## 3 Experimental Set-Up
### Experimental Matrix and Pre-Test Analysis
#### 3.1.1 Preliminary Fluid Dynamics and Thermal Simulation of the Test Mock-Up Analysis  
The main objective of this analysis was to determine if FWMU can provide relevant information on the behavior of a DEMO blanket first wall under a LOFA event as modeled in [14]. Having U-shaped channels (in contrast to one-way cooling channels for DEMO FW) it is important to select the right inlet and outlet configuration that results in a temperature distribution similar to the that of a DEMO blanket under steady state conditions. Additionally, in terms of dynamic behavior, it is important to check if the mock-up can produce similar transients when the flow rate changes as the one obtained in [14]. In order to clarify these two points, using Ansys CFX, a two-channel model was created to simulate the transient behavior of the temperature field in a blanket FW during a LOFA event: one channel operating at its nominal parameters while the other is subject to a partial (50% flow rate drop from the nominal value) or total LOFA event, similarly to the cases investigated in [14]. To reduce the calculation time for transient simulations, a simplified two-channel model was created (Figure 1c). Thus, by omitting the inlet and outlet nozzles, hexahedral structured mesh could be used for the two-channel model. As it will be discussed later on, this simplification has an impact on the flow and heat transfer of the first segment of the channel (in the flow direction), and the simulation of the second (return) segment was close to the results obtained when the inlet and outlet nozzles were included in model. The DEMO blanket FW has a single passage counter-flow cooling scheme, the coolant entering the FW on one side and exiting on the opposite side, in each two adjacent channelshelium flowing in opposite directions. For the FWMU, due to the U-shape of the cooling channels, before moving to further investigations, we needed to see which flow configuration provided results closer to the temperature fields calculated in [14]. To clarify this aspect, two different flow configurations have been investigated: a counter-flow pattern where the flows in any two neighboring cooling channels have opposite directions, and a co-current configuration where the coolant in two adjacent flow paths has the same flow direction. In both cases, periodic boundary conditions were applied on both sides of the model to account for the presence of neighbor channels. The channels were heated by applying a constant heat flux of 300 kW/m\({}^{2}\) on a length of 642 mm on the \"plasma-facing\" side. This length corresponds to the straight channel length between the 90\({}^{\circ}\) inlet and outlet nozzles and the beginning of the 180\({}^{\circ}\) bend. For this comparison, the simulation considered steady-state cooling and heating conditions, the same initial conditions assumed at the initiation of the LOFA event. With the coolant flowing in both channels at its nominal values (50 g/s), the temperature distribution on the heated wall in case of counter-flow arrangement was more uniform than the co-flow case, and the surface temperature range was about 10 \({}^{\circ}\)C less, as can be seen in Figure 3. In addition to that, the counter-current flow configuration generated a surface temperature distribution closer to the one expected in the DEMO blanket [3,14]; therefore, it was selected as the reference cooling scheme for the experimental campaign. For this reason, only the results for this case are presented and discussed in the following. Note that, as compared to the simulations conducted in [14], for these calculations, the loading conditions are somewhat lower (300 kW/m\({}^{2}\) as compared to 500 kW/m\({}^{2}\)). However, the flow rate was also reduced from 66 g/s to 50 g/s, which resulted in a maximum surface temperature that was only 20 \({}^{\circ}\)C lower than in the case modeled in [14].