Exploration of Heat Transfer Characteristics of Metal Foam and Its Application in HVAC Industry

The world's energy demand is growing at a rate of about 1.3% per year, and energy conservation and emission reduction is one of the country's important development strategies, so energy-saving equipment in the field of energy applications (heat conversion, heat storage, and heat utilization) is attracting a lot of attention.

Metal foamis a material processing technology proposed by the American scientist Sosnick in 1948, a kind of pore formed by the Metal ligament-type skeleton and pore units interconnected with open cells, each open cell (pore) includes 12~14 pentagonal or hexagonal surfaces, the material toaluminum, steel, nickel, copper, ceramics, and metal alloys, which has the excellent characteristics of both the continuous-phase metal and the characteristics of discrete-phase pores.
Metal Foams are characterized by the following key parameters:
1. The pore diameter is the average diameter of the openings and ranges from 0.1 to 10 mm;
2. Skeleton diameter is the average diameter of the metal skeleton;
3. pore density is the number of pores per inch and is related to cell size, pore size, and rib size;
4. porosity ε is the volume fraction of pores obtained from mass and volume measurements of the sample, ranging from 80% to 99%, and can be categorized into open and closed pores according to whether the pores are permeable or not.
Due to the characteristics of fluid flow and heat exchange, the application in the energy field is mostly open pore type, which has a continuous and unobstructed three-dimensional pore structure inside, and there are pores between the solid skeleton, and the fluid can flow inside the pores, with porosity ε>80%, large specific surface area (1000~5000 m2/m3), large thermal conductivity, and good heat exchange performance, which is mostly used in the heat exchanger, electronic cooling, and fuel cell. In energy facilities it is used in heat exchangers with embedded foam metal, solar collectors, solar receivers, heat accumulators and so on.
According to the structural characteristics and physical features of metal foam, investigate the functionalized application of metal foam in the field of HVAC, respectively, from heat transfer, heat collection, heat storage 3 aspects of its main applications and characteristics of the study, the main findings are as follows:
(1) the heat exchanger embedded in the metal foam is a highly efficient compact heat exchanger, which can increase the convective heat transfer coefficient by 3~9 times; different materials (copper, aluminum, nickel) can significantly improve the ability to strengthen the heat transfer, and different materials can make the heat transfer coefficient is different; foam metal in the theory of heat transfer in the porosity, pore density, and permeability of the pore scale is to determine the heat transfer of the main parameters of the effective; the parameters in the rate of heat transfer, the pressure drop, the flow of the coupling of heat transfer coefficient is the main parameter; parameters in the rate of heat transfer, pressure drop, flow of the solid Coupling of the heat transfer coefficient of the main parameters need to be determined, is the design of the heat exchanger is the main indicator parameters.
(2) Metal foam is used in solar collector facilities, due to the metal foam used in this high temperature situation, the heat radiation effect is obvious and dominant. The coupling effect of heat conduction and radiation heat transfer at high temperatures in the heat transfer process, as well as the specificity of the structure of the metal foam, makes the factors affecting the effective thermal conductivity of the metal foam and the heat transfer mode multiple and complex. Commonly used are direct absorption solar collectors and volumetric solar collectors, in which the absorption collector is the influence of metal foam on solar energy absorption is dominant, between the metal, foam and fluid heat transfer indirectly affects the performance of solar energy receiver; volumetric collector is the use of air flow and solar energy flow into the device, the geometry and the material configuration of the metal foam occupies the main factors.
(3) Metal foam is used in thermal energy storage devices due to the continuity of the structure and high efficiency of thermal conductivity, using its high thermal conductivity, high porosity and high specific surface area, and phase change materials with large heat storage capacity, simple structure and no need to consume additional energy, etc., the advantages of the superposition of the composition of composite phase change materials, enhance the heat transfer of paraffin, strengthen the thermal conductivity of phase change materials, the study of the improvement of the foam metal in the phase change material Improvement of the foam metal in the phase change material arrangement, change the porosity and pore scale, selection of materials, etc. on the composite system of the phase change response time, temperature distribution, heat storage time and heat transfer has an important impact on the subsequent optimization of the design of the equipment to provide a certain theoretical basis.