Numerical simulation of flow in a natural draft wet cooling tower – The effect of radial thermofluid fields

Abstract

This paper presents a two-dimensional axisymmetric two-phase simulation of the heat and mass transfer inside a natural draft wet cooling tower with particular emphasis on determining the extent of the non-uniformities across the tower. The water droplets in the spray and rain zones are represented with droplet trajectories written in Lagrangian form. The heat and mass transfer in the fill is represented using source terms implemented with Poppe style transfer coefficients. These coefficients are defined through empirical correlations which capture the functional dependence of the system. The model has the capability to represent non-uniformities in fill layout and water distribution, which traditional one-dimensional models are unable to resolve. The results show a largely uniform velocity profile across the tower radius with the greatest non-uniformity occurring at the outer edge of the tower. The water outlet temperature was found to vary by 6 K (∼40%) between the tower centre and exterior under reference tower conditions. This is largely due to the rise in air temperature and humidity through the rain zone. The considerable non-uniformity of heat transfer across the tower is shown to be tied to the performance of the rain zone. The effect of tower inlet height on radial non-uniformity is surprisingly small but the model is very sensitive to changes in water flow rate. At small fill depths the water temperatures entering the rain zone tend to be higher, which in turn slightly increases the heat transfer rate here and the over-all non-uniformity in the tower.