Parametric study on spray cooling system for optimising nozzle design with pre-cooling application in natural draft dry cooling towers

• Results from parametric nozzle design optimisation to maximise spray systems performance.
• Insight into effects of droplet velocity and spray pattern on spray cooling performance.
• Identification of spray characteristics required to attain full evaporation.

Pre-cooling of inlet air with water spray is proposed for performance enhancement of natural draft dry cooling towers (NDDCTs) during high ambient temperature periods. Previous experiments showed promising results on cooling enhancement using spray cooling. Spray nozzles characteristics are an essential element for designing spray cooling systems. A parametric analysis is conducted on the effect of different spray characteristics parameters and various hollow-cone droplet size distribution patterns on a spray cooling system performance for nozzle design optimisation. A validated Eulerian–Lagrangian 3-D model with a novel nozzle representation method for hollow-cone nozzles was used to perform the analysis. The results can help prepare design principles of effective spray nozzles for spray cooling systems. The results showed a clear trend of increasing spray cooling efficiency as droplet velocity increases due to better spray dispersion. 15% improvement in cooling performance was achieved by increasing droplet velocity from 20 to 80 m/s. A spray nozzle with high injection velocity, small droplet size distribution, large cone angle and a droplet size pattern of mean droplet size increasing towards the spray periphery was found to be the best for sprays with Dv90 ≤ 50 μm. Complete evaporation was successively achieved within 5 m distance using a spray with droplet size distribution of Dv90 = 20 μm and droplet velocity of 120 m/s at an air velocity of 1 m/s.