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Research Papers: Flows in Complex Systems

Computational Fluid Dynamics Analysis of Cooling Tower Inlets

[+] Author and Article Information
H. C. R. Reuter1

Department of Mechanical and Mechatronic Engineering,  University of Stellenbosch, Private Bag X1, Matieland, 7602, South Africahreuter@sun.ac.za

D. G. Kröger

Department of Mechanical and Mechatronic Engineering,  University of Stellenbosch, Private Bag X1, Matieland, 7602, South Africa

1

Corresponding author.

J. Fluids Eng 133(8), 081104 (Aug 23, 2011) (12 pages) doi:10.1115/1.4004454 History: Received May 26, 2011; Accepted June 06, 2011; Published August 23, 2011; Online August 23, 2011

Cooling tower inlet losses are the flow losses or viscous dissipation of mechanical energy affected directly by the cooling tower inlet design, which according to the counterflow natural draft wet-cooling tower performance analysis example given in Kröger (Kröger, 2004, Air-Cooled Heat Exchangers and Cooling Towers: Thermal-Flow Performance Evaluation, Pennwell Corp., Tulsa, OK), can be more than 20% of the total cooling tower flow losses. Flow separation at the lower edge of the shell results in a vena contracta with a distorted inlet velocity distribution that causes a reduction in effective fill or heat exchanger flow area. In this paper, a two-dimensional (axi-symmetric) computational fluid dynamic (CFD) model is developed using the commercial CFD code ANSYS FLUENT, to simulate the flow patterns, loss coefficients and effective flow diameter of circular natural draft cooling tower inlets under windless conditions. The CFD results are compared with axial velocity profile data, tower inlet loss coefficients and effective diameters determined experimentally by Terblanche (Terblanche, 1993, “Inlaatverliese by Koeltorings,” M. Sc. Eng. thesis, Stellenbosch University, Stellenbosch, South Africa) on a cylindrical scale sector model as well as applicable empirical relations found in Kröger, determined using the same experimental apparatus as Terblanche. The validated CFD model is used to investigate the effects of Reynolds number, shell-wall thickness, shell wall inclination angle, fill loss coefficient, fill type, inlet diameter to inlet height ratio and inlet geometry on the flow patterns, inlet loss coefficient and effective diameter of full-scale cooling towers. Ultimately, simple correlations are proposed for determining the cooling tower inlet loss coefficient and inlet effective flow diameter ratio of full-scale cooling towers excluding the effect of rain zones and the structural supports around the cooling tower entrance.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Natural draft counterflow dry- or wet-cooling tower inlet flow patterns

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Figure 2

Cooling tower sector model for measuring inlet losses and effective flow area [3]

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Figure 3

Main dimensions and boundary definitions of the CFD flow domain of a cylindrical cooling tower

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Figure 4

Effect of physical size and grid size on the velocity profile downstream of the fill for Kfi  = 12.2 and di /Hi  = 10

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Figure 5

Effect of the turbulence model on the velocity profile downstream of the fill for Kfi  = 12.2 and di /Hi  = 10 for the experimental apparatus

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Figure 6

Comparison between experimental and CFD data showing the effect of Kfi and di /Hi on the velocity profile downstream of the fill for the experimental apparatus

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Figure 7

Experimental [4] and CFD axial velocity data for di /Hi  = 15 and Kfi  = 6.6

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Figure 8

Comparison between CFD axial velocity profile data for orthotropic and isotropic fill resistance; square and rounded (ri /di  = 0.02) inlets; Kfi  = 6.8/6.6; and di /Hi  = 15

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Figure 9

Visually observed flow patterns at the tower inlet for orthotropic fill resistance

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Figure 10

CFD pathline flow patterns at the tower inlet (di /Hi  = 10) for orthotropic fill resistance

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Figure 11

Effects of different variables on the inlet loss coefficient and effective diameter for square inlets and orthotropic fill resistance

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Figure 12

Effects of different variables on the inlet loss coefficient for square inlets and isotropic fill

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Figure 13

Effects of different variables on the inlet loss coefficient for rounded inlets (ri /di  = 0.02) and orthotropic fill resistance

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Figure 14

Effects of different variables on the inlet loss coefficient for rounded inlets (ri /di  = 0.02) and isotropic fill resistance

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Figure 15

Effects of a protruding platform above the air inlet on the inlet loss coefficient and effective diameter for orthotropic fill resistance

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Figure 16

CFD pathline flow patterns and vector diagrams for a square, round and protruding platform inlet for di /Hi  = 10 and Kfi  = 12.2

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Figure 17

Schematic of a NDCT inlet showing the difference between inlet and fill diameter

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