Study of Water Droplet Behavior in Hot Air Layer in Fire Extinguishment

Evaporation of water droplets while traveling in hot air layer will be studied. The air-droplet system is analyzed by solving the mass, momentum and energy conservation equations for each phase. The droplet phase is described by the Lagrangian approach. Two conditions of air flow in the smoke layer are assumed. Firstly, as commonly used in modeling fire suppression by water spray, the smoke layer is assumed to be quiescent. Secondly, both gas cooling effect and air entrainment in the water spray cone are included. The properties of gas phase related to evaporation are specific heat capacity, thermal conductivity and dynamic viscosity. All these are evaluated by the one-third rule. The Runge-Kutta algorithm is used to solve the ordinary differential equation group for the droplet motion with heat transfer. Droplet positions, velocities, temperatures and diameters are calculated while traveling in the hot air reservoir. The effects of air temperature, water vapor mass fraction, thickness of hot air reservoir, and initial diameter on the droplet behavior are analyzed. The quantity of heat absorbed by a single droplet is calculated. Results are then calculated for a water spray by taking it has many droplets. The cooling effect of the water vapor produced is considered. Water spray consisting of small droplets should absorb more heat while acting on the hot air layer. The ratio of the heat for vaporization to the total heat absorbed by water can go up to 0.9 when all the droplets are evaporated. Limited experimental data are selected to verify the mathematical model. Predicted results are useful for studying fire suppression by water mist system.