A critical flaw of the direct gas Brayton system is that it is susceptible to a single point failure in which the working fluid will leak out of the system. For this reason, gas leakage due to failure of a recuperator material is a mission-threatening concern. Because most composite materials are porous, the current design of a C/C recuperator core should incorporate metallic side- and end-walls used for pressure-vessel containment of the working fluid. The design must comply with the ASME boiler and pressure vessel codes. Considerations for the pressure vessel should be focused on the load-sharing and strain compatibility of the metal and composite under pressure cycling, thermal strain compatibility of the metal and composite under thermal cycling, the stress-rupture and creep capability of the composite under long-term pressurized loads.
There is also a concern over inter-passage gas leakage between C/C plates, which will reduce overall efficiency. One method of prevention is to densify the C/C composite with multiple impregnation/pyrolysis/CVI cycles. The high-density composite will also provide better thermal conductivity. Another idea that is under consideration is to insert thin metallic foils between C/C sections while still improving the overall performance from an all metallic core. Improved assembly methods and brazing techniques are necessary to develop this technology.
Thermal conductivity and density values for C/C composites are important recuperator design considerations, although wide variations in the data exist for carbon-carbon materials. Carbon-carbon composites present the opportunity to tailor thermo-physical properties, which can be controlled by the different fiber, matrix, or processing options available. For instance, in-plane conductivity is greater than through-plane conductivity; therefore, fiber orientation could be tailored to increase thermal performance in a specific direction. Carbon-carbon processing methods can affect conductivity values such that a more graphitic matrix exhibits higher conductivities than an isotropic matrix. Typically, a pitch-based fiber or matrix will exhibit a graphitic microstructure and have higher thermal conductivity values than a phenolic-resin.