Abstract
An investigation on the extinction with water sprays of fires of liquids burning in cylindrical vessels has been carried out. The blow-off tests by the wind sent from the right above position of burning liquids have also been carried out to clarify the effect of the air-flow in the spray on extinction. Eleven vessels of various dimensions and sixteen different liquids were used. Sprays were produced from six swirl nozzles and two impinging jet nozzles. Through these tests, the following two mechanisms are suggested to account for the blow-off :
1) dilution of the flammable vapor from the liquid to a concentration lower than that required to sustain flame ;
2) cooling the liquid surface when the stable flame as shown in Fig. 9 is formed.
As the mechanism of extinction by means of water sprays are considered the following factors in addition to the above-mentioned mechanisms of blow-off by the entrained air :
3) dilution of the vapor layer by the formation of steam at the flames, vapor zone, vessel wall and liquid ;
4) cooling the flames, vapor zone, vessel wall and liquid by the formation of steam and by spray drops
5) for the water-soluble liquid, dilution of the surface layer to the concentration where the liquid is no longer flammable.
The extinction will not occur by any single factor of those mentioned above but by the combination of some of them.
When the preburn time is short and the depth h of the liquid surface below the edge of the vessel wall is large, the stable flame which will not easily be blown off is formed with such liquids as gas oil, ethanol, dioxane etc., whereas, at long preburn times and small h, the vapor zone becomes thick and the fire can be extinguished in a very short time (within 5 sec), by applying the wind faster than the critical speed, due to the dilution of flammable vapor. The fires of such liquids as gasoline, benzol etc., the vapor layers of which are thick from the start of ignition, are likely to be extinguished quite rapidly by the dilution of the vapor zone above the critical wind speeds which are greater than those of the former liquids. The fire is extinguished at the lower entrained air velocity with fine sprays than with coarse sprays. The extinction time for the water-insoluble liquid and the average concentration at the moment of extinction for the water-soluble liquid are mainly dependent upon the drop size and the entrained air velocity.
