Numerical Investigation of Physicochemical Processes Occurring During Water Evaporation in the Surface Layer Pores of a Forest Combustible Material

Abstract

A numerical investigation of the physicochemical processes occurring during water evaporation from the pores of the surface layer of a forest combustible material has been carried out. The characteristic features of the suppression of the thermal decomposition reaction of a combustible material with water fi lling fullyits pores and formation of a water fi lm over its surface have been determined. The characteristic times of suppression of thermal decomposition reactions under various environmental conditions and the thickness and kinds of forest combustible material (birch leaves, pine and spruce needles, etc.) have been established.Introduction. The results of investigations [1][2][3] show that the heat content of the region of direct gas-phase combustion of typical forest combustible materials (FCM), which may consist of birch leaves, as well as pine and spruce needles, can be more than ten times the heat content of the FCM surface layer heated to high temperatures (>600 K). No more than 3% of water required to suppress the fl ame directly in the zone of gas-phase combustion is needed to reduce the temperature ofpyrolyzing FCM to T d = 400-500 K (the values correspond to the start of thermal decomposition [1-3]). The characteristic dimensions of this gas-phase zone can attain 10-15 m [3], whereas the heated FCM layer thickness under conditions of forest fi res amounts to no more than 0.1 m (usually this value varies within the range from 0.02 to 0.06 m [4,5]). The conditions of suppression (termination) of the thermal decomposition reaction of typical FCMs with their surface layers exposed to water (especially to fi nite-size droplets and fi lms) have inadequately been studied to date. The reason is that the mechanism underlying the suppression of reaction can change substantially when water passes into the FCM pores or spreads over its surface (as deduced from the analysis of the results of numerous investigations, for example [6][7][8], of heat and mass transfer processes occurring in moist wood and FCM). It is of interest to carry out a numerical analysis of the processes of heat transfer, phase transformations, and of chemical reaction (response) on suppression of the FCM thermal decomposition reaction and under different heat transfer conditions at the water-FCM interface (especially during the passage of water with a temperature of T >T d into the FCM pores).The aim of the present work is the numerical investigation of the physicochemical processes that occur during water evaporation in the pores of typical FCMs (birch leaves, pine and spruce needles) and the establishment of the characteristic time of suppression of the FCM thermal decomposition reaction.Formulation of the Problem. In formulating the problem of heat transfer it was assumed that a certain extensive mass of water (a large aggregate of droplets) had passed through the fl ame, with the liquid spreading uniformly over the FCM surface (Fig. 1). We considered three variants of materials: birch leaves, pine and spruce needles. We analyzed two p...

Forest Fuel Drying, Pyrolysis and Ignition Processes during Forest Fire: A Review

“…Furthermore, there is a research direction in the literature devoted to the study of the pyrolysis process during liquid evaporation [129][130][131]. These developments contribute to a deeper understanding of the processes occurring directly during pyrolysis and its suppression.…”

Fire extinction of forest fuels by droplets and water film

“…This knowledge is important for development of an appropriate mathematical models and the new techniques of localization and suppression of forest fires. In particular, the experimental database can be applied for development of the models of the FCM fire extinction under different heat transfer conditions, like described in [5,6].…”

“…This is due to the fact that a water mass (fire-extinguishing liquid) usually covers a quite limited FCM surface area [3]. In addition, recent theoretical studies [5,6] have shown that when large water masses are locally discharged to a fire site, about 90-95% of the liquid is involved in firefighting inefficiently. In this case, almost entire volume of water passes through the flame combustion zone without evaporation.…”

“…Most of the flame energy is absorbed due to the heat effect of water vaporization (2.26 MJ/kg). The flue gas temperature is reduced to 300-500 K relatively to the initial flame temperature (which corresponds to the average fire temperature and is about 1170 K) [5,6]. The heat content of the gas-phase combustion of typical FCM (birch leaves, pine and spruce needles) is ten times greater than the heat content of the FCM superficial layer heated up to high temperatures (over 600 K) [5,6].…”

“…The flue gas temperature is reduced to 300-500 K relatively to the initial flame temperature (which corresponds to the average fire temperature and is about 1170 K) [5,6]. The heat content of the gas-phase combustion of typical FCM (birch leaves, pine and spruce needles) is ten times greater than the heat content of the FCM superficial layer heated up to high temperatures (over 600 K) [5,6]. In order to reduce the temperature of the FCM pyrolysis to 350-500 K (corresponding to the start of thermal decomposition), it is required no more than 3% of water that is needed for suppression of gas-phase combustion [5].…”