Abstract: Based on the basic physic laws and the idea of two-zone model of fire，a mathematical model of the downward smoke displacement in fire under the effect of water droplets is constructed by analyzing the interaction between a single droplet and hot air flow and by investigating the impact of water droplets on the smoke layer of fire． Then，the constructed model is compared with the measured data，finding that the error of the constructed model is ±0. 3 m． The modeling results show that (1) increasing the flow rate of nozzles and decreasing the size and spray angle of water droplets can increase the downward smoke displacement; (2) under the same spray condition，the downward smoke displacement is easy to occur in a thin and low-temperature fire smoke layer; (3) when the thick-ness of the smoke layer is less than a critical value，the downward smoke displacement distance will greatly increase;and (4) the operation pressure of the commonly-used sprinkler nozzle with a 12. 7 mm orifice should not be larger than 20 m，so that the impact of the water droplets on the downward smoke displacement can be effectively reduced．

Key words: nozzles, water droplet, fire, smoke, downward displacement