Numerically investigating fire suppression mechanisms for the water mist with various droplet sizes through FDS code

With rapid progress in computer capability recently, it becomes feasible to investigate the sophisticated phenomena related to the fire, especially for the interaction of fire and water spray, by way of the computational fluid dynamics (CFD). In this paper, a fire simulation CFD code_FDS is used to numerically investigate the different droplet sizes on the fire suppression/extinguishment mechanisms. The CFD models adopted in the FDS are first assessed against the previous experimental work of Kim and Ryou. The droplet size interested is varied from 100 μm to 1000 μm that is located within the droplet size range for a water mist. Based on the sensitivity simulations with different droplet sizes, the dependency of fire extinguishing time on the discharged droplet size can be obtained. The fire extinguishing time decreases with the decreasing droplet size for a mist with relatively fine droplet size since both the evaporation cooling and the oxygen displacement are the dominant mechanisms of fire suppression. However, this trend is reverse for a mist with larger-size droplets for the sake that the direct cooling of flame is the major suppression mechanism. These conclusions are also confirmed by comparing the simulation distributions of gas temperature, oxygen concentration, and steam concentration after the mist actuation and just before the fire extinguishment.

► CFD simulation is one of the key requirements to implement the NFPA 805 in fire protection program of NPPs.
► Different fire suppression mechanisms for a water mist with various droplet sizes are investigated through FDS code.
► A mist performance is increased with the decreasing droplet size as its size is < 500 micrometer.
► A mist performance is increased with the increasing droplet size as it is large than 500 micrometer.