Over the past two decades, a large number of metallic foams have been developed. In recent years research on this multi-functional material class has further intensified. However, despite their unique properties only a limited number of large-scale applications have emerged. One important reason for this sluggish uptake is their high cost. Many cellular metals require expensive raw materials, complex manufacturing procedures, or a combination thereof. Some attempts have been made to decrease costs by introducing novel foams based on cheaper components and new manufacturing procedures. However, this has often yielded materials with unreliable properties that inhibit utilization of their full potential. The resulting balance between cost and performance of cellular metals is probed in this editorial, which attempts to consider cost not in absolute figures, but in relation to performance. To approach such a distinction, an alternative classification of cellular metals is suggested which centers on structural aspects and the effort of realizing them. The range thus covered extends from fully stochastic foams to cellular structures designed-to-purpose.

This paper reviews the available methods to study thermal applications with open-cell metal foam. Both experimental and numerical work are discussed. For experimental research, the focus of this review is on the repeatability of the results. This is a major concern, as most studies only report the dependence of thermal properties on porosity and a number of pores per linear inch (PPI-value). A different approach, which is studied in this paper, is to characterize the foam using micro tomography scans with small voxel sizes. The results of these scans are compared to correlations from the open literature. Large differences are observed. For the numerical work, the focus is on studies using computational fluid dynamics. A novel way of determining the closure terms is proposed in this work. This is done through a numerical foam model based on micro tomography scan data. With this foam model, the closure terms are determined numerically.

Two differently-produced open-cell aluminum foams were compared to a commercially available finned heat sink. Further, an aluminum plate and block were tested as a reference. All heat sinks have the same base plate dimensions of four by six inches. The first foam was made by investment casting of a polyurethane preform and has a porosity of 0.946 and a pore density of 10 pores per linear inch. The second foam is manufactured by casting over a solvable core and has a porosity of 0.85 and a pore density of 2.5 pores per linear inch. The effects of orientation and radiative heat transfer are experimentally investigated. The heat sinks are tested in a vertical and horizontal orientation. The effect of radiative heat transfer is investigated by comparing a painted/anodized heat sink with an untreated one. The heat flux through the heat sink for a certain temperature difference between the environment and the heat sink’s base plate is used as the performance indicator. For temperature differences larger than 30 °C, the finned heat sink outperforms the in-house-made aluminum foam heat sink on average by 17%. Furthermore, the in-house-made aluminum foam dissipates on average 12% less heat than the other aluminum foam for a temperature difference larger than 40 °C. By painting/anodizing the heat sinks, the heat transfer rate increased on average by 10% to 50%. Finally, the thermal performance of the horizontal in-house-made aluminum foam heat sink is up to 18% larger than the one of the vertical aluminum foam heat sink.

It is long known that for high-velocity fluid flow in porous media, the relation between the pressure drop and the superficial velocity is not linear. Indeed, the classical Darcy law for shear stress dominated flow needs to be extended with a quadratic term, resulting in the empirical Darcy–Forchheimer model. Another approach is to simulate the foam numerically through the volume averaging technique. This leads to a natural separation of the total drag force into the contribution of the shear forces and the contribution of the pressure forces. Both representations of the total drag lead to the same result. The physical correspondence between both approaches is investigated in this work. The contribution of the viscous and pressure forces on the total drag is investigated using direct numerical simulations. Special attention is paid to the dependency on the velocity of these forces. The separation of the drag into its constituent terms on experimental grounds and for the volume average approach is unified. It is shown that the common approach to identify the linear term with the viscous forces and the quadratic term with the pressure forces is not correct.

In studies using computational fluid dynamics software, very often a uniform air stream is applied as an inlet boundary condition of a heat exchanger. In actual applications, however, the inlet flow conditions are not uniform. Therefore, the effect of nonuniformities on the thermal performance is characterized in a wind tunnel for a commercially available plate water/air heat exchanger. Three nonuniform flow conditions are investigated. The heat exchanger is 275 mm wide and 295 mm high. Three nonuniformities are created by placing a plate 10 cm upstream of the heat exchanger: The first one covers the right-hand side of the heat exchanger, the second one covers the top half of the heat exchanger, and the last obstruction consists of a circular hole of 150 mm diameter in the middle of a plate. Only the circular obstruction has a significant influence on the heat transfer rate: The external convective resistance is up to 25% higher compared to the uniform case. The measurement results presented in this study can be used by other researchers to validate numerical simulations with nonuniform inlet conditions.

The influence of the geometry of open-cell aluminum foam on the thermohydraulic behavior in channel flow is investigated. The mean cell diameter and the strut cross-sectional surface area are chosen as geometrical parameters, ranging respectively between 1.2 and 5.2 mm and between 0.0125 and 0.17 mm². The flow arrangement and the operating conditions are fixed. A numerical model is implemented in a commercial solver, based on volume averaging theory. The model is validated against experimental data. The porous properties, which take the sub-REV scaled physics into account, are written as a function of both geometrical parameters. The thermohydraulic characteristics of 16 well-chosen foams are used to build a surrogate model. An ordinary Kriging model is used for this, indicating that the root mean square error of interpolated results is lower than 0.6 and 3% for, respectively, heat transfer and total pressure. The resulting heat transfer and total pressure difference are nondimensionalized by dividing them by the results obtained from an empty channel. The relative increment of the pressure drop is an order of magnitude higher than the increment observed for heat transfer. Consequently, the applied performance evaluation criterion (defined as the ratio of dimensionless heat transfer versus total pressure) is mainly influenced by the hydraulic performance. For the given application, a clear optimum is found. The proposed method allows performing the parameter study with acceptable computational cost with a sufficient level of detail from an engineering perspective.