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Research Papers: Gas Turbines: Cycle Innovations

Scaling of Gas Turbine From Air to Refrigerants for Organic Rankine Cycle Using Similarity Concept

[+] Author and Article Information
Choon Seng Wong

Department of Mechanical Engineering,
University of Canterbury,
Private Bag 4800,
Christchurch 8041, New Zealand
e-mail: choon.wong@pg.canterbury.ac.nz

Susan Krumdieck

Professor
Department of Mechanical Engineering,
University of Canterbury,
Private Bag 4800,
Christchurch 8041, New Zealand
e-mail: susan.krumdieck@canterbury.ac.nz

1Corresponding author.

Contributed by the Cycle Innovations Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 16, 2015; final manuscript received September 15, 2015; published online November 17, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(6), 061701 (Nov 17, 2015) (10 pages) Paper No: GTP-15-1340; doi: 10.1115/1.4031641 History: Received July 16, 2015; Revised September 15, 2015

Similitude, or similarity concept, is an essential concept in turbomachinery to allow the designer to scale a turbine design to different sizes or different working fluids without repeating the whole design and development process. Similarity concept allows the testing of a turbomachine in a simple air test bench instead of a full-scale organic Rankine cycle (ORC) test bench. The concept can be further applied to adapt an existing gas turbine as an ORC turbine using different working fluids. This paper aims to scale an industrial gas turbine to different working fluids, other than the fluid the turbine was originally designed for. The turbine performance map for air was generated using the 3D computational fluid dynamics (CFD) analysis tools. Three different approaches using the similarity concept were applied to scale the turbine performance map using air and generate the performance map for two refrigerants: R134a and R245fa. The scaled performance curves derived from the air performance data were compared to the performance map generated using CFD analysis tools for R134a and R245fa. The three approaches were compared in terms of the accuracy of the performance estimation, and the most feasible approach was selected. The result shows that complete similarity cannot be achieved for the same turbomachine with two different working fluids, even at the best efficiency point for particular expansion ratio. If the constant pressure ratio is imposed, the location of the optimal velocity ratio and optimal specific speed would be underestimated with calculation error over 20%. Constant Δh0s/a012 was found to provide the highest accuracy in the performance estimation, but the expansion ratio (or pressure ratio) is varying using different working fluids due to the variation of sound speed. The differences in the fluid properties and the expansion ratio lead to the deviation in turbine performance parameters, velocity diagram, turbine's exit swirl angle, and entropy generation. The use of Δh0s/a012 further limits the application of the gas turbine for refrigerants with heavier molecular weight to a pressure ratio less than the designed pressure ratio using air. The specific speed at the best efficiency point was shifted to a higher value if higher expansion ratio was imposed. A correction chart for R245fa was attempted to estimate the turbine's performance at higher expansion ratio as a function of volumetric flow ratio.

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Figures

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Fig. 1

Overall procedure of CFD simulation using ansys turbomachinery package

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Fig. 2

Comparisons of the three similarity approaches and the CFD analysis in term of velocity ratio for R134a

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Fig. 3

Comparisons between the three similarity approaches and the CFD analysis in terms of specific speed for R134a

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Fig. 4

Comparisons of scaled performance from air data and CFD result for R245fa as a function of specific speed

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Fig. 5

Effect of machine Reynolds number on turbine's performance using R134a

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Fig. 6

Blade loading and density at blade surface using air at pressure ratio of 5.7 (a), R245fa at pressure ratio of 4.0 (b), and R134a at pressure ratio of 2.7 (c)

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Fig. 7

Distribution of relative Mach number in the meridional plane and distribution of absolute flow angle at the trailing edge using air at pressure ratio of 5.7 ((a) and (b)), R245fa at pressure ratio of 4.0 ((c) and (d)), and R134a at pressure ratio of 2.7 ((e) and (f))

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Fig. 8

Range of applicability using Eq. (6) for R134a (a) and R245fa (b)

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Fig. 9

Performance map of R245fa at different specific speeds and pressure ratios

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Fig. 10

Deviation of best efficiency point and the corresponding specific speed at increasing volumetric flow ratio

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