Analysis of the thermal exposure and ignition propensity of a lignocellulosic building material subjected to a controlled deposition of glowing firebrands

Abstract

A series of experiments was carried out in a bench-scale wind tunnel where firebrand piles of controlled coverage density and geometry were deposited onto a flammable and non-flammable substrate, Western Red Cedar (WRC) and Kaowool PM, respectively. Back substrate temperature, combustion heat release rate, modified combustion efficiency, and gaseous flame ignition and extinction statistics were collected. For a firebrand pile covering 5 × 10 cm2 area of the WRC substrate at 0.16 g cm−2 density, the probability of the substrate ignition increased from 11 to 55% with the air flow velocity increasing from 0.9 to 2.4 m s−1. The probability decreased upon further increase in the air flow. At 2.4 m s−1, the probability of the substrate ignition decreased to 11% when the firebrand coverage density was reduced to 0.06 g cm−2. Qualitatively similar trends were observed for the maximum Kaowool PM back surface temperature and peak heat release rate. The WRC's maximum contribution to the combustion heat release rate was found to be < 12% of the maximum contribution of the firebrand pile. Almost all WRC surface ignitions were observed to occur at the air flow facing edge of the firebrand pile. When the length of the air flow facing edge was increased by a factor of two, the probability of ignition nearly doubled.