## 2 Experimental Setup  
A custom-build test rig for uniaxial compression was set up with the objective of investigating the mechanical properties of a specific packed bed at elevated temperatures and to validate the particle discrete simulation model. The uniaxial compression test rig enabled the compression of a packed bed in a cylindrical container in axial direction, while both the axial pressure and the axial strain of the packed bed were recorded. The stainless steel-made container (Figure 2) measured 0.5 meters in diameter as well as 0.5 meters in height, which resulted in a maximum filling capacity of 0.098 m\({}^{3}\). Heating wires with a total of 7.5 kW of heating power were wrapped around the outer container wall and allowed for packed bed temperatures up to 600 \({}^{\circ}\)C. The container was thermally isolated by using insulation wool of 0.2 m thickness for the cylindrical outer wall. Additionally, two refractory bricks were mounted on top of the cover plate for thermal isolation and to protect the load sensor from high temperatures. A hydraulic press with a potential of 160 tons of pressure weight was used for mechanical loading. The exerted forces were recorded by a load censor produced by HBM. The axial displacement of bed  
Figure 2: Cylindrical pressure cell with heating wires (left) and filled in particles (right). height was measured by using four laser sensors (manufactured by WAYCON) positioned on top of the cover plate and periodically spread at a 90-degree offset (see Figure 3). Technical details on the measurement devices can be found in Table 1. In order to prevent impairment of measurement precision during operation at elevated temperatures, the measuring instruments were thermally decoupled by ceramic encapsulation. In addition, there was an active cooling system installed on top of the cover plate to reduce the temperature at the contact point of the punching tool and the cover plate. The test procedure starts with the filling of the selected packed bed into the pressure cell, followed by the positioning of the cover plate on top of the packed bed. For testing at elevated temperatures, the packed bed is heated by the heating wires until isothermal conditions inside of the packed bed are reached. Afterwards, pressure is applied stepwise on the cover plate until a predefined strain is reached. After that, the pressure is kept at a constant level for 15 seconds. The test procedure continues with the reversed sequence as the packed bed is decompressed by subsequently releasing the pressure from the cover plate. These compression-decompression cycles are repeated six times and the test run is completed. After each completed test run, the pressure cell is refilled with a new packed bed batch for a new test run. The substantial material properties of the investigated packed bed are listed in Table 2.  
The given particle size was chosen in order to be representative of typical particle sizes for large-scale applications in the range of 10 mm to 100 mm, while still ensuring a sufficient number of particles over the length and height of the test rig. The particle size distribution of the utilized pebbles was nearly uniform, with small variances due to the fabrication process. While the Young's modulus of the bulk material significantly changed at elevated temperature, the temperature dependencies of the other material properties can be neglected. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Load Sensor** & **Displacement Sensor** \\ \hline Measurement error & \(\pm\) 0.025 \% & \(\pm\) 0.16 \% \\ Measurement resolution & \(\pm\) 0.579 kN & \(\pm\) 10 \(\upmu\)m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Technical details of measurement devices.