Session: 34-09 High-fidelity CFD – General

Submission Number: 179124

Direct Numerical Simulation of the Transonic Flow in the Trailing Edge Region of an Organic-Fluid Cascade 

The design of Organic Rankine Cycle (ORC) expanders still relies on low- and medium-fidelity methods, primarily based on correlations originally developed for conventional air-driven steam turbines. However, the non-ideal behaviour of organic fluids, combined with the presence of transonic and supersonic flows, introduces complexities that may not be adequately captured by these models. Among the various sources of inefficiency, trailing-edge losses are particularly relevant, constituting a substantial portion of the total losses in transonic and supersonic turbines. These losses are strongly influenced by the base pressure, the pressure downstream of the blade trailing edge, which plays a critical role in determining the flow dynamics at the cascade outlet. Accurate prediction of these losses, essential for the optimisation of ORC expanders, requires a deeper understanding of base pressure phenomena. Although empirical correlations for base pressure exist for turbines operating with air, supported by extensive experimental campaigns on single plates and linear cascades, similar analyses for organic fluids in transonic and supersonic flow regimes are missing in the open literature.

Since experimental investigations of supersonic expansions of organic fluids remain technically challenging, this study aims at characterising the trailing-edge flow field through Direct Numerical Simulations (DNS). The simulations were carried out using the 3DNS code, analysing a simplified geometry inspired by Sieverding’s classical experiments on the base pressure problem. This configuration reproduces the flow in the final part of a transonic blade cascade, enabling detailed investigation of the base region and the wake area. The setup of the simulations is described, including the design of the computational domain, the choice of boundary conditions representative of ORC applications, the selection of the siloxane MM as working fluid and the integration of a non-ideal thermodynamic model into the solver.

The numerical analysis focuses on the influence of the Reynolds number and inlet turbulence intensity on the boundary-layer transition and wake development. Both instantaneous and time-averaged flow fields are examined to describe the evolution of the base region, the shock structure and the turbulent length scales. The reliability of the results is ensured through a rigorous grid independence study. Simulations are conducted for both MM and air to assess the influence of fluid non-ideality on the flow behaviour. Finally, DNS results are compared with RANS predictions, providing insight into the main limitations of RANS models in capturing flow features and the impact of such limitations in predicting the performance of blade cascades.

Presenting Author: Marco Oliveti Politecnico di Milano

Presenting Author Biography: Marco Oliveti graduated in Mechanical Engineering at Politecnico di Milano in 2022. He is presently PhD student at Politecnico di Milano. His main research interests are the experimental and computational analysis of turbine cascades, as well as the whole machine aerodynamic design, applied to Organic Rankine Cycle turbo-expanders for applications in binary geothermal plants.

Authors:

Marco Oliveti Politecnico di Milano
Andrew P. S. Wheeler University of Cambridge
Giacomo Persico Politecnico di Milano