# Induction Heating of a Fluidized Pebble Bed: Numerical and Experimental Analysis
## 3 Experimental Setup  
The objective of the experimental study is on the one hand to verify the performance of the proposed PtH concept for the application in a Brayton battery. On the other hand, to provide the data for the validation of the introduced electrothermal model. The concept verification, model validation and investigation is accompanied with numerical parameter studies presented in Section 4. A testing facility was designed and setup to investigate the PtH concept of inductively heated packed bed in a vertical fluidized column. The experimental test rig consists of a middle-frequency converter, a transformer, an induction heating coil and a flow system for circulating air through the test section. A schematic of the testing facility and the measuring points is shown in Figure 7. The test section assembly is depicted in Figure 6.  
Figure 6: Experimental setup without exterior insulation layers and a glass tube instead of used Al2O3 ceramic tube between induction coil and pebble bed. Figure 7: P&ID flow chart for the experimental test rig based on DIN 19277. The experiment makes contradictory demands on the materials to prevent parasitic heating of the surrounding components. Therefore, only ceramic insulators made of Al2O3 were used inside and outside the test section, see Figure 6. Only the spheres in the test section were made of electrically conductive material to promote heating. Different configurations of sphere diameter and total number of spheres were tested in this setup. The geometries and materials used for individual components of the test section are summarized in Table 2. To prevent an electric current between spheres and to address the assumption made for Equation (13), the spheres were additionally coated in an electrically insulating and HT resistant coating. This experiment is subject to losses which can be categorized into leakage losses, thermal losses, and electrical losses. To minimize leakage, a HT gasket [30] was used to fill in axial gaps in the construction. Additionally, the facility was designed to benefit from additional sealing pressure when heated through thermal expansion of the mid-section in the vertical direction. Thermal losses were minimized by adding two layers of insulation material [26] between the tube walls and the coil as well as between the coil and the environment. Moreover, the temperature increase of the cooling water inside the inductor coil was measured with thermocouples at the inlet and outlet. Finally, the power loss to the coil was evaluated via direct voltage and current measurement at the coil terminals shown in Figure 6. The frequency was returned by the power converter, which is an input parameter for the electromagnetic simulation. Five thermocouples were installed at the outlet of the test section. All the other thermocouples inside the packed bed structure were located according to Figure 7. These were spread out in the radial and vertical directions to capture the distribution of temperature as it varies in the pebble bed. Furthermore, three thermocouples were placed at the outer wall of the facility to determine heat loss to the environment. This measurement is introduced in Table 3, where uncertainties of the measuring instruments are additionally reported. The highest measurement uncertainties were caused by the current measurement of the Rogowski coil (\(\pm\)2.5%) and from the power measurement at the converter (\(\pm\)2.0%). The output error for the measured key performance indicators (KPIs) in Figures 8 and 9 was estimated using the common Gaussian error propagation method.