The Organic Rankine Cycle (ORC) represents an emerging technology aimed at exploiting lower temperature heat sources, like waste heat in industrial processes or exhaust heat in engines, more efficiently due to the lower evaporation temperatures of ORC working fluids compared to steam in conventional Clausius-Rankine cycles. A further advantage of ORC working fluids is the high density in the relevant pressure and temperature range, which leads to smaller and more cost-effective turbines.

A supersonic axial, single stage impulse ORC turbine generator unit operated with an ethanol-water mixture was successfully designed and tested \[1\] as an efficiency booster for internal combustion engines. The stage consists of 8 stator passages with a Laval nozzle design and 33 rotor blades. The flow is accelerated inside the stator and enters the rotor with supersonic speed. The maximum power output of 18.3 kW is achieved at a rotational speed of 110,000 1/min. The shroud diameter is about 63 mm. A key feature of the turbine design is the use of partial admission control/regulation to ensure high efficiency levels over a wide operating range. In \[1\], first 3D-CFD results were also presented, comparing full admission computations with subsequent corrections based on the empirical Horlock \[2\] model to account the partial admission losses to the experiments.

This work presents a detailed numerical investigation of the flow field in the experimental configuration described above aiming at assessing the reliability of empirical correlations for partial admission losses and paves the way to deriving knowledge-based improved loss models. Both a full annulus and a reduced 120°-model of the turbine are considered. For the RANS-based computations a hybrid CPU/GPU 3D-CFD in-house fully-compressible solver is used and the results are carefully compared to experimental data. Since the turbine operates near the critical point of the working fluid, modeling of real gas properties is of fundamental importance to achieve accurate results. For this purpose, the Peng-Robinson equation of state is adopted. A multi-parameter Equation of state approach as provided by the REFPROP library has been tested as well and the accuracy of the thermodynamic models and their influence on the results will be assessed. Finally, the computational efficiency of the solution method has been further enhanced by means of an innovative spline-based tabulation approach.

References

\[1\]        J. R. Seume et al, “Design and test of a 10kW ORC supersonic turbine generator”. Journal of Physics: Conference Series Vol. 821, 2017.

 

\[2\]        J. H. Horlock, Axial-Flow Turbines, Butterworths, London, 1966.


Numerical Investigation of a Partially Loaded Supersonic ORC Turbine Stage

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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