A new underground fire-fighting system has been developed on the basis of a gas-liquid mixing-type nozzle which enables liquid nitrogen to form as very fine droplets and to be evaporated in the ventilation atmosphere. By using this system, it has become possible to evaporate a large quantity of liquid nitrogen in short times without any additional heat source near the fire area. Through the model gallery experiments using this system, open fires have been extinguished successfully within the restriction of a ventilation speed of up to 2 m/s. Furthermore, a satisfactory suppression effect has been proved through full-scale open-fire experiments in a timber-lined gallery. Thus, it is concluded that the newly developed fire-fighting system can be put into practice.
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