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Research Papers

Use of a Shroud and Baffle to Improve Natural Convection to Immersed Heat Exchangers

[+] Author and Article Information
Sandra K. S. Boetcher1

Department of Mechanical Engineering,  Embry-Riddle Aeronautical University, Daytona Beach, FL 32114sandra.boetcher@erau.edu

F. A. Kulacki, Jane H. Davidson

Department of Mechanical Engineering,  University of Minnesota, Minneapolis, MN 55455

1

Corresponding author.

J. Sol. Energy Eng 134(1), 011010 (Nov 29, 2011) (7 pages) doi:10.1115/1.4005089 History: Received January 24, 2011; Revised August 26, 2011; Published November 29, 2011; Online November 29, 2011

Optimizing heat transfer during the charge and discharge of thermal stores is crucial for high performance of solar thermal systems for domestic and commercial applications. This study models a sensible water storage tank for which discharge is accomplished using a heat exchanger immersed in the storage fluid. The heat exchanger is a two-dimensional isothermal cylinder in an adiabatic enclosure with no initial stratification. An adiabatic shroud and baffle whose geometry is parametrically varied is placed around and below the cylinder. Transient numerical simulations of the discharge process are obtained for 105  < RaD  < 107 , and estimates of the time needed to discharge a given fraction of the initial stored energy are obtained. We find that a short baffle is least effective in increasing heat transfer rates. The performance benefit is greatest early in the transient discharge period when the buoyant flow in the store is strongest. As with all flow control devices, the benefit decreases as energy is extracted from the tank and the temperature difference driving the flow decreases. The use of a shroud increases the transient Nusselt number by as much as twentyfold.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Thermal store with immersed heat exchanger operating in discharge mode

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Figure 2

The two-dimensional cylinder with a circular shroud and a straight baffle of width DB and thickness tB

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Figure 3

The baffle-shroud combination and the solution domain

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Figure 4

Baffle shapes considered in the present study. The heights H of the baffles are (a) 0D, (b) 0.375D, (c) 0.75D, (d) 1.125D, and (e) 1.5D.

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Figure 5

Average Nusselt number for RaD  = 105 , (a) L/D = 10 and (b) L/D = 30

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Figure 6

Average Nusselt number for RaD  = 106 , (a) L/D = 10 and (b) L/D = 30

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Figure 7

Average Nusselt number for RaD  = 107 , (a) L/D = 10 and (b) L/D = 30

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Figure 8

Fractional energy discharge for RaD  = 105 , (a) L/D = 10 and (b) L/D = 30

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Figure 9

Fractional energy discharge for RaD  = 106 , (a) L/D = 10 and (b) L/D = 30

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Figure 10

Fractional energy discharge for RaD  = 107 , (a) L/D = 10 and (b) L/D = 30

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Figure 11

Streamline diagrams at τ = 0.135 (maximum heat transfer at the fluctuating stage) for RaD  = 106 and shroud height for (a) 0D, (b) 0.75D, and (c) 1.5D

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Figure 12

Centerline velocity in the baffle, RaD  = 106 , (a) H/D = 0, (b) H/D = 0.75, and (c) H/D = 1.5

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