Behavior of Downslope Fire Spread Through a Pine Needle Fuel Bed

Abstract

The downslope fire spread is one of the typical wildfire spread phenomena but has attracted less attention during the past wildfire research. In our prior work (Fire Technology, 2018, https://doi.org/10.1007/s10694-018-0740-0), downslope fire spread over thermally thin fuel bed was investigated. However, if the fuel bed was thermally thick, the combustion interface inside the fuel bed would be different and therefore could affect the heat transfer process. This work investigated the downslope fire spread over thermally thick fuel bed. The experiments were conducted by using pine needle fuel beds with 1.2 kg/m2 fuel load and the slopes of −30°, −20°, −10°, 0°, and 10°. The combustion interface shape was identified from the temperature profiles of five vertical thermocouples inside the fuel bed and found to tilt backward from the vertical direction of the fuel bed for all slopes, indicating the fuel bed was thermally thick. Experimental results showed that the rate of spread (ROS) decreased firstly and then increased from 10° to −30°, same as that reported in the literature and in our prior work for thermally thin fuel bed. Based on experimental observations, a two-dimensional model of energy balance valid for thermally thick fuel bed was developed. The prediction revealed that the fuel is heated mainly by flame radiation throughout the whole process but the combustion zone radiation plays more important role in the near flame region, which is similar with that reported for thermally thin fuel bed in our prior work.