Abstract

Recuperators with design temperatures at and above 800 °C can further increase the thermal efficiency of supercritical CO2 power cycles by enabling higher turbine exhaust temperatures. Mar-M247 is a well-suited nickel-based superalloy for high temperature service due to its high creep strength that prevents excessive material thickness being required for pressure containment. While not compatible with the diffusion bonding process due to its low ductility, additive manufacturing using a high-speed laser directed energy deposition (DED) process presents a promising solution with the ability to produce non-conventional flow channels for enhanced heat transfer. A design process is presented that includes aerothermal and mechanical evaluation to maximize performance within the constraints of the manufacturing process. The conceptual design stage evaluates the feasibility of numerous heat transfer concepts from a unit cell perspective. A 2-D heat transfer network and pressure drop code allows prediction of flow distribution in each passage and its effect on overall thermal performance. Established literature correlations, along with CFD simulation, inform the prediction of heat transfer coefficients and friction factors for the flowpaths and enhancement features in the heat exchanger core. Sizing codes for the heat exchanger distribute wall thicknesses for pressure containment according to creep life data for Mar-M247. Mechanical evaluation using FEA modeling with the intent of the ASME BPVC Section VIII, Div 2 assesses the operational safety of the design. The capabilities of the laser DED process and the build strategy for minimizing total build time is discussed, along with the results of build trials that evaluate the settings of the powder nozzle and laser and their effect on deposition rate and susceptibility to build defects. Major considerations affecting the core geometry include the overhang angle of passage structures and the alignment of enhancement features for time efficient builds. The presented detailed design features annular finned passages that take advantage of helical flow paths to distribute the flow from separated headers to shared heat transfer surfaces. Performance predictions for the recuperator at a 50 kW scale provide insights on the feasibility of the AM process to produce recuperators on a commercial scale that extend existing operating envelopes.

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