Aerodynamics of Centrifugal Turbine Cascades

The centrifugal turbine architecture represents a promising solution for Organic Rankine Cycle (ORC) Systems, in the small-to-medium power range. ORC expanders operate with very high volumetric expansion ratios, which can be better accomplished in a centrifugal machine thanks to the increase of passage area along the ow path. Furthermore the centrifugal arrangement allows for assembling a multiplicity of stages in a relatively compact machine, thus reducing the expansion ratio per stage, with benecial eects on the performances in both design and o-design operating conditions. A preliminary design exercise proposed by the authors [1] has recently shown the po- tential of multistage centrifugal turbines, composed by a succession of xed nozzles and rotors, for medium power applications (about 1MWel). By limiting the ow regime to transonic or slightly supersonic conditions, purely converging ducts can be used with a certain degree of post-expansion, if necessary; the resulting ow conguration, char- acterized by weak oblique shocks, leads to a promising design eciency above 85% ; the absence of converging-diverging ducts allows to negotiate a certain degree of power control without the onset of normal shocks which, instead, result in a dramatic increase of aerodynamic losses in axial ORC turbines. These promising features, however, were based on estimates performed with preliminary design tools. In this perspective, the aerodynamic performances of centrifugal cascades represent the most critical issue, since very few information are available in literature for centrifugal turbines. In absence of experimental data on this kind of machines, the correlations used to estimate the aerodynamic losses and the ow deviation must be assessed with high-delity computational methods. In the present work the aerodynamics and performance of centrifugal turbine cascades are studied by applying a three-dimensional CFD model. The study is focused on the sixth stage of the transonic centrifugal turbine proposed in [1]. At rst a simple but eective blade design technique, introduced by the authors in [2], is recalled and applied to the present conguration. The resulting stage is then analyzed focusing on the prole aerodynamics, considering both stator and rotor blade rows (in this latter case discussing the eects of the Coriolis acceleration). Finally the morphology of secondary ows and the three-dimensional eects of the channel aring are investigated.