Predicting thermal performance of a closed-wet cooling tower for chilled ceilings

Computational fluid dynamics (CFD) is applied to the prediction of thermal performance of a prototype closed-wet cooling tower for chilled ceilings. The prediction involves the two-phase flow of gas and water droplets. This is based on an earlier investigation by the authors in which CFD prediction was compared with experimental measurement of the thermal performance for a large industrial closed-wet cooling tower. In the present investigation, the accuracy of CFD modelling of the pressure loss for fluid flow over the heat exchanger is assessed for a range of flow velocities applied in closed-wet cooling towers. The predicted pressure loss for single-phase flow of air over the heat exchanger is in good agreement with the empirical equation for tube bundles. One of the advantages of using CFD is that the effect of flow interference on the pressure loss of single- and two-phase flow over the heat exchanger can be predicted. The present work shows that the performance of the prototype cooling tower can be improved by several methods depending on the requirements for energy efficiency and/or minimum use of air or water. The methods include (1) optimising the air flow rate and the mass flow ratio of spray water to supply air so as to increase the effectiveness of evaporative cooling, (2) installing baffles under the heat exchanger to distribute air and water flows uniformly, and (3) reducing the transverse pitch (tube centre-to-centre distance normal to the flow direction) of the existing heat exchanger to enhance heat transfer between tubes and air.