Study of Droplet Vaporization of Water Spray in Building Fire Suppression

摘要

Evaporation of water droplets while traveling in hot air layer is studied for simulating water spray in building fire suppression. 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. The gas phase is assumed to be quiescent, which is commonly used in modeling fire suppression by a water spray. The Runge-Kutta algorithm is used to solve the ordinary differential equation group for predicting 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 behaviors 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. The analytical results show that water spray consisting of small droplets should absorb more heat while acting on the hot air layer. In addition, 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. Results are useful for studying fire suppression by water mist system.