CFD analysis of Part-Load operation of an Open-Circuit Induced-Draft cooling tower

Evaporative towers are the most widely used systems for cooling water in industrial processes. These systems, however, are characterized by high energy and water consumptions. Therefore, technologies and operating strategies for optimization of these devices are highly recommended. The heat exchange, which takes place inside an evaporative tower is very complex and not easy to model. In this work, an induced-draft evaporative tower is modelled by means of computational fluid dynamics (CFD), which is relatively new in this field of application. A numerical model, based on the adoption of sub-models for the heat exchange between the water droplets and humid air in the filling region, is developed and validated with experimental data, laying the foundation for further analysis in the future. The achieved results can be considered reliable and accurate as the numerical uncertainty is approximately 4 %. The study aims at evaluating and analyzing the thermo-fluid-dynamic quantities that characterize the heat exchange, with reference to water outlet temperature, cooling capacity, and evaporated water flow rate. In particular, partial load operation are analyzed. The conditions refer to the simultaneous reduction of water and air flow rates. The study shows that both at full load operation and by reducing water and air flow rates to 60 % of the design value while maintaining a constant water/air ratio, the tower process parameters are kept very close to the design ones. Under these conditions, the evaporated water is halved (-0.05 kg/s instead of -0.1 kg/s maintaining air flow at full load). Moreover, by adopting lower water and air flow rates a significant reduction of the electric power requested by the fan, from 15 kW at full load to about 8 kW at part load, is achieved.