14th European Conference on Turbomachinery Fluid dynamics & Thermodynamics
The design of Organic Rankine Cycle (ORC) turbines often demands to deal with transonic flows as a result of cycle efficiency requirements and the matching of the temperature profiles with heat source and sink, as well as the nature of organic fluids, often featuring large molecular weight. Consequently, the use of convergent-divergent turbine stators has established as a widespread solution in the published literature for both axial and radial-inflow machines. For what concerns the latter layout in particular, design guidelines are still limited. The present work shows the results of an investigation on a series of ORC radial-inflow convergent divergent nozzles which differ by vane solidity and design outlet flow angle. These stators are designed by fitting into a radial-inflow turbine stator the divergent portion of a sharp-edged minimum length nozzle designed by means of the Method of Characteristics (MoC) adapted to dense gases. The geometries are analysed by means of steady-state RANS CFD calculations and the results are used to assess the influence of the design parameters on the nozzle losses and downstream flow field uniformity. Finally, some examples are presented concerning the effect of the optimized solution on both stator – rotor interaction and turbine performance.