COMPARISON OF IGNITION AND COMBUSTION CHARACTERISTICS OF WESTERN RED CEDAR AND ORIENTED STRAND BOARD EXPOSED TO FIREBRAND PILE DEPOSITION

Abstract

Some of the most devastating consequences of the increasing occurrence of large wildfires throughout the world are the acres of land burned and the number of structures lost. Firebrand exposure has been identified as one of the main mechanisms of how wildfires spread as well as an ignition source for structural components. A bench-scale experimental procedure was developed to study the ignition process of Western Red Cedar (WRC) and Oriented Strand Board (OSB), two common materials used in the construction of outdoor decks. To study the combustion process of these materials, they were loaded into a wind tunnel and exposed to a constant wind velocity of 1.4 m s-1, 2.4 m s-1, or 2.7 m s-1 and a glowing firebrand pile coverage density of either 0.06 g cm-2 or 0.16 g cm-2. All tests were also conducted using Kaowool PM, an inert ceramic fiberboard, in order to quantify the heat feedback of the isolated firebrand pile as well as differentiate the contributions of WRC and OSB to the combustion process from that of the firebrand pile. Surface ignitions on the combustible materials were determined visually and characterized by time to ignition after deposition, burn duration, and location of ignition events. Back surface temperature profiles were collected using an infrared camera. Results from gas analyzer measurements were used to compare the combustion characteristics of the WRC, OSB, and Kaowool PM under the same conditions through heat release rate (HRR) and modified combustion efficiency (MCE) profiles. Additional tests were conducted under a single airflow of 2.4 m s-1 and firebrand pile coverage density of 0.16 g cm-2 yet rotated the orientation of firebrand deposition onto the board by 90 degrees, doubling the leading edge length of the firebrand pile. A series of tests also varied the airflow in the tunnel for a comparison between the surface ignition characteristics and the temperature profiles of the firebrand pile between continuous and intermittent wind exposure for a 2.7 m s-1 airflow and a 0.16 g cm-2 firebrand pile coverage density. Results included a higher probability of ignition on WRC than OSB under all continuous wind conditions, higher peak temperatures achieved with an increasing airflow up to 2.4 m s-1, and combination smoldering-flaming mode of combustion for the system, whether that be the firebrand pile alone or the firebrand pile deposited onto WRC or OSB. It was also found that changing the firebrand pile deposition orientation leading edge length by a factor of two doubled the number of surface ignitions observed on both WRC and OSB. Compared to the continuous wind condition, gusting the airflow velocity caused an increase in the number of ignitions by a factor of 14 on WRC and 19 on OSB, yet each saw a decrease in the burn duration by a factor of at least 4.