# Experimental Characterization of Thermal Conductivity with a New Compact Hot-Box Prototype
## 2 Materials and Methods
### Experimental Cell Architecture  
A layered format was used to construct the experimental cell:  
1. The outer layer was composed of plywood with a thickness of 1.5 cm and a thermal conductivity of 0.13 W/m\(\cdot\)K, forming the external enclosure.
2. The intermediate layer was made of expanded polystyrene with a thickness of 20 cm, a density of 20 kg/m\({}^{3}\), and a thermal conductivity of 0.0358 W/m\(\cdot\)K.
3. The inner layer was composed of hardboard sheets with a thickness of 1 cm and a thermal conductivity of 0.10 W/m\(\cdot\)K.  
The external envelope was fully sealed with lateral and top covers that fit together using embedded slots. The geometric dimensions of the cell prototype are listed in Table 1.  
The prototype consists of two chambers, identified as the hot chamber and the cold chamber, with dimensions provided in Table 1. Both chambers were separated by a specimen holder, constructed with an outer layer of 1.5 cm plywood and an interior filled with expanded polystyrene to enhance insulation (Figure 1). Two functions are served by the specimen holder:  
1. The airflow between the hot and cold chambers is separated, ensuring both physical and thermal division.
2. The specimen is held, ensuring direct contact between the circulating air in both chambers and the specimen surfaces (Figure 1).  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Dimensions (H \(\times\) W \(\times\) L)** \\ \hline Experimental cell (Prototype) & 81 cm \(\times\) 61 cm \(\times\) 122 cm \\ Hot chamber & 33 cm \(\times\) 20 cm \(\times\) 61 cm \\ Cold chamber & 26 cm \(\times\) 20 cm \(\times\) 44 cm \\ Specimen carrier & 75 cm \(\times\) 60 cm \(\times\) 10 cm \\ \hline \hline \end{tabular}
\end{table}
Table 1: Geometry of the experimental cell.  
Figure 1: Illustrative diagram of the experimental cell.  
The specimen holder consisted of two separate symmetrical pieces: one acting as the base and the other as the closure. The test specimen was placed in the center of the holder using machine slots in both the base and closure pieces. The holder was designed to accommodate panel specimens with dimensions of 25 cm \(\times\) 25 cm \(\times\) Z cm, where Z can vary up to a maximum thickness of 10 cm.  
As shown in Figure 1, the specimen holder was integrated into the experimental cell enclosure through lateral sliding slots. This embedded coupling ensured a tighter seal and minimized heat losses between the chambers in accordance with the construction recommendations of the standard [9]. To install a specimen in the holder, the base piece was first assembled within the envelope. Then, the specimen was placed in the slot of the base piece. To compensate for possible sealing irregularities between the holder and the specimen, the lateral edges were covered with a thin elastomeric foam layer (3-5 mm thick), preventing potential air leaks between chambers and thermal bridging. Finally, the closure piece was inserted into the envelope to fully separate the chambers.  
During the commissioning process, various insulation thicknesses for the intermediate layer were tested. The optimal thickness was determined using thermographic techniques, identifying thermal bridges and temperature distribution through infrared imaging, as shown in Figure 2. As can be illustrated in Figure 2, the atmosphere operation conditions of the prototype in the laboratory, where \(\epsilon\) is the emissivity, Max, maximum temperature, Min, minimum temperature, Prom, average temperature. Additionally, the experimental cell was equipped with four wheels to facilitate easy mobility.