Unsteady operation of a highly supersonic Organic Rankine cycle turbine

Journal article (2016)

Authors

E. Rinaldi Energy Technology - Mechanical, Maritime and Materials Engineering
Rene Pecnik Energy Technology - Mechanical, Maritime and Materials Engineering
Piero Colonna Flight Performance and Propulsion - Aerospace Engineering
Research Group
Energy Technology (Mechanical, Maritime and Materials Engineering) (TU Delft)
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Published Date

2016

Language

English

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Faculty

Mechanical, Maritime and Materials Engineering

Department

Process and Energy

Research Group

Energy Technology

Abstract

Organic Rankine cycle (ORC) turbogenerators are the most viable option to convert sustainable energy sources in the low-to-medium power output range (from tens of kWe to several MWe). The design of efficient ORC turbines is particularly challenging due to their inherent unsteady nature (high expansion ratios and low speed of sound of organic compounds) and to the fact that the expansion encompasses thermodynamic states in the dense vapor region, where the ideal gas assumption does not hold. This work investigates the unsteady nonideal fluid dynamics and performance of a high expansion ratio ORC turbine by means of detailed Reynolds-averaged Navier-Stokes (RANS) simulations. The complex shock interactions resulting from the supersonic flow (M≈2.8 at the vanes exit) are related to the blade loading, which can fluctuate up to 60% of the time-averaged value. A detailed loss analysis shows that shock-induced boundary layer separation on the suction side of the rotor blades is responsible for most of the losses in the rotor, and that further significant contributions are given by the boundary layer in the diverging part of the stator and by trailing edge losses. Efficiency loss due to unsteady interactions is quantified in 1.4% in absolute percentage points at design rotational speed. Thermophysical properties are found to feature large variations due to temperature even after the strong expansion in the nozzle vanes, thus supporting the use of accurate fluid models in the whole turbine stage.