Organic rankine cycles (ORC) are renowned to be attractive energy conversion systems for the thermal energy sources in the small-to-medium power range. A critical component in the ORC technology is the turbo-expander; the difficulties involved in the accurate thermodynamic modeling of organic fluids and, especially, the complex gasdynamic phenomena that are commonly found in ORC turbines may result in relatively low efficiency and in performance reduction at partial loads. In this perspective, a relevant path of development can be outlined in the evaluation of nonconventional turbine architectures, such as the radial-outward or centrifugal turbine. In the present work, a critical evaluation of the feasibility of multistage transonic centrifugal turbines for ORC systems is presented. To support this study, a two-step design procedure, specifically oriented to ORC turbines, was developed. The methodology includes a 1D mean-line code coupled to an external optimizer to perform a preliminary design of the machine. The selected configurations are then verified with a CFD (computational fluid dynamics)-based throughflow solver, able to deal with any flow regime and to treat fluids described by arbitrary equations of state. The overall procedure is applied to the design of two different turbines of the same target power of about 1 MW, the former representing a transonic six-stage turbine and the latter a supersonic three-stage turbine. The two machines are characterized by very different shape and comparable performances. The results are extensively discussed in terms of both overall data and detailed flow fields.

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