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Master of Science / Department of Mechanical and Nuclear Engineering / Akira T. Tokuhiro / The Secure Transportable Autonomous Reactor – Liquid Metal system combines a Generation IV nuclear reactor with an advanced Supercritical Carbon Dioxide (S-CO[subscript]2) Brayton power conversion cycle. The Brayton cycle was selected as the power conversion cycle due to its high efficiency, small turbomachinery size, and competitive cost due to reduced complexity as compared to a traditional Rankine cycle. Overall system thermal efficiency is closely tied to the performance of the precooler and recuperators. The Printed Circuit Heat Exchanger (PCHE) manufactured by Heatric is being considered for use as both the precooler and recuperator in the STAR-LM system due to its high effectiveness, wide temperature and pressure operating range, small size, and low cost. PCHEs have been used primarily in the hydrocarbon processing industry to date, and are relatively new in being considered for nuclear applications. In this study, a PCHE is investigated using S-CO[subscript]2 and water as the heat transfer media in conditions relevant to the precooler in the STAR-LM system. Experiments conducted with small temperature differences across the PCHE revealed that the heat transfer coefficient is strongly correlated with the temperature-dependent specific heat near the pseudocritical point. The STAR-LM precooler outlet temperature is near the pseudocritical point, making this region of interest to this work. Testing was conducted to determine the effect of property variation near the precooler outlet in conditions with large temperature differences in the PCHE. These tests revealed that maintaining the precooler outlet temperature near the pseudocritical point does not have a significant effect on heat transfer coefficients in the PCHE under large temperature difference test conditions. Computational Fluid Dynamics (CFD) models were developed to simulate fluid flow and heat transfer in the PCHE. A 2D, 4-channel, zig-zag model was found to reproduce the outlet temperatures to within approximately 15% relative error. The 3D straight channel model reproduced the experimental data to within 3% relative error for the cases simulated. Both of these models predicted the water side outlet temperatures to within 20% relative error.

A substantial raise in recuperator effectiveness has been established in the past by improving the fabricating and joining configurations regarding the manufacturing of compact recuperators. Further advancement of state-of-the-art recuperators requires providing for increased temperatures and pressures. 1bis can only be achieved by incorporating high temperature materials into the recuperator design. Although many high temperature materials have been identified in past research, less of these can be utilized in new concepts due to difficulties regarding fabricating and joining. However recently, in an independent study, a tungsten-copper alloy was identified through detailed material selection methods as a suitable material for high temperature applications. The validity of tungsten-copper regarding fabricating and joining, to establish a leak tight structure still needs to be demonstrated. The aim of the study is to carry out a comprehensive review of existing recuperator technologies and design methodologies as well as to investigate the manufacturability of tungsten-copper for use in a recuperator design of limited size. More specifically, the objectives entail the following: (1) The comprehensive review of existing recuperator technologies and recuperator design methodologies, (2) The design and fabrication of a recuperator of limited size using tungsten-copper as a heat transfer material and (3) The determination of the feasibility of fabrication of the design and the applicability of the selected W -eu alloy in the design. The fabrication technique that is presented in the design entailed the use of 2.Irm tungsten carbide drill bits to machine the correct recuperator profile, while the recuperator unit was joined by utilizing a mechanical fastening system. Although diffusion bonding was initially identified as the ideal joining technique for the recuperator of this research, restrictions and limitations relating to the use of diffusion bonding has lead to the identification of a fastening system as the technique used. Evaluation of the fabricated recuperator revealed that several factors were outside the initially specified values, inter alia the flatness tolerance of recuperator plate geometries and machined slots precision. These factors contributed to a leaJdng recuperator structure when tested. The most likely contributing factors for the latter relate to non-conforming tolerances achieved in the fabricated design, residual stresses induced by the machining process as well as design issues relating to the recuperator plate geometries. The design and fabrication of a recuperator of limited size using tungsten-copper as a heat transfer material, requires re-evaluation. Similar work will ensure a design of a high quality when provision is made for advanced surface fmishing of machined parts (notably the recuperator plate geometries), slight modifications to the design as well as stress relieving of machined components for the purpose of eliminating any residual stresses thatJnight be present. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2009.

A substantial raise in recuperator effectiveness has been established in the past by improving the fabricating and joining configurations regarding the manufacturing of compact recuperators. Further advancement of state-of-the-art recuperators requires providing for increased temperatures and pressures. 1bis can only be achieved by incorporating high temperature materials into the recuperator design. Although many high temperature materials have been identified in past research, less of these can be utilized in new concepts due to difficulties regarding fabricating and joining. However recently, in an independent study, a tungsten-copper alloy was identified through detailed material selection methods as a suitable material for high temperature applications. The validity of tungsten-copper regarding fabricating and joining, to establish a leak tight structure still needs to be demonstrated. The aim of the study is to carry out a comprehensive review of existing recuperator technologies and design methodologies as well as to investigate the manufacturability of tungsten-copper for use in a recuperator design of limited size. More specifically, the objectives entail the following: (1) The comprehensive review of existing recuperator technologies and recuperator design methodologies, (2) The design and fabrication of a recuperator of limited size using tungsten-copper as a heat transfer material and (3) The determination of the feasibility of fabrication of the design and the applicability of the selected W -eu alloy in the design. The fabrication technique that is presented in the design entailed the use of 2.Irm tungsten carbide drill bits to machine the correct recuperator profile, while the recuperator unit was joined by utilizing a mechanical fastening system. Although diffusion bonding was initially identified as the ideal joining technique for the recuperator of this research, restrictions and limitations relating to the use of diffusion bonding has lead to the identification of a fastening system as the technique used. Evaluation of the fabricated recuperator revealed that several factors were outside the initially specified values, inter alia the flatness tolerance of recuperator plate geometries and machined slots precision. These factors contributed to a leaJdng recuperator structure when tested. The most likely contributing factors for the latter relate to non-conforming tolerances achieved in the fabricated design, residual stresses induced by the machining process as well as design issues relating to the recuperator plate geometries. The design and fabrication of a recuperator of limited size using tungsten-copper as a heat transfer material, requires re-evaluation. Similar work will ensure a design of a high quality when provision is made for advanced surface fmishing of machined parts (notably the recuperator plate geometries), slight modifications to the design as well as stress relieving of machined components for the purpose of eliminating any residual stresses thatJnight be present. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2009.

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