Ultra Low Temperature Microturbine for Magic Angle Spinning System

Abstract

We investigate the fluid dynamics of a microturbine system that is applied in a device for chemical and biological analysis—a so-called magic angle spinning (MAS) nuclear magnetic resonance (NMR) probe. The present system is utilized in a wide temperature range from 45 K to 293 K. Pressurized air, nitrogen, or helium are used to drive a Pelton type microturbine. This turbine is mounted on a MAS rotor with a diameter between 0.7 mm and 3.2 mm. The rotor system is equipped with a pressurized gas bearing that is operated by the same gas species as the turbine. Computational fluid dynamics (CFD) simulations have been performed and compared with fluid dynamics measurements of the MAS system for different diameters, temperatures, and spinning rates between 23 kHz and 120 kHz. To our knowledge, this work is the first comprehensive CFD and experimental study of such a wide temperature range that has been carried out for microturbines with pressurized gas bearings. The results show good agreement between measurements and CFD simulations with appropriate (real) gas models, i.e., the ideal gas model for air at room temperature, Peng–Robinson model for nitrogen at 105 K, and ideal gas model for helium at 45 K.