Fire spread characteristics of dry–wet aged fir under environmental pressure

Abstract

Ancient buildings hold significant historical value. Over time, they are exposed to the natural environment, during which the wooden materials that constitute these structures can age, affecting their fire safety. To address this, this paper employs a research method combining experimental analysis and theoretical analysis, focusing on the commonly used fir wood in ancient building construction. By conducting dry‒wet ageing experiments to prepare samples of varying ageing degrees, various experimental methods, including industrial analysis, elemental analysis, X-ray diffraction, scanning electron microscopy, and thermal property testing, are employed to investigate the fundamental physicochemical properties of fir wood at different ageing levels. An experimental system for measuring fire spread parameters is independently constructed to analyze the influence of environmental pressure on the fire spread process of aged wood. The more the process deepens, the deeper the amount of ageing, the more water and volatile matter in the fir wood, the decrease, content fixed ile. The carbon increases, ash content first rises and then falls, and the content of C, H, and O elements changes slightly. The surface color gradually darkens, and the density linearly decreases; the crystallinity of cellulose decreases, cell walls thin out, intercellular distances shorten, and cell structures deform and rupture, leading to surface warping, cracking, and deformation. The time it takes for local high temperatures to spread to low-temperature areas increases, with thermal conductivity becoming dominant, enhancing heat transfer capabilities. The reduction in pressure leads to an intensified phenomenon of flame body fragmentation and jumping, causing inconsistent flame spread across the wood boundary, elongation and widening of the flame body, a gradual decrease in the trend of flame height and width changes, relatively longer pyrolysis zone lengths, and a narrowing of the peak temperature width of gas–solid phase in aged fir. The fire spread rate of fir initially increases and then decreases as environmental pressure decreases, and first increases, then decreases, and finally increases again with increasing ageing degree. Under low pressure, the fire spread rate of aged fir continues to increase, while under high pressure, the oxygen concentration produced by fir combustion decreases, leading to incomplete combustion processes and a reduction in the fire spread rate of aged fir.