Influence of Supersonic Nozzle Design Parameters on the Unsteady Stator-Rotor Interaction in Radial-Inflow Turbines for Organic Rankine Cycles

Abstract

Abstract Organic Rankine Cycle (ORC) technology represents an interesting option for improving the efficiency of existing power plants and industrial processes as well as exploiting renewable and renewable-equivalent energy sources. The use of Radial-Inflow Turbine (RIT) for ORC plant sizes below 100 kW is promising, although the application remains challenging. In fact, the single stage arrangement imposed by economic constraints and hence the large expansion ratio, together with the large molecular weight, which characterizes organic fluids, usually result in highly supersonic flows, so making the use of transonic stators often mandatory. Particularly, the influence of RIT stator design parameters on losses and the level of unsteadiness seen by the subsequent rotor is still scarcely addressed in published literature. Previous work by the authors investigated the effect of some stator design parameters on stator loss and downstream circumferential uniformity. The present work investigates the effect of the convergent-divergent stators design parameters and the resulting downstream flow field non-uniformity on the unsteady stator-rotor interaction and loss generation in ORC Radial-Inflow Turbines. To this end, two stator and rotor configurations which differ by the stator design parameters (i.e., discharge metal angle and number of vanes) have been tested by means of 3D unsteady CFD calculations accounting for real-gas properties. The results show that larger stator-rotor interaction is present for the case featuring higher vane count and lower outlet metal, which also features the largest fluctuations of power output and pressure force on blade, together with a substantially lower average total-to-static efficiency.