## 5 Case Study of Firebrand Accumulation  
In this section, a different approach is addressed to evaluate the firebrands action, in which a wood corner wall that has its surface in contact with firebrands is used (Figure 20). In this case, the standard fire design curves are also used [52], namely, ISO 834, external fire curve (EFC) [52], and a standard firebrand accumulation curve (FAC) [38]. The numerical model is a 2D wall with dimension of 100 cm \(\times\) 100 cm with 15 cm of thickness, using the properties with temperature dependence (thermal conductivity, specific heat, and density) from Eurocode 5:1-2 [58]. The \"jump\" in the specific heat from 99-120 \({}^{\circ}\)C was replaced into a linear variation (Figure 21) and introduced the latent heat (\(L=\Delta H_{\mathit{vap}}u\)), using the heat of vaporization (\(\Delta H_{\mathit{vap}}\)) of 2.260 kJ/kg with different percentages (0%, 3%, 6%, 12%, and 20%) of moisture content (\(u\)) in order to make sure that the heat from water vaporization is taken into account during the non-linear finite element analysis [59,60]. It is worth noting that for 0% of initial moisture content the specific heat used was presented in Figure 21a, which is the default values of Eurocode 5:1-2 [58], and no latent heat was used, but this does not mean that there is no moisture content because the Eurocode has already taken this into account with the peak \"jump\" in the specific heat. In addition, the latent heat with different moisture content helps to understand the thermal behaviour for different scenarios, and all percentages addressed are related to structural design (service classes 1 and 2). While employing modified specific heat to account for phase changes during the heating of a material is a valid approach, it can potentially give rise to numerical challenges, especially when dealing with scenarios characterised by a narrow phase change temperature range and high moisture content [60]. The ABAQUS [51] software tackles this issue by using latent heat property, instead of relying solely on modified specific heat. In cases where substantial latent heat effects are present, a significant disparity in the tangent terms of the internal energy arises between the interior and exterior of the phase change temperature range. To counteract this, in the initial iterations of the solution for a given time step, the tangent term is transformed from its original form to a secant approximation [51]. In light of these considerations, the decision was made to adopt the more robust latent heat approach. Figure 20: Example of a wood dwelling porch with firebrand accumulation, units in m.  
Figure 23: Fire design curves. Figure 24: (**a**) Mesh adopted, (**b**) prescribed boundary heat flux for the firebrands, (**c**) prescribed boundary flux using ambient temperature with \(h_{f}=35\) W/ (m\({}^{2}\)K), (**d**) prescribed boundary flux using ambient temperature with \(h_{f}=4\) W/ (m\({}^{2}\)K), and (**e**) adiabatic surfaces. Figure 26: Numerical maximum surface temperatures using (**a**) Wood Pallet, (**b**) Wood Pallet/Charcoal, (**c**) Pine, and (**d**) Eucalyptus with different moisture contents. Figure 25: Temperature distribution in the model for wood pallet at 5000 s (in Kelvin). The joint observation of these plots leads to the following comments: (i) the firebrand maximum temperature measured was from the wood pallet firebrands, and wood pallet/charcoal firebrands reached around 300 \({}^{\circ}\)C for all results at approximately 1000 s; (ii) the initial moisture content affects considerable the maximum temperature and behaviour, which using \(u=0\%\), shows that the temperature distribution stays almost in the middle of the results from 6% and 12% (see Figure 27d), meaning that the specific heat in Eurocode admits an initial moisture content around 9% [58].