The Effect of Water Droplets on Premixed Flames
It is well known that water is an effective agent for fire extinguishing due to thermal effects related to the heat of vaporization. Specifically, the appropriately chosen water spraying can increase the flame suppression efficiency, which depends on the size of the droplets. For example, large particles can pass through the flame reaction zone with partial evaporation, and this effect leads to the increase of the water amount required for extinguishment. At the same time, small droplets can evaporate before the flame front and not create significant heat loss from the reaction zone required for flame extinction.
The aim of this work was the theoretical study of the effects of droplet sizes and concentration on burning velocity and extinction of premixed flames. The mathematical model used in this paper is an extension of the classical theory of laminar flame to the case of premixed flame with water particles. The model assumes that all droplets have the same size. It was assumed that droplets begin to evaporate when the temperature of the mixture attains the boiling point. Coefficient of heat losses spent on evaporation of the liquid depends on the size and concentration of droplets in the mixture. Depending on the value of heat losses and initial characteristics of liquid phase in fresh mixture, droplets can completely vaporize either before or behind the flame front. In this paper we take into account both cases as well as the transition between them.
In the case when droplets completely vaporize before flame front, using a partition into four areas: the first - fresh mixture heating zone from an initial to the boiling temperature; the second - the zone from the boiling temperature to the complete evaporation of the droplets; the third - the zone from the complete evaporation of the droplets to the flame front; the fourth - the zone of the combustion products.
In the case where the drops do not completely evaporate, flying via flame front, using the following partition into four areas: the first - fresh mixture heating zone from an initial to the boiling temperature; the second - the zone from the boiling temperature to the flame front; the third - the zone from the flame front to the complete evaporation of the droplets; the fourth - the zone of the combustion products without droplets.
The dependencies of the flame propagation velocity and the flammability limits on the properties of initial mixture with water droplets were obtained. The similarity of analyzed regimes of flame propagation with nonadiabatic flame propagation in narrow channel and with the flame stabilized in flat porous burner was discussed.