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How to Study Thermal Applications of Open-Cell Metal Foam

Heat exchangers play a vital role in various thermal management applications across transportation, domestic, and industrial sectors. These devices significantly impact safety, environmental quality, and energy consumption. Air is commonly used as a working fluid in heat exchangers due to its ubiquitous nature. However, the low thermal conductivity of air results in a significant air-side thermal resistance, which can account for over 80% of the total thermal resistance in air-based heat exchangers. Therefore, reducing this resistance can lead to significant improvements in performance, resulting in cost, space, material, and energy savings. Previous research has explored the impact of fluid characteristics, flow arrangements, material selection, and heat transfer surface area extension, leading to considerable advancements in heat exchanger technology.

In forced convection, the objective of heat transfer enhancement techniques is to optimize the product of the heat transfer coefficient and the heat transfer surface area per unit volume, while minimizing the pressure drop on the air-side. This leads to the development of various fin designs, which are often tailored to specific applications. Currently, the interrupted fin design, such as louvered fins or slit fins, is considered the most advanced type for non-corrosive environments. Additionally, the implementation of vortex generators in the vicinity of wake zones of the tube can further enhance heat transfer performance.

Silver Foam (2)

In the context of natural convection, a similar optimization methodology is employed, albeit with increased complexity. This is due to the fact that flow resistance has an impact on local temperatures and heat transfer coefficients. Additionally, radiation plays a significant role in this scenario. Various types of fins, such as pin fins and plain fins with different shapes (particularly the inverted trapezoidal fin), can enhance the airflow over the heat sink compared to a plain rectangular fin. Moreover, the orientation of the fins can be further optimized. For instance, the implementation of a flared pin design, as suggested by CoolInnovations, can greatly enhance the heat transfer performance. Another potential optimization strategy involves perforating the fins to increase their porosity.

The optimization process of “conventional” fins includes the use of porous media, such as packed beds of spheres or open-cell metal foam, which have porosities of around 60%. The interconnected struts of the foam form both cells and pores.

Open-cell foam is known to have many interesting structural and functional properties:

  • High porosity (higher than 80%). Typically, the porosity can go up to 95%. High porosity results in a low weight application.
  • High interstitial surface area per unit volume.
  • Good impact energy absorption.
  • Excellent fluid mixing due to tortuous flow paths.
  • Hybrid manufacturability: different foam materials (e.g., Ni, Cu, Al) can be sandwiched into one foam panel.
  • Shapeable in three dimensions (obtainable via casting and/or co-casting techniques).
  • Visually appealing.

The amalgamation of these benefits has led to the utilization of metal foam in various applications, including the Porifera LED system.

The investigation of thermal applications involving open-cell metal foam can be effectively conducted through experiments or computational fluid dynamics (CFD). Both approaches possess their own advantages and disadvantages. Experiments offer reliability and can serve as evidence for industrial clients or as a basis for subsequent academic research.

Copper Foam 1 (6)

On the other hand, generally for thermal foam applications, a large number of parameters influence the thermal characterization of an application, amongst others:

  • Type of open-cell metal foam: This includes the material and manufacturing technique, on the one hand, and the thickness of the foam, on the other hand. Both of these parameters will affect the effective solid conductivity and heat transferring surface area.
  • Geometrical characterization., as discussed later in this work.
  • Orientation under which the metal foam sample is placed: Metal foam is generally not isotropic, depending on the manufacturing technique.
  • Bonding methods: Commonly, this is achieved with a high conductive epoxy or by brazing/soldering. Although epoxy contact is the easiest to establish, it results in an inferior thermal contact resistance, which is especially problematic for forced convective applications.
  • Cutting method: Machining can result in plastic deformation of struts at the foam edges, creating a local porosity variation. This deformation will also influence the amount of struts that are available for contact with a substrate when bonded together.
  • Effects of boundary conditions at the interface of a foam; especially of importance when comparing lumped parameters like permeability determined from two test sections with a completely different cross-section.
  • Specific construction of the test rig.
  • Effect of radiation: Determination of radiative properties is of great importance in buoyancy-driven convection and high temperature applications.
  • Effect of fouling.

Considerable resources have been allocated to the computational fluid dynamics (CFD) analysis of open-cell foam due to the impracticality of conducting comprehensive parameter testing. The predominant focus of numerical investigations lies in the application of volume averaging theory (VAT) and the determination of closure terms. It is important to note that experiments and numerical simulations are typically intertwined, as the latter necessitates validation against empirical data.

Copper Foam
Copper Foam

Although open-cell metal foam has been studied in numerous applications, it is not widely used in practical thermal applications. One possible application for metal foams would be as a replacement of fins in fin and tube heat exchangers. However, the thermal performance of a heat exchanger is not just determined by the volume, heat transfer coefficient and the surface-to-volume ratio, but also by the flow arrangement. For example, for a plate-fin heat exchanger, the fluid stream is split into individual streams (more or less) and flows through the heat exchanger with reduced mixing (individual streams between the fins). Such plate-fin heat exchangers can be seen as unmixed on both sides of the plate-fin. In the case of a foam matrix, on the other hand, the fluid on the foam side is effectively mixed. Whether a configuration can be regarded as mixed or unmixed has an impact on the heat exchanger effectiveness.

The complete characterization of open-cell foam is achieved through mCT scans, encompassing both microscopic and macroscopic parameters such as s0 and porosity. Additionally, a hybrid model for foam characterization is proposed, requiring only three parameters measured via mCT scans to create a foam model. This foam model accurately calculates the same macroscopic parameters as a full mCT scan. The focus of this work is on VAT modeling for numerical analysis, and a novel approach for determining closure terms is introduced. Closure terms are exclusively determined numerically using a foam model based on mCT data. This paper discusses the modeling of these closure terms, treating permeability and the inertial coefficient as viscous and pressure forces acting on a representative volume of the foam, rather than material properties. Experimental results are compared with this specific modeling approach, demonstrating good agreement. Furthermore, the paper provides insights into the potential use of open-cell metal foam in heat exchangers. Heat exchangers with low NTU values and/or a C∗ value of 0 (such as condensers and evaporators) may exhibit superior or comparable thermal performance to plain fins, for example.

 

 

 

 

 

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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