Aerodynamic Optimization of a Microturbine Inserted in a Magic-Angle Spinning System

Abstract

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) probe—is investigated. The drive fluid is pressurized air at ambient temperature provided by nozzles aligned on an intake spiral, driving a Pelton-type microturbine. Computational fluid dynamics (CFD) simulations have been performed and compared with fluid dynamics measurements of the MAS system with 1.3 mm rotor diameter for spinning rates between 23 kHz and 67 kHz. The main optimization criteria of the MAS system are rotor speed and turbine stability and not primarily efficiency, which is standard for turbomachinery applications. In the frame of fabrication tolerances, a sensitivity study has been carried out by varying the nozzles diameter and the nozzle position relative to the rotor. The presented fluid dynamics study of the microturbine system includes the analysis of local fluid flow values such as velocity, temperature, pressure, and Mach number, as well as global quantities like forces and driven torque acting on the turbine. Comparison with the experimental results shows good agreement of the microturbine efficiency. Furthermore, the parameter study of the nozzle diameter reveals optimization potential for this high-speed microturbine system employing a smaller nozzle diameter.

Ultra Low Temperature Microturbine for Magic Angle Spinning System

“…The sliding mesh method allows adjacent grids to slide relative to each other along the interface surface, whereas the coupling between the two regions is achieved by interpolating the flow variables on the cell faces. In our previous paper [27] both the MRF and the SMM approach were validated by performing three-dimensional simulations of the microturbine system. For steady-state simulations, the main problem was to calculate physical variable values which are independent of the rotor position relative to the nozzles.…”

“…In order to avoid any doubt, simulations with a density-based application were also carried out, as mentioned in Ref. [27], and no differences in the flow fields were found. The steady-state simulations were mainly carried out by choosing upwind discretization for the advection terms and a linear scheme for the gradient terms.…”

“…In the past, experimental testing in an empirical manner has played an important role, whereas computational fluid dynamics (CFD) analysis is not very wide spread in the design of MAS systems, cf. [24][25][26][27]. Recently, Sesti et al [26] reported CFD simulations of NMR systems at very low temperature (i.e., 6 K).…”

“…Herein it is shown that the power generated by the microturbine is in the range of 6 W and the efficiency is about 20%, which is rather low compared to industrial Pelton turbine systems for power generation [31,32]. Herzog et al [27] investigated the MAS system in more detail employing CFD simulations compared with experimental measurements. They contributed to the optimization directions for the air propelled turbine design for the rotation speed of 23 kHz to 67 kHz.…”

Thermo-mechanical analysis of a probe for electron paramagnetic resonance spectroscopy operating at cryogenic temperatures

“…This analysis helps to gain insight into the fluid dynamics of the MAS stator, optimize turbine shapes and air delivery ducts to design MAS rotors and stators for higher MAS frequencies. [19][20][21][22] In this article, we focus on the thermo-mechanical analysis of a high-field EPR probe operating at cryogenic temperatures using FEA. The thermo-mechanical analysis plays a key role in the design process as EPR probes are often subjected to large stresses at low temperatures.…”