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Research Papers: Natural and Mixed Convection

Natural Convection From Horizontal Cylinders at Near-Critical Pressures—Part II: Numerical Simulations

[+] Author and Article Information
Gopinath R. Warrier

e-mail: gwarrier@ucla.edu

Vijay K. Dhir

Henry Samueli School of
Engineering and Applied Science,
Mechanical and Aerospace
Engineering Department,
University of California, Los Angeles,
Los Angeles, CA 90095

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the Journal of Heat Transfer. Manuscript received September 9, 2011; final manuscript received June 17, 2012; published online December 28, 2012. Assoc. Editor: Ali Ebadian.

J. Heat Transfer 135(2), 022502 (Dec 28, 2012) (10 pages) Paper No: HT-11-1438; doi: 10.1115/1.4007673 History: Received September 09, 2011; Revised June 17, 2012

A numerical investigation of laminar natural convection heat transfer from small horizontal cylinders at near-critical pressures has been carried out. Carbon dioxide is the test fluid. The parameters varied are: pressure (P), (ii) bulk fluid temperature (Tb), (iii) wall temperature (Tw), and (iv) wire diameter (D). The results of the numerical simulations agree reasonably well with available experimental data. The results obtained are as follows: (i) At both subcritical and supercritical pressures, h is strongly dependent on Tb and Tw. (ii) For Tw < Tsat (for P < Pc) and Tw < Tpc (for P > Pc), the behavior of h as a function of Tw is similar; h increases with increase in Tw. (iii) For P > Pc and large Tw (Tw > Tpc), natural convection heat transfer occurring on the cylinder is similar that observed during film boiling on a cylinder. The heat transfer coefficient decreases as Tw increases. (iv) For subcritical pressures, the dependence of h on D is h ∝ D0.5 in the range 25.4 ≤ D ≤ 100 μm. For larger values of D (500–5000 μm), h ∝ D0.24. (v) For supercritical pressures, the dependence of h on D is h ∝ D0.47 in the range 25.4 ≤ D ≤ 100 μm. For larger values of D (500–5000 μm), h ∝ D−0.27. (vi) For a given P, the maximum heat transfer coefficient is obtained for conditions where Tb < Tpc and Tw ≥ Tpc. Analysis of the temperature and flow field shows that this peak in h occurs when k, Cp, and Pr in the fluid peak close to the heated surface.

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References

Figures

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Fig. 1

Property variation as a function of temperature for CO2 [8] (a) subcritical pressure (P = 6.99 MPa) and (b) supercritical pressure (P = 8.10 MPa)

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Fig. 2

Computational domain

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Fig. 3

Effect of number of grid points (a) varying Nη and (b) varying Nθ

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Fig. 4

Comparison of numerical results with experimental data for subcritical pressures, P = 7.34 MPa, 10 °C ≤ Tb ≤ 35 °C, D = 100 μm

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Fig. 5

Effect of Tb on h for subcritical pressures (P = 7.34 MPa, D = 100 μm)

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Fig. 6

Effect of pressure for D = 25.4 μm, Tb = 10 °C (P < Pc)

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Fig. 7

Variation of h with D for subcritical pressures

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Fig. 8

Comparison of numerical results with experimental data for supercritical pressures, P = 8.10 MPa, 10 °C ≤ Tb ≤ 33.3 °C, and D = 100 μm

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Fig. 9

Effect of bulk liquid temperature for supercritical pressures

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Fig. 10

Effect of pressure for D = 100 μm, Tb = 10, and 31 °C (P > Pc)

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Fig. 11

Effect of diameter for supercritical pressures, P = 7.50 MPa, Tb = 25, 31, and 35 °C

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Fig. 12

Variation of Tw,peak as a function of Tb and P (P > Pc)

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Fig. 13

Radial profiles of T, ∂T/∂y and relevant fluid properties

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