Heat transfer properties of open cell metal foams

Metal foam is a kind of multifunctional composite metal material containing a certain number, a certain size of pores, and a certain porosity in the metal matrix. Foam metal has unique structural characteristics such as large specific surface area, large porosity, and small density, and has multifunctional composite characteristics such as heat dissipation (open-cell), heat insulation (closed-cell), sound absorption and vibration damping, lightweight, high specific strength, electromagnetic shielding, etc., and it is a kind of structural and functional material integrating the properties of thermophysics, mechanics, acoustics, and electricity.
For open-cell metal foam, the high porosity and complex three-dimensional mesh structure make it have very good heat dissipation and high heat transfer efficiency, which has a very broad application prospect in the field of compact heat exchangers, microelectronic device cooling, and so on. For the heat transfer properties of open-cell foam metal, its research status is investigated and analyzed from the application areas of compact heat exchangers.
Metal foam for compact heat exchangers
The enhanced heat transfer properties of metal foam have been generally recognized. When the gas or liquid flows through the holes, the heat is transferred and interchanged in the form of forced convection between the metal foam with an open-cell structure and the fluid. On the one hand, the complex three-dimensional mesh structure of the metal foam makes the nonlinear effect of the fluid, and the degree of turbulence is also strengthened, thus promoting the solid surface to the fluid in the high local heat transfer; on the other hand, the foam metal large specific surface area is also an important reason for the metal has good heat transfer performance. At present, the study of metal foam for compact heat exchanger transfer performance mainly focuses on air-side enhanced heat transfer and refrigerant-side enhanced heat transfer.
For the air-side enhanced heat transfer properties of metal foam, metal foam is used instead of fins for heat dissipation in applications where traditional fins are used, such as cooling electronic components, plate heat exchangers, and finned heat exchangers. In compact heat exchangers, the flow heat transfer properties are currently studied mainly for copper and aluminum foam metals with pore densities generally less than 40% and porosities of 70% or more.
At a certain pore size, the heat transfer rate increases with the increase of porosity and dominates. On the other hand, for a certain porosity, the higher the pore density, the lower the heat transfer efficiency, which may be due to the excessively large pore size. The pore structure parameters of porosity, pore density, and pore diameter are the main factors affecting the heat transfer performance of metal foams. For the application of metal foam in cooling electronic components, the effects of metal foam porosity and pore diameter on the heat dissipation effect were investigated by orthogonal experiments. The results show that under forced convection, the heat dissipation performance of foam metal is related to the pore structure, in which the pore size has the greatest influence.
Some research results show that although the foam metal has a strong role in enhancing heat transfer, the growth of resistance is not negligible along with the growth of heat exchanger performance. The heat transfer efficiency of foam metal is significantly improved at smaller pressure loss, and the pressure loss increases with the increase of pore density. The results of the study are instructive for the design and fabrication of lightweight and compact high-efficiency heat exchangers. An experimental study of foam metal heat exchangers and finned heat exchangers of the same size was carried out, and it was found that under the same test conditions, the foam metal heat exchanger was less efficient than the finned heat exchanger, and the pressure loss and friction coefficient were higher than those of the finned heat exchanger. The porous foam metal has an obvious strengthening effect on convective heat transfer, and the strengthening effect decreases with the increase of Reynolds number; the porous foam metal makes the flow resistance of fluid increase obviously, but the multiplication of resistance increase decreases with the increase of Reynolds number. Comprehensive strengthening of convective heat transfer and increase in the pressure drop in the flow of two aspects can be seen, porous foam metal is more suitable for the fluid with smaller flow rate occasions. Foam metal has the role of air-side enhanced heat transfer, but heat transfer performance enhancement at the same time, the resistance also increased, and the overall performance is worse than the traditional fins. Research shows that foam metal is suitable for low-flow rare occasions. Therefore, foam metal air-side enhanced heat transfer needs to consider the application occasions, and can not blindly carry out the traditional fin heat dissipation alternative research.

The metal foam can also play a role in enhancing the heat transfer of refrigerant by phase change. The results show that when the porosity is certain, the heat transfer performance doubles as the pore density decreases from 20 to 40 PPI and the pore size decreases, due to the finer cell structure with a larger surface area and more intense perturbation. At high flow rates, the boiling heat transfer coefficient maintains a steady and slow increase with increasing dryness. With the gradual increase in operating pressure, the boiling heat transfer phenomenon at low dryness is similar to ground boiling, i.e., the heat transfer is intensified with the increase in pressure, but the pressure rises to a certain extent plays a deteriorating role in the boiling heat transfer. The heat transfer coefficient of copper foam tubes is about three times higher than that of light tubes. In the case of pure refrigerant, the presence of foam metal enhances the flow of boiling heat transfer, and the heat transfer coefficient is increased by up to 185%. In the case of oil-containing conditions, the effect of metal foam to strengthen heat transfer is weakened; under the same conditions, compared with 5PPI foam metal, 10PPI foam metal can increase the heat transfer coefficient by up to 0.6 times, and smaller pore size can improve the flow boiling heat transfer coefficient.
The effects of mass flow rate and two-phase fluid dryness on flow condensation pressure drop and heat transfer coefficient were comprehensively analyzed. The results show that the pressure drop of the inner wall-filled annular metal foam tube is much larger than that of the light tube, and the pressure drop increases rapidly and nonlinearly with the increase of mass flow rate and dryness. The condensation heat transfer coefficient of the inner wall-filled annular metal foam tube is larger than that of the light tube, and the heat transfer coefficient increases with the increase of mass flow rate and dryness, and the flow condensation heat transfer coefficient of this type of reinforced tubes is about twice as much as that of the light tube. Because its capillary effect is obvious, the pore density of the 130PPI reinforced tube is the best effect, for the light tube 3.06 times, 40PPI tube is mainly similar to the structure of the low-ribbed tube as a means of strengthening.
In the compact heat exchanger, metal foamcan be fully utilized to strengthen the role of heat transfer, but the study showed that the resistance characteristics of the foam metal growth are also very significant, the need for comprehensive performance to assess the application of foam metal in the compact heat exchanger. In addition, in the presence of oil-containing lubricants, foam metals can lead to deterioration of heat transfer, and the enhanced heat transfer of metal foams for phase change heat transfer of refrigerants needs to be considered in light of the effects of lubricants.