Design Space Exploration of a Beam Flexible Hub Concept for an Inside-Out Ceramic Turbine Using a Simplified Rotordynamic Finite Element Model

This paper presents a new flexible hub design for the inside-out ceramic turbine (ICT) rotor configuration. This configuration is used in microturbines to integrate ceramic blades in order to increase turbine inlet temperature (TIT), which leads to higher cycle efficiency values. The ICT uses an outer composite rim to load the ceramic blades in compression by converting the centrifugal loads of the blades into hoop stresses in the composite rim. High stresses in the composite rim lead to high radial displacement of the blades. This displacement is compensated by using flexible hub in order to maintain the contact with the blades. However, hub flexibility can lead to rotordynamic problems as heavy hub deformation will induce high stresses in it. Thus, stresses in the hub are induced by both rotordynamics and centrifugation, requiring a multi-objective design process, which has yielded geometries that limited, until now, the blade tip speed to 358 m/s. In this paper, a simplified rotordynamics finite element model of a flexible hub is developed to allow quick design iterations. Using the model, a design space exploration of this hub concept is done while considering centrifugation and rotordynamics. Experimental validation is conducted on a simplified ICT prototype up to 129 krpm, i.e., an equivalent blade tip speed of 390 m/s. Finally, predictions from the experimentally calibrated model show that the tested prototype hub could reach a blade tip speed of 680 m/s.