Ignition of Forest Combustible Materials in a High-Temperature Medium
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Journal of Engineering Physics and Thermophysics, Vol. 93, No. 5, September, 2020
HEAT AND MASS TRANSFER IN COMBUSTION PROCESSES IGNITION OF FOREST COMBUSTIBLE MATERIALS IN A HIGH-TEMPERATURE MEDIUM N. V. Baranovskii and V. A. Kirienko
UDC 536.6
The ignition of typical forest combustible materials (birch leaves) at temperatures characteristic of a crown forest fire was investigated. The dependence of the delay time of ignition of a birch-leaf sheet on the temperature of the surrounding medium was determined, and the mechanism of ignition of leaves in a high-temperature medium was revealed. A mathematical model of classification of forest combustible materials on the basis of experimental data on their properties has been developed. Keywords: ignition, forest combustible material, crown forest fire, ignition delay time, experiment, high-temperature medium, mathematical model. Introduction. Of the forest fires, the crown fire is more dangerous. Crown fires happen predominantly in coniferous forests and mixed forests in which the coniferous trees are prevailing. However, as experiments with birch leaves have shown, even though these leaves have a low heat conductivity, they ignite easily and, therefore, represent a very fire-hazardous material. In a crown forest fire, a combustible is formed by needles, leaves, and thin twigs of trees [2], and their trunks and branches of large diameter do not burn in this fire. Because of this, the front of such a fire, representing a high-temperature medium, moves over the crowns of trees. Individual deciduous trees in a coniferous forest can increase the rate of propagation of a crown fire in it. The problem on the propagation of a crown fire in a mixed forest remains to be solved. Therefore, of importance is the study of the ignition of deciduous trees in a high-temperature medium [3]. In [4], the ignition of dead cluster-pine needles and oak leaves by an external heat source was investigated. Such ignition of forest combustible materials (FCM) is realized most frequently in the case where they are heated by a radiant heat flow or a convective heat flow. Sources of ignition of FCMs can be firebrands and heated particles. In [5], the ignition of bush leaves was investigated depending on their thickness and moisture content. Data on the ignition of living elements of plants are necessary for the simulation of the propagation of a fire from the ground of a forest to the branches of trees or brushwoods in it. The pyrolysis of 23 kinds of FCMs was investigated on [6]. It was established that the leaves of different species differ insignificantly in their combustion properties, and marked differences were detected between the FCMs of different types. The influence of the shape, thickness, and orientation of leaves on the delay time and temperature of their inflammation was determined in [7]. In [8], the ignition of branches of coniferous trees by a fixed heat source was experimentally investigated, and, in [9], an experiment on the ignition of vegetable combustibles under identical c
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