3D FLUID DYNAMIC ANALYSIS OF A HIGH LOADED CENTRIFUGAL ROTOR FOR MINI ORC POWER SYSTEMS

asme-orc2015 Tracking Number 125

Organic Rankine Cycle (ORC) power systems are a well-established technology for the conversion of thermal energy sources in the small-to-medium power range. In the last few years, efforts have been devoted to the development of mini ORC (mORC) power systems (5- 30 kWe) for waste heat recovery from truck engines, or distributed conversion of concentrated solar radiation. In these high-temperature applications the expander is arguably the most critical component. Due to the high expansion ratio, turbo-expanders are typically preferred. Recently, a multi-stage radial-outflow turbine (ROT) configuration for ORC power systems has been studied. However, even if the authors preliminarily demonstrated that ROT may allow for compact and efficient expanders [1], some research questions are still open. Notably, the key point is the fluid dynamic design of the first stages, which are subject to severe flow conditions (very high flow deflection, low aspect ratio of the blades and with high tip clearance losses). This work thus proposes a novel design methodology for centrifugal cascades, specifically targeted to the first rotor of the mORC centrifugal turbine described in Ref. [1]. Blades are initially designed using a novel in-house Turbomachinery Blade Modeler (BM), then performance is verified by means of 3D CFD simulation on unstructured grids using the solver SU2, recently extended also in-house to treat non ideal compressible fluid flows [2]. Results show that traditional blade design rules for axial cascades are not directly extendable to centrifugal profiles and new design guidelines are needed. Moreover, the 3D performance of the cascade has also been investigated by taking into account tip clearance and secondary loss mechanisms. Finally, an accurate comparison with the mean-line code predictions is provided. As expected, the outcome of the study reveals moderate discrepancy between the CFD results and the mean-line code. This suggests that in case of non-conventional machines a more tight integration of design tools of increasing fidelity may be convenient.