3. Applications

Due to their uniquely promising potentials, high-porosity metal foams have been increasingly utilized in a variety of engineering and industrial applications. Such applications are diversely increasing day by day, making it quite hard to categorize them into particular groups. Thus, it is aimed herein to present an overview of their most recent applications without intensely going into details.

Their ability to meet the increasing daily demands to effectively transfer, exchange, or dissipate heat has attracted researchers and manufacturers to utilize them as a successful alternative to traditional heat transport media. For example, the experiments conducted by Boomsma et al. [11] showed that the thermal performance offered by the compressed open-cell aluminum foam heat exchangers is usually two to three times higher than that achieved through the commercially available heat exchangers, while they require comparable pumping power. Similarly, Mahjoob and Vafai [12] pointed out that despite the potential increase in pressure loss, utilizing metal foams in heat exchangers leads to a substantial enhancement in heat transfer, which can compensate the increase in pressure drop.

Therefore, metal foam heat exchangers have emerged recently in various practical sectors. Among them, metal foams were used as alternative to find extended surfaces utilized in removing heat from geothermal power plants, where metal foam heat exchangers offer superior thermal performance compared to conventional finned surfaces, at no extra cost resulting from the pressure drop and/or material weight [13]. In this regard, wrapping a thin layer of foam around the surfaces of tubes was proposed to enhance the heat transferred from/to them with little increase in the pressure drop produced [14–16]. Despite the higher pressure loss resulted from the increase in foam layer thickness, it was observed that the exterior convective resistance is reduced significantly, and hence, a considerable transfer enhancement is achieved. Also, the overall performance attained through using the foam-covered tubes was comparable to that achieved by the helically finned tubes at the low levels of inlet velocities and far superior at the higher velocities.

In the context of HVAC&R applications, metal foams have been presented as a promising candidate to replace the conventionally finned heat exchangers. Dai et al. [17] compared the heat transfer performance of a flat-tube, louvered-fin heat exchanger with that obtained using an identical foam heat exchanger. The analytical results revealed that for the same fan power and heat transfer performance, the metal foam heat exchanger is significantly more economical in both size and weight for a wide range of design requirements. In another comparison study [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 thermal duty, and hence, a smaller fan can perform what is required.

8. They have the potential to be used for radiation shielding [9].

to ensure a constant stress throughout the deformation.

transfer, which can compensate the increase in pressure drop.

cost via the metal sintering route.

184 Porosity - Process, Technologies and Applications

3. Applications

going into details.

superior at the higher velocities.

9. Good-impact energy absorption [10], where the structure of metal foam makes it possible

10. Have attractive stiffness/strength properties and can be processed in large quantity at low

Due to their uniquely promising potentials, high-porosity metal foams have been increasingly utilized in a variety of engineering and industrial applications. Such applications are diversely increasing day by day, making it quite hard to categorize them into particular groups. Thus, it is aimed herein to present an overview of their most recent applications without intensely

Their ability to meet the increasing daily demands to effectively transfer, exchange, or dissipate heat has attracted researchers and manufacturers to utilize them as a successful alternative to traditional heat transport media. For example, the experiments conducted by Boomsma et al. [11] showed that the thermal performance offered by the compressed open-cell aluminum foam heat exchangers is usually two to three times higher than that achieved through the commercially available heat exchangers, while they require comparable pumping power. Similarly, Mahjoob and Vafai [12] pointed out that despite the potential increase in pressure loss, utilizing metal foams in heat exchangers leads to a substantial enhancement in heat

Therefore, metal foam heat exchangers have emerged recently in various practical sectors. Among them, metal foams were used as alternative to find extended surfaces utilized in removing heat from geothermal power plants, where metal foam heat exchangers offer superior thermal performance compared to conventional finned surfaces, at no extra cost resulting from the pressure drop and/or material weight [13]. In this regard, wrapping a thin layer of foam around the surfaces of tubes was proposed to enhance the heat transferred from/to them with little increase in the pressure drop produced [14–16]. Despite the higher pressure loss resulted from the increase in foam layer thickness, it was observed that the exterior convective resistance is reduced significantly, and hence, a considerable transfer enhancement is achieved. Also, the overall performance attained through using the foam-covered tubes was comparable to that achieved by the helically finned tubes at the low levels of inlet velocities and far

In the context of HVAC&R applications, metal foams have been presented as a promising candidate to replace the conventionally finned heat exchangers. Dai et al. [17] compared the heat transfer performance of a flat-tube, louvered-fin heat exchanger with that obtained using an identical foam heat exchanger. The analytical results revealed that for the same fan power and heat transfer performance, the metal foam heat exchanger is significantly more economical in both size and weight for a wide range of design requirements. In another comparison study 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 performance of such heat exchangers incomparable, that is, OSP = O(10<sup>2</sup> ) (Figure 3) (Alhusseny 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 configuration as a recuperator in gas turbine systems.

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

semiconductor, manufacturing, and aerospace manned flight [4]. Similarly, they have been proposed as an effective way to enhance the heat dissipated from heavy-duty electrical generators through filling their rotating cooling passages either fully or partially with open-cell metal foams [23–25]. The value of the this proposal was inspected by introducing an enhancement factor as the ratio of heat transported to the pumping power required, that is, Nu Re pin�<sup>p</sup> ð Þ<sup>e</sup> <sup>=</sup>ρu<sup>2</sup> in , and comparing it with the corresponding values from a previous work regarding turbulent flow in a rotating clear channel [26], as shown in Figure 4 [25], where it was confirmed that the proposed enhancement is practically justified and efficient.

composite opticals having the ability to allow unprecedented heat transfer. In the experiments conducted by Williams et al. [37], a porous insert material (PIM) formed of high porosity foam was proposed to improve the swirl stabilization in LPM combustion systems. It was found that using a reticulated foam insert results in mitigating the thermoacoustic instability effectively as well as reducing the combustion noise over the entire frequency range for a wide range of the

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

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187

As most of the phase change materials (PCMs) used for latent heat thermal energy storage (LHTES) possess poor thermal conductivity, the charging/discharging rate achieved will be quite modest. To overcome this deficit, high porosity metal foams have been suggested by Zhao et al. [38] as an effective means to improve the PCMs' overall thermal conductivity, leading to enhance the heat transported and, hence, promote the PCM melting and solidification, as can be seen in Figure 5 (Alhusseny et al. [39]). This concept has been extensively investigated later to further improve the performance achieved, to name a few, the works

presented for low-temperature [40–42] and high-temperature LHTES systems [43–45].

performance is still in progress [47, 48] and requires further optimizations.

Figure 5. Time development of charging process of paraffin-copper foam composite vs. pure paraffin.

Open-cell aluminum foams are considered as promising lightweight materials for γ-ray and thermal neutron shielding materials. The data collected experimentally by Xu et al. [46] reveal that filling the foam with water results in improving the mass attenuation coefficients compared to the nonfilled samples. Overall, following such a proposal to achieve high shielding

Overall, there is a variety of applications where high-porosity metal foam can be utilized successfully. For further applications, ERG Materials & Aerospace [4] lists diverse sorts of applications whether in daily life or military industries. For example, open-cell metal

design parameters considered.

They have also been utilized effectively in electronic cooling, where various configurations of metal foam heat sinks have been suggested [27–33] as an effective alternative to the traditional heat sinks incorporated in electronic devices.

In similar context, utilizing metal foams to improve internal cooling of turbine blades is of increasing interest. Filling a radially rotating serpentine channel with open-cell aluminum foam is proposed as an effective way to improve the overall efficiency of the cooling process [34]. Recently, heat transport enhancement along a 180�. round channel was proposed through placing multiple aluminum foam blocks alternately along the flow path [35]. It was found that using discrete foam blocks increases the heat transported by 74–140% compared to what the empty channel yields. In addition, it was observed that staggering the foam blocks vertically is more desirable for improving the overall system performance.

Due to their capability to transport heat effectively, metal foams are shaped into rings placed between the combustor and the turbine section of a jet engine in order to homogenize the temperature profiles of the gases leaving the combustor and, hence, to improve the overall efficiency of turbojet engines [36]. To provide a stable isothermalized platform for the airborne laser communication systems, ERG Materials & Aerospace [4] have fabricated aluminum foam

Figure 4. Influence of ΔT on the overall system performance; at S = 2, Ω = 500 rpm, and Tc1 = 20�C.

composite opticals having the ability to allow unprecedented heat transfer. In the experiments conducted by Williams et al. [37], a porous insert material (PIM) formed of high porosity foam was proposed to improve the swirl stabilization in LPM combustion systems. It was found that using a reticulated foam insert results in mitigating the thermoacoustic instability effectively as well as reducing the combustion noise over the entire frequency range for a wide range of the design parameters considered.

semiconductor, manufacturing, and aerospace manned flight [4]. Similarly, they have been proposed as an effective way to enhance the heat dissipated from heavy-duty electrical generators through filling their rotating cooling passages either fully or partially with open-cell metal foams [23–25]. The value of the this proposal was inspected by introducing an enhancement factor as the ratio of heat transported to the pumping power required, that is,

turbulent flow in a rotating clear channel [26], as shown in Figure 4 [25], where it was

They have also been utilized effectively in electronic cooling, where various configurations of metal foam heat sinks have been suggested [27–33] as an effective alternative to the traditional

In similar context, utilizing metal foams to improve internal cooling of turbine blades is of increasing interest. Filling a radially rotating serpentine channel with open-cell aluminum foam is proposed as an effective way to improve the overall efficiency of the cooling process [34]. Recently, heat transport enhancement along a 180�. round channel was proposed through placing multiple aluminum foam blocks alternately along the flow path [35]. It was found that using discrete foam blocks increases the heat transported by 74–140% compared to what the empty channel yields. In addition, it was observed that staggering the foam blocks vertically is

Due to their capability to transport heat effectively, metal foams are shaped into rings placed between the combustor and the turbine section of a jet engine in order to homogenize the temperature profiles of the gases leaving the combustor and, hence, to improve the overall efficiency of turbojet engines [36]. To provide a stable isothermalized platform for the airborne laser communication systems, ERG Materials & Aerospace [4] have fabricated aluminum foam

confirmed that the proposed enhancement is practically justified and efficient.

, and comparing it with the corresponding values from a previous work regarding

Nu Re pin�<sup>p</sup> ð Þ<sup>e</sup> <sup>=</sup>ρu<sup>2</sup>

in

186 Porosity - Process, Technologies and Applications

heat sinks incorporated in electronic devices.

more desirable for improving the overall system performance.

Figure 4. Influence of ΔT on the overall system performance; at S = 2, Ω = 500 rpm, and Tc1 = 20�C.

As most of the phase change materials (PCMs) used for latent heat thermal energy storage (LHTES) possess poor thermal conductivity, the charging/discharging rate achieved will be quite modest. To overcome this deficit, high porosity metal foams have been suggested by Zhao et al. [38] as an effective means to improve the PCMs' overall thermal conductivity, leading to enhance the heat transported and, hence, promote the PCM melting and solidification, as can be seen in Figure 5 (Alhusseny et al. [39]). This concept has been extensively investigated later to further improve the performance achieved, to name a few, the works presented for low-temperature [40–42] and high-temperature LHTES systems [43–45].

Open-cell aluminum foams are considered as promising lightweight materials for γ-ray and thermal neutron shielding materials. The data collected experimentally by Xu et al. [46] reveal that filling the foam with water results in improving the mass attenuation coefficients compared to the nonfilled samples. Overall, following such a proposal to achieve high shielding performance is still in progress [47, 48] and requires further optimizations.

Overall, there is a variety of applications where high-porosity metal foam can be utilized successfully. For further applications, ERG Materials & Aerospace [4] lists diverse sorts of applications whether in daily life or military industries. For example, open-cell metal

Figure 5. Time development of charging process of paraffin-copper foam composite vs. pure paraffin.

foams are commonly used as energy absorber in aerospace and military applications, air/ oil separators in aircraft engine gearboxes, baffles to prevent sudden surges in liquids while being penetrated by solid frames, and breather plugs in applications requiring fast equalization of pressure changes. Also, they are utilized in electrodes, fuel cells, bone researches, micrometeorite shields, optics/mirrors, windscreens and so on. Manufacturers' data and the open literature can be further dug for much more applications where highporosity metal foams have outperformed and/or achieved considerable savings in the expenses required.

Calmidi [50]:

df dp ¼ 2

df dp

area equal to that of the representative pore.

ffiffiffiffiffiffiffiffiffiffiffiffiffi <sup>9</sup> � <sup>8</sup><sup>ε</sup> <sup>p</sup> 2ε

> 1 <sup>χ</sup> <sup>¼</sup> <sup>π</sup> 4ε

cos

< : 4π 3 þ 1

1 � 1:18

<sup>χ</sup> <sup>¼</sup> βε 1 � ð Þ 1 � ε

With regard to predicting the pressure drop produced in fluid flows across high-porosity metal foams, a variety of models have been developed, which can be classified into two main categories. The first encompasses those investigations interested in estimating the pressure drop by means of the foam friction factor. Among them is the model presented by Paek et al. [54] for the friction factor as a function of pore Reynolds number (Eq. (7)). Also, the empirical correlations established by Liu et al. [55] offer friction factor estimation for airflow via alumi-

4.3. Models developed for estimating pressure drop across open-cell metal foams

num foams for a wide range of porosity and various flow regimes (Eq. (8)).

<sup>3</sup> cos �<sup>1</sup> <sup>8</sup>ε<sup>2</sup> � <sup>36</sup><sup>ε</sup> <sup>þ</sup> <sup>27</sup>

( ) " #

G

ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð Þ 1 � ε 3π r 1

!<sup>2</sup> 8

ð Þ <sup>9</sup> � <sup>8</sup><sup>ε</sup> <sup>3</sup>=<sup>2</sup>

9 = ;

<sup>1</sup>=<sup>3</sup> (6)

1 <sup>χ</sup> <sup>¼</sup> <sup>3</sup> 4ε þ

Du Plessis et al. [51]:

Bhattacharya et al. [52]:

Yang et al. [53]:

4.2. Models developed for predicting tortuosity of high-porosity metal foams

¼ 1:18

The tortuosity, defined as the total tortuous pore length within a linear length scale divided by the linear length scale in the porous medium [49], was modeled by Du Plessis et al. [51] as a function of porosity only (Eq. (4)). However, experiments conducted by Bhattacharya et al. [52] indicated that the accuracy of tortuosity model proposed by Du Plessis et al. [51] is limited for higher levels of pore density; hence, a tortuosity formulation that accurately covers a wider range of porosity and pore densities was established in terms of porosity and shape function G (Eq. (5)). Recently, an analytical model was proposed by Yang et al. [53] (Eq. (6)), as a simple function of both foam porosity and a pore shape factor. The shape factor β is defined as the ratio of the representative pore perimeter to the perimeter of a typical reference circle with an

ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð Þ 1 � ε 3π r 1

> ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð Þ 1 � ε 3π r 1

<sup>G</sup> (2)

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

<sup>G</sup> (3)

(4)

189

(5)
