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Research Papers

Discharge of Thermal Storage Tanks via Immersed Baffled Heat Exchangers: Numerical Model of Flow and Temperature Fields

[+] Author and Article Information
Yan Su, Jane H. Davidson

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

J. Sol. Energy Eng 130(2), 021016 (Apr 11, 2008) (7 pages) doi:10.1115/1.2856012 History: Received July 05, 2007; Revised July 09, 2007; Published April 11, 2008

A model of a thermal storage tank in which stored energy is extracted via an immersed heat exchanger is presented and used to predict transient temperature and velocity fields in tanks with and without baffles. The heat exchanger is modeled as a porous medium within the storage fluid. A simple cylindrical baffle that creates an annular space in which a coiled tube heat exchanger is positioned provides a modest increase in the rate of energy extraction compared to a tank with no baffle. The improved discharge rate is attributed to an increase in the flow speed across the heat exchanger. A baffle with greater hydraulic resistance slows the flow and reduces performance.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Sketch of cylindrical water storage tanks: (a) no baffle, (b) simple cylindrical baffle, and (c) complex baffle that uses the simple baffle plus hydraulic resistance elements to slow the flow

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Figure 2

Two-dimensional computational domain; the heat exchanger is treated as a porous medium within the fluid and is indicated by the shaded area: (a) no baffle, (b) simple cylindrical baffle, and (c) complex baffle that uses the simple baffle plus hydraulic resistance elements in the bulk storage fluid, r*=r∕D and z*=z∕D

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Figure 3

Transient isotherms and streamlines for storage tanks with and without baffles for RaD,0=1.8×1010: (a) no baffle, (b) simple baffle, and (c) complex baffle; dimensionless times of 10, 100, and 400 correspond to 69s, 690s, and 2769s

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Figure 4

Transient isotherms and streamlines for storage tanks with and without baffles for RaD,0=1.8×1011: (a) no baffle, (b) simple baffle, and (c) complex baffle; dimensionless times of 10, 100, and 300 correspond to 22s, 220s, and 661s

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Figure 5

Fraction of the energy discharged for (a) no baffle, (b) simple baffle, and (c) complex baffle

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Figure 10

Water temperature in the heat exchanger zone

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Figure 9

Drag coefficient of the simple (top) and complex baffle (bottom)

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Figure 8

Transient velocity in the heat exchanger zone

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Figure 7

Nusselt numbers for RaD,0=1.8×1011

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Figure 6

Nusselt numbers for RaD,0=1.8×1010

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