4 .8.3 Brayton System Parametrics 4.8.3.1 Working Fluid Selection Parametric descriptions of the Brayton turbomachine set, the recuperator and the cooler were prepared by the Garrett Corporation. They determined that the working fluid should be a Zenon- Helium gas. The first baseline system included the selection of a Xenon-Helium gas mixture working fluid with a molecular weight of 8 instead of Helium. This selection was based on the heat exchanger specific weight being equal with the turbocompressor variations as shown. The Xenon-Helium turbocompressor incorporates a lighter and shorter rotor which is more amenable to use of hydrostatic gas bearings. Longer turbine blading will result in increased efficiency potential. Figure 4-34 shows the layout of the first baseline system. Fig. 4-34. Xenon-Helium Mixture Results in Lighter and Smaller Turbomachine 4.8.3.2 Parametrics A baseline design was first prepared. Then the effects of changes in significant parameters were calculated. These effects are called influence coefficients. Specific weight influence coefficients were generated for the turbocompressor, recuperator and cooler components. These influence coefficients were developed for both metallic (ASTAR 81 IC) and ceramic turbine structures. The metallic turbocompressor specific weight increases with turbine inlet temperature as shown in Figure 4-35 because of the additional material Fig. 4-35. Specific Mass Variation With Temperature (Example Influence Coefficient) required to contain the high pressure working fluid at the increased temperature. The recuperator, cooler and ceramic turbocompressor specific weights decrease with increased T3 due to the reduction in cycle volumetric flow. Note that the silicon carbide turbine material has a constant design stress of 10,000 psi (68.95 MN/m^) at temperatures up to 3000°F (1922°K). The density of silicon carbide is approximately 20 percent of the ASTAR 81 IC material baselined for the metallic turbine structure. Tables 4-7 through 4-25 contain the influence coefficients used in the study.
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