EXTINGUISHING OF COMBUSTIBLE LIQUID BY ATOMIZED WATER

The fire control mechanism of combustible liquids (CL) with high flashpoint is determined by physical and chemical composition of liquid as well as dispersion of water drops. Fire extinguishing is possible only if the burning zone has been cooled to the extinction temperature regardless of the cooperation behaviour of sprayed water and combustible liquid. It could be achieved by various methods:
at the expense of evaporation of water drops directly in the burning zone;
reduction of heat emission in the burning zone;
dilution of gas mixture with water vapour;
reduction of the inflow speed of air-vapor mixture by the surface cooling or by dilution of combustible liquid with water. The surface is given a liquid – vapour interface, and the issue of the surface cooling is ambiguous for stating the boundary conditions in a heat exchange problem in the process of extinguishing of CL by cooling its surface. In theory cooling of the thinnest liquid layer bordering the liquid – vapour interface to the flashpoint will be enough to fulfill the conditions of fire extinction. Though, the minimal thickness of this layer is not to be larger than evaporating molecules, and the maximal one is defined by the variation diagram of fundamental system parameter — density. This very parameter indicates liquid – vapour transition. The term “hot boundary liquid layer” should be used instead of the term “liquid – vapour interface”. It is natural that in conditions of heat convection in the boundary layer heated to the higher temperature than the flashpoint, one will manage to cool the surface to the flashpoint only after excessive heat accumulated in hot boundary layer has been removed. The flying speed and the size of water drops are very high; therefore, provided that the thickness of the boundary layer heated to the flashpoint does not exceed 1.0–1.5 mm one can imagine that the process of the surface cooling by water will lead to intensive mixing of liquids in the boundary layer and convective heat and mass exchange between the hot boundary layer and deep layers of the liquid. The latter effect denotes an additional factor that contributes to the cooling of the boundary layer during extinguishing of CL whereas it mixes up with “cold” underlying layers of the liquid. When the CL is being quenched with gross water, the flame torch disappears only after the boundary layer has been cooled to the flashpoint, therefore the moment of fire suppression coincides the moment when the surface reaches this temperature. An average temperature of the boundary layer in steady state of combustion is maintained due to dynamic equilibrium of established heat flows from the flame torch to the surface, from the boundary layer into the depths of the liquid and as a result of fuel evaporation and constant burnout velocity. If dispersed water is supplied to the burning zone, it disturbs the established dynamic equilibrium. The dispersed water dramatically reduces the intensity of a radiant heat flow from the flame torch to the surface, meanwhile the temperature of water drops increseas. When the CL is being quenched with finely dispersed water, its drops do not reach the surface of the liquid, they evaporate in the burning area and pre-flame area. As a result, the temperature of gas phase and the speed of vapour inflow decrease synchronously which respectively reduces the intensity of heat release. In this case reduction of burnout velocity is synchronous to the cooling of the flame torch, and the quantitative criterion of fire extinction is the temperature at which the flow of the evaporating liquid is not concentrated enough to keep burning, i. e. the liquid appears to be in the mixture under lower explosive limit (LEL).