GRAPHITE FOAM PERFORMANCE

Graphite Foam Performance as Heat Transfer Medium... 

The preliminary investigation showed that lower pressures result in significantly higher heat fluxes. It s possible to approaeh 50 W/em while maintaining the wall temperature below 85 °C with little optimization. A more eomprehensive parameterization study will be addressed such as pore size, geometry, and other effeets as the limits of graphite foam evaporator performance [34-35]. 

Coursey, J. S., 2003, Performance and Parametric Investigation of a Graphite Foam Thermosyphon Evaporator , MSME Thesis, University of Maryland, College Park... 

Having conducted the initial research on the graphite foam and having performed the scoping parametric analyses from neutronics and thermal-hydraulic perspectives, it was necessary to focus on a particular application that would (1) demonstrate the viability of the overall concept and (2) require a reasonably stmctured design analysis process that would synthesize those important parameters that influence the concept the most as part of a feasible, working reactor system. 

A small physical model was constructed, as well, using the graphite foam and electric heaters coupled with a computer model to benchmark codes, heat transfer, and overall material performance. 

The graphite foam s performance is central to this design concept. Characterizing the foam both in terms of structural and physical properties is required. Examining the effects on stmcture, dimensional stability, thermal conductivity, thermal expansion, and strength under irradiation is equally important. 

The current manufacturing process for graphite foams induces a preferred aligrunent of the ligaments of the foam in the Z direction. This alignment translates into an anisotropic behavior of the properties of the foam (i.e., the thermal conductivity in the Z-direction is 3-4 times that of the X- or Y-direction). Work has been directed toward improving the manufacturing process to eliminate/minimize the anisotropic behavior of the foam. Additionally, techniques have been developed to speed up the characterization process of foam properties. Some of the work performed includes ... 

The substrates tested alone have substantially different values. Polycarbonate (1/4 inch) structural foam has an of 27.5, modified-polyphenylene oxide (1/4 inch), 84.4, and RIM polyurethane (1/2 inch), 173.3. These values compare with 164.4 for 1/4 inch hardboard and 139.1 for 1/4 inch plywood. A comparison of graphite, nickel, and copper/aciylic coatings on polycarbonate and modified-polyphenylene oxide substrates illustrate a dramatic result. Despite a factor of 3 difference in substrate performance, the Q and Fs values for the coated samples are very similar. The Q for the modified-polyphenylene oxide samples are 0.7 to 0.5 that of the uncoated sample. One would expect a similarity in Fs for the coated sample, but such a reduction in Q is dramatic. Both Q and Fs are determined by the 2 mil surface. 

Carbon electrodes are commercially available in many forms. These include plates, foams, felts, cloths, fibers, spherical and other particles suitable for beds or powders. Graphite or amorphous carbons exhibit quite different performances. Porosity, surface area and pretreatment are important variables to be considered in designing carbon electrodes. 

Optimization of foam density uniformity, batch consistency, and thermal properties was performed and showed that a foaming rate of 3.5°C/min and a graphitization rate of l°C/min produced the best foam structures. 

---