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Liquid nitrogen which is a cryogenic fluid can be used as a fire extinguishing agent. This is because liquid nitrogen vaporizes rapidly under atmospheric condition, which results in cooling of burning materials and its surroundings in firefighting, and also the volume of the vaporized nitrogen becomes approximately 700 times larger than that of its liquid state, which results in reduction of oxygen concentration in air or fuel concentration in combustion zone. Moreover, liquid nitrogen has no water damage and can extinguish fires more cleanly than dry chemical extinguishing agents. However, it is difficult to delivery liquid nitrogen over long distance from the extinguishing equipment to fires through the surrounding air because of its rapid vaporization. If liquid nitrogen is filled into a capsule and the capsule wall suppresses heat transfer from the surroundings to the liquid nitrogen, it will be easy to transport liquid nitrogen to the targeted fire area without rapid vaporization. Moreover, by using the capsule, it may be possible to increase the extinguishing effectiveness of liquid nitrogen. In the present study, in order to clarify the fundamental characteristics of flame extinguishment by using the capsule filled with liquid nitrogen, extinguishing experiments of a methane-air jet diffusion flame have been performed. The spherical hollow ball made of ice was used as the extinguishing capsule and the ice capsule was formed by rotating casting machine. The wall thickness and the outer diameter were 2 mm and 20 mm, respectively. The filling volume of liquid nitrogen was 20 cm3. The ice capsule was dropped freely from the height of 800 mm and impacted on the aluminum plate, in which the round burner to form a jet diffusion flame was embedded. The extinguishing probability was measured by varying the distance from the impact point of the ice capsule and the flame. The extinguishing processes were recorded with high-speed camera. Moreover, for comparison, the same extinguishing experiments have been performed using the ice capsule filled with water. As a result, liquid nitrogen shows the higher extinguishing effectiveness than water because liquid nitrogen droplets released from the ice capsule to the collided horizontal plate can travel over the longer distance due to the Leidenfrost effect and liquid nitrogen can be vaporized rapidly under room temperature condition. The extinguishable range of the ice capsule filled with liquid nitrogen is less than 200 mm.
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