Research Papers: Gas Turbines: Turbomachinery

Performance Metric for Turbine Stage Under Unsteady Pulsating Flow Environment

[+] Author and Article Information
Jinwook Lee

Gas Turbine Laboratory,
Department of Aeronautics and Astronautics,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: jinwook@mit.edu

Choon S. Tan

Gas Turbine Laboratory,
Department of Aeronautics and Astronautics,
Massachusetts Institute of Technology,
Cambridge, MA 02139

Borislav T. Sirakov, Hong-Sik Im, Martin Babak, Denis Tisserant

Honeywell Turbo Technologies,
Torrance, CA 90504

Chris Wilkins

Hawthorne, CA 90250

1Corresponding author.

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received December 2, 2016; final manuscript received December 8, 2016; published online March 7, 2017. Editor: David Wisler.

J. Eng. Gas Turbines Power 139(7), 072606 (Mar 07, 2017) (7 pages) Paper No: GTP-16-1562; doi: 10.1115/1.4035630 History: Received December 02, 2016; Revised December 08, 2016

In a turbine stage for a vehicular turbocharger or a pulse detonation engine (PDE) system, the power extraction process is inherently unsteady due to a highly pulsating flow delivered from the upstream combustor. Characterizing the operating performance of such a turbine stage would call for defining unsteady efficiency on a physically rigorous basis. Since the instantaneous efficiency can be calculated as the fraction of the actual power to the unsteady ideal power, an expression for the unsteady ideal power from the turbine stage is first derived by applying mass conservation and the first/second law of thermodynamics for the turbine stage. The newly derived expression elucidates the distinction from the quasi-steady situation in that the storage effect of mass/energy/entropy over the turbine stage is no longer negligible compared to the flux of mass/energy/entropy at the inlet and outlet. The storage effect resolves the previously reported physical inconsistency that the instantaneous efficiency can be a value of above unity or below zero; an erroneous result associated with defining the efficiency based on a quasi-steady basis. As the reduced frequency of the inlet pulsation of the turbine stage becomes larger than unity, the mass/energy/entropy accumulation rate over the turbine stage becomes significant compared to the mass/energy/entropy influx rate. Then, the definition of the efficiency based on a quasi-steady assumption loses its applicability. In this paper, the role of mass/energy/entropy storage rate in the unsteady ideal power is assessed in order to underpin the inconsistency in the previous quasi-steady approach. The utility of the unsteady efficiency definition is elucidated for the case of a turbocharger turbine stage subjected to high inlet flow pulsation.

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Fig. 1

Twinscroll type turbocharger turbine stage a cross section of the twin scroll is shown with the turbine wheel. The compressor wheel (barely seen) is on the opposite side. Figure courtesy of Honeywell Turbo Technologies.

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Fig. 2

Turbine stage computational domain with 7.5 × 106 finite volume elements. Volute (with two inlets), turbine wheel, and diffuser (with outlet) are indicated in the figure.

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Fig. 3

Pulsating stagnation pressure inlet boundary condition normalized by constant outlet static pressure—each line (blue and red) indicates the boundary condition for each inlet

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Fig. 4

Volute mesh configuration with 3.4 × 106 finite elements—hybrid type mesh is used with structured mesh for initial straight section and unstructured mesh for latter curved section

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Fig. 5

Turbine wheel mesh configuration with 2.7 × 106 finite elements—structured mesh is used for the entire domain

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Fig. 8

Computational setup assessment in terms of total-to-static efficiency under steady operations using industrial best practice (several times lower in mesh density than the one used in this work)

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Fig. 9

Unsteady ideal powers based on various forms—note the quasi-steady form is physically inconsistent (lower than actual power for some instances)

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Fig. 10

Instantaneous efficiencies based on various forms—note the physically inconsistent behavior of quasi-steady from (efficiency beyond unity)

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Fig. 11

Instantaneous efficiencies based on various forms—Note severe numerical noise of storage rate form

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Fig. 12

Work normalized by ideal cycle work—all forms have similar trend of work accumulation during the cycle

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Fig. 6

Diffuser mesh configuration with 1.3 × 106 finite elements—hybrid type mesh is used with unstructured mesh for the lower intricate section and structured mesh for the upper straight section

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Fig. 7

Computational setup assessment in terms of corrected mass flow under steady operations using industrial best practice (several times lower in mesh density than the one used in this work)



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