Enhancing the vaporization and secondary atomization of two-liquid droplets for fire suppression

Abstract

Large-scale fires of various origins are most often contained and suppressed using fine water spray systems with a droplet size of 50–300 μm. Enhancing the extinguishing performance of these systems is an urgent task as this will reduce the amount of irreversible damage fires cause to forests, buildings, and compartments. Finer droplet size of the extinguishing liquid sprayed directly in the combustion zone prevents droplet entrainment, thus increasing the firefighting efficiency and reducing the extinguishing liquid consumption. However, too little is still known about alternative ways to intensify the vaporization and secondary atomization of fine water sprays in order to disrupt the chemical reactions responsible for combustion. Experimental findings on enhancing the secondary atomization and phase change of sprayed water with added vegetable oils for fire suppression are presented. Experiments on water droplets with sunflower, rapeseed, and tall oils were performed in the ethanol burner flame zone. The radii of two-liquid droplets were 0.85 ± 0.05 mm. The concentration of explosion-triggering additives to water was constant at 10 ± 2 vol.% in all the experiments. The addition of 10 vol.% rapeseed oil to water allows one to reduce the local gas temperature around the droplet up to 250 K in case of micro-explosion of an isolated droplet and up to 450 K in case of micro-explosion of a group of droplets. The experimental findings were processed to obtain approximations of the relationships established in the experiments: micro-explosion delay times, droplet lifetimes, and normalized evaporation surface area after breakup. A physical model is presented describing the processes involved in the experiments. An additional series of experiments involved suppressing a laboratory-scale wood crib fire using water and water/rapeseed oil emulsion to compare the fire suppression times and the extinguishing liquid consumption in both cases. The suppression times and the extinguishing liquid consumption were on average 15% lower when using a water/rapeseed oil emulsion compared with pure water. Practical recommendations are given on how to provide the intense disruption of extinguishing liquids with added vegetable oils to accelerate fire suppression. Two-liquid droplets exploding in the flame zone are established as a possible alternative to water capsules, extinguishing missiles, and cryogenic technologies for suppressing large-scale fires.