Energy transition has become one of the most discussed topics of the current times. Turbomachinery still plays a key role in the power generation field, but the machines must meet very high standards in terms of performance levels, compactness, and cost reduction. In this perspective, new design configurations need to be explored. This paper presents a procedure for the aerodynamic design of a radial-axial stage turbine for geothermal applications. Based on a real set of possible power plant conditions, a dedicated CFD campaign has been carried out to assess the performance of the radial-axial machine — at design and off-design conditions. The main goal of the presented work is to exploit the benefit of combining radial and axial stage architectures, providing a compact, flexible, and efficient machine, capable to maintain high performance levels even at off-design conditions, which is a crucial requirement for renewable energy applications. Since organic fluids with high molecular weight are used in these applications, in addition to the common design procedure based on RANS CFD analyses, real-gas calculations are mandatory to correctly predict the behavior of the machine. All the CFD analyses are performed using the TRAF code, while the design of the radial-axial stages is carried out based both on in-house and commercial tools as per Baker Hughes best practice. The paper highlights the feasibility of the unusual radial-axial architecture as an effective way to combine the benefits of both the radial and the axial configurations, resulting in an efficient, robust, and compact machine.

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