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Research Papers

Unsteady Operation of a Highly Supersonic Organic Rankine Cycle Turbine

[+] Author and Article Information
Enrico Rinaldi

Process & Energy Department,
Delft University of Technology,
Leeghwaterstraat 39,
Delft 2628 CB, The Netherlands
e-mail: erinaldi@mech.kth.se

Rene Pecnik

Process & Energy Department,
Delft University of Technology,
Leeghwaterstraat 39,
Delft 2628 CB, The Netherlands
e-mail: r.pecnik@tudelft.nl

Piero Colonna

Propulsion & Power,
Delft University of Technology,
Kluyverweg 1,
Delft 2629 HS, The Netherlands
e-mail: p.colonna@tudelft.nl

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received June 27, 2015; final manuscript received June 14, 2016; published online July 19, 2016. Assoc. Editor: Li He.

J. Turbomach 138(12), 121010 (Jul 19, 2016) (9 pages) Paper No: TURBO-15-1127; doi: 10.1115/1.4033973 History: Received June 27, 2015; Revised June 14, 2016

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.

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References

Figures

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Fig. 1

Turbine real geometry (top) and computational domain (bottom)

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Fig. 2

Normalized area distribution in the rotor channel. ϕ=0 and ϕ=1 indicate the inlet and the outlet of the rotor passage, respectively.

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Fig. 3

Steady-state pressure field calculated from (a) quasi-2D simulations with an augmented number of rotor blades (45 instead of 43); and extracted from a steady-state 3D simulation by Harinck et al. [35] at (b) 50%, (c) 35%, and (d) 65% span

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Fig. 4

Snapshot of the magnitude of the pressure gradient field: (a) ω = 24 krpm and (b) ω = 28 krpm

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Fig. 5

Time evolution of the magnitude of the pressure gradient field at 28 krpm over half-time period. The contour scale is the same as in Fig. 4.

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Fig. 6

Time-averaged (solid lines) and maximum and minimum values (dashed lines) of pressure on the rotor and stator blades calculated at 28 krpm. Quantities are functions of the radial distance r=x2+y2  from the axis of rotation.

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Fig. 7

Local frame of reference for blade loads calculation

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Fig. 8

Time evolution of the blade forces and torque. Time-averaged values are also reported (dashed lines).

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Fig. 9

Time-averaged entropy increase Δs = (s − sin)/sin

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Fig. 10

Snapshot of the entropy increase from unsteady simulation at 28 krpm. Contour scale is the same as in Fig. 9. Vectors represent the relative velocity field.

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Fig. 11

Deviation of total-to-static efficiency from the time-averaged value at 28 krpm. Values are in absolute percentage points.

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Fig. 12

Instantaneous entropy generation rate [36] at 25.8 krpm. The value is minimum for light shade and maximum for dark shade.

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Fig. 13

Average conditions at the stage inlet, outlet, and stator/rotor interface at 24 krpm. Contours indicate the deviation from the ideal gas law, 1 − P/ρRT.

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Fig. 14

Percentage deviations of cP, λ, μ, and Pr with respect to the circumferentially averaged values at the stator/rotor interface

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