Appliance Design - March 2009 - (Page 26)

METALS The pores and struts of metal foam such as this from the Fraunhofer Institute can be customized to enhance certain mechanical properties such as thermal conductivity. Metal foam created at Northwestern University. The university has helped develop new foams including this bulk metallic glass foam that retain an amorphous structure when cooled rapidly from the molten state. the gas to create cells in a controlled manner. Mixing metal powder with a foaming agent then heating the mixture into a slurry is another option. With the casting method, an open-cell ceramic mold can be created by mixing ceramic material with a heat-soluble material such as wax or polymer. Molten metal infiltrating the mold would then burn out the heat-soluble material, and then the ceramic material would also be removed. Casting is slower than other methods, but offers more control and customization. M-pore, Dresden, Germany, a manufacturer of metal foams, casts their metal foams to customer specifications, providing foam with specified sizes for the struts and cells. Dieter Girlich, founder of M-pore, says that the uniqueness of the material requires not just an understanding of foam’s mechanical properties, but also a completely different way of thinking about design and construction using the material. For example, in a traditional heat-exchanger application, the cabinet is usually built around the exchanger and the fins so that the fins are oriented in line with airflow. With metal foam, fluid flow direction is not as important because air flows through the cells from many different directions. Because of this increased flow, a heat exchanger can be used to achieve Density [g/cm3] Poisson ratio E modulus [N/mm2] Compression strength Heat conductivity [W/mK] Heat expansion [ K-1 ] 3300 7 11 4500 10 15 0,5 0,6 increased heat transfer levels, or, conversely, a smaller heat exchanger can be used to achieve the same heat transfer levels. The struts and pore sizes can influence fluid flow, Girlich says. Smaller struts, referred to as fine struts, are often specified for filtering applications, while thicker struts are used to obtain greater surface area and aid in heat transfer efforts. Of course, these decisions may require compromises. Thicker struts aid heat transfer, but can cause a high-pressure drop. Conversely, thinner struts, with big pores and lots of space between the struts, can cause a low-pressure drop. This might require a more powerful pressure pump to push fluid through the foam, says Girlich. Choices must also be made between materials when they each have their own benefits. Bekaert, a Belgian manufacturer of heat exchangers and other industrial products, conducted internal studies about the best metal foam materials for use in compact heat exchangers. The company compared copper and aluminum foam. They found that although copper has a higher thermal conductivity, for some applications it is too expensive, too heavy, and, in some environments, too prone to corrosion. For them, cast open-cell aluminum foam is the better material for compact heat exchangers, 0,7 6000 13 20 α20 100°C 0,8 0,31-0,34 7600 17 25 9500 20 31 11400 25 38 16300 29 45 0,9 1,0 1,1 provided the application does not require entrance temperatures higher than 400 DegC. Aiming for the lowest possible weight and dimensions, Bekaert found that the use of open cell aluminum foam resulted in an increase in heat transfer of 25 percent. The thermal capabilities of open-cell foams are a key benefit of metal foams, says Mark Heamon, market manager for Porvair Advanced Materials, Hendersonville, N.C. The company produces MetFlame metal foam that is used in a gas, infrared burner for barbeque grills, as well as in commercial applications such as a high-volume production of tortilla chips, and in textile and paper drying applications, says Heamon. MetFlame is made from an open-cell reticulated, iron-based high-temperature alloy (FeCrAlY) foam. In testing, the average surface temperature of the burner plate for the MetFlame emitters operated at as much as 50 DegC hotter than ceramic tiles at all gas input rates above 160 kW/m2. One reason for this was that the materials captured and held onto the waste heat from the escaping exhaust gases, which helped it achieve higher operating temperatures from the same gas input. Compared to metal-fiber burners, the MetFlame burners achieved up to a 200 DegC higher surface temperature than knit- = 23,9 x 10-6 This table details select mechanical properties of aluminum foam. Source: Fraunhofer Institute 26 applianceDESIGN March 2009 www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - March 2009 Contents Editorial Shipments/Forecasts News Watch Transmitting Power Wirelessly Promises to Reduce the Ever-Growing Tangle of Cords and Cables Found in the Modern Home. There Are Different Technological Approaches for Achieving this Goal, and Some Are Already on the Market. Electrochemical Capacitors Can Provide an Extra Peak-Power Boost in Battery-Operated Appliances, Allowing Product Designs to Have Smaller Battery Packs. Metallic Foams Put the Properties of Metals into Lightweight Packages. Applications for this Emerging Technology Include Gas Burners, Heat Exchangers, and Electronics Housings. Hall-Effect Switches Are Compact and Provide a High Degree of Reliability and Durability as They Virtually Eliminate Mechanical Wear, Shock, and Contact Oxidation. Blowing Agents for Polyurethane Foam Insulation Face Increased Scrutiny Due to Global Warming Concerns, but Alternatives with Lower GWP Values May Provide Solutions to Meet These Challenges. Design Marts Association Report: AHAM Advertisers' Index

Appliance Design - March 2009

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