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Research Papers

Computational Models for Predicting Cooling Tower Fill Performance in Cross-Counterflow Configuration

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

D. G. Kröger

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

1

Corresponding author.

J. Thermal Sci. Eng. Appl 4(2), 021003 (Apr 16, 2012) (9 pages) doi:10.1115/1.4006028 History: Received July 04, 2011; Revised November 18, 2011; Published April 16, 2012; Online April 16, 2012

In cooling towers packed with trickle or splash fills, which have anisotropic flow resistance, the air flow through the fill is oblique or in cross-counterflow to the water flow, particularly at the cooling tower inlet when the fill loss coefficient is small or when the fill hangs down into the air inlet region. This results in that the fill Merkel number or transfer characteristic for cross-counter flow is between that of purely counter- and crossflow fills. When using CFD to model natural draught wet-cooling tower performance for isotropic fill resistance, two- or three-dimensional models are therefore required to determine fill performance. In this paper, the governing fundamental partial differential equations are derived in cylindrical and Cartesian coordinates to determine the cooling water temperature, water evaporation rate, air temperature, and air humidity ratio in two-dimensional cross-counterflow fills for both saturated and supersaturated air. To solve these equations, a relation is proposed to determine Merkel numbers for oblique air flows by linear interpolation and extrapolation of purely cross- and counterflow Merkel numbers in terms of the air flow angle. This model is compared to analytical Merkel numbers obtained for different air flow angles using a single drop trajectory model. A linear upwind computational model and an Eulerian FLUENT ® model are developed to evaluate fill performance characteristics from test data and to model fill performance in cooling towers, respectively. The results of these two models are compared and verified with a FLUENT Euler–Lagrange model, showing minor deviations.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Air inlet of a natural draft wet-cooling tower packed with splash grids

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Figure 2

Elementary control volume in an axisymmetrical circular cooling tower

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Figure 3

Vertical section through the elementary control volume of a cross-counterflow fill region in a circular cooling tower

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Figure 4

Control volume of a cross-counterflow fill region in a rectangular cooling tower per unit width

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Figure 5

Example of a 3 × 3 cell computational grid for cross-counterflow fill

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Figure 6

Computational domain used to compare the different models (dimensions in millimeters)

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Figure 7

Contour plots for an air flow angle of ϕ = 45 deg for the domain depicted in Fig. 6

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