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Research Papers

An Investigation of Real Gas Effects in Supercritical CO2 Centrifugal Compressors

[+] Author and Article Information
Nikola D. Baltadjiev

MIT Gas Turbine Laboratory,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: nikola@alum.mit.edu

Claudio Lettieri

MIT Gas Turbine Laboratory,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: lettieri@mit.edu

Zoltán S. Spakovszky

MIT Gas Turbine Laboratory,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: zolti@mit.edu

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received December 17, 2014; final manuscript received December 28, 2014; published online February 25, 2015. Editor: Ronald Bunker.

J. Turbomach 137(9), 091003 (Sep 01, 2015) (13 pages) Paper No: TURBO-14-1319; doi: 10.1115/1.4029616 History: Received December 17, 2014; Revised December 28, 2014; Online February 25, 2015

This paper presents a comprehensive assessment of real gas effects on the performance and matching of centrifugal compressors operating in supercritical CO2. The analytical framework combines first principles based modeling with targeted numerical simulations to characterize the internal flow behavior of supercritical fluids with implications for radial turbomachinery design and analysis. Trends in gas dynamic behavior, not observed for ideal fluids, are investigated using influence coefficients for compressible channel flow derived for real gas. The variation in the properties of CO2 and the expansion through the vapor-pressure curve due to local flow acceleration are identified as possible mechanisms for performance and operability issues observed near the critical point. The performance of a centrifugal compressor stage is assessed at different thermodynamic conditions relative to the critical point using computational fluid dynamics (CFD) calculations. The results indicate a reduction of 9% in the choke margin of the stage compared to its performance at ideal gas conditions due to variations in real gas properties. Compressor stage matching is also impacted by real gas effects as the excursion in corrected mass flow per unit area from inlet to outlet increases by 5%. Investigation of the flow field near the impeller leading edge at high flow coefficients shows that local flow acceleration causes the thermodynamic conditions to reach the vapor-pressure curve. The significance of two-phase flow effects is determined through a nondimensional parameter that relates the time required for liquid droplet formation to the residence time of the flow under saturation conditions. Applying this criterion to the candidate compressor stage shows that condensation is not a concern at the investigated operating conditions. In the immediate vicinity of the critical point however, this effect is expected to become more prominent. While the focus of this analysis is on supercritical CO2 compressors for carbon capture and sequestration (CCS), the methodology is directly applicable to other nonconventional fluids and applications.

Copyright © 2015 by ASME
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Figures

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Fig. 1

Focus of paper is the in-depth assessment of internal flow behavior as the critical point is approached

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Fig. 2

Isentropic real gas expansion in a two-dimensional convergent nozzle

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Fig. 3

Real gas effect on corrected flow per unit area

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Fig. 4

2D radial impeller flow field at MU2 = 0.75 and design flow coefficient for real gas CO2

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Fig. 5

Polytropic efficiency of the 2D radial impeller at MU2 = 0.75

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Fig. 6

Excursions in corrected mass flow from inlet to outlet of the 2D radial impeller

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Fig. 7

Supercritical CO2 compressor stage

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Fig. 8

Reduction in choke margin of compressor when approaching the critical point

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Fig. 9

Excursions in corrected mass flow per unit area between inlet and outlet of the compressor stage

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Fig. 10

Relative Mach number field at 85% span for inlet stagnation pressure of 140 bar and φ/φd = 1.36

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Fig. 11

Vorticity field for inlet stagnation pressure of 140 bar and φ/φd = 1.36 after one impeller revolution

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Fig. 12

Supersonic patch due to negative incidence at impeller leading edge for high flow coefficient leads to thermodynamic state below saturation

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Fig. 13

Enthalpy of CO2 based on EOS model (top) is extrapolated to define the gas property for the metastable phase (bottom)

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Fig. 14

Volume of metastable fluid at impeller leading edge for candidate CO2 compressor

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Fig. 15

Analysis of condensation approaching critical point

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Fig. 16

Contours of compressibility factor

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Fig. 17

Contours of compressibility function βpT

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Fig. 18

Contours of compressibility function βTp

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Fig. 19

Contours of isentropic pressure exponent ns

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Fig. 20

Contours of isentropic pressure exponent ms

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Fig. 21

Contours of fundamental derivative

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