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Research Papers

Investigation of Shroud Geometry to Passively Improve Heat Transfer in a Solar Thermal Storage Tank

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

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

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING. Manuscript received July 20, 2012; final manuscript received September 8, 2013; published online November 26, 2013. Assoc. Editor: Werner Platzer.

J. Sol. Energy Eng 136(1), 011017 (Nov 26, 2013) (7 pages) Paper No: SOL-12-1182; doi: 10.1115/1.4025708 History: Received July 20, 2012; Revised September 08, 2013

A shroud and baffle configuration is used to passively increase heat transfer in a thermal store. The shroud and baffle are used to create a vena contracta near the surface of the heat exchanger, which will speed up the flow locally and thereby increasing heat transfer. The goal of this study is to investigate the geometry of the shroud in optimizing heat transfer by locally increasing the velocity near the surface of the heat exchanger. Two-dimensional transient simulations are conducted. The immersed heat exchanger is modeled as an isothermal cylinder, which is situated at the top of a solar thermal storage tank containing water (Pr = 3) with adiabatic walls. The shroud and baffle are modeled as adiabatic, and the geometry of the shroud and baffle are parametrically varied. Nusselt numbers and fractional energy discharge rates are obtained for a range of Rayleigh numbers, 105 ≤ RaD ≤ 107 in order to determine optimal shroud and baffle configurations. It was found that a baffle width of 75% of the width of the heat exchanger provided the best heat transfer performance.

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Figures

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Fig. 1

Schematic diagram of an immersed heat exchanger in a thermal store

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Fig. 2

Solution domain for an isothermal cylinder with a shroud and baffle situated in a thermal storage tank

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Fig. 3

Schematic diagram of (a) the three-dimensional situation, and (b) the dimensions for the isothermal cylinder and adjacent shroud-baffle situated in a thermal storage tank

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Fig. 4

Average Nusselt number versus dimensionless time for RaD = 105

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Fig. 5

Average Nusselt number versus dimensionless time for RaD = 106

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Fig. 6

Average Nusselt number versus dimensionless time for RaD = 107

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Fig. 7

Velocity vector diagrams for RaD = 105 at τ = 2 for (a) W = 0.25 D, (b) W = 0.75 D, and (c) W = 1.25D

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Fig. 8

Fractional energy discharge versus dimensionless time for RaD = 105

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Fig. 9

Fractional energy discharge versus dimensionless time for RaD = 106

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Fig. 10

Fractional energy discharge versus dimensionless time for RaD = 107

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Fig. 11

Schematic diagram showing the shroud angle

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Fig. 12

Average Nusselt number versus dimensionless time for RaD = 105, W/D = 0.25, and various shroud angles

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Fig. 13

Average Nusselt number versus dimensionless time for RaD = 106, W/D = 0.25, and various shroud angles

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Fig. 14

Average Nusselt number versus dimensionless time for RaD = 107, W/D = 0.25, and various shroud angles

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