Suppression of forest fuel thermolysis by water mist

Abstract

The processes involved in the suppression of thermolysis in typical forest fuel (mixtures of birch leaves, asp branches, spruce needles) have been experimentally researched. The suppression was achieved by exposing forest fuel (FF) to water mist with different drop sizes. The drop radius was adjusted in the range between 50 µm and 500 µm. The spray intensity ranged from 0.01 to 0.065 l/(m2 s). The experiments were performed on a simulated fire source: an ad-hoc cuvette (100 mm in diameter, initial sample thickness 60 mm) where the FF sample was placed. The times necessary for complete FF combustion and the times necessary for suppressing the thermolysis of FF with water mist have been determined. It has been shown that the times of FF thermolysis suppression with fine aerosol are significantly shorter as compared to using a large-drop flow with the density identical to that of fine mist. The dependences between the water evaporation area in the combustion zone and the typical sizes of injected drops at identical spray density have been determined. The relation of energy spent on water evaporation and the heat accumulated in the combustion zone has been determined. The physical principles of a more efficient use of fine water mist for suppressing FF thermolysis (at constant spray density) have been determined. The conditions have been determined, under which a large-droplet mist can be used efficiently in gas-vapor-drop fire suppression technologies.