Development of an axial microturbine for a portable gas turbine generator

Abstract

A miniature gas turbine is under development with the aim of generating electrical energy from fuel. This system consists of a compressor, combustion chamber, turbine and generator. The turbine is a single-stage axial impulse turbine (Laval turbine) with a rotor diameter of 10 mm, made of stainless steel using die-sinking electro-discharge machining. It has been tested with compressed air to speeds up to 160 000 rpm and generates a maximum mechanical power of 28 W with an efficiency of 18.4%. When coupled to a small generator, it generates 16 W of electrical power, which corresponds to an efficiency for the total system of 10.5%. The power density is mainly limited by the maximal speed of the ball bearings. The main losses are the blade profile losses and the exit losses. Higher speeds can considerably reduce the exit losses and therefore increase efficiency and power density. An improved turbine has been tested at temperatures up to 360 °C and generates up to 44 W of electrical energy with a total efficiency of 16%. A 20 mm diameter centrifugal compressor matching the pressure and flow characteristics of the turbine has been designed and is currently under construction.

Effect of electro discharge machining (EDM) on the AISI316L SS white layer microstructure and corrosion resistance

“…The EDM surface corrosion resistance has encountered little interest [19][20][21][22][23] despite of its importance for many applications of metallic components manufactured by this process and used in more or less aggressive environments typically in aerospace, nuclear power plant and biomedical dental implant industries [1][2][3][24][25][26][27][28]. Furthermore, the available data on the corrosion resistance of finished surface by EDM is incomplete and sometimes controversial to provide any meaningful conclusion.…”

Limits, Modeling and Design of High-Speed Permanent Magnet Machines

“…Reliable gas turbines that utilize PM generators are available on the market [40]. Still, there is a great interest in academia for developing reliable high-speed generators that would keep pace with newly developed turbines which are capable of rotating at speeds beyond 1 million rpm [41,42]. A good example of such an effort in academia is work on high-speed generators in ETH Zurich [43].…”

“…Hence, the flux linkage becomes: 42) which, after substituting expressions for n 1 and θ (equations (3.2), (3.3) and (3.9)), yields:…”

“…where: 42) and: When the bearings' stiffness is small compared to the rotor stiffness (the soft-mounted case) the stiffness of the bearings has influence only on values of the low-order, rigid critical speeds. Values of the higher order critical speeds, which certainly include flexural modes, are primarily influenced by the shaft slenderness.…”

“…Ball bearings limited the speed of a gas turbine designed at University of Leuven to 160.000 rpm [147]. The authors suggested that special ball bearings would allow speeds over 200.000 rpm; however, air bearings would be needed for the desired speed of 420.000 rpm.…”