Assessment of Cooling Tower Discharge Recirculation and Dispersion Using CFD Techniques

The performance of cooling towers, whether operating by themselves, or in close vicinity of other cooling towers can be adversely affected by the re-ingestion of the cooling tower discharge into the tower intakes. The recirculation of the discharge from a wet cooling tower raises the wet bulb temperature of the air entering a wet cooling tower. Current design strategies, often account for this discharge re-ingestion issue, through a conservative adjustment to the far field ambient wet bulb temperature to calculate the actual intake wet bulb temperature. Critical applications, such as those related to nuclear safety applications where there is concern about cooling tower performance, may require more accurate and comprehensive assessment of the recirculation and dispersion of cooling tower discharge. Gaussian plume models alone are of limited use when dealing with discharges in the vicinity of large structures. This paper discusses the use of a computational fluid dynamics approach to evaluate worst case discharge recirculation effects in cooling towers. The bounding design values of tower intake wet bulb temperature increase due to recirculation (ingestion of tower’s own discharge), and interference (ingestion of another interfering tower’s discharge), are calculated considering the various conditions of cooling tower operation, ambient temperature, humidity and wind conditions. The RANS CFD model used in the study is evaluated against published experimental data for flow over bluff bodies at high Reynolds numbers, and experimental data on buoyant jets in cross flow.