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Session: Turbo expanders II
Session starts: Tuesday 08 October, 14:00
Presentation starts: 15:00
Room: Van Weelde Zaal

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David Pasquale (Università degli Studi di Brescia)
Matteo Pini (Politecnico di Milano)
Giacomo Persico (Politecnico di Milano)
Antonio Ghidoni (Università degli Studi di Brescia)
Stefano Rebay (Università degli Studi di Brescia)

Abstract:
During the last decade, Organic Rankine Cycle (ORC) turbogenerators have become very attractive for the conversion of low-temperature thermal energy sources in the small-to-medium power range. Complex gasdynamic phenomena and strong real-gas effects in the thermodynamic behavior of the working fluid usually characterize the thermo-fluid-dynamics of ORC turboexpanders. The use of Computational Fluid Dynamics (CFD) codes coupled with accurate thermo-physical property models is crucial to correctly model the flow expansion and therefore to achieve high-efficiency ORC turboexpanders. The design of ORC turbines is particularly challenging for small power output machines (up to a few hundreds of kWe); in these applications compactness is crucial and single-stage turbines represent a typical solution. As a result, fully supersonic flow conditions are typically adopted in these machines, and dedicated design techniques must be applied to avoid the onset of strong shocks in the stator-rotor gap. In the present work an automated procedure to design single-stage centripetal turbines for ORC systems is presented. The design methodology involves a two-step procedure coupled to a global optimization strategy in order to define the optimal degree of reaction, size and the flow path of the machine. First the mean-line code zTurbo and a Genetic Algorithm (GA) are used to perform a preliminary design of the machine. The mean flow surfaces of the selected configurations are then optimized by means of the CFD-based throughflow solver TzFlow and a metamodel-assisted GA. This latter design strategy was successfully applied to the design of an axial compressor. Both numerical codes are coupled with the most accurate equations of state for the thermophysical description of the fluid. The overall procedure is applied to design two turbines with a power output of about 50 and 250 kW. The results are extensively discussed.