Conceptual layout design for a two-shaft pebble bed micro model configuration
Venter, Luttig De Villiers
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The pre- and inter-cooled recuperative closed Brayton cycle can be configured to be either a single or a multi-shaft arrangement. In comparison to the single shaft, multi-shaft arrangements have not been used widely in the closed cycle environment. Therefore, during the development of a three-shaft Pebble Bed Modular Reactor (PBMR), a Pebble Bed Micro Model (PBMM) was constructed to illustrate the envisaged PBMR control methodologies. The PBMM now offers the opportunity to investigate other shaft arrangements. The main objective of this study is to perform a conceptual layout design for a two-shaft PBMM configuration. The major steps performed in the conceptual layout design of the two-shaft PBMM are preliminary studies, thermodynamic design point studies and turbo machine selection. During the thermodynamic design point studies the influence of turbocharger selection on the cycle performance were investigated. Two optimum-values for Overall Pressure Ratio (OPR) were found, one for maximum cycle efficiency and one for maximum cycle power output. This, together with the requirement for the OPR to be equally shared between the Low- Pressure Compressor (LPC) and the High-pressure Compressor (HPC), was used to identify suitable turbocharger pairs. In order to evaluate the compressor operating points of the turbochargers, the unique matching characteristics for the pre- and inter-cooled recuperative closed Brayton cycle were derived. Final turbocharger selection was performed in Flownex. The turbocharger pair that enjoyed the highest cycle efficiency at thermodynamic conditions similar to that of the three-shaft PBMM configuration was suggested as the preferred turbocharger configuration. Unexpectedly the turbochargers selected for the two-shaft configuration were found to be identical to that of the three-shaft PBMM configuration. This led to a comparison of matching characteristics of the two-shaft configuration to that of the three-shaft configuration. The close proximity of both configurations to the choking region of the LPT ensured the close proximity of their operating points on the HPT. The fact that the HPC operating point is only a function of the HPT operating point caused close proximity of the HPC operating points. The requirement for flow compatibility that must exist between the NPC and LPC limited the operating point of the LPC for both configurations to the same flow compatibility line, which ensured the close proximity of the LPC operating points.
- Engineering