Benefits and Challenges of the Inside-Out Ceramic Turbine: An Experimental Assessment

Distributed aircraft propulsion has renewed the interest in power-dense, high-efficiency power packs. Ceramic turbomachinery could be a major enabler, although no successful design has been achieved in microturbine rotors. Rotor blade loading is tensile and a hurdle for successful conversion to ceramics. The inside-out ceramic turbine (ICT) rotor uses the superior compressive properties of monolithic ceramics by supporting ceramic blades against a structural composite rotating shroud. This enables low stress levels throughout the blade, increasing reliability and extending service life. An experimental demonstration of two ICT designs was conducted with 15-kW scale prototypes to identify critical issues: design A, a flexible hub that clamps blades against the structural shroud and design B, a sliding-blade configuration that allows free displacement of the blade. The flexible-hub design was tested up to 1000°C. Rotor integrity was preserved, but local blade cracking occurred. The sliding-blade design was successfully tested up to 1100°C for over 1 hour at a tip speed of 350 m/s with no issue. Tensile loading at the ceramic/metallic interfaces remains the key challenge to address. Reducing friction should overcome blade cracking and allow the proposed ICT to reach the targeted temperature of 1275°C and tip speed of 425 m/s.