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Technical Briefs

Negatively Buoyant Plume Flow in a Baffled Heat Exchanger

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

Department of Mechanical and Energy Engineering, Center for Advanced Scientific Computing and Modeling (CASCaM), University of North Texas, Denton, TX 76203sandra.boetcher@unt.edu

F. A. Kulacki, Jane H. Davidson

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

1

Corresponding author.

J. Sol. Energy Eng 132(3), 034502 (Jun 21, 2010) (7 pages) doi:10.1115/1.4001471 History: Received September 04, 2009; Revised January 24, 2010; Published June 21, 2010; Online June 21, 2010

A numerical simulation of transient two-dimensional negatively buoyant flow into a straight baffle situated below an isothermal circular cylinder in an initially isothermal enclosure is presented for both an adiabatic and a highly conducting baffle for Rayleigh numbers from 106 to 107. Results show the effects of baffle offset, width, and length on the point where viscous flow develops and on velocity profiles within the baffle. Results are interpreted to guide the design of straight baffles to reduce destruction of stratification in thermal stores using an immersed heat exchanger. The preferred geometry is a low-conductivity baffle of width equal to the effective width of the heat exchanger and 15 or more cylinder diameters in length to ensure nearly fully developed flow at the baffle outlet.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Vertical storage tank with an immersed coil heat exchanger located at the top of the tank for energy discharge. The coil heat exchanger is positioned in the annular space between an open-ended cylindrical baffle and the tank wall.

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Figure 2

Two-dimensional diagram of the cylindrical heat exchanger with a straight baffle of length L, width DB, and thickness tB offset from the cylinder center by δ

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Figure 3

Solution domain for numerical studies of a cylinder-baffle system situated in a thermal storage tank. The isothermal cylinder is held at temperature T0. The initial temperature of the fluid is Ti.

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Figure 4

Streamline patterns for Ra=106 and DB/D=1: (a) δ/D=1, (b) δ/D=1.5, and (c) δ/D=2

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Figure 5

Streamlines at the quasisteady state, Ra=106 and δ/D=1

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Figure 6

Velocity profiles at the quasisteady state within a baffle with δ/D=1 at various axial X locations: (a) Ra=106, conducting baffle; (b) Ra=106 adiabatic baffle; (c) Ra=107, conducting baffle; and (d) Ra=107, adiabatic baffle

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Figure 7

Comparison of dimensionless centerline velocities versus dimensionless distance X at the quasisteady state for the conducting and adiabatic baffles with δ/D=1: (a) RaD=106, (b) RaD=5×106, and (c) RaD=107

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