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Thermal application of metal foam

Copper Foam
Copper Foam

The increasing demands of society and industry have led to a projected annual rise of 1.3% in global energy consumption until 2040. Consequently, there is a growing emphasis on energy conservation and the efficient utilization of resources. In the realm of thermal applications, there is a need for heat conversion, storage, and utilization devices that are lightweight, compact, and highly efficient.

Metal foam is a material that was first developed by Sosnick in 1948. This material is composed of a network of interconnected open cells, which are held together by metal ligaments. The base materials used in commercial production of metal foam include aluminum, steel, nickel, copper, ceramics, and metal alloys. The open cells in metal foam are typically composed of 12-14 pentagonal or hexagonal faces. The material is characterized by several key parameters, including the average diameter of open cells (dp), the average diameter of ligaments (df), the number of pores per inch (PPI), which can range from 5-100, and the void volume fraction (porosity, ε), which is measured by the weight and volume of the sample and can range from 80%-99%.

Metal foam is a material that possesses low density, high mechanical strength, stiffness, and a large specific surface area. Due to these properties, it has found widespread use in various industrial applications, including heat exchangers, electronic cooling, fuel cells, catalytic reactors, aerospace planes, and air-cooling. Of particular interest is its high permeability and thermal conductivity, which has led to extensive research on its potential for enhancing heat transfer in recent years.

In thermal applications, the thermal and flow characteristics of metal foam are concerned to improve the thermoshydraulic performance.

  1. Flow properties of metal foam

Metal foam is a common type of porous media, and its penetration behavior can be described by the Darcy model. The Darcy model takes into account the viscous force within the porous media and establishes a linear relationship between pressure drop and velocity. However, for packed bed as a porous media, the permeability K is no longer monotonic with the particle diameter, and the viscous shear force on the wall must be considered. To address this, the Darcy-Brinkman model proposed by Brinkman is utilized to describe the penetration in porous media. Compared to soil, metal foam exhibits a larger permeability, resulting in a much higher velocity. The Forchheimer model considers the inertial effect of the velocity quadratic term. For numerical analysis of fluid flow in metal foam, the Darcy-Brinkman-Forchheimer model is widely employed.

  1. 2. Thermal properties of metal foam

In the initial investigations of heat transfer in metal foam, the fluid and porous phases were regarded as homogeneous and isotropic, with no regard for heat convection between the foam ligament and fluid. The intensive investigation of the thermal and flow properties of metal foam has led to its widespread application in the field of thermal energy. Metal foam is now commonly utilized in various thermal energy applications, including compact heat exchangers, solar thermal facilities, and thermal energy storage.

  1. Numerical simulation in pore scale

Metal foam is characterized by its random open cells. In order to determine the physical properties of metal foam, the cells are often assumed to be regular three-dimensional micro-structures based on their morphology. These ideal micro-models include the two-dimensional hexagonal structure, cubic cell model, three-dimensional dodecahedron unit cell, three-dimensional tetrakaidecahedron unit cell, and Kelvin cell model. Volume-averaged semi-empirical equations can be used to simulate the flow and thermal characteristics of metal foam by a macroscopic approach based on these ideal micro-models. However, this method may overlook the pore scale details, and therefore, microscopic approaches are applied to accurately simulate the intricate geometry of metal foam.

In recent decades, researchers have utilized ideal cell models to construct periodic cell structures of metal foam. Boomsma et al. employed the 14-sided tetrakaidecahedron cell (Kelvin model) to examine the flow characteristics in metal foam. The simulations by volume-averaged method should consider the wall effects that generate flow resistance.

As previously discussed, direct numerical simulation of metal foam provides detailed information on wall effects, interfacial heat transfer coefficients, and variations in foam shape. However, this approach requires significant computational resources. Future research may benefit from utilizing a direct numerical simulation approach based on a real foam model, which would enable more accurate determination of microstructure and physical parameters.

Copper Foam 1 (11)

  1. Compact heat exchangers fully filled with metalfoam

In the fields of chemical engineering, aerospace engineering, and electronic production cooling, heat exchangers are required to possess qualities of lightness, compactness, and efficiency. To meet these demands, compact heat exchangers have been developed, which incorporate metal foam as a key component. These heat exchangers can be fully or partially filled with metal foam.

We aims to provide an overview of the thermal and flow characteristics of two categories of heat exchangers. The first category comprises tube and plate heat exchangers that are fully filled with metal foam and possess a regular shape. The second category includes finned tube and tube-in-tube heat exchangers that are fully filled with metal foam and possess a complex shape.

Compact heat exchangers that are fully embedded with metal foam have been found to significantly enhance the heat transfer rate, albeit at the expense of a substantial pressure drop. In comparison to finned heat exchangers, the performance evaluation criterion (PEC) of heat exchangers that are fully embedded with metal foam is inferior. Therefore, it is imperative to optimize the configuration of the metal foam to achieve optimal performance.

Nickel Foam (6)

  1. Compact heat exchangers partially filled with metal foam

Optimization of metal foam partially filled in the heat transfer region has been identified as an effective method for improving heat exchanger performance. In order to determine the effective area of metal foam, investigations were conducted on velocity and temperature distributions. A range of arrangements for plate heat exchangers and tube sides of tube heat exchangers were proposed. The coupling effects of porous and clear fluid regions were explored in regular space, with investigations conducted on metal foam/porous media inserted at the core and wall of the channel. Results indicated that an increase in foam height could enhance heat transfer rate, however, after half height of the channel, the heat transfer enhancement was found to be insignificant. An experimental investigation on heat sink with metal foam revealed that a foam height of 22% fan diameter and 50% channel height resulted in the best heat transfer performance.

In contrast to conventional methods of partially filling channels, the insertion of metal foam blocks represents a novel approach to optimizing heat transfer performance, as illustrated in Figure 6. Chen et al. devised a configuration in which four metal foam blocks were evenly spaced within a horizontal channel and fully filled in the vertical direction. As previously established, velocity distribution within metal foam is more uniform than in a free channel. The intermittent flow occurring in the foam region and free region disrupts the development of the boundary layer and reduces pressure drop. The effects of foam microstructure in this arrangement are similar to those observed in channels that are fully filled with metal foam. However, the thermo-hydraulic performance of channels that are partially filled with metal foam in both horizontal and vertical directions differs. The presence of metal foam blocks results in flow blockage, with foam height and permeability being two critical parameters. Increasing foam height reduces the free flow region and forces fluid into the foam region. In channels with larger foam height, increasing permeability enlarges the pressure drop and enhances heat convection. Conversely, increasing foam length leads to a larger boundary layer, particularly in metal foam blocks with smaller height and larger permeability.

The utilization of metal foam wrapped around the tube in tubular heat exchangers is an effective technology for optimizing the shell side and reducing flow resistance. In the case of a bare circular cylinder, it is well-known that the wake is periodic with vortex shedding occurring at Reynolds numbers beyond 40 due to the buoyancy and viscous effects. Figure 7 illustrates that the presence of porous media wrapped around the cylinder alters the onset of vortex shedding, resulting in changes to the thermal characteristics. The vorticity generated by the buoyancy effect is negative downstream of the cylinder. Consequently, the fluid penetrates the porous region from the front stagnation point up to a certain angle, leading to a higher rate of heat and mass transfer. In the downstream region, heat conduction dominates the heat transfer process. The thickness and permeability of the porous layer play a critical role in determining the heat transfer rate. Beyond a certain critical value, heat transfer is dominated by heat conduction due to the low velocity in the porous media. Below the critical value, heat transfer is dominated by heat convection occurring in both the porous and clear regions.

The tube bundle is a commonly utilized form of heat exchanger in various industrial applications. In order to achieve compactness, metal foam has been identified as a potential solution due to its ability to provide a larger specific surface area compared to commercial fins. It has been observed that the complete filling of gaps in the tube bundle with metal foam leads to a significant enhancement in heat transfer, albeit at the cost of a substantial increase in pressure drop. To mitigate this issue, the use of metal foam as an interconnector in an in-line tube bundle layout has been proposed, which can reduce recirculation and vortices and subsequently decrease pressure drop. However, the resulting enhancement in heat transfer is only marginal. As a result, studies have been conducted on the use of metal foam-wrapped cylinders, which have led to the design of tube bundles wrapped with metal foam for improved heat exchanger performance.

  1. Solar collectors utilizing metal foam or porous media for non-concentration purposes

The advancement of solar energy has led to the widespread utilization of energy conversion facilities, such as solar collectors and solar receivers, which convert solar radiant energy into thermal energy. These facilities are often accompanied by thermal energy storage systems, which are commonly employed to provide steam and hot water for industrial or commercial purposes. Despite its potential, solar energy is considered a low-quality energy source due to its low conversion efficiency. To address this issue, metal foam with high thermal performance has been employed to enhance the efficiency of solar facilities, as reported in reference. Solar thermal facilities can be classified into two types: concentrating and non-concentrating.

Solar collectors of the non-concentrating type are designed for the purpose of utilizing intermediate-low temperature. Among the various types of non-concentrating solar collectors, the flat plate solar collectors are commonly employed in both civilian and industrial sectors. Nevertheless, the distribution of solar energy flux density is non-uniform, and the efficiency of convection between the radiant absorber plate and fluid is low. To address this issue, metal foam has been investigated as a potential heat enhancement device for application in solar collector pipes/channels.

  1. Metal foam-based thermal energy storage facilities are a promising technology for efficient energy management

Renewable energy sources, such as solar and wind energy, generate thermal energy that requires storage in thermal energy storage facilities before being released. The classification of thermal energy storage is based on the storage modes of materials, which include sensible heat storage and latent heat storage. Sensible heat storage has been developed for commercial applications, such as concentrating solar power plants and urban heating, for several decades. Materials such as water/steam, molten salt, and concrete with low-cost and thermal stability are utilized for different temperature ranges. However, these materials suffer from low thermal capacity and large volume, which is a common problem. On the other hand, latent heat storage utilizes materials with larger thermal capacity, but the low heat transfer rate limits the thermal storage efficiency and charge/discharge time. To enhance heat transfer performance of phase change materials (PCMs), metal foams with ultra-light isotropic structures, continuous metal ligaments, and high thermal conductivity are employed. Researchers have compared metal foam and expanded graphite embedded in PCMs and found that metal foam performs better due to its continuous matrix. Therefore, understanding the effect mechanism of metal foam properties is crucial in designing thermal energy storage systems.

We provides a review of heat exchangers that incorporate metal foam. The design of these heat exchangers for industrial applications was optimized based on the thermos-hydraulic characteristics of metal foam. The paper presents exploratory work on the use of these heat exchangers in compact heat exchanger systems, solar thermal facilities, and thermal energy storage applications.

The theories of heat transfer and fluid flow in metal foam have been developed at a macroscopic scale, considering metal foam as a porous medium. Simulations at the pore-scale have confirmed the accuracy and applicability of these theories. Compact heat exchangers that are fully embedded with metal foam can be classified into three categories: plate/tube heat exchangers, tube-in-tube heat exchangers, and tube bundles. When fully filled with metal foam, heat convection is the dominant term that determines the heat transfer process. The microstructure parameters of metal foam, such as porosity, pore density, and permeability, are used to optimize the heat transfer performance. Increasing the permeability and porosity can reduce the heat transfer rate and pressure drop. However, there is a trade-off between heat transfer enhancement and an increase in pressure drop. This paper presents correlations between heat transfer rate and pressure drop in several types of heat exchangers that are fully embedded with metal foam.

Nickel Foam (2)

Compact heat exchangers that are partially embedded with metal foam are optimal for reducing pressure drop. This paper presents experimental and numerical studies on rectangular channel arranged metal foam blocks, tubes, and tube bundles wrapped with metal foam. The porous region and clear fluid region are coupled to affect the flow and temperature field. The conversion mechanism of heat transfer is carried out, where heat conduction is the dominant term at higher dimension permeability. The correlations between heat transfer rate and pressure drop are concluded to guide the design of heat exchangers. Several bonding methods for the point contact of metal foam are presented, with the powder-sintering method being the most stable method with a minimum thickness bonding layer. It should be noted that particle deposition in metal foam increases pressure drop and causes heat transfer deterioration.

Metal foam is also utilized in solar thermal facilities, combining radiation properties. For non-concentrating solar collectors, geometries of solar collectors and configurations of metal foam are presented. For concentrating solar receivers, the effects of metal foam on solar absorbing are the dominant term in direct solar receivers, while heat transfer between metal foam and fluid dominates the indirect solar receiver performance. The heat transfer mechanism of foam structure in solar thermal facilities should receive more attention in further work.

The utilization of metal foam in thermal energy storage can improve the heat transfer rate and reduce the charge/discharge periods due to the continuity of structure and high effective heat conductivity. The large volume fraction of metal foam ensures stable energy density. A series of optimal designs of thermal energy storage facilities with metal foam are presented. With the addition of metal foam, thermal energy storage facilities have a better commercial future compared to electrochemical energy storage and mechanical energy storage.

 

Picture of Lu

Lu

Our materials research team from Tsinghua University postdoctoral researcher lin and Harbin Institute of Technology researcher Mu, Nanjing University of Technology researcher Wei, they share their expertise in foam metal materials article.

About HGP

WE were established in 2003, located in the Gaoxin Zone of Guangdong-Guangxi Cooperation Special Experimental Zone, covering an area of 70 mu, with a plant of about 30,000 square meters, with more than 170 employees, is an advanced new material technology enterprise integrating research and development, production and sales.

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