Rotor for a micro gas turbine
With the development of renewable energies, the European environmental policy aims at decreasing fossil fuel consumption and pollutant emissions, thus emphasizing the need for reliable provision of energy at peak loads. Stationary gas turbines supply power very flexibly and produce comparatively little emissions because of their high efficiency. Micro gas turbines are predestinated for local and independent energy conversion with combined heat and power generation. Recent research and development activities have been focused on decreasing emissions and fuel consumption of such turbomachines. This can be achieved by increasing the efficiency through a higher operating temperature or a lower amount of cooling. Both approaches result in significantly higher turbine component temperatures. Metal alloys are already operating at their physical limits in terms of temperature and cannot tolerate any significant increases. Hence, substitution of metal turbine parts by highperformance ceramic materials can offer tremendous benefits.
A silicon nitride (Si3N4) rotor for a radial-flow micro gas turbine with a capacity of 30 kWel was developed within the scope of a Fraunhofer project. The ceramic rotor exhibits long-term stability up to 1200 °C at maximum operating loads and can be mass-produced.
This project was a collaboration of five Fraunhofer institutes: IKTS (material development, fabrication), IPK (tool production, final shaping), SCAI (simulation, shape optimization), IFF (testing, lean gas tests), and IWS (bonding, coating).
Si3N4 high-performance ceramics are suitable for rotating parts and high thermomechanical loads because of their excellent mechanical properties from room temperature up to 1400 °C. Dependent on chemical composition, sintering and after-treatment, specific properties can be amplified. To adapt the material properties to the operational stresses and to optimize the component design a repetitive adjustment of both is necessary.
The illustration of a realistic profile of operational demands via simulative coupling of thermal and (fluid-)mechanical loads done by Fraunhofer SCAI was the groundwork for material development. Based on this data, specific development aims could be defined. The adjustment of the material properties was done by a targeted design of the grain boundary. This led to high strength as well as high oxidation resistance and fatigue strength up to 1200 °C.
|Material data for micro gas turbine rotor|
|Fracture toughness||6,8 MPa m1/2|
|Strength||~ 1000 MPa|
|Fatigue strength at 1200 °C||~ 500 MPa|
The near-net-shape process of ceramic injection molding (CIM) was used for fabrication. This method is very suitable for the production of high quantities with low loss of material. In this process, a heated thermoplastic compound composed of ceramic powders and an organic binder (feedstock) is pressed into a mold cavity under high pressure to form a near-net-shaped part. The large volume of the rotor (148 cm³) imposed numerous demands on the mold cavity and the feedstock, with the greatest challenge proving to be the debinding process. This problem was solved by an innovative combination of chemical and thermal treatment of the part to enable sintering of defect-free rotors.
To evaluate the technological competitiveness of such large injection molded ceramic components, rotors have also been fabricated via green machining for comparison. After balancing, rotors have undergone burst-tests at rotational speeds up to 20% higher than in maximum application conditions. After minor structural modifications of the Capstone® C30 gas turbine located at Fraunhofer IFF Magdeburg, a rotor was installed and tested successfully up to maximum speed (96.000 rpm).