Fundamental and Applied Energetics
ONERA and micro energy sources : the micro gas turbine engine
First development
In a first step, thanks to the financial support of DGA, ONERA could start the study of the micro gas turbine engine, through three technological bricks, in other words three key points in terms of technology or scientific know-how, that must be managed to be able to develop a micro gas turbine engine:
- micro combustion, the source of energy of the micro gas turbine engine;
- guiding system which is adapted to the very high rotation speeds;
In this first phase, only two departments of ONERA were involved: DEFA (Fundamental and Experimental Energetics) and DAAP (Applied Aerodynamics). Moreover, two external partners were also implied: University of Lodz (Poland) for the high rotating speeds guiding system, and SilMach for the MEMS fabrication.
Here are some examples of the achievements of these studies:
First micro combustion chamber under operation (volume: 850 mm3).
Numerical simulation with an ONERA code (CEDRE) of H2-air combustion in the first micro combustion chamber.
Study of combustion in small volumes (also called micro combustion) was led both experimentally and numerically. Indeed, a micro combustion chamber was designed, fabricated and tested through various campaigns, for H
2-air combustion. For that matter, a specific test rig was created.
Meanwhile, 3D numerical simulation calculations were led, to get a better understanding of the phenomena, and analyse the flow and combustion structure inside of the chamber. For this study, we used CEDRE, the code developed by ONERA.
- Guiding system adapted to very high rotation speeds, journal and thrust aerodynamic bearings:
Scale 5 journal and thrust aerodynamic bearings: tilting pad journal bearings, spiral grooves thrust bearings and corresponding test rig.
Extreme downscaling of the compressor and turbine involves very high rotation speeds, in the range of 900 000 rpm. At such speeds, no classic journal and thrust bearing can be used. That’s why aerodynamic gas journal and thrust bearings were studied and developed by ONERA. This type of system uses a tiny part of the main air flow to create aerodynamic forces, which provide the sustentation of the rotating parts.
A first study was led at scale 5. A system of aerodynamic bearings was designed, fabricated and tested through various successful campaigns. In particular, 100 000 rpm was achieved. In a second step, a study at scale 1 was led, with an 8 mm diameter turbine impeller. The parts, which correspond to the turbine impeller and the aerodynamic bearings, were fabricated in Silicon. Tests could be led only at low speeds, partly because of the low mechanic characteristics of the machined Silicon.
Taking account of the machining precisions demanded at this scale, ONERA decided to take advantage of a micro fabrication method used in micro electronics industry. The latter is the physico-chemical etching of Silicon: the DRIE. Etching depth of 400 µm was achieved, with extremely high aspect ratios. Eventually, a turbine impeller (8 mm in diameter) was fabricated, and so were the associated aerodynamic bearings.
