Three-dimensional Unsteady Stator-rotor Interactions in a High Expansion ORC Turbine

orc2019 Tracking Number 31

Organic Rankine cycle (ORC) power systems are a viable alternative to convert low- to-medium grade heat sources into electrical power, typically at temperatures between 120 to 350 ° C. Instead of using steam as the working fluid, the system operates with an organic compound that can effectively convert waste heat or solar energy into mechanical power, as it allows for fewer turbine stages and a dry expansion. However, this expansion process takes place in the dense-vapour region, where the ideal gas assumption is invalid. In the development of ORC turbogenerators, the expander is the most critical component due to its direct impact in the overall system performance. The present work investigates the three-dimensional (3-D) unsteady simulation of a high expansion radial inflow turbine which operates with toluene (C 6 H 5 -CH 3 ). The unsteady Reynolds-averaged Navier-Stokes equations are solved with a multi-parameter equation of state, to account for the non-ideal fluid properties. To account for the unsteady stator-rotor interactions, a fully conservative flux assembling technique for the treatment of non-matching mesh interfaces is implemented. The expansion process considered has a large expansion ratio (~100) which results in a highly supersonic flow in the stator exit (Mach ~2.7). To capture the shock waves, a finite volume scheme is used with an approximate Riemann solver. The novelty of this work is that it presents for the first time a detailed analysis of the unsteady phenomena (shock waves, viscous wakes, and shockwave-boundary layer interaction) in an ORC turbine, including the stator/rotor interaction, by means of 3-D calculations. The simulations indicate strong three-dimensional and unsteady effects, especially in the rotor blade passage. Unsteady shock waves emanating from the trailing edge of the stator interact with the bow shock at the leading edge of the blade and a separation bubble in the suction side of the blade. These loss mechanisms need to be considered when predicting the stage performance. Moreover, the three-dimensional effects clearly indicate that the blade profile needs to be adjusted at different span-wise locations to reduce entropy losses (produced by e.g. flow separation, shock waves, and/or secondary flows) and increase the power output.