Performance improvement of a radial orc turbine by means of automated CFD design

There is a growing interest in organic Rankine cycle (ORC) turbogenerators because of their ability to efficiently utilize external heat sources at low-to-medium temperature in the small-to-medium power range. High-temperature ORC turbines typically operate at very high pressure ratio and expand the organic working fluid in the dense-vapor thermodynamic region, thus requiring computational fluid dynamics (CFD) solvers coupled with accurate thermodynamic models for their performance assessment and design. In this paper we present a steady-state three-dimensional viscous CFD study of the Tri-O-Gen ORC radial turbine, including the radial nozzle, the rotor and the diffuser. The turbine operates with toluene as the working fluid, whose accurate thermophysical properties are obtained with a look-up table approach. Based on the 3D simulation results, together with a 2D fluid dynamic optimization procedure documented elsewhere, an improved nozzle geometry is designed, manufactured and experimentally tested. Measurements show it delivers 5 kW_e or 4% more net power output, as well as improved off-design performance.