High-Porosity Metal Foams: Potentials, Applications, and Formulations (original) (raw)
Related papers
Thermophysical properties of high porosity metal foams
In this paper, we present a comprehensive analytical and experimental investigation for the determination of the eective thermal conductivity (k e), permeability (K) and inertial coecient (f) of high porosity metal foams. In the ®rst part of the study, we provide an analysis for estimating the eective thermal conductivity (k e). Commercially available metal foams form a complex array of interconnected ®bers with an irregular lump of metal at the intersection of two ®bers. In our theoretical model, we represent this structure by a model consisting of a two-dimensional array of hexagonal cells where the ®bers form the sides of the hexagons. The lump is taken into account by considering a circular blob of metal at the intersection. The analysis shows that k e depends strongly on the porosity and the ratio of the cross-sections of the ®ber and the intersection. However, it has no systematic dependence on pore density. Experimental data with aluminum and reticulated vitreous carbon (RVC) foams, using air and water as ¯uid media are used to validate the analytical predictions. The second part of our paper involves the determination of the permeability (K) and inertial coecient (f) of these high porosity metal foams. Fluid ¯ow experiments were conducted on a number of metal foam samples covering a wide range of porosities and pore densities in our in-house wind tunnel. The results show that K increases with pore diameter and porosity of the medium. The inertial coecient, f, on the other hand, depends only on porosity. An analytical model is proposed to predict f based on the theory of ¯ow over blu bodies, and is found to be in excellent agreement with the experimental data. A modi®ed permeability model is also presented in terms of the porosity, pore diameter and tortuosity of our metal foam samples, and is shown to be in reasonable agreement with measured data.
Due to the characteristics of large surface area-to-volume ratio and inter-connected ligament structure, open-cell metal foams are promising materials for enhancing heat transfer in forced convection and have been researched for thermal applications in thermal management systems, air-cooled condensers and compact heat sinks for power electronics. However, the tortuous complex flow path inside metal foams leads to relatively higher pressure drop, which requires larger system pumping power. Hence, it is important to study the heat transfer performance of metal foam compared to its flow resistance characteristics.
Influence of pore density on thermal development in open-cell metal foam
Experimental Thermal and Fluid Science, 2017
Herein heat transfer measurements due to water flow in commercial open-cell aluminum foam confined by a cylindrical shell, which was heated by a constant heat flux, are described. Two kinds of commercial foam were tested: 10 and 40 pores per inch (ppi). Measurements included wall temperature along flow direction as well as average inlet and outlet temperatures of water. Flow rates ranged from Darcy to Forchheimer regimes. The wall temperature along the foam, as well as the local Nusselt number lucidly displayed thermal entry effects leading to thermal fully-developed ρ density (kg.m-3) Subscripts f fluid b bulk (mean) value e effective w wall Highlights • Nusselt numbers were obtained for Darcy and Forchheimer flows • Thermal entry length was determined and was significant for 10, 20 and 40 ppi • Thermal entry length differs from analytical predictions • Thermal entry length in metal foam is weak function of pore density
Metal foams as novel materials for air-cooling heat exchangers
2012
High-porosity metal foams have thermal, mechanical, electrical, and acoustic properties making them attractive for various engineering applications. Due to their large surface-area-to-volume ratio, tortuous flow path, and relatively high thermal conductivity they are being considered for a range of heat transfer applications. In this experimental study, open-cell aluminum metal foam is considered as a replacement for conventional louvered fins in brazed-aluminum heat exchangers. Closed-loop wind tunnel experiments are conducted to measure the pressure drop and heat transfer performance of metal-foam heat exchangers. In addition to characterizing the air-side pressure-drop and heat transfer performance, issues related to condensate drainage and frost formation are considered. The main performance obstacle for the application of metal foams is the relatively high pressure drop occurring for velocities typical to air-cooling applications. This high pressure drop results in larger air-side fan power requirements if metal foams are used as a "drop-in replacement" for louver fins. On the other hand, the heat transfer performance of the metal foams far surpasses that of conventional louvered fins, reaching two to three times the heat transfer coefficient of conventional fins. Smaller pore sizes provide larger surface area per unit volume and enhanced mixing, resulting in higher heat transfer. This excellent heat transfer performance means that alternate deployments of the metal foam are possible to manage fan power, while achieving comparable thermal performance. The experimental data are presented in terms of friction factors and Colburn j factors, and design correlations are developed to predict heat exchanger performance. Under wet-surface conditions, water retention can be an important problem for louvered-fin operation. Surprisingly, metal foams have water drainage behavior superior to that of conventional fins. The effects of geometry, porosity, surface treatment, and orientation on water drainage have been analyzed. iii ACKNOWLEDGEMENT I am heartily thankful to my supervisor, Professor Anthony Jacobi, whose encouragement, guidance and support from the initial to the final level enabled me to develop an understanding of the subject. I offer my regards and blessings to all of those who supported me in any respect during the completion of the project. I am grateful to Zhengshu Dia, Prof.Chen Qi, Prof.Young-Gil Park and Jessica Bock for their help and suggestions. I would like to show my gratitude to ARTI (Air Conditioning and Refrigeration Technology Institute) for financing the project. Lastly, I am thankful to my mother and sisters for their help. Kashif Nawaz iv TABLE OF CONTENTS Chapter 1: Water retention behavior of aluminum metal foams…………... 1 1.1. Introduction..………………………………………………………… 1 1.2. Dynamic dip testing of open-cell metal foams...…………………….. 5 1.3. Conclusion…………………………………………………………... 1.4 References ……………………………………………………………. Chapter 2: Pressure drop for air flow through open-cell foams…………. 14 2.1. Introduction…………………………………………………………. 14 2.2. Experimental results………………………………………………… 2.3. Modeling the pressure drop performance…………………………… 39 2.4. Conclusion…………………………………………………………... 2.5. References…………………………………………………………… 50 Chapter 3: Heat transfer performance of metal foams ……….………….. 56 3.1. Introduction………………………………………………………….. 56 3.2. Experimental results…………………………………………………. 3.3. Modeling the heat transfer performance.……………………………. 77 3.4. Conclusion…………………………………………………………… 3.5. References……………………………………………………………. 83 Appendix A: Sample manufacturing……………………………………...
The thermally-developing region in metal foam with open pores and high porosity
Thermal Science and Engineering Progress, 2017
The thermal development phenomenon in foam-like conductive porous media, e.g. open-cell metal foam, is investigated via simulation. A typical cell of commercial metal foam was geometrically idealized while maintaining its actual structural features as much as possible. Then, clones of the idealized cell were virtually connected to each other to form a sizable vertical sheet representing a slice of metal foam. This one-cell thick sheet had a length of 33 cells (flow direction) and a height of 15 cells. Because of symmetry, this sheet could represent flow and heat transfer in a semi-infinite metalfoam channel sandwiched between two heated parallel plates separated by a distance twice the sheet's height. The pertinent mass, momentum and heat equations were solved directly at the microscopic level; and the temperature fields were obtained for various approach velocities using a commercial numerical package. The results provided insight into the thermal development region in general, and the thermal entry lengths were determined for air flow at various speeds. The thermally fully-developed Nusselt number was correlated with Reynolds number. To substantiate the results, 2 comparisons were made to analytical predictions; and qualitative contrasts were made to experimental data from the literature.
Light-metal foams – some recent developments
Metallic foams have now reached the maturity of development in terms of process stability, materials properties and costs required for industrial applications. The fine-tuning of the manufacturing process carried out in the past few years has been responsible for this success. Nowadays, Al foam panels as large as 2.5 m × 1.5 m in area and having a uniform pore structure are commercially available. In parallel to this development, new processing routes are being explored, including foaming with novel blowing agents, foaming by application of under-pressure, foaming of scrap and of other metals such as magnesium. Some of the steps of these developments are reviewed.
On the effective thermal conductivity of metal foams
Journal of Physics: Conference Series, 2014
Knowing the effective thermal conductivity is essential in order to design a metal foam heat transfer device. Beside the experimental characterization tests, this quantity can be deduced from empirical correlations and theoretical models. Moreover, CFD (Computational Fluid Dynamics) and numerical modeling in general, at the pore scale, are becoming a promising alternative, especially when coupled with a realistic description of the foam structure, which can be recovered from X-ray computed microtomography (µ-CT). In this work, a review of the most relevant correlations and models published in the literature, usable for the estimation of the effective thermal conductivity of metal foams, will be outlined. In addition, a validation of the models with the experimental values available in the literature will be presented, for both air and water as working fluids. Furthermore, the results of a strategy based on µ-CT -CFD coupling at the pore level will be illustrated.
The Development of Aluminium Foams for Enhanced Heat Transfer
2017
A novel replication technique for the production of open-celled aluminium foam has recently been devised and is undergoing commercial development by the company Constellium. The technique allows close control over the pore size and shape; a feature that is uncharacteristic of metal foam production methods in general and control to such an extent is unprecedented. The method provides an excellent pathway for the exploration of pore geometry/heat transfer behaviour relations, which is the objective of this study. This also aligns with the commercial goals of Constellium as heat transfer applications have been identified as a key market for their foams. Based on the technique; the focus of this work was the development of a laboratory protocol to allow the production of aluminium foam samples with a range of different mesostructures. The heat transfer behaviour, including permeability, of foams with differing matrix metal, pore size, pore aspect ratio and pore shape were examined under forced convection conditions. Decreasing pore size was found to provide enhanced heat transfer, although for pores <3mm the benefit was outweighed by a large decrease in permeability. Small changes in pore shape as a result of preform compaction during processing may be exploited to provide improved heat transfer without reducing permeability. Elongation of pores provided no enhancement of heat transfer or permeability.