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Persico, Giacomo
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Persico
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Giacomo
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Giacomo Persico
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Experiments on Supersonic ORC Nozzles in Linear Cascade ConfigurationOliveti, Marco; Manfredi, Marco; Persico, Giacomo; Spinelli, Andrea; Gaetani, Paolo; Dossena, VincenzoIn organic Rankine cycles (ORCs), the turbo-expander represents a critical component due to the major impact of its efficiency on working fluid selection, cycle layout and overall plant performance and profitability. Also, its design is complicated by large expansion ratios, by the demand of operational flexibility and by the thermo-physical characteristics of the working fluid and non-ideal gas effects. This typically leads to turbines with low number of stages and transonic/supersonic flow regimes. For these reasons, the ORC turbine design relies on advanced aerodynamic models and high-fidelity tools based on computational fluid dynamics (CFD). The verification of high-fidelity tools requires accurate fluid thermodynamic models and experimental data concerning canonical flows, since experiments on non-ideal flows within ORC turbine cascades are still missing in the literature. To fill this gap, a novel experiment has been designed at Politecnico di Milano on an ORC supersonic linear cascade, aimed at characterizing the flow field within the bladed and semi-bladed portion of the channels, at the trailing edge where shock/fan systems arise, and downstream the cascade, by retrieving the pitch-wise total pressure loss distribution. This paper reports the outcomes of an experimental campaign focused on the expansion of hexamathyldisiloxane (MM) within the cascade. Initial commissioning tests were performed using nitrogen at different pressure levels. Finally, the experimental data gathered during a campaign carried out with MM in non-ideal conditions are presented and compared with CFD simulations, allowing to assess real gas effects on the trailing edge shock pattern and pressure distribution through the cascade.- SHAPE OPTIMIZATION OF A sCO2 CENTRIFUGAL COMPRESSOR STAGERomei, Alessandro; Gaetani, Paolo; Persico, GiacomoThe design of a centrifugal compressor for supercritical carbon dioxide (sCO2) power cycle must account for non-ideal gas effects and the possible occurrence of two-phase flows. Shape optimization techniques combined with computational fluid-dynamic (CFD) simulations can produce optimized designs while inherently coping with the peculiar flow characteristics near the thermodynamic critical point. This work presents the first shape-optimization attempt of such non-conventional compressors. The compressor stage includes the impeller and the vaneless diffuser and starts the compression close to the critical point. The impeller blade angle distributions and meridional channel are parameterized with Bezier control points, which grant a local shape control within the optimization routine. The pinch of the vaneless diffuser is optimized as well. The validated CFD solver accounts for both non-ideal effects and two-phase homogeneous flows under the assumption of thermodynamic equilibrium and barotropic fluid. The constrained optimization problem is solved with genetic algorithms. To reduce the computational cost, surrogates for the objective function and constraints are trained over a limited number of CFD results. The surrogate accuracy is improved throughout the optimization process by adding optimal stage geometries to the initial training samples. The optimized geometry shows an appreciable efficiency increase (0.7 percentage points) while delivering the design pressure ratio. Although performing better in the design condition, the operating range of the compressor is altered by optimization. This finding leaves the door open for future optimizations that include both design and off-design operating points in the definition of the objective function and constraints.
