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Research Papers: Natural and Mixed Convection

Optimum Operating Conditions for Subcritical/Supercritical Fluid-Based Natural Circulation Loops

[+] Author and Article Information
Ajay Kumar Yadav

Assistant Professor
Department of Mechanical Engineering,
National Institute of Technology,
Karnataka, Surathkal,
Mangalore 575025, Karnataka, India
e-mail: ajaykyadav@nitk.edu.in

Souvik Bhattacharyya

Professor
Department of Mechanical Engineering,
Indian Institute of Technology Kharagpur,
Kharagpur 721302, India
e-mail: souvik.iit@gmail.com

M. Ram Gopal

Professor
Department of Mechanical Engineering,
Indian Institute of Technology Kharagpur,
Kharagpur 721302, India
e-mail: ramg@mech.iitkgp.ernet.in

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received August 2, 2014; final manuscript received September 18, 2015; published online June 14, 2016. Assoc. Editor: Ali Khounsary.

J. Heat Transfer 138(11), 112501 (Jun 14, 2016) (9 pages) Paper No: HT-14-1510; doi: 10.1115/1.4031921 History: Received August 02, 2014; Revised September 18, 2015

Natural circulation loop (NCL) is simple and reliable due to the absence of moving components and is preferred in applications where safety is of foremost concern, such as nuclear power plants and high-pressure thermal power plants. In the present study, optimum operating conditions based on the maximum heat transfer rate in NCLs have been obtained for subcritical as well as supercritical fluids. In recent years, there is a growing interest in the use of carbon dioxide (CO2) as loop fluid in NCLs for a variety of heat transfer applications due to its excellent thermophysical environmentally benign properties. In the present study, three-dimensional (3D) computational fluid dynamics (CFD) analysis of a CO2-based NCL with isothermal source and sink has been carried out. Results show that the heat transfer rate is much higher in the case of supercritical phase (if operated near pseudocritical region) than the subcritical phase. In the subcritical option, higher heat transfer rate is obtained in the case of liquid operated near saturation condition. Correlations for optimum operating condition are obtained for a supercritical CO2-based NCL in terms of reduced temperature and reduced pressure so that they can be employed for a wide variety of fluids operating in supercritical region. Correlations are also validated with different loop fluids. These results are expected to help design superior optimal NCLs for critical applications.

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Figures

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Fig. 1

Schematic of the NCL employed in the model

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Fig. 2

Meshing of a cross section (fluid part only)

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Fig. 3

Variation in heat transfer rate with ΔT for (a) subcritical CO2, (b) supercritical CO2 at 313 K, (c) supercritical CO2 at 323 K, and (d) supercritical CO2 at 333 K

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Fig. 4

Variation of thermophysical properties with pressure

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Fig. 5

Variation of specific heat of CO2 with pressure for different temperatures

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Fig. 6

Variation of optimum pressure of CO2 with temperature

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Fig. 7

Variation of Rayleigh number with pressure for different operating temperatures

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Fig. 8

Variation of modified Rayleigh number with pressure for various operating temperatures

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Fig. 9

(a) Comparison of developed correlations in terms of heat transfer rate and (b) variation of heat transfer rate near the obtained pressure employing correlation based on Rayleigh number

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Fig. 10

Validation of obtained result with experimental data for turbulent flow

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