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Technical Briefs

Computational Study of a High-Expansion Ratio Radial Organic Rankine Cycle Turbine Stator

[+] Author and Article Information
John Harinck

Process and Energy Department, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, 2628 CA Delft, The Netherlandsj.harinck@tudelft.nl

Teemu Turunen-Saaresti

Fluid Dynamics Laboratory, LUT Energy, Faculty of Technology, Lappeenranta University of Technology, 53850 Lappeenranta, Finlandteemu.turunen-saaresti@lut.fi

Piero Colonna

Process and Energy Department Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, 2628 CA Delft, The Netherlandsp.colonna@tudelft.nl

Stefano Rebay

Department of Mechanical Engineering, University of Brescia, 25123 Brescia, Italyrebay@ing.unibs.it

Jos van Buijtenen

 Tri-O-Gen B.V., Nieuwenkampsmaten 8, 7472 DE Goor, The Netherlands

J. Eng. Gas Turbines Power 132(5), 054501 (Mar 03, 2010) (6 pages) doi:10.1115/1.3204505 History: Received December 30, 2008; Revised May 08, 2009; Published March 03, 2010; Online March 03, 2010

There is a growing interest in organic Rankine cycle (ORC) turbogenerators because they are suitable as sustainable energy converters. ORC turbogenerators can efficiently utilize external heat sources at low to medium temperature in the small to medium power range. ORC turbines typically operate at very high pressure ratio and expand the organic working fluid in the dense-gas thermodynamic region, thus requiring computational fluid dynamics (CFD) solvers coupled with accurate thermodynamic models for their performance assessment and design. This article presents a comparative numerical study on the simulated flow field generated by a stator nozzle of an existing high-expansion ratio radial ORC turbine with toluene as working fluid. The analysis covers the influence on the simulated flow fields of the real-gas flow solvers: FLUENT , FINFLO , and ZFLOW , of two turbulence models and of two accurate thermodynamic models of the fluid. The results show that FLUENT is by far the most dissipative flow solver, resulting in large differences in all flow quantities and appreciably lower predictions of the isentropic nozzle efficiency. If the combination of the kω turbulence model and FINFLO solver is adopted, a shock-induced separation bubble appears in the calculated results. The bubble affects, in particular, the variation in the flow velocity and angle along the stator outlet. The accurate thermodynamic models by Lemmon and Span (2006, “Short Fundamental Equations of State for 20 Industrial Fluids,” J. Chem. Eng. Data, 51(3), pp. 785–850) and Goodwin (1989, “Toluene Thermophysical Properties From 178 to 800 K at Pressures to 1000 Bar,” J. Phys. Chem. Ref. Data, 18(4), pp. 1565–1636) lead to small differences in the flow field, especially if compared with the large deviations that would be present if the flow were simulated based on the ideal gas law. However, the older and less accurate thermodynamic model by Goodwin does differ significantly from the more accurate Lemmon–Span thermodynamic model in its prediction of the specific enthalpy difference, which leads to a considerably different value for the specific work and stator isentropic efficiency. The above differences point to a need for experimental validation of flow solvers in real-gas conditions, if CFD tools are to be applied for performance improvements of high-expansion ratio turbines operating partly in the real-gas regime.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) The stator and rotor of the Tri-O-gen ORC turbine. The main pump, the rotor of the electrical generator, and the rotor of the turbine are mounted on the same shaft. (b) Coarse 2D computational grid (11,264 cells) of the stator nozzle for the viscous simulations. The figure aspect ratio is deformed since the blade design is a confidential property of the manufacturer.

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Figure 2

Comparison of scaled Mach number fields of the flow through the turbine stator nozzle, as calculated by (a) FLUENT and (b) FINFLO viscous flow solvers, respectively. Both flow solvers employ the k−ε turbulence model and the Lemmon–Span thermodynamic model for toluene. The Mach number fields are scaled by one outlet value for reasons of confidentiality requested by the manufacturer.

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Figure 3

Comparison of scaled Mach number fields of the flow through the turbine stator nozzle, as calculated by (a) inviscid FLUENT and (b) inviscid ZFLOW solvers, respectively. Both flow solvers employ the Lemmon–Span thermodynamic model for toluene. The Mach number fields are scaled by one outlet value for reasons of confidentiality requested by the manufacturer. Streamlines and equidistant isolines of total pressure are also indicated.

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Figure 4

Comparison of scaled Mach number fields of the flow through the turbine stator nozzle, as calculated by (a) FLUENT and (b) FINFLO , both employing the k−ω turbulence model. The latter combination predicts a shock-induced separation bubble.

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Figure 5

Comparison of (a) the outflow angle and (b) outflow velocity, both with respect to a reference value calculated with FINFLO using different turbulence models. Both quantities are plotted as a function of the circumferential rotor angle, where the range corresponds to exactly one periodic stator nozzle outlet. The outflow angle is calculated from the radial direction (the lower the value, the more radial is the flow).

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