## 2 Previous Experimental Campaign for Firebrand Accumulation  
All experiments described here were conducted at the laboratory of the research institute CERIS-IST in Portugal [38]. The experimental setup, the wood specimens analysed, and the measured temperatures are described below. In order to simulate the firebrands, five different types of wood were characterised, which are the most common in Portugal: Eucalyptus, Pine, Branches, Wood Pallets, and Charcoal. All specimens were kept in a low-humidity room during the summer to achieve a low moisture content. In this context, the spot fire formation due to heat transfer by firebrand accumulation is a complex process that depends on several factors. These factors include the size of the firebrands, their state (temperature, smoldering, glowing, or flaming), the properties of the contact surface (temperature, density, porosity, and moisture content), as well as environmental conditions (temperature, humidity, and wind velocity). A standard firebrand geometry of 30 mm \(\times\) 30 mm \(\times\) 5 mm was chosen, with the exception of branches that were assigned a diameter of 6 mm and length of 30 mm (Figure 2). It is crucial to clearly characterise the firebrand because their geometry defines the behaviour of the heat transfer distribution on the contact surface. Similarly, it specifies the amount of energy available in the build-up [49]. Additionally, cases where wood types were mixed with charcoal were considered because, in a wildfire, the vegetation layer is composed of different fuel elements, so it is necessary to understand the thermal behaviour of unburned wood with already-burned wood (charcoal). The experimental procedure was followed according to [37], using 200 g of wood in each test. For the sake of correctly collecting the temperature of the contact surface, the first step was to keep all wood specimens in a grill until they reached the pyrolysis state and  
Figure 2: Standard geometry for specimens [38]. then carefully transfer them into the metal plates, in which thermocouples were installed. A 150 mm diameter basket was used to control the deposition area because other diameters had already been experimented with [37], and this diameter better quantified the accumulation shape. Regarding the steel plates, three layers of 5 mm each were used, whose properties at elevated temperatures are well-known. Each plate was instrumented with three thermocouples welded (RS PRO Type K Thermocouple, 2 m length and 1/0.3 mm diameter) in order to record the heat flux due to firebrand accumulation on the surface (direct contact with the firebrands) and its transmission to the bottom plates. The parameters adopted for temperature data acquisition were a frequency of 1 Hz and 90 min of test duration. The thermocouples were placed in the centre of the steel plate, with a small distance between them. The experimental setup is displayed in Figure 3.  
Figure 4 presents the maximum temperature over time for all types of wood used, including a combination of woods as it occurs in a real wildfire, and it also displays the average maximum temperature of all specimens for thermocouples on the top side of the top steel plate, which was in direct contact with the firebrands. Figure 4: Maximum temperatures for the specimens. Figure 3: Schematic of the experimental setup [38]. The maximum temperature was in the range from 123 \({}^{\circ}\)C to 432 \({}^{\circ}\)C. The average temperature is not admissible to design a fireproof structure because there is variability in the experimental values correlated with the wood utilised (see Figure 4). The authors did an extensive review of firebrands, gathering information about the firebrand's internal temperature, surface temperature, firebrand radius, firebrand weight, heat flux, and duration, which can be consulted in [38].