# Study of the Influence of Heat Flow on the Time to Ignition of Spruce and Beech Wood
## 3 Results and Discussion
Other researchers evaluated the behaviour spruce during and after various thermal loading [13; 61; 62; 63]. For example, [PERSON] et al. [55] assessed the influence of a higher heat flux on spruce wood's (_Picea abies_ L.) behaviour. The heat fluxes of 15, 20, 25, and 30 kW\(\cdot\)m\({}^{-2}\) were used. They determined the fire-technical properties of the wood, such as the mass burning rate, charring thickness, and charring rate, as well as the chemical composition (contents of the extractives, lignin, cellulose, holocellose). The same procedure for evaluating the experiment was followed for the beech wood samples as well. Images of the samples exposed to a 5 kW radiant panel heat are presented in Figure 12, and those exposed to a 10 kW radiant panel heat are shown in Figure 13.  
Figure 6.0 Visual observations of the influence of different heat fluxes on residues of spruce wood samples after burning at a radiant panel power of 10 kW. (**a**) Thermally stressed sample of spruce wood with a heat flux of 90 kW\(\cdot\)m\({}^{-2}\) at 17 s; (**b**) sample exposed to a heat flux of 62 kW\(\cdot\)m\({}^{-2}\), burning with flames; (**c**) residue after non-flaming combustion under a heat flux of 47 kW\(\cdot\)m\({}^{-2}\) for 10 min. Figure 6.1 Visual observations of the influence of different heat fluxes on residues of beech wood samples after burning at a radiant panel power of 5 kW. (**a**) Thermally stressed beech sample with a heat flux of 91 kW\(\cdot\)m\({}^{-2}\), but immediately after its ignition, the flame was extinguished to document the initial phase of flaming combustion; (**b**) beech sample exposed to a heat flux of 74 kW\(\cdot\)m\({}^{-2}\); (**c**) Heat flux of 64 kW\(\cdot\)m\({}^{-2}\) to which the sample was thermally exposed for 10 min. This is the residue after flaming combustion. [PERSON] et al. [63] focused their attention on the influence of size and temperature of the heating on the auto-ignition of beech (fagus sylvatica) and spruce (picea abies) wood cubes. The experiments were conducted in a furnace at five isothermal temperatures (240 \({}^{\circ}\)C, 270 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, 330 \({}^{\circ}\)C, and 360 \({}^{\circ}\)C) with four cube sizes (5 mm, 10 mm, 15 mm, and 20 mm). The point of pyrolysis was identified at 360 \({}^{\circ}\)C. After pyrolysis, a combination of internal heating and heterogenous oxidation reactions on the surface lead to ignition and combustion. [PERSON] et al. [64] carried out an experiment involving three distinct wood species, namely spruce, alder, and beech, which were combined in various arrangements. Various parameters were measured during combustion, namely time to ignition (TTI), mass loss rate (MLR), the heat release rate (HRR), the peak heat release rate (pHRR), the peak average rate of heat emission (MARHE), the effective heat of combustion (EHC), and the average rate of heat emission (ARHE) [50]. The time to ignition (TTI) was consistent, occurring between the first and second minute across all tested wood species and combinations. This result was similar to that in our research, but this study used the cone calorimeter method. The results of the experiments indicate that as the heat flux decreases, the ignition time of the individual samples burning with flames increases. This fact can be explained by the gradual heating of the sample exposed to a constant heat flux. The temperature rises, leading to the release of volatile substances. If the exposure to heat flux continues, the temperature may reach a critical value, or the gas concentration may be high enough to cause ignition. It is evident that the higher the heat flux, the sooner ignition occurs. Despite identical conditions and exposure to heat flux, we have found that the type of wood, characterized by different densities, significantly influences the material's behaviour during the ignition phase and, consequently, it also affects its contribution to fire development.