Flame extinction and burning behaviour of timber under varied oxygen concentrations

Abstract

Self-extinction plays a key role in enabling the safe use of timber structures in mid-and high-rise construction, where structural integrity during and after a fire is key. To date, the influence that oxygen concentration may have in the conditions that lead to self-extinction has not been studied in depth. Also, the thermal conditions of timber after self-extinction has occurred have also not been carefully studied. During fires, oxygen concentrations will defer from those during ambient conditions. Within this study, the testing conditions were carefully controlled to investigate the circumstances before and after the occurrence of self-extinction; considering a range of heat exposures and oxygen concentrations in the air surrounding the test sample. A statistically sound number of repeat tests was done to estimate the occurrence of self-extinction. For samples tested in air containing 21% oxygen, self-extinction always occurs for an incident heat flux of 30 kW/m 2 but never occurs for 40 kW/m 2 ; with the critical mass loss rate that leads to self-extinction being 3.65±0.4 g/m 2 s. The study concluded that lower oxygen concentrations result in an increase of the critical mass loss rate leading to self-extinction and an increase of the minimum incident heat flux leading self-extinction.

Burning dynamics and in-depth flame spread of wood cribs in large compartment fires

“…A critical mass flux, , for flaming extinction of softwood is shown for reference. This value was obtained under bench-scale testing conditions of timber blocks and generally ranges within 3.5 to 4.0 g/m -2 .s -1 [65][66][67] across different species of wood and experimental apparatus. Thus, is a useful reference to highlight non-physical values from the experimental data.…”

Understanding flame extinction in timber under external heating using high-activation energy asymptotics

“…Following, the results derived from this model will be used to characterize the flame structure and to determine the conditions under which flame extinction is reached. Finally, the results obtained from the theoretical analysis will be combined with earlier experimental data obtained by Cuevas et al with timber as solid fuel [9].…”

“…This study better characterized the heat input and performed the tests under different oxygen concentrations in the surrounding atmosphere. It was found that timber self-extinguished when the mass-loss rate of the samples decreased below critical values of 3.4±0.4, 3.7±0.6, and 3.8±0.6 g/m 2 s, when tested under atmospheres featuring 21%, 19%, and 17% oxygen content in volume, respectively [9]. The discrepancy in the results obtained from these studies is caused by the variability in species, experimental set-ups, and degree of control of the boundary conditions [15].…”

“…Nevertheless, most of the research work consists of bench-scale experimental studies. Within this context, the works of Emberley et al [6], Bartlett et al [7], Crielaard et al [8], and more recently, Cuevas et al [9] have delivered critical conditions for self-extinction. These conditions have either been characterized as a minimum heat input or as a critical mass-loss rate.…”

Fire performance and regulatory considerations with modern methods of construction

“…ETH Zurich in Switzerland (e.g Frangi et al 2008;Klippel et al 2014;Hozjan et al 2019; . Voulpiotis et al 2021), and the University of Queensland in Australia (e.g Emberley et al 2017;Cuevas et al 2020;Nothard et al 2022)…”