Profile Loss Prediction for Organic Rankine Cycle Turbines: An Experimental Case Study

Authors

Leander Hake  - Muenster University of Applied Sciences, Germany
Stephan Sundermeier - Muenster University of Applied Sciences, Germany
Stefan aus der Wiesche - Muenster University of Applied Sciences, Germany

Abstract

Predicting profile losses for turbine cascades is relevant for the turbomachinery industry. Since the pioneering work of Ainley and Mathieson in 1951, several different loss models and loss prediction systems have been developed for steam and gas turbines. Although sophisticated and complex computational fluid dynamics methods are now widely available, mean line models are still necessary for the preliminary design and optimization of turbines. Specific loss models are able to predict losses for steam and gas turbines to a reasonable extent, but relatively little is known regarding their performance for organic Rankine cycle (ORC) turbines. An inspection of the open literature and the current industrial practice shows that the conventional loss systems derived originally for steam and gas turbines are applied to ORC turbines without any experimental proof. Considering the non-perfect gas behavior of organic vapors and their high density and low speed of sound levels, the unproven use of conventional loss models for ORC turbine applications seems risky. In addition to the thermodynamic complexity of organic vapors, the impact of blade roughness and turbulence behavior introduces further challenges for loss models dealing with ORC turbines. The present contribution presents the outcome of an experimental investigation of profile losses of a transonic turbine cascade placed in the test section of a closed-loop organic vapor wind tunnel (CLOWT). This test facility at Muenster University of Applied Sciences enables the investigation of high subsonic and transonic organic vapor flows under realistic ORC turbine flow conditions at elevated pressure and temperature levels. The so-called VKI blade profile from the open literature was chosen for the cascade, and the working fluid was Novec 649. Pitot probes and hot wire anemometry were employed to measure the flow field up- and downstream of the cascade. Details of the unsteady flow caused by the trailing edge of the blades and the turbulent spectrum were investigated using hot-wire anemometry. The new organic vapor flow results were compared with literature data obtained for air and the prediction of conventional loss models. It was found that under certain thermodynamic conditions, traditional loss models can still predict the loss of ORC turbines reasonably well.