Mechanical design and rotordynamic analysis of the ORCHID turbine

The ORCHID turbine is a laboratory single-stage 10 kW high-speed (∼100 krpm) radial-inflow turbine for high-temperature/high-efficiency organic Rankine cycle (ORC) systems, designed at the Aerospace Propulsion and Power laboratory of Delft University of Technology. It will be installed within a test section of the organic Rankine cycle hybrid integrated device (ORCHID) facility, the setup for fundamental and applied studies on ORC technology currently in operation in the same lab. Experimental data from future measurement campaigns will be employed to validate design and performance prediction tools and to develop best practices for operating these unconventional machines, whose most notable features are the ultra-high expansion ratio (>40), highly supersonic flow in the stator (Ma > 2), and the large flow deflection within the impeller channels (>90). This article presents the mechanical design and the rotordynamic assessment of the ORCHID turbine rotor, and best practices for the numerical evaluation of the rotordynamic characteristics and stability of high-speed ORC turbine rotors. The emphasis is on three main aspects: i) the modeling and quantification of the damping characteristics of a squeeze film damper (SFD) cartridge for the selected turbocharger ball bearings supporting the turbine shaft; ii) the characterization of the stiffness and damping coefficients of a custom designed pocket gas seal, performed using 3D RANS simulations; iii) the analysis of linear elastic rotordynamic simulations using a finite beam element model of the turbine rotor. Results of the rotordynamic simulations show that the rotor would operate in the sub-critical regime with respect to the first bending mode. The influence of model parameters such as the rotor eccentricity and the orbit radius was included in the analysis by conducting a parametric study of the SFD fluid forces. The Campbell diagrams show that shaft bending frequencies are significantly affected by changes in the SFD dynamic response depending on the rotational speed. The influence of the stiffness of rolling bearing elements and of the destabilizing forces due to the impeller and gas seal cross-coupled stiffness was also evaluated. The rigidity of the rolling elements slightly affects the shaft bending mode, while large values of the destabilizing forces cause a rotordynamic instability of the first rigid mode.