# Study of the Influence of Heat Flow on the Time to Ignition of Spruce and Beech Wood
###### Abstract  
Achherence to fire safety regulations for wood is one of the most important tasks in its use in structural and architectural applications. This article deals with determining the influence of heat flux on the ignition process of spruce (_Pican abies_ L. Karst.) and beech wood (_Fagus sylvatica_ L.). The heat flux was generated by an electric radiant panel. The analysed parameters included the ignition time of the spruce and beech wood samples, the influence of wood density, and sample moisture, and the course of sample combustion, both with and without flame, was observed. The heat flux was maintained at constant values, depending on the distance of the examined sample from the panel, along with the specific power of the radiation panel. The power of the radiation panel was set to constant values of 5 kW and 10 kW. The samples were placed at distances of 50, 70, 100, 150, and 200 mm from the heat source, and heat fluxes in the range of 13-92 kW\(\cdot\)m\({}^{-2}\) were observed. At a power of 5 kW and a heat flux of 64 kW\(\cdot\)m\({}^{-2}\), neither the sample of beech nor that of spruce wood, placed at the distance of 100 mm from the radiation panel, exhibited flaming combustion. The ignition time for the beech wood was approximately twice that of the spruce wood, likely due to the higher average wood density. It can be stated that wood density, as one of the main factors, significantly influences the ignition phase of burning. The statistical analysis examined variables including wood type, radiant panel output, distance, and heat flux in relation to ignition time. The analysis revealed a significant difference between ignition time and distance (\(p\)-value = 0.0000, H = 37.51583) as well as between ignition time and heat flux (\(p\)-value = 0.0000, H = 37.69726). Similarly, the time to ignition for all tested beech wood samples was longer than for spruce wood. b 2024 14 May 202416 May 2 while gases mixed with air create a flammable mixture. The presence of flames in the gas phase indicates successful ignition and is internally linked to the heat absorbed by solid materials [20; 21]. The flammable mixture can ignite if the fuel-air concentration reaches the appropriate level, along with the required gas temperature. As the surface temperature increases, the charred layer reacts with oxygen from the surrounding environment. The fire continues until the fuel or air supply is exhausted [22; 23]. This fact has been confirmed by [PERSON] et al. [24] and [PERSON] et al. [25]. The initial degradation can be attributed to the loss of moisture content, followed by some degree of dehydration (ca. up to 200 \({}^{\circ}\)C), and the main chain pyrolysis of both the amylose and amylopectin fractions, yielding leucoglucose and various other volatile fragments (CO, CO\({}_{2}\), CH\({}_{4}\)) [25]. During combustion, cellulose, hemicellulose, and lignin, which are the primary components of wood, undergo dehydration, depolymerization, and thermal decomposition [26; 27; 28]. A significant amount of research has been dedicated to identifying the characteristics of the initial stage of a fire to prevent fire disasters and mitigate fire hazards. Numerous experiments and models [29; 30; 31; 32; 33] have been developed to study the pyrolysis and combustion behaviour of wood, taking into account various factors such as external sources of energy, materials, and environmental conditions. The significance of experimental studies lies in understanding how materials behave during fires. Ignition is the ability of a flammable material to ignite under the action of an external thermal initiator and under defined test conditions according to [34; 35]. According to ISO 3261 [36], ignition is the ability of a material to ignite. The process of ignition is characterized by the time to ignition of a sample, which depends on the thermal characteristics of materials, the temperature, the sample's conditions (size, humidity, orientation), and the critical heat flux [37].