Aerodynamic design and analysis of centrifugal turbine cascades

The centrifugal turbine architecture represents a promising solution for Organic Rankine Cycle (ORC) Systems, in the smallto- medium power range. The large volumetric expansion ratios occurring in ORC turbines complicate the design of the turboexpander, making the centrifugal arrangement worth of interest with respect to conventional architectures. A new-concept centrifugal turbine has been recently proposed by the authors, based on the design of Ljungstr¨om but implementing a stator-rotor arrangement, which allows for multi-stage assembly without compromising compactness. To properly evaluate the potential of the centrifugal turbine solution, reliable data on cascade aerodynamic performances are required, but they are still lacking in the open literature. In this paper the aerodynamics of radial-outward turbine cascades is discussed, on the basis of Computational Fluid-Dynamics (CFD) simulations. A weakly transonic operating condition is selected and for that different classes of profiles are tested. Results show that the intrinsic diverging shape of the radialoutward configuration complicates the blade design; if the flow deflection is not properly controlled along the streamwise direction, the bladed duct can result in a converging-diverging channel, leading to unexpected chocked flows and shocks even in weakly transonic configurations. The indications achieved by the comparison between different blade profiles are gathered to define guidelines for the design of novel elliptic profiles, which allow to control the flow acceleration process, providing high aerodynamic efficiency. Off-design performances of the elliptic profiles are finally addressed by studying the response of the cascade to different expansion ratios and high incidence angles.