Investigating the Influence of a Helium-nitrogen Gas Mixture on the Performance of a Closed Brayton Cycle Axial Compressor for Sustainable Energy System

Helium, the preeminent coolant for closed Brayton cycles, is difficult to compress. It requires a high number of compressor stages, which leads to an increase in the size and mass of the plant. This issue not only complicates the system design but also introduces dynamic challenges in power-conversion units. Historically, helium compressors, such as those in the Oberhausen-II (50 MW) and JAEA (300 MW) plants, have required a substantial number of stages, for example 25 for high-pressure and 35 for low-pressure. This research evaluates how the mixture of helium with nitrogen as an inert gas affects the performance of an axial compressor from a thermodynamic perspective. The use of helium-nitrogen mixtures with molecular weights below 16 g/mol results in extraordinary improvements in the heat transfer coefficient alongside increased compressor loading. The heat transfer coefficient reaches 4.2% higher when utilizing a 13 g/mol helium-nitrogen mixture. Therefore, a compressor is designed to process this mixture to evaluate its operational characteristics using Ansys CFX. The introduction of a helium-nitrogen mixture with 13 g/mol molecular weight creates higher blade loading, which produces an elevated total-outlet-pressure ratio of 1.1315, demonstrated through numerical simulation for a single stage. As a result, the compressor stages needed for the helium-nitrogen mixture are reduced to only 31.4% of the number required for pure helium to reach the desired outlet pressure. The combination of a helium-nitrogen mixture provides an advantageous compression solution for high-temperature-gas-cooled-reactor (HTGR) plants because it yields more compact and affordable turbo-compressors compared to pure helium systems.