Fire extinction of forest fuels by droplets and water film

Abstract

The paper presents the experimental results about fire extinction in the model fire sites of forest combustible materials. We use the high-speed video recording tools during the experiments, as well as the system of multiphase media diagnostics based on panoramic optical flow visualization techniques. The presented experiments prove that it is possible to extinguish the forest combustible material (FCM) by small amount of water. The study considers several ways to stop FCM thermal decomposition: by individual large water droplets (1-3 mm), by an aerosol flow (the droplet size is from 0.05 mm to 0.12 mm), or through water film formation on the FCM surface. Typical durations of the FCM thermal decomposition and time of fire suppression are determined for various conditions of interaction with water. The experimental results identify which amount of water is enough to extinguish the FCM by different ways of water transfer to the reacting surface layer. Furthermore, it is estimated how the component composition and the properties of the tested FCM mixtures effects onto the characteristics of the investigated processes. The residual fraction of the FCM was evaluated by comparison of initial and final (after extinguishing) mass of the sample.

Features of propagation of droplets of water and special water-based compositions in a sample of forest fuel material

“…In the early works (in particular, [7][8][9][10][11][12]) the authors have shown results of experimental and theoretical research of suppression of burning of typical forest fuel materials (FFM) of drops, films, aerosols, unsprayed water arrays (including with the use of specialized fire extinguishing additives). The main focus of the research during 2014-2016 was on the creation of technology for extinguishing forest fires by droplet flows, distributed in time and space.…”

“…Such flows can be created with specialized polydisperse spraying of water into the combustion zone from different heights relative to the surface of the forest. It has been found [7][8][9][10][11][12] that effective conditions for localizing the forest material combustion become possible with temperature decrease in the gas phase above the material surface as well as with blocking the oxidizer access to the combustion zone and the outflow of thermal decomposition products from the sample material into the gas phase. These conditions may be fulfilled by controlling the completeness of evaporation of the extinguishing liquid in the gas phase and on the surface of the pyrolyzing material.…”

“…The average size and concentration of droplets reaching the surface of pyrolyzing materials are predicted using data [9,10]. These data were used in experiments [11,12] to establish the characteristics of combustion localization. According to the results of studies [7][8][9][10][11][12], it was concluded that the optimal consumption of fire extinguishing fluid in the combustion zone with the provision of conditions for complete suppression of pyrolysis of forest materials in real practice is possible with the use of a group (at least 5-8) of aircraft and helicopters, simultaneously carrying at least 30-50 tons of liquid.…”

“…Ensuring such conditions for many of the emergency services difficult. Therefore, the rational stage of development the results of research [7][8][9][10][11][12] has been the development of theories on the creation of effective protective strips that prevent the spread of fronts of pyrolysis and combustion. These stripes can be generated when the moisture of forest litter.…”

Droplet evaporation on a heated structured wall

“…The growth of the droplet diameter of easily volatile liquids leads to significant intensification of evaporation due to steam convection [30]. Gas-droplet flows are widely used for fire extinction and in solving environmental problems [31][32][33]. The measurement technique of the thermal imager is presented in [34].…”