numerical simulation of convection heat transfer in high porosity open-cell metal foam under rotating conditions

High porosity open-cell metal foam can be used in rotating machinery to improve its performances. The interstitial heat transfer coefficient is a key parameter to describe heat transfer in porous media under local thermal non-equilibrium conditions. A pore-scale numerical simulation is performed to obtain the interstitial heat transfer coefficient between air and high porosity open-cell metal foam under rotating conditions in this study. To this end, the convective heat transfer in a radially rotating channel filled with Weaire-Phelan foam cell is investigated numerically. The interstitial heat transfer coefficients are calculated at different Reynolds numbers, Rotation numbers, foam porosities and locations. And the correlation for porous-fluid interstitial heat transfer Nusselt number is established. The results show that the increasing of Reynolds and Rotation number will gradually increase the interstitial heat transfer Nusselt number. The location also has a positive effect on the interstitial heat transfer coefficient. However, the interstitial heat transfer Nusselt number gradually decreases with the increase of porosity. The established correlation can predict the interstitial heat transfer Nusselt number with a deviation within 10% at the studied ranges of the corresponding parameters. It can also be applied to the cases for larger location, larger channel size and larger Rotation number.

Effective thermal conductivity of high porosity open-cell metal foams

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

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.

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.

Simulation intricacies of open-cell metal foam fin subjected to convective flow

An exact 3D digital image of 10 PPI open-cell metal foam matrix used in this study has been generated through X-ray micro-computed tomography. The precise foam geometrical model, in combination with the commercial computational software, facilitates synthesizing typical thermal applications of metal foam subjected to convective fluid flow and creates unique opportunity of probing some of the finer fluid flow aspects usually beyond the scope of most of the experimental investigations. Simulation of fluid flow through 10 PPI open porous metal foam subjected to constant terminal temperature has been carried out and validated a priori with the experimental data. The convective flow simulation, being computationally intensive, has been carried out in a small piece of metal foam to eventually extrapolate the data over the original length. The practical limitations of foam modelling leading to the deviations between experimental data and the predicted results have been explained in details. Subsequently, the temperature and flow fields have been probed to understand the complicated flow physics around the metal foam strut. This study gives a better insight of the state of the art research and development in the field of high porosity open-cell metal foam. The computational predictions have also been compared at length with the analytical outcomes based on the simple cubic model of metal foam. This work, in all, provides the end-user an overview of the associated intricacies while considering the computational, experimental and simplified models for open-cell metal foams when used as extended heat transfer surfaces or fins.

Experimental and numerical modeling of metal foam passive radiator at low temperatures

ABSTRACTSuitability of high porosity open-cell metal foams as radiant fins in passive cryogenic radiators has been studied. Copper foam having porosity and pore density has been used for this purpose. The details of the experimental set-up devised to measure the radiative cooling capacity of metal foam have been presented. The tests have been conducted in the temperature range of attained by foam when subjected to a radiative background of liquid nitrogen temperature (~). Concurrently, the radiative cooling capacity has been predicted computationally using the three-dimensional X-ray micro-computed tomographic image of the tested foam sample.

Direct Simulation of Interstitial Heat Transfer Coefficient Between Paraffin and High Porosity Open-Cell Metal Foam

The interstitial heat transfer coefficient (IHTC) is a key parameter in the two-energy equation model usually employed to investigate the thermal performance of high porosity open-cell metal foam/paraffin composite phase change material. Due to the existence of weak convection of liquid paraffin through metal foam during phase change process, the IHTC should be carefully determined for a low Reynolds number range (Re = 0–1), which however has been rarely addressed in the literature. In this paper, a direct simulation at foam pore scale is carried out to determine the IHTC between paraffin and metal foam at Re = 0–1. For this purpose, the cell structures reflecting realistic metal foams are first constructed based on the three-dimensional (3D) Weaire–Phelan foam cell to serve as the representative elementary volume (REV) of metal foam for direct simulation. Then, by solving the Navier–Stokes equations and energy equation for the REV, the influences of Reynolds number (Re), Prandtl number (Pr), foam porosity (ε), and pore density (PPI) on the dimensionless IHTC, i.e., the Nusselt number Nuv, are investigated. According to the numerical results, a correlation of Nuv at Re = 0–1 is proposed for metal foam/paraffin composite material, which covers both diffusion-dominated interstitial heat transfer region (Re ≤ 0.1) and convection-dominated interstitial heat transfer region (0.1 < Re ≤ 1). Finally, the applicability of this correlation in the two-energy equation model for solid–liquid phase change of paraffin in metal foam is validated by comparing the model predicted melting front with that of experimental observations made in this study. It is found that the IHTC correlation proposed in this study can be used in the two-energy equation model for well predicting the phase change process of paraffin in metal foam.

Cooling capacity of high porosity open-cell metal foams as passive cryogenic radiators

This work presents an innovative avenue for employment of high porosity open-cell metal foams as extended heat transfer surfaces in passive cryogenic radiators. Metal foams are known for being light in weight and possess high surface area density. In contrast to a solid surface, porosity of metal foams makes it feasible for penetration of radiation thereby resulting in higher radiatively interactive surface area. Two 20 PPI metal foams made of copper and aluminum with 94.9% and 90.3% porosity respectively have been chosen for this study. A laboratory-scale test rig measures the radiative cooling capacity of metal foams in vacuum (10−6mbar) subjected to liquid nitrogen environment. Heat load to the foam has been provided by means of convective fluid loop. Simultaneously, a theoretical model based on radiation-conduction fin analysis has been developed to predict the foam cooling capacity at a specified temperature. The required radiation heat transfer coefficient has been obtained from a previous experiment wherein the foam samples are freely suspended in similar conditions but with no heat load. Lastly, performance of the foams under study has been expressed in terms of a commonly used performance parameter (surface area/cooling capacity) for passive cryogenic radiators.

Geometric Mean of Fin Efficiency and Effectiveness: A Parameter to Determine Optimum Length of Open-Cell Metal Foam Used as Extended Heat Transfer Surface

High porosity open-cell metal foams have captured the interest of thermal industry due to their high surface area density, low weight, and ability to create tortuous mixing of fluid. In this work, application of metal foams as heat sinks has been explored. The foam has been represented as a simple cubic structure and heat transfer from a heated base has been treated analogous to that of solid fins. Based on this model, three performance parameters namely, foam efficiency, overall foam efficiency, and foam effectiveness have been evaluated for metal foam heat sinks. Parametric studies with varying foam length, porosity, pore density, material, and fluid velocity have been conducted. It has been observed that geometric mean of foam efficiency and foam effectiveness can be a useful parameter to exactly determine the optimum foam length. Additionally, the variation in temperature profile of different foams heated from one end has been determined experimentally by cooling these with atmospheric air. The experimental results have been presented for open-cell metal foams (10 and 30 PPI) made of copper/aluminium/Fe–Ni–Cr alloy with porosity in the range of 0.908–0.964.

Prospective of employing high porosity open-cell metal foams in passive cryogenic radiators for space applications

Passive cryogenic radiators work on the principle of dissipating heat to the outer space purely by radiation. High porosity open-cell metal foams are a relatively new class of extended surfaces. These possess the advantages of high surface area density and low weight, characteristics which the space industry looks for. In case of radiative heat transfer, the porous nature of metal foams permits a deeper penetration of the incident radiation. Consequently, the heat transfer area participating in radiative heat exchange increases thereby enhancing the heat transfer rate. However, effective heat conduction in between the foam struts reduces as a result of the void spaces. These two conflicting phenomenon for radiation heat transfer in metal foams have been studied in this work. Similar to the foam conduction-convection heat transfer analysis, a conduction-radiation heat transfer model has been developed for metal foams in analogy with the conventional solid fin theory. Metal foams have been theoretically represented as simple cubic structures. A comparison of the radiative heat transfer through metal foams and solid fins attached to a surface having constant temperature has been presented. Effect of changes in foam characteristic properties such as porosity and pore density have also been studied.

Radiation heat transfer in high porosity open-cell metal foams for cryogenic applications

Radiation heat transfer in high porosity open-cell metal foams subjected to cryogenic temperatures has been studied. This study helps designing metal-foam-based devices exposed to radiation of deep space at extremely low temperature. Aluminium and copper foam samples with more than 90% and 95% void volume respectively and pore size in the range of 10–30 PPI have been studied. Suspending the foam samples freely inside a vacuum chamber (at 10−6mbar) and maintaining the chamber wall at liquid nitrogen temperature, the cool down time–temperature profiles has been recorded for nearly 5.5h, wherein different foams reach temperatures in the range of 119–128K. Simultaneously, theoretical model based on lumped-capacity analysis has been developed to simulate the transient behaviour of foam samples. Simple cubic structure foam model has been used to estimate the view factor. The experimental outcome could be predicted by the theoretical model with reasonable accuracy. Additionally, cool-down history of an aluminium foam sample has been compared to a solid body of same weight so as to appreciate their radiative potential at low temperatures.

Flow boiling visualization and heat transfer in metal-foam-filled mini tubes – Part I: Flow pattern map and experimental data

High-porosity open-cell metal foams are well known to enhance the heat transfer mechanism in rectangular or circular channels. Their high surface area to volume ratio makes them a great candidate for manufacturing high-performance small-scale heat exchangers. This two-part experimental study investigated the two-phase flow boiling inside a circular copper mini tube. In Part I, the flow pattern was visualized by high-speed imaging in glass tubes. Flow pattern maps, the heat transfer coefficient, and pressure drop are presented for mean vapor quality of 0.1–0.7, heat flux of 20–40kW/m2, and mass flux of 400–700kg/m2s. The experiments were also performed without the metal foam in the mini tube for comparison to the original data. In this range of experimental conditions, the metal foam increased the heat transfer coefficient up to 3.2 times. Also, as expected, the metal foams adversely affected the pressure drop inside the tubes.

A new prediction model for the effective thermal conductivity of high porosity open-cell metal foams

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.

An Analytical Unit Cell Model for the Effective Thermal Conductivity of High Porosity Open-Cell Metal Foams

We present an analytical model for the effective thermal conductivity of fluid-saturated metal foams with open cells. For high porosity ranges $$(\varepsilon \ge 0.9)$$ , the model is derived based on a realistic representative unit cell (a tetrakaidecahedron with cuboid node) under the assumption of parallel heat conduction along the highly tortuous cell ligaments and the saturating fluid. Good agreement with existing experimental data as well as the present measurements of open-cell aluminum foams saturated with either air or water validates the present model. More realistic and reasonable node size is estimated and compared with other model predictions. Ligament shape and pore size (PPI) are found to have little influence upon the effective thermal conductivity of the bulk porous media. Further, the influence of the fluid phase as well as the interactive heat conduction between solid and fluid phase on the overall effective thermal conductivity is quantified.

A simplistic analytical unit cell based model for the effective thermal conductivity of high porosity open-cell metal foams

We present a simplistic yet accurate analytical model for the effective thermal conductivity of high porosity open-cell metal foams saturated in a low conducting fluid (air). The model is derived analytically based on a realistic representative unit cell (a tetrakaidecahedron) under the assumption of one-dimensional heat conduction along highly tortuous-conducting ligaments at high porosity ranges (ε ⩾ 0.9). Good agreement with existing experimental data suggests that heat conduction along highly conducting and tortuous ligaments predominantly defines the effective thermal conductivity of open-cell metal foams with negligible conduction in parallel through the fluid phase.