Abstract: Residual surface fires and underground smoldering fires after forest fires are the main causes of secondary forest fires. To prevent secondary forest fires, it is necessary to clean up forest fires and smoldering fires. As the best substitute for halon fire extinguishing agent, water mist is widely used in the field of fire protection. In this paper, an impingement split-flow high-pressure water mist nozzle suitable for extinguishing forest fires and smoldering fires is designed. Fluent is used to simulate the nozzle flow area, and the nozzle hole diameter, nozzle length and nozzle spray cone angle are analyzed to spray the water. The influence of water mist speed and spray protection radius is testified by experiments. Fluent simulation results show that the velocity of water mist is significantly affected by the nozzle hole diameter，nozzle length and increases with the increase of the nozzle hole diameter and the decrease of the nozzle length; the spray protection radius is significantly affected by the nozzle hole diameter and the nozzle spray cone angle, and increases with the growth of both. Design-Expert is used to analyze the best parameter combination of water mist nozzles, and the parameters are substituted into Fluent to establish a VOF-to-DPM （volume of fluid-to-discrete phase model） water mist nozzle spray model for simulation. The water mist nozzles with similar parameters are selected for experimental verification, and the spray protection is measured by the experiment. The error between the spray protection radius and the Fluent simulation result is 0.8%, which proves that the optimization and simulation results are reasonable.

Key words: high pressure water mist nozzle, Fluent, smoldering fire, spray protection radius, Design-Expert