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Research Papers

# Investigation of Buoyancy Effects on Heat Transfer Characteristics of Supercritical Carbon Dioxide in Heating Mode

[+] Author and Article Information
Sandeep R. Pidaparti, Mark M. Mikhaeil

Georgia Institute of Technology,
George W. Woodruff School of Mechanical Engineering,
Atlanta 30332, GA

Jacob A. McFarland

Department of Mechanical Engineering,
University of Missouri,
Columbia 65211, MO

Mark H. Anderson

Department of Engineering Physics,

Devesh Ranjan

Georgia Institute of Technology,
George W. Woodruff School of Mechanical Engineering,
Atlanta 30332, GA,
e-mail: devesh.ranjan@me.gatech.edu

1Corresponding author.

Manuscript received September 23, 2014; final manuscript received December 20, 2014; published online May 20, 2015. Assoc. Editor: Dmitry Paramonov.

ASME J of Nuclear Rad Sci 1(3), 031001 (May 20, 2015) (10 pages) Paper No: NERS-14-1043; doi: 10.1115/1.4029592 History: Received September 23, 2014; Accepted January 12, 2015; Online May 20, 2015

## Abstract

Experiments were performed to investigate the effects of buoyancy on heat transfer characteristics of supercritical carbon dioxide in heating mode. Turbulent flows with Reynolds numbers up to 60,000, at operating pressures of 7.5, 8.1, and 10.2 MPa, were tested in a round tube. Local heat transfer coefficients were obtained from measured wall temperatures over a large set of experimental parameters that varied inlet temperature from 20 to 55°C, mass flux from 150 to $350 kg/m2s$, and a maximum heat flux of $65 kW/m2$. Horizontal, upward, and downward flows were tested to investigate the unusual heat transfer characteristics due to the effect of buoyancy and flow acceleration caused by large variation in density. In the case of upward flow, severe localized deterioration in heat transfer was observed due to reduction in the turbulent shear stress and is characterized by a sharp increase in wall temperature. In the case of downward flow, turbulent shear stress is enhanced by buoyancy forces, leading to an enhancement in heat transfer. In the case of horizontal flow, flow stratification occurred, leading to a circumferential variation in wall temperature. Thermocouples mounted 180° apart on the tube revealed that the wall temperatures on the top side are significantly higher than the bottom side of the tube. Buoyancy factor calculations for all the test cases indicated that buoyancy effects cannot be ignored even for horizontal flow at Reynolds numbers as high as 20,000. Experimentally determined Nusselt numbers are compared to existing correlations available in the literature. Existing correlations predicted the experimental data within $±30%$, with maximum deviation around the pseudocritical point.

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## Figures

Fig. 2

Schematic of the experimental facility and the test section

Fig. 1

Variation of thermophysical properties of CO2 in the supercritical region

Fig. 3

Effect of operating pressure on heat transfer for downward flow, G=195  kg/m2s, QPS′′=13.5  kW/m2

Fig. 4

Effect of flow configuration on heat transfer for p=8.1  MPa, G=195  kg/m2s, QPS′′=24  kW/m2, Tin=46°C

Fig. 5

Effect of inlet temperature on the wall temperatures for p=7.5  MPa, G=320  kg/m2s, and QPS′′=24  kW/m2

Fig. 9

Calculated Nusselt number using Mokry et al. correlation

Fig. 7

Normalized Nusselt number versus Jackson’s buoyancy parameter, Bu

Fig. 8

Normalized Nusselt number versus Jackson’s buoyancy parameter, Boj

Fig. 6

Effect of heat flux on downward flow heat transfer for p=7.5  MPa and G=195  kg/m2s

Fig. 10

Calculated Nusselt number for downward flow

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