Application of Porous Media in Heat-Transfer and Mass-Transfer Enhancements: A Brief Review

Abstract

The heat and/or mass transfer is crucial in various energy conversion and storage systems such as heat exchangers and energy storage systems, since they highly affect the efficiency of energy conversion and transport. Enhancing the heat and/or mass transfer within these systems is the most important means to improve system efficiency. Porous media have found wide application in enhancing the heat conduction, mass diffusion, or both, for different energy conversion and storage systems. In this chapter, a brief review on the application of different porous media for transport enhancement in various systems was made, indicating that using porous media is capable of enhancing the transport ability appreciably, sometimes being up to hundreds of times in some physical problems. This review could provide some insight into the transport enhancement design of various energy conversion and storage systems, which is especially important in the background of carbon neutralization.

1. Introduction

Porous media structures are widely existing around us, like soil, rocks, wood, and many other solid objects. Some features of these porous structures, like large surface area, low-density, permeability, selectivity, adsorptivity, and thermal conductivity, have inspired us to explore their potential applications. Some artificial porous media with specified porous structure, like activated carbon, zeolites, and sponges, metal foams, have been designed for various purposes. These natural and synthetic porous materials have received a wide range of applications, including filtration and separation, chemical reaction, adsorption, heat exchanger, hydrogen storage, fuel cells, and so forth.

This chapter, however, will focus on the applications of the porous media in heat-transfer and mass-transfer enhancements. The enhancements in heat and mass transfer have been attracting significant attention, especially in the background of carbon neutralization, since they are capable of enhancing the energy efficiency and thus reducing the carbon emission. The unique characteristics of porous media, such as large surface area, rich interconnected pore structure, and high thermal conductivity, make them good candidates to enhance the heat and mass transfer in various energy systems.

Therefore, this chapter will make a brief review on where and how the porous media have been employed to enhance the heat and mass transfer. To the authors’ knowledge, there is no systematic review on the application of porous media in terms of heat-transfer and mass-transfer enhancements. So, we hope this chapter would provide a reference to the researchers or engineers when they are developing innovative solutions to enhance the heat and/or mass transfer so as to enhance the efficiency and performance of various industrial processes.

2. Application of porous media in heat-transfer enhancement

Enhancement in heat transfer by porous media can be realized from increase in either thermal conductivity, convective heat transfer, or radiation heat transfer. However, the radiation is not discussed in this chapter due to space limit. Therefore, in this part, some applications of porous media to enhance the heat transfer will be discussed from the heat conduction and convection aspects.

2.1 Heat conduction enhancement by porous media

The solid matrix of some porous media has larger thermal conductivity than the working medium (e.g., phase change materials), so the working medium can be embedded in some porous media for the enhancement in thermal conductivity and thus the thermal performance. The applications with this regard mainly associated with latent-heat energy storage systems.

Phase change materials (PCMs) are usually of low thermal conductivity, which seriously restrains the energy storage efficiency. Since the solid matrix of porous media bears large thermal conductivity, the PCMs can be embedded in the porous matrix, and therefore, the high thermal conductivity of porous media enables efficient heat transfer between the PCMs and the heat exchangers. The porous media used for this application are mainly divided into two groups, graphite-based porous media and metal foams.

The widely used graphite-based porous media are mainly divided into two groups: graphite foam and expanded graphite (EG). Graphite foam is usually produced by a foaming process that creates interconnected pores within the graphite material, thus creating a highly porous structure. However, expanded graphite is produced by treating the natural graphite with chemicals or high temperatures. This treatment causes the layers of graphite to separate and expand, creating a porous structure with a high surface area. The resulting expanded graphite can be used to embed PCMs to produce composite PCMs with large conductivity.

Zhong et al. [1] prepared four composite PCMs by paraffin wax and mesophase pitch-based graphite foams (GFs) with different thermal properties and pore sizes. The polarizing optical microscopy (POM) pictures of the polished surface of the four composite PCM samples are shown in Figure 1. After the test, they found that the thermal diffusivity of the paraffin-GFs can be enhanced by 190, 270, 500, and 570 times, respectively, as compared with that of pure paraffin wax. Zhong et al. [2] synthesized three kinds of porous heterogeneous composite PCMs with EG and binary molten salts (LiNO₃-KCl, LiNO₃-NaNO₃ and LiNO₃-NaCl), as shown in Figure 2. They found that the heat conductivity of all binary molten salts was enhanced by 4.9–6.9 times after impregnation with EG. Sari et al. [3] prepared different paraffin/EG composites with the EG mass fraction of 2, 4, 7, and 10%. Heat conductivities of the pure paraffin and the composite PCMs were measured as 0.22, 0.40, 0.52, 0.68, and 0.82 W/(mK), respectively, indicating that the heat conductivity of the paraffin/EG composite was enhanced by approximately 2–4 times compared to that of pure paraffin.

Figure 1.

POM pictures of the polished surface of the four filled foam samples [1].

Figure 2.

SEM photographs of porous heterogeneous composite phase change materials: (a) LiNO₃-KCl/EG, (b) LiNO₃-NaNO₃/EG, and (c) LiNO₃-NaCl/EG [2].

Another porous media usually used to enhance the heat conductivity of PCMs are metal foams, which are typically produced through a process called foaming, where a mixture of metal powder, a blowing agent, and a binder are heated or chemically treated to create a foam-like structure. Metal foams find wide applications in latent heat thermal energy storage with PCMs.

Xiao et al. [4] prepared the paraffin/nickel foam and paraffin/copper foam PCMs, each of which was designed with three different pore sizes (5PPI, 10PPI, and 25PPI), as shown in Figure 3. Their results indicated that the heat conductivity of the paraffin/nickel foam composite was nearly three times larger than that of pure paraffin, while the paraffin/copper foam composite was 15 times larger. Fleming et al. [5] investigated the heat-transfer enhancement of a shell-and-tube latent heat thermal storage unit, as shown in Figure 4, by the addition of open-cell aluminum foam. Their results showed that the inclusion of aluminum foam significantly enhances the heat transfer coefficient by 100% for the melting process and by 20% for the solidification process. It was also found that the natural convection during melting was suppressed by the foam, but the suppression was still outweighed by the thermal conductivity enhancement.

Figure 3.

Images of metal foam and paraffin/metal foam composite PCMs with different pore sizes: (a) nickel foams, (b) paraffin/nickel foam composite PCMs, (c) copper foams, and (d) paraffin/copper foam composite PCMs (I: 5PPI, II: 10PPI, III: 25PPI) [4].

Figure 4.

The dual-pass heat exchangers with and without foam and the complete storage unit (without lid) insulated with vacuum insulation panels [5].

Some other research regarding the application of porous materials/foams in PCMs for the heat-transfer enchantment purpose can be referred to Ref. [6].

2.2 Convection heat-transfer enhancement by porous media

The enhancement in convection heat transfer by porous media attributes to a few factors: Firstly, the porous media have larger specific surface area, providing more contact points between liquids and solids and thus enhancing the convection heat transfer; secondly, the interconnected pores in porous media allow for more efficient transport of fluids through the material than solids, leading to heat-transfer enhancement by convection, which carries heat away from the hot regions and distributes it throughout the material; thirdly, the porous structure at the wall surface in a flow stream causes some micro local flow, which enhances the heat transfer coefficient; and fourthly, the porous structure provides more nucleation sites for boiling, and enhances the surface wettability and thus promotes the formation of thin liquid films for condensation, both leading to increased heat transfer coefficients; the abovementioned four aspects of porous media collectively make them effective in improving heat transfer performance.

The metal foams find good application in tubes to enhance the convection heat transfer. These metal foam embedded tubes are widely used in various heat exchangers due to high heat transfer efficiency. Huang et al. [7] inserted a porous media in the core of a tube for the heat-transfer enhancement purpose, as shown in Figure 5. Their results showed that the heat transfer rate in the tube with inserted porous media whose diameter approaches the diameter of the tube is about 1.6–5.5 times larger than those of the smooth tube cases in laminar, transitional, and turbulent flows. Ming et al. [8] studied the similar problem, finding that inserting the metal foam in the tube brings a notable increase of Nusselt number and that good heat transfer performance was achieved if the diameter of the metals foam approaches the tube diameter.

Figure 5.

Schematic diagram of the problem [7].

Metal foams are also used in other geometries to enhance the heat transfer. For example, Vazifeshenas et al. [9] carried out both numerical and experimental research to assess the thermo-hydraulic behavior of open aluminum metal foam sheet adjacent to the heated wall in role of polymer electrolyte membrane (PEM) fuel cell end plate shown in Figure 6. The result showed that open aluminum metal foam is good for heat transfer and that the heat transfer rate is positively proportional to the porosity of the foam.

Figure 6.

Single-cell metal foam adjacent to electrical heater plate [9].

Another application of porous media for heat-transfer enhancement is associated with boiling. Bai et al. [10] investigated the enhancement of boiling of anhydrous ethanol in the porous-coated microchannels. In their research, three porous-coated microchannels were prepared by fabricating metallic porous coating with copper particles of different diameters in the bottom of the microchannels by a solid-state sintering technology, as shown in Figure 7. As indicated by their results, a dramatic enhancement of flow boiling heat transfer in the porous-coated microchannels was obtained, due to the preferable bubble nucleation condition of porous coatings. Ma et al. [11] investigated the pool boiling heat transfer on the surface of different kinds of multilayer gradient aperture porous copper they prepared, as shown in Figure 8. They found that bubble nucleation on the porous surface emerges at much lower heat flux (50% lower) than that of the smooth surface.

Figure 7.

(a) Schematic of solid-state sintering process; (b) SEM images of porous-coated microchannels (#1, #2, and #3) [10].

Figure 8.

Macro-morphology and SEM photos of the top and bottom surfaces of the 4-layer sample: (a) top view where the pore is 100 mesh; (b) bottom view where the pore is 300 mesh [11].

3. Application of porous media in mass-transfer enhancement

The mass-transfer enhancement by porous media attributes to the porous interconnected voids within their structure, which can be of various sizes and shapes and allow for the stronger flow of fluids than that through the low-permeability solids.

The most well-known application of porous media in mass-transfer enhancement is in crude oil and natural gas production industry. Sometimes, the crude oil reservoir or natural gas reservoir is of low permeability; some artificial fractures have to be constructed by a technique called hydraulic fracturing, which forms an interconnected porous zone, as shown in Figure 9, and enhances the flow of crude oil or natural gas in the reservoir. Geometries of various fractures can be seen in Figure 10.

Figure 9.

Intersection of hydraulic fractures with natural horizontal fractures in multistage horizontal well [12].

Figure 10.

Different fractures of the reservoir [13].

Another typical example of the application of porous media in mass-transfer enhancement, actually both heat- and mass-transfer enhancements, is in the exploitation of geothermal energy from dry rocks. There is a huge amount of heat energy stored in the dry rock, but the dry rock is of pretty low permeability, through which the fluid hardly flows to extract heat energy. An enhanced geothermal system (EGS) [14], as shown in Figure 11, was proposed to exploit the thermal energy in these dry rocks, by drilling wells into hot rock and fracturing the rock sufficiently to enable the water to flow between the wells. The fluid flows along permeable pathways, picking up heat, and exits the reservoir through production wells. The fractures allow for the circulation of the fluid through the heat reservoir, increasing the contact area between the fluid and the hot rock. This enhances the mass transfer as well as heat transfer from the rock to the fluid. The characteristics of fractures have a great impact on the heat transfer performance, details of which can be found in Refs. [15, 16].

Figure 11.

EGS cutaway diagram [14].

Mass-transfer enhancement by porous media also finds application in hydrogen storage. Among various hydrogen storage methods, hydrogen storage in metal hydride (e.g., magnesium hydride) is a very promising technique due to large storage capacity [17, 18]. However, one of the most important factors influencing the storage efficiency is the transport of hydrogen in the metals that are of pretty low-permeability, because the hydrogen has to transport within the metal and chemically react with the metals in the bulk of the material. To solve this problem, the metal powder, which behaves like a porous media, is used instead of solid metal. The widely used hydrogen storage system based on metal powder is shown in Figure 12. The hydrogen can transport through the metal powder and chemically react with it, and the produced hydride is still in a powder form, which also allows the hydrogen to flow through it. The porous structure formed by the metal powder effectively enhances the mass transport of the hydrogen to ensure the chemical reaction. A real application of this metal powder (Mg) for hydrogen storage is shown in Figure 13. As can be seen, the stack of Mg/MgH₂ presents obvious porous structure.

Figure 12.

Geometrical configuration of (a) hydrogen tank and (b) the reactor [19].

Figure 13.

Sketch of the tank equipped with a copper fin heat exchanger fitting the compacted discs (a) and stack of MgH2 D 5 wt.% ENG (b) [20].

In addition, porous media can also be used to enhance the mass transfer in electrode of fuel cells or other batteries. These porous electrodes can provide a larger surface area for fuel transport and thus electrochemical reactions, improving the efficiency and performance of the energy storage or conversion devices.

4. Conclusions

Porous media have found wide applications in enhancing heat and mass transfer. The first method to enhance heat transfer by porous media is to increase the thermal conductivity of the material, which is widely used in latent thermal energy storage to improve the heat conductivity of the PCMs. The porous materials used in this regard mainly include two types: graphite-based porous materials (e.g., expanded graphite or graphite foams) and metal foams. The second method for enhancing heat transfer can be realized by enhancing the convective heat transfer, which finds wide applications in heat exchangers, boiling or condensation, and other fields. The enhancement of convective heat transfer is mainly attributed to the larger surface area of porous materials, rich flow channels, formation of local disturbances, and easier formation of bubble nucleation. The enhancement of mass transfer by porous media is, however, attributed to the permeability, which allows effective mass transport inside the porous media. The well-known applications of porous media for mass-transfer enhancement are in crude oil and natural gas extractions and enhanced geothermal energy exploitation. By creating artificial fractures and thus increasing the permeability of the porous media, mass transfer is enhanced in the reservoirs. Another application of porous media for mass-transfer enhancement can be found in hydrogen storage by metal hydride. The porous powder of metals, instead of solid metals, is used to enhance penetration of hydrogen into the interior of materials and undergo chemical reactions with solid materials. Many more applications of porous media in heat and mass transfer are yet to be discovered.

Acknowledgments

The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (Nos. 51606117) and Natural Science Foundation of Shanghai (No. 20ZR1423300).

Conflict of interest

The authors declare no conflict of interest.