ANALYSIS OF THE PROCESS OF EXTINGUISHING OF A FLAME OF FLAMMABLE LIQUIDS BY DISPERSE EXTINGUISHING AGENTS AND LOW EXPANSION FOAMJournal: Pozharovzryvobezopastnost/Fire and Explosion Safety (Vol.25, No. 2)
Publication Date: 2016-02-25
Ability of fire extinguishing agents to absorb half of the heat flow escaped while burning, that can lead to flame extinction, raises doubts in calculation of the thermal balance of a powder and, further, of sprayed water. It turned out that the balance of heat has a big advantage towards heat escaped during flame combustion. Amount of powder or sprayed water become insufficient to absorb half of the heat flow of flame. This work contains data and results of complex experimental and theoretical researches of the process of extinguishing of the flame of hydrocarbons by various types of fire extinguishing agents, including sprayed water, fire extinguishing powder, reactive freon and low expansion foam. On the basis of experimental researches of the process of extinguishing by means of streams of dispersed extinguishing agent and low expansion foam it is offered and substantiated the "local" mechanism of suppression of a flame of liquids by extinguishing agents. Existence of the minimum on curves of dependence of specific consumption on flow rate in the area of optimal flow rate of extinguishing agent is experimentally shown and theoretically substantiated. It is also offered two-stage mechanism of suppression. The first stage consists in interaction of the stream of extinguishing agent with part of a flame jet which is in contact with front of extinguishing agent. The flow of extinguishing agent interacts only with a small (local) area of the flame. Fire extinguishing agent allows not only to absorb half of the heat from flame jet in a local part, but also to cool it practically up to the temperature close to water boiling point. It leads to termination of burn directly on the front of movement of a fire extinguishing stream. Such "local" mechanism of interaction of sprayed water stream leads to evaporation of larger or smaller part of sprayed stream depending on its dispersion degree. The second stage of suppression process provides spreading of water vapor, as a layer of certain thickness, over the burning surface. The higher dispersion degree of sprayed water, the larger volume of water vapor and higher it's covering rate of burning surface. Such extinguishing mechanism is also implemented in case of application of sprayed freon, sprayed powder, solid aerosol and low expansion foam. Standard equations for calculation of time and specific consumption of different types of extinguishing agents during suppression of flammable liquids are proposed. Satisfactory congruence of experimental results with data obtained by calculation using formulas which correspond to the "local" mechanism of suppression of a flame by fire extinguishing substances is shown.
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