Abstract: Abstract: In order to study the fire extinguishing effect and fire extinguishing mechanism of aqueous film?forming foam(AFFF) against boil?over oil fire, a small?scale boil?over oil fire test device was set up to study the fire extinguishing effect under different application rates of foam solution.The results show that when AFFF foam extinguishing agent is used to extinguish boil?over oil fire, after the foam was applied to the boil?over oil surface at high temperature, the foam disappears rapidly via the action of evaporation, the oil surface presents a violent boiling state, the flame height rises sharply, and the heat radiation intensity increases greatly. The fire extinguishing mechanism of AFFF is cooling effect. Firstly, it directly cools the surface boil?over oil temperature; secondly, it indirectly cools the bottom oil temperature through the thermal convection inside the oil. When the surface oil temperature is cooled below the spontaneous combustion point, the flame goes out. The optimum application rate of foam solution for AFFF to quench boil?over oil fire was determined as 25 L/（min·m2）.

Key words: boil?over oil, aqueous film?forming foam, fire fighting mechanism, cooling, application rate of foam solution