Development concerns (3)-Fabrication challenges of carbon carbon composite

The major heat exchanger components of a plate-fin, cross-flow core include corrugated fins, parting sheets, and channel edge closures. As mentioned, the use of C/C composites allows each component to be tailored for thermal and structural performance through materials and processing selection. Although this option exists, it is often labor intensive or processing becomes a cost limiting factor. For these reasons, a plain plate-fin design has been envisioned for the C/C recuperator core.

The corrugated plate-fins require both thermal and structural design considerations. High in-plane thermal conductivity is most important, therefore fiber orientation in the direction of conductivity must be considered for optimization. High-thermal conductivity fibers and a dense graphitic matrix will also aid thermal performance. The increase in performance by fiber selection and orientation could ease the concern over plain-fin geometry and help reduce the projected volume penalty. Also, since heat transfer surface area within the core dominates performance, the fin packing density must be enhanced. Reducing the plate-fin thickness to increase packing density is required but is limited by fabricability concerns.

The parting sheets serve primarily a structural function and are the principal leakage boundary between the hot and cold airflow streams. For the parting sheets, through-plane conductivity is important and sheets should be as thin as possible while meeting thermal, structural, and permeability requirements. Since C/C composites are inherently porous, additional processing steps must be utilized to increase the composite density. The use of thin metallic foils between the parting sheets may help to prevent leakage, but incorporates more challenges to the fabrication concern. The metallic foils must have a low coefficient of thermal expansion and high-temperature capabilities, and must meet compatibility requirements with the composite and gas Brayton system.

Channel edge closures serve a structural function by acting as load carrying members and leakage boundaries. For the conceptual recuperator design, the side- and end-walls are expected to be metallic to meet the critical permeability requirements. The composite core and metallic pressure boundary must be compatible and conform to the ASME boiler and pressure-vessel code.

Fabrication of a C/C heat exchanger core has been demonstrated for aircraft applications. Conventional core design mimics the procedure used to produce metallic heat exchangers by brazing each corrugated layer to the flat parting sheet on both the top and bottom to form a layer. Each layer is brazed together to form the core. The study showed that fin heights varied up to 6%, with only the high points attaching to the parting sheet, thus reducing structural performance. The study concluded that control over composite fabrication is essential and must be further developed.

An innovative approach to minimize fabrication and cost, while maintaining thermal and structural performance has also been evaluated. This integral plate-fin design took advantage of the unique fabrication possibilities of C/C composite materials. Instead of processing each layer of the HX core individually and brazing them together, the integral design allows each alyer to be constructed and co-cured as an integral unit. The size of the Prometheus recuperator may prevent the use of this technology.

The study also demonstrated C/C material optimization in the HX design. Commercial materials and existing processes were used to reduce time and cost. Three standard processing methods were evaluated to give favorable properties. The high thermal conductivity process used more costly steps to produce a graphitic composite from a pitch-matrix prepreg. The structural process was less expensive and optimized mechanical properties by pyrolyzing a phenolic-matrix prepreg without graphitization. Finally, the mixed method pyrolized and graphitized a phenolic-resin prepreg to provide intermediate strength and thermal properties.

It is expected that current C/C recuperator fabrication development by Allcomp will take these problems into consideration. Innovative approaches to reduce cost and ease of fabrication may be utilized while maintaining quality.

 

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