Heat transfer in soils beneath a spreading fire.

摘要

This study was undertaken to examine the heat transfer in soils beneath a spreading fire. In particular the aim was to determine the depth of lethal heat penetration in soils, defined as the depth at which 60{dollar}spcirc{dollar}C is achieved for a particular fire. This investigation was carried out in four specific steps.; In the first step the effect of test fires on the depth of lethal heat penetration was determined experimentally and theoretically. The experimental results indicated that for any fuel bed configuration, as the time of heating and the total heat transferred to the soil increases, the depth of lethal heat penetration increases. Heat flux distribution with time at the soil surface does not have a large influence on the depth of lethal heat penetration; however, the amount and duration of convection heat transfer does have a significant effect on the depth of lethal heat penetration.; In the second step of this investigation fires were set over various types of soils. The physical properties of the soils were determined experimentally. The depth of lethal heat penetration, in general, increased with an increase in thermal conductivity of the soil for a particular test fire, giving a correspondingly higher value for the thermal diffusivity and the quantity {dollar}Ksbrho C{dollar} (since the thermal conductivity varied significantly more than either the bulk density or the heat capacity of the soils).; In the third step the experimental depth of lethal heat penetration in Ottawa sand of various moisture distributions beneath a spreading fire was determined. The results showed that the depth of lethal heat penetration decreased with an increase in soil surface moisture content for a given fire intensity and time of heating. The data indicated that as the fuel loading density increases the depth of lethal heat penetration increases.; The fourth and last step consisted in determining the moisture content below which an organic layer undergoes complete combustion for various fuel loading densities above the layer. The results showed that the organic layer can either reduce or increase the depth of lethal heat penetration. The major variable in determining either a reduction or an increase in this important quantity is the moisture content of the layer; however, the thickness of the layer is also an important variable.; In conclusion, the depth of lethal heat penetration in soils beneath a spreading fire were found to be dependent on the characteristics of the fuel bed, the physical properties and moisture content of the soil, and the properties, moisture and thickness of the organic material located between the surface spreading fire and the soil. (Abstract shortened by UMI.)