2. Potentials

High-porosity metal foams are usually porous media with low density and novel structural and thermal properties. This sort of media is mainly formed from multi-struts interconnected to each other at joint nodes to shape pores and cells (see the SEM image in Figure 1) (Liu et al. [1]). They offer very high porosity (ε ≥ 0.89), light weight, high rigidity and strength, and a large surface area, which make them able to recycle energy efficiently. This capacity to transport a large amount of heat is attributed to their superior thermal conductivity compared to ordinary fluids and high surface-area density (surface area per a given volume of metal foam) as well as enhanced convective transport (flow mixing) due to the tortuous flow paths existing within them, as shown in Figure 1 (Zhao [2]). Also, their open-cell structure makes them even less resistant to the fluids flowing through them, and hence, the pressure drop across them is much less than it is in the case of fluid flow via packed beds or granular porous materials.

182 Porosity - Process, Technologies and Applications

Open-cell metal foams were first invented by the ERG Materials and Aerospace Corp. in 1967, and since then, they have been continuously developed. This invention was patented to Walz [3], where the manufacturing processes were based on an organic preformation cast. However, this invention was originally intended for only classified military and aerospace applications. Accordingly, nonclassified applications had not made use of this technology until the mid of 1990s, when it has become generally available for industrial applications. Since then, other manufacturers have joined the global competition in this industry. To name a few, M-Pore GmbH in Germany, the French company Alveotec, and Constellium from Netherlands are currently making open-cell metal foams on a large scale for a wide range of applications.

The traditional way of casting open-cell metal foams is still adopted by ERG Materials and Aerospace [4] as well as M-Pore GmbH [5], where the foams are cast with an investment process based on polyurethane preformation. As the fabrication process is affected by gravity, the foams resulted will be shaped from oval rather than spherical cells, as illustrated in Figure 2 (De Schampheleire et al. [6]). Alveotec [7] and Constellium, on the other hand, use a different way called leachable bed casting, in which metal is cast over a stack of soluble spheres to shape out the interconnecting open-cells desired. The spheres used are usually made out of either salt

Figure 1. Open-cell metal foams: SEM image of the structure (left); mechanism of flow mixing (right).

Open-cell metal foams possess unique characteristics, making them a promising candidate for plenty of practical and engineering applications. Among these potentials are the following:


[18], it was observed that the heat transfer rate offered by metal-foam heat exchangers is up to six times better than that in the case of the bare-tube bundle with no extra fan power. Also, it was found that if the dimensions of the foamed heat exchanger are not fixed, that is, the frontal area can be manipulated, metal-foam heat exchangers outperform the louvered-fin heat exchanger. In other words, a smaller metal-foam heat exchanger can be used for the same

High-Porosity Metal Foams: Potentials, Applications, and Formulations

http://dx.doi.org/10.5772/intechopen.70451

185

Employing high-porosity metal foams to improve the thermal effectiveness of counterflow double-pipe heat exchangers has been the subject of increasing interest recently. Xu et al. [19] pointed out that to achieve high thermal effectiveness, that is, greater than 0.8, porosity and pore density should be in the range of (ε < 0.9) and (ω > 10 PPI), respectively. Furthermore, Chen et al. [20] observed that despite the increase occurred in the pressure drop, using metal foams results in a remarkable heat transfer enhancement (by as much as 11 times), which leads to a considerable improvement in the comprehensive performance, that is, up to 700%. More recently, an innovative double-pipe heat exchanger was proposed [21, 22] through using rotating metal foam guiding vanes fixed obliquely to force fluid particles to flow over the conducting surface while rotation. Furthermore, the conducting surface itself was covered with a metal foam layer to improve the heat conductance across it. To optimize the performance achieved, an overall performance system factor, that is, OSP, was introduced as the ratio of the heat exchanged to the total pumping power required. Overall, the negligibly small pumping power required compared to the amount of heat exchanged makes the overall

seny et al. [22]). It was also observed that while increasing the temperature difference from 30 to 300C, the overall performance achieved can be improved up to 200–300% depending on the Re\* value. This outcome indicates the promising prospects to utilize the proposed configura-

Now, utilizing metal foam can offer as more as twice the cooling effectiveness obtained by the traditional finned heat exchangers. Thus, such a sort of heat exchangers is widely employed today in medical and medicinal products, defense systems, industrial power generation plants,

Figure 3. The change of the overall system performance OSP with the rotational speed Ω and characteristic temperature

) (Figure 3) (Alhus-

thermal duty, and hence, a smaller fan can perform what is required.

performance of such heat exchangers incomparable, that is, OSP = O(10<sup>2</sup>

tion as a recuperator in gas turbine systems.

difference ΔT for ε = 0.9 and ω = 10 PPI.

10. Have attractive stiffness/strength properties and can be processed in large quantity at low cost via the metal sintering route.
