Discharge and dispersion analysis of water mist sprays

Water mist is an increasingly popular suppression choice for many industrial and residential applications. At the same time, Computational Fluid Dynamics (CFD) analysis is becoming an accepted method for evaluating fire hazards and fire protection design performance. Water mist system design and development would benefit greatly from CFD based analysis. However, the fire suppression problem is extremely complex concerning many aspects of fluid dynamics, heat transfer, and combustion. For accurate simulations, CFD models describing these physics are required along with establishing appropriate practices for computational problem definition. The current study focuses on atomization and dispersion. Spray measurements, models, and simulations are presented and evaluated for high pressure (above 80 bar) water mist systems. One representative commercially available mist injector (645 J12C B1 by PNR Italia S.r.l.) was selected and drop size measurements were performed in a large quiescent room providing basic discharge characteristics. Furthermore, novel atomization models coupled with the NIST Fire Dynamics Simulator (FDS) were used to predict the initial spray and its subsequent dispersion for a posteriori comparisons with measurements. The primary modeling and measurement challenges related to fire suppression spray dispersion have been identified in this study and are discussed in some detail along with suggested practices for improved simulation of water mist sprays.