Indirect evaporative cooling (IEC) is a wellknown sustainable cooling technology that can potentially replace mechanical cooling equipment and operate as an efficient energy recovery unit or standalone cooling system. However, the technology relies heavily on water evaporation in the wet channels to cool the fresh air in the adjacent channel. Although many moisture-absorbing materials have been proposed for IEC to improve the surface wettability, the uneven water membrane coverage caused by inappropriate nozzle parameters is still a great problem that degrades the evaporation. Therefore, it is critical to optimize the nozzle parameters in water spray systems and arrange the nozzles reasonably above the IEC for more efficient evaporation performance. In this research, a novel 3D simulation model describing the vapor and liquid phase distribution from the spray system was developed to allow a comprehensive parametric study based on Computational Fluid Dynamics (CFD). A test rig of an open-circuit wind tunnel was built to measure the actual wetted areas on the surface of a single channel pair for model validation. The results based on the verified simulation model showed that the water membrane coverage ratio could achieve the optimal value of 0.21 as the droplet diameter was 0.05 mm. Furthermore, the wet-bulb efficiency of the IEC with a size of 260 × 400 × 400 mm 3 could reach a maximum value of 52%, supported by two spray nozzles 160 mm apart with a droplet size of 0.05 mm.