Experimental Study of the Effect of Thermally Inert Structures on Fire Spread in Discrete Fuel Arrays

Thermally inert structures (like concrete and cement components) are commonly present in the wildland-urban interface (WUI) and affect the fire spread behavior. In this study, five different plan area fractions (0, 0.04, 0.09, 0.16, and 0.25) and five different spacings (5.9–17.5  cm, with a 2.9  cm increment) of thermally inert structure distributions in discrete fuels were investigated, with each experimental case repeated three times. The results showed that increasing the plan area fraction caused a non-monotonic variation in the rate of fire spread (ROS). As the plan area fraction increased, the ROS changed by 3.8 %, 13.3 %, −7.4 %, and −34.9 %, while the peak heat flux changed by −27.0 %, −38.7 %, −48.1 %, and −76.5 %, respectively. Increasing the spacing from 5.9  cm to 11.7  cm led to a maximum ROS increase of 40.0 %, followed by a decline, whereas the peak heat flux consistently increased by up to 87.8 % as the spacing widened to 17.5  cm. During most of the analyzed preheating stage, the contribution of radiation was higher than that of convection, whereas convective heat transfer exhibited a marked enhancement at ignition. Based on the distribution patterns of thermally inert structures, two semi-empirical models for radiative heat flux and ROS were developed to describe their influence within the experimental range.