Performance Predictions of Axial Turbines for Organic Rankine Cycle (ORC) Applications Based on Measurements of the Flow Through Two-Dimensional Cascades of Blades

Abstract

The Organic-Rankine-Cycle (ORC) offers a great potential for waste heat recovery and use of low-temperature sources for power generation. However, the ORC thermal efficiency is limited by the relatively low temperature level, and it is, therefore, of major importance to design ORC components with high efficiencies and minimized losses. The use of organic fluids creates new challenges for turbine design, due to real-gas behavior and low speed of sound. The design and performance predictions for steam and gas turbines have been mainly based on measurements and numerical simulations of flow through two-dimensional cascades of blades. In case of ORC turbines and related fluids, such an approach requires the use of specially designed closed cascade wind tunnels. In this contribution, the specific loss mechanisms caused by the organic fluids are reviewed. The concept and design of an ORC cascade wind tunnel are presented. This closed wind tunnel can operate at higher pressure and temperature levels, and this allows for an investigation of typical organic fluids and their real-gas behavior. The choice of suitable test fluids is discussed based on the specific loss mechanisms in ORC turbine cascades. In future work, we are going to exploit large-eddy-simulation (LES) techniques for calculating flow separation and losses. For the validation of this approach and benchmarking different sub-grid models, experimental data of blade cascade tests are crucial. The testing facility is part of a large research project aiming at obtaining loss correlations for performance predictions of ORC turbines and processes, and it is supported by the German Ministry for Education and Research (BMBF).