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Research Papers: Gas Turbines: Turbomachinery

Empirical Models for a Screw Expander Based on Experimental Data From Organic Rankine Cycle System Testing

[+] Author and Article Information
Vamshi Krishna Avadhanula

Department of Mechanical Engineering,
Alaska Center for Energy and Power,
University of Alaska Fairbanks,
Fairbanks, AK 99775
e-mail: vkavadhanula@alaska.edu

Chuen-Sen Lin

Department of Mechanical Engineering,
Alaska Center for Energy and Power,
University of Alaska Fairbanks,
Fairbanks, AK 99775

1Corresponding author.

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received November 21, 2013; final manuscript received December 14, 2013; published online January 9, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(6), 062601 (Jan 09, 2014) (8 pages) Paper No: GTP-13-1423; doi: 10.1115/1.4026303 History: Received November 21, 2013; Revised December 14, 2013

The screw expander discussed in this work was part of a 50 kW organic Rankine cycle (ORC) system. The ORC was tested under different conditions in heat source and heat sink. In conjunction with collecting data for the ORC system, experimental data were also collected for the individual components of the ORC, viz. evaporator, preheater, screw expander, working fluid pump, and condenser. Experimental data for the screw expander were used to develop the two empirical models discussed in this paper for estimating screw expander performance. As the physical parameters of the screw expander discussed in this article are not known, a “black-box” approach was followed to estimate screw expander power output, based on expander inlet and outlet pressure and temperature data. Refrigerant R245fa was used as the working fluid in the ORC. The experimental data showed that the screw expander had ranges of pressure ratio (2.70 to 6.54), volume ratio (2.54 to 6.20), and power output (10 to 51.5 kW). Of the two empirical models, the first model is based on the polytropic expansion process, in which an expression for the polytropic exponent is found by applying regression curve-fitting analysis as a function of the expander pressure ratio and volume ratio. In the second model, an expression for screw expander work output is found by applying regression curve-fitting analysis as a function of the expander isentropic work output. The predicted screw expander power output using the polytropic exponent model was within ±10% of experimental values; the predicted screw expander power output using the isentropic work output model was within ±7.5% of experimental values.

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Figures

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Fig. 1

Schematic of organic Rankine cycle

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Fig. 2

Schematic of a screw expander with the main components labeled [3]

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Fig. 3

Expansion path for ideal polytropic expansion process (4-a-b-5), expansion path with blowdown (4-a′-b′-c′-5), and expansion path with blowback (4-a′-b″-c″-5)

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Fig. 4

Screw expander volume ratio (rv) and refrigerant mass flow rate with respect to screw expander pressure ratio (rp)

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Fig. 5

Screw expander inlet pressure (state 4) with respect to screw expander pressure ratio (rp)

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Fig. 6

Screw expander power output (WSE) and isentropic efficiency (ηS) with respect to screw expander pressure ratio (rp)

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Fig. 7

Comparison of predicted data with experimental data for polytropic exponent (nc) in model I

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Fig. 8

Comparison of predicted data with experimental data for screw expander power output (WSE) in model I

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Fig. 9

Comparison of predicted data with experimental data for screw expander power output (WSE) in model II

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