Metal Foam Flow Research Articles

Characteristics of oscillating liquid flow in foam-like highly-porous media: An experimental study

Characteristics of oscillating water flow in open-cell metal foam were obtained experimentally. The foam had 20pores per inch and a porosity of 87.6%. Three flow displacements 74.35, 97.2 and 111.53mm were applied in the flow frequency range 0.116–0.696Hz. Steady-state experiment was also conducted and the permeability and form/inertial drag coefficient were obtained. The range of flow velocity covered Darcy and Forchheimer flow regimes. The effect of flow displacement and frequency on important variables is presented and discussed. The appropriately defined friction factor correlated well with the Reynolds number for steady-state and oscillating flows, with the friction factor for oscillating flow being higher. The current results were compared to other studies from the literature employing oscillating air and water in various kinds of porous media. Some agreements and disagreements are stated and discussed.

Long-domain simulation of flow in open-cell mesoporous metal foam and direct comparison to experiment

Open-cell metal foam is a class of modern mesoporous media that possesses high thermal conductivity, large accessible surface area per unit volume and high porosities (often greater than 90%). When a fluid passes through the foam, the internal structure of the foam, which is web-like, produces a complex flow field including flow reversal and vigorous mixing. All of these attributes make metal foam a very attractive core for many engineering applications, e.g. heat exchangers, filtration devices and reactors. The rather complex and intrinsically random architecture of the foam is extremely difficult to capture exactly. In this paper, we use a unit cell geometrical model to numerically investigate the flow field and pressure drop inside commercial open-cell aluminum foam. The Navier–Stokes equations are solved directly, and velocity and pressure fields are obtained for various approach velocities using a commercial numerical package. The details of the modeling process are given in this paper. The pressure drop results are compared to the Forchheimer equation, from which the permeability and form drag coefficient are calculated. Comparisons to experimental data were also carried out. The commercial foam that was used in the experiment had 10 pores per inch and porosity of 90% approximately. Air was forced to flow inside the foam using an open-loop wind tunnel. Good agreement between the modeling and experimental results are obtained for low velocities, with the agreement becoming poorer for larger velocities. The results for the low-velocity range are encouraging and lend confidence to the modeling approach, which paves the way for investigating other phenomena inside the foam, using the same unit cell, e.g. heat transfer. The limitations of the models are outlined and discussed.

High resolution microtomography-based CFD simulation of flow and heat transfer in aluminum metal foams

The necessity for devising more effective heat transfer technologies and innovative materials, capable of increasing performances while keeping power consumption, size and cost at reasonable levels, is well recognized. Under this prospect, metal foams are good candidates for improving the thermal efficiency of heat transfer devices and allowing, at the same time, the use of smaller and lighter equipments. For design purposes, the proper characterization and quantification of transport and thermal properties of metal foams is fundamental but far from simple. This lack of information constitutes a fundamental limit in the employment of metal foams in practical and industrial applications. Nowadays, besides classical transport models and correlations, computational fluid dynamics (CFD) at the pore scale, although challenging, are becoming a promising approach for recovering the transport properties of the medium, especially if coupled with a realistic description of the foam structure. In order to precisely describe the microstructure of the foams, a 3D approach based on the X-ray computed microtomography (μ-CT) technique can be adopted. In this work, the results of high resolution μ-CT-based CFD simulations, performed on three different open-cell aluminum foams samples, will be illustrated. The results demonstrate that open-cell aluminum foams are effective means for enhancing heat transfer. Moreover, the procedure proved that μ-CT is a valid tool for capturing the peculiar details of the foam structure, thus to overcome the limits associated to the use of economical, but simplified, geometric models.

Comments on two analyses of thermal non-equilibrium Darcy–Brinkman convection in cylindrical porous media

The problem of forced convection heat transfer in porous media using the non-thermal equilibrium two-equation model is revisited analytically. Darcy-extended Brinkman momentum equation is used to describe the velocity profile inside the porous medium. The solution steps for the volume-averaged equations are outlined concisely to allow easy comparison to existing solutions in the literature. Using a case of air flow in open-cell metal foam, the fluid temperature is obtained by the current solution and by a solution existing in the literature. The pre-existing solution produced a physically-unexplained behavior. What seemed to be an error in the pre-existing solution was identified and described in detail. Robust solutions are useful in parametric studies, initial engineering design and for calibration of numerical solutions of more complex cases.

The Effective Pore Diameter of a Three-Dimensional Numerical Model for Estimating Heat and Fluid Flow Characteristics in Metal Foams

A three-dimensional numerical model is proposed to determine heat and fluid flow characteristics in metal foams. A series of full three-dimensional numerical calculations was performed to reveal complex three-dimensional ve- locity, pressure and temperature fields within three-dimensional porous structures of the metal foams. These numerical re- sults are processed to obtain the macroscopic characteristics such as the permeability, inertia, dispersion and interstitial heat transfer coefficients. An effective pore diameter concept has been proposed to correlate the resulting heat and fluid flow characteristics with available empirical correlations. In this study, we shall conduct a numerical study on heat and fluid flow in metal foam using a three-dimensional nu- merical model of periodical structure. In order to capture irregularities in real foams, quantities calculated at specific flow angles are ensemble-averaged over the flow angle. A rational way to evaluate the effective pore diameter, which is used as the characteristic length of present three-dimensional numerical model, is proposed. Permeability, Forchheimer coefficient, thermal dispersion and volumetric heat transfer coefficient are determined by spatially averaging micro- scopic numerical results. The validity of the present numeri- cal model and the effective pore diameter are examined by comparing the numerical results with available empirical correlations. Furthermore, an interesting relationship be- tween the thermal dispersion conductivity and the volumet- ric heat transfer coefficient is elucidated.

Effects of flow field design on the performance of a PEM fuel cell with metal foam as the flow distributor

In this work, we report the improvements made on the PEM fuel cell with metal foam as the flow distributor. The comparison in polarization curve is made between the PEM unit cell with different metal foam flow field designs and the PEM unit cell with graphite bipolar plate as flow distributor. The experimental results show that after using improved metal foam flow field designs, the fuel cell's performance increases. Our results show that, in the PEM unit cell with single zone metal foam, the convection is weak at side corners. Dividing the metal foam into multiple regions and using multiple inlets effectively improves gas distribution. AC impedance measurement was also performed to study impedance characteristics. The Nyquist and Bode plots confirmed that Ohmic resistance, activation resistance, and mass transfer resistance of metal foam fuel cell are all smaller than that of conventional PEM unit cell.

Experimental air heat transfer and pressure drop through copper foams

This paper presents experimental heat transfer coefficient and pressure drop measurements carried out during air forced convection through five different copper foam samples. The specimens present a different number of pores per inch: 5, 10, 20, and 40 PPI and different porosities, which vary between 0.905 and 0.934. The tests were run by varying the air mass flow rate in the range between 0.006 and 0.012 kg s −1, which corresponds to the air frontal velocity from 2.5 to 5 m s −1. Two different heat fluxes, imposed by means of an electrical heater were investigated: 25.0 and 32.5 kW m −2. The collected heat transfer and pressure drop data were analyzed to obtain the global heat transfer coefficient, the normalized mean wall temperature, the pressure gradient, permeability, inertia coefficient, and drag coefficient. The experimental heat transfer measurements reported in the present work increase the knowledge in heat transfer and fluid flow in metal foams.

A two-permeability approach for assessing flow properties in metal foam

One of the key properties for understanding the fluid flow and pressure drop in metal foam is the permeability. This paper investigates a non-traditional approach for evaluating this critical transport property by considering the physics of energy dissipation, i.e., the viscous dissipation and form drag and the fact that they are linked. The current work emphasizes the necessary initial step of clearly distinguishing between different flow regimes prior to evaluating the permeability. This distinction is usually either absent or overlooked in several previous studies, which may lead to significant errors. The approach presented in this study is applied to wind-tunnel steady-state unidirectional pressure-drop measurements for airflow through isotropic open-cell aluminum foam samples having different pore densities. The results show that the porous medium exhibits two different permeabilities—one is calculated and only valid in the Darcy regime, while the other is calculated and only valid in the Forchheimer regime.

Pressure drop and friction factor of steady and oscillating flows in open-cell porous media

Open-cell metal foams are often used in heat exchangers and absorption equipment because they exhibit large specific surface area and present tortuous coolant flow paths. However, published research works on the characteristics of fluid flow in metal foams are relatively scarce, especially for the flow oscillation condition. The present experimental investigation attempts to uncover the behavior of steady and oscillating flows through metal foams with a tetrakaidecahedron structure. In the experiments, steady flow was supplied by an auto-balance compressor and flow oscillation was provided by an oscillating flow generator. The pressure drop and velocity were measured by the differential pressure transducer and hot-wire sensor, respectively. The friction factor of steady flow in metal foam channel was analyzed through the permeability and inertia coefficient of the porous medium. The results show that flow resistance in the metal foams increases with increasing form coefficient and decreasing permeability. The empirical equation obtained by the present study indicates that the maximum friction factor of oscillating flow through the tested aluminum foams with specific structure is governed by the hydraulic ligament diameter-based kinetic Reynolds number and the dimensionless flow amplitude.

Interstitial Heat Transfer Coefficient and Dispersion Conductivity in Compressed Metal Foam Heat Sinks

Abstract This work experimentally and numerically investigates the heat transfer in uncompressed∕compressed metal foams. Experiments were conducted to obtain the thermal characteristics of a rectangular channel filled with aluminum foams using air as the fluid medium. The experimental data reveal that the uncompressed sample has a larger Nusselt number (Nu) than the compressed sample. The 0.93 porosity sample has the largest average Nu followed by the 0.7 porosity sample; the 0.8 porosity sample has the worst average Nu. The experimental data concerning the 0.93 porosity samples (uncompressed samples) were consistent with the numerical predictions obtained using the model for high-porosity metal foam, reported elsewhere. Finally, a numerical model to simulate flow and heat transfer characteristics in compressed metal foams is presented and the interstitial heat transfer coefficient and dispersion conductivity were semi-empirically determined.

Characteristics of oscillating flow through a channel filled with open-cell metal foam

An experimental study was performed to investigate the characteristics of oscillating flow through a channel filled with open-cell metal foam with a fully inter-connected pore structure. Detailed experimental data of oscillating flow pressure drops and velocities for a wide range of oscillatory frequency and the maximum flow displacement were presented. A correlation equation for the maximum friction factor of metal foams subject to oscillating flow was obtained and compared with the results for channels inserted with wire-screens obtained by other investigators. The results showed that oscillating flow characteristics in an open-cell metal foam are governed by a hydraulic ligament diameter based kinetic Reynolds number Re ω( Dh) and the dimensionless flow displacement amplitude A Dh . The effects of kinetic Reynolds number on the variations of pressure drop and flow velocity in metal foam are more significant than that of the dimensionless flow displacement amplitude. The maximum friction factor of oscillating flow in open-cell metal foams is much smaller than that of oscillating flow in wire-screens for large flow displacement amplitudes.

Determination of energy efficiency for a direct methanol fuel cell stack by a fuel circulation method

Abstract A method of fuel circulation with a fixed amount of fuel was employed to investigate a direct methanol fuel cell (DMFC) stack that was built with metal foam flow fields for the air and fuel flows. The stack power output increases significantly with environmental temperature from 20 to 40 °C. The average peak power per cell at 40 °C is 26 mW cm−2 per cell. The average discharge voltage per cell at peak power does not change with temperature but remains at 0.3 V. The energy output of the stack was determined at constant current or constant voltage with a fixed amount of methanol to feed the anode of the stack until the fuel was consumed. The results by constant current discharge show that at higher temperature the stack has remarkably higher energy output; while at the same temperature only a suitable magnitude of discharge current can achieve the highest energy output. The results by constant voltage discharge show that the Faradic efficiency is 86%, and the energy efficiency is 17% at 30 °C.

Material flow in metal foams studied by neutron radioscopy

Two kinds of experiments are presented in this paper: In the first lead alloy foams were generated in a furnace by expanding a foamable precursor material containing metal and a blowing agent. Vertical columns of liquid metal foam were scanned with a beam of neutrons while recording the time-dependent local neutron transmission. The resulting transmission profiles reflect the kinetics of material redistribution in liquid metallic foams under the influence of gravity (drainage). In the second experiment pre-fabricated solid lead foams were re-melted in a furnace. Neutron transmission profiles were also obtained in these experiments. Results of each type of experiment are presented and compared with theoretical predictions for the density profile of aqueous foams.

Heat transfer efficiency of metal honeycombs

The efficiency of micro-cell aluminium honeycombs in augmenting heat transfer in compact heat exchangers is evaluated using analytical models. For convective cooling, the overall heat transfer rate is found to be elevated by about two order of magnitudes when an open channel is designed with an aluminium honeycomb core. The performance is comparable to that achieved by using open-celled aluminium foams, but attributed to different mechanisms. At low Reynolds numbers (<2000) , the flow is essentially laminar in honeycombs, in contrast to the largely turbulent flow in metal foams; this deficiency in fluid dynamics is compensated for by the superior surface area density offered by honeycombs over foams. Another advantage of designing heat sinks with honeycombs is the relatively small pressure drop experienced and minimal noise generated by the laminar flow. The overall heat transfer rate of the heat sink is maximised when the cell morphology of the honeycomb is optimised. However, the optimal cell morphology is not constant but dependent upon the geometry and heat transfer condition of the heat sink as well as the type of convective cooling medium used. For air cooling, the optimal relative density of the honeycomb is about 0.1. Other related effects, such as cell orientation and double cell wall thickness, are discussed.