NUMERICAL ANALYSIS OF THE THERMAL-HYDRAULIC PERFORMANCE OF A NATURAL DRAFT COOLING TOWER

Abstract

In the present research, a two dimensional mathematical model governing the flow of moist air, and the conservation of energy and mass has been set up for an evaporative natural draft counter flow cooling tower. Based on the latest experimental studies and theoretical analyses, the physical models of the cooling tower have been greatly improved. The new models, which describe the motion, heat and mass transfer process of water drops, take into account of the effect of non-spherical shape on their resistance to the flow and heat and mass transfer properties. Four different models representing typical drag coefficient of water drops have been investigated and their results are compared. A new point thermal-hydraulic model for natural draft cooling towers has also been developed for the computation of the total mass flow rate of the inlet air. The evaporation loss of water is considered by introducing a well-defined evaporation coefficient, which can be easily evaluated. The coupled non-linear governing equations are solved by finite difference method with a region extension technique. Staggered grid system, SIMPLE algorithm, linearization of source terms, under-relaxation, block correction method and other general numerical skills have been adopted in the solution. The convergence process has been explored to obtain the right solution. The effect of the grid density on the solution has been carried out and the proper grid layout is determined for the present simulation. Field test data of the simulated cooling tower has been used to validate the present simulation scheme. The average difference between the predicted and measured outlet water temperature is only 0.26°C for the 15 operating points simulated in this study. Considering the uncertainty of the field test data and the approximation made in the models, this agreement is good. The distributions of the moist air velocity components, density, pressure, enthalpy and moisture content, the water temperature and its mass flux in the fill and rain regions inside the cooling tower have been predicted and carefully analyzed. The global parameters that may be obtained include the average outlet water temperature, the average temperature and humidity of the moist air at the exit, the total water loss due to evaporation and the total heat transferred between the water and air stream. The influences of the ambient dry and wet-bulb temperatures, the relative humidity, and the atmospheric pressure on the cooling tower performance have been investigated. Parametric studies have also been carried out on some important geometric parameters of the cooling tower, which include the tower height, inlet port height, base radius, the position and thickness of the fill packing. Finally, the hourly performance of a natural draft-cooling tower under the ambient condition of Singapore has been predicted. It can be concluded that based on the improved physical and mathematical models and the well-organized numerical scheme, the flow and transport processes inside a large natural draft cooling tower has been successfully simulated and analyzed. The presently developed and validated computer program can be used as a tool for the design optimization of cooling towers and predictions of their performance.