Small scale radial inflow turbine performance and pre-design maps for Organic Rankine Cycles

While small scale ORCs are currently dominated by volumetric expanders, the use of turbomachines is reconsidered due to their high efficiency and power density. Yet, suitable performance maps, which ensure an accurate starting point for the turbine design, are still missing for small scale turbines. This paper proposes a new non-dimensional performance map tailored for small scale turbines. The map is generated using an experimentally validated 1D code and adapted to small scale ORCs applications. A new polynomial fit is proposed, which accounts for the pressure ratio, since it is suggested to have a strong influence on the shape of the map. Through the analysis of the turbine losses, the underlying phenomena shaping the efficiency map are explained. A sensitivity analysis of the geometrical dependencies on the map shows a strong impact of the shroud to tip radius ratio, and explains why the Ns-Ds surface of the presented map is smaller than the original one. Compared to the experimentally validated 1D model the new map yields prediction errors below 4%.