Within the framework of the Future Circular Collider (FCC) currently being investigated at CERN, the entire cryogenic cycle required to maintain a temperature level of the superconducting magnets near absolute zero had to be revised with respect to the existing Large Hadron Collider (LHC). Indeed, stringent heat load constraints, coming from a four times longer accelerator ring and twice higher magnetic fields, imposed a major revision of the entire cryogenic cycle. In particular, a novel pre-cooling cycle had to be developed for this purpose. This led to a closed-loop cryogenic cycle operating with a mixture of helium and neon, also called Nelium.To better understand the challenges and opportunities associated with the design and operation of radial compressors with such light gases, a closed loop test facility has been designed, built and commissioned at the ITSM. The test facility has been developed to operate with air as well as with helium-neon gas mixtures of varying mixing ratios ranging from pure neon to pure helium.The test facility includes a preconditioning unit, in which the helium-neon gas mixture is prepared and its exact gas composition verified by means of a binary gas analyser. The preconditioned gas is then admitted to a closed test loop, where a high speed motor supported by gas bearings directly drives a radial compressor impeller. The test rig architecture allows to adjust the gas mass inside the test loop as well as compressor inlet temperature and pressure as desired, which enables to measure and compare the compressor performance at different Reynolds number.This paper starts by providing an overview of the test rig facility by presenting its architecture, the components used together with a measurement uncertainty analysis and a description of the operating procedure. Experimental results are then presented starting with air to validate the test rig operation and inlet boundary conditions. Moreover, the compressor motor being liquid cooled, a heat transfer between the coolant and the operating fluid leads to a modification of the operating fluid temperature and with it the measured aerodynamic efficiency. A model derived for all operating gases has been developed to remove this effect from measurements and is presented in this paper. Finally, after having validated the aerodynamic performance of a newly designed centrifugal compressor stage, the Reynolds number effect is investigated for different gas mixtures and compared to models presented in the literature.

Experimental and Numerical Similitude Study Using a Novel Turbocompressor Test Facility Operating With Helium-Neon Gas Mixtures

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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