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Research Papers: Gas Turbines: Vehicular and Small Turbomachines

Special Challenges in the Computational Fluid Dynamics Modeling of Transonic Turbo-Expanders

[+] Author and Article Information
Filippo Rubechini

e-mail: filippo.rubechini@arnone.de.unifi.it

Andrea Arnone

Department of Industrial Engineering,
University of Florence,
via di Santa Marta,
3 Firenze 50139,Italy

Roberto Biagi

GE Oil & Gas,
via Felice Matteucci 2,
Firenze 50127, Italy

Contributed by the Vehicular and Small Turbomachines Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received June 27, 2013; final manuscript received July 1, 2013; published online August 30, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 135(10), 102701 (Aug 30, 2013) (8 pages) Paper No: GTP-13-1188; doi: 10.1115/1.4025034 History: Received June 27, 2013; Revised July 01, 2013

High pressure ratio turbo-expanders often put a strain on computational fluid dynamics (CFD) modeling. First of all, the working fluid is usually characterized by significant departures from the ideal behavior, thus requiring the adoption of a reliable real gas model. Moreover, supersonic flow conditions are typically reached at the nozzle vanes discharge, thus involving the formation of a shock pattern, which is in turn responsible for a strong unsteady interaction with the wheel blades. Under such circumstances, performance predictions based on classical perfect gas, steady-state calculations can be very poor. While reasonably accurate real gas models are nowadays available in most flow solvers, unsteady real gas calculations still struggle to become an affordable tool for investigating turbo-expanders. However, it is emphasized in this work how essential the adoption of a time-accurate analysis can be for accurate performance estimations. The present paper is divided in two parts. In the first part, the computational framework is validated against on-site measured performance from an existing power plant equipped with a variable-geometry nozzled turbo-expander, for different nozzle positions, and in design and off-design conditions. The second part of the paper is devoted to the detailed discussion of the unsteady interaction between the nozzle shock waves and the wheel flow field. Furthermore, an attempt is made to identify the key factors responsible for the unsteady interaction and to outline an effective way to reduce it.

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Figures

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Fig. 1

View of the computational mesh

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Fig. 2

Expected, measured, and computed (unsteady) stage performance

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Fig. 3

Unsteadiness at midspan for three different nozzle openings (a) nozzle (b) wheel

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Fig. 4

Impact of computational model on performance at varying nozzle opening

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Fig. 5

Unsteadiness at midspan for two different nozzle geometries (a) nozzle (b) wheel

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Fig. 6

(a) Averaged entropy rise across the stage and (b) instantaneous entropy rise contours on a blade-to-blade surface of the wheel at midspan

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Fig. 7

Space-time diagrams for two different nozzle geometries, static pressure distribution on nozzle–-wheel circumferential interface over a vane passing period

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Fig. 8

Space-time diagrams for two different nozzle geometries, static pressure distribution on wheel blade surface at midspan over a vane passing period

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Fig. 9

Unsteady pressure spectra at nozzle–wheel interface plane, circumferential and time harmonic modes (rotating reference frame)

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Fig. 10

Circumferential distribution of instantaneous static pressure at midspan nozzle–wheel interface t/T = 0

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