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Research Papers: Natural and Mixed Convection

# Natural Convection From Horizontal Cylinders at Near-Critical Pressures—Part I: Experimental Study

[+] 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

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), 022501 (Dec 28, 2012) (10 pages) Paper No: HT-11-1437; doi: 10.1115/1.4007672 History: Received September 09, 2011; Revised June 17, 2012

## Abstract

An experimental study of free convection heat transfer from horizontal wires to carbon dioxide at near-critical pressures has been performed. In the experiments, platinum wires ranging in size from 25.4 μm to 100 μm and a nichrome 60/20 wire of 101.6 μm diameter were used. The pressure (P) and bulk temperature (Tb) of the fluid were varied in the range: 6.34 MPa ≤ P ≤ 9.60 MPa and 10 °C ≤ Tb ≤ 33.3 °C, respectively. The wall temperature (Tw) was systematically increased from Tb + 0.1 °C to 250 °C. Visual observations of the fluid flow were made using a high speed camera. The similarity between natural convection heat transfer at Tw < Tsat (for P < Pc) and Tw < Tpc (for P > Pc), as well as the similarity between film boiling at Tw > Tsat (for P < Pc) and natural convection heat transfer at Tw > Tpc (for P > Pc), was demonstrated. The dependence of the heat transfer coefficient on the wire diameter was found to be h$∝$D−0.5, for both P < Pc and P > Pc. The bulk fluid temperature is introduced as a new reference temperature for the calculation of fluid properties. Correlations have been developed to predict the natural convection heat transfer coefficient at both subcritical and supercritical pressures. The developed correlations predict almost all the experimental data from the current study and those reported in the literature to within ±15%.

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## Figures

Fig. 1

Some properties of carbon dioxide at: (a) P = 6.99 MPa, (b) 8.10 MPa

Fig. 2

Schematic of the experimental apparatus

Fig. 3

(a) Variation of q and (b) h with Tw for supercritical and subcritical pressures

Fig. 4

(a) Variation of h with (Tw − Tb) for various Tb and P = 7.34 MPa, (b) variation of h with (Tw − Tb) for different P and Tb = 10 °C, and (c) variation of h with (Tw − Tb) for various wire diameters

Fig. 5

Variation of h with Tw for different Tb and fixed P: (a) P = 7.50 MPa, (b) P = 8.10 MPa, (c) P = 9.60 MPa

Fig. 6

Variation of h with (Tw − Tpc) for different P and fixed Tb: (a) Tb = 10 °C, (b) Tb = 31 °C, (c) Tb = 33.3 °C

Fig. 7

Variation of (Tw,peak − Tpc) with (Tpc − Tb) for various P and Tb

Fig. 8

Variation of h with the Tw for different P, Tb, and D

Fig. 9

Visual observation: (a) threadlike columns flow pattern and (b) disordered flow pattern

Fig. 10

Influence of the wire material on the variation of h with Tw, for different P and Tb

Fig. 11

(a) Nu as a function of (Tw − Tb) / Tb, (b) Ra as a function of (Tw − Tb) / Tb and (c) Nu as a function of Ra for various property evaluation methods

Fig. 12

Comparison between experimental and predicted values of h for P < Pc

Fig. 13

Comparison between experimental and predicted values of the Nu for Tb < Tsat, at subcritical pressures

Fig. 14

Comparison between experimental and: (a) predicted values of h, for P = 8.10 MPa, Tb = 33.3 °C, D = 25.4 μm, (b) predicted values of Nu for Tw < Tpc, Tb < Tpc, P > Pc, (c) predicted values of Nu for Tw > Tpc, Tb < Tpc, P > Pc

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