## 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
Figure 3: Surface temperature distribution for the two channels model at a surface heat flux of 300 kW/m\({}^{2}\): (**a**) co-flow configuration; (**b**) counter-flow configuration. The arrows indicate the coolant flow direction in each channel segment. Figure 4 shows a comparison of the surface temperature profile for the two-channel model (Figure 3b) with the one obtained for a single-channel simulation that has inlet and outlet nozzles included into the model (Figure 1b). From this figure it can be seen that the absence of the nozzles in the model had an impact mostly on the first leg of the flow path, as can be seen in Figure 4. The profile, taken along a path on the loaded surface that follows the coolant flow, had a more pronounced slope when the nozzle was modeled as compared to the other case. This behavior can be explained by the impact of the entrance effects associated with the change in the cross-section between the nozzle zone and the channel on the heat transfer, as the strong flow mixing in that area locally increased the heat transfer coefficient. However, this is true only for the first leg of the flow path; the profiles on the exit leg (after the 180\({}^{\circ}\) turn) showed only minimal differences between the three simulated channels. The same situation can also be observed in Figure 5, which shows the evolution of the temperature, pressure, and velocity along the channel center line starting from the inlet position of each channel. Thus, looking at the coolant temperature evolution along the channels, it can be seen that, for the two-channel model, the reduced coolant mixing at the inlet, due to the absence in the model of the cross-section change associated with the nozzle-channel junction, moved downstream, at which point the helium in center channel started to heat up. The final helium temperature was, however, only marginally lower for the two-channel model as for the one where the nozzles are included. In both models, passing through the U-turn resulted in an increased flow mixing and, consequently, in an increase in the coolant temperature in the central region. This increase was, however, only local, and the flow recovered its stratified temperature profile about two channel diameters downstream from the bend. This is why the central channel temperature had an apparent drop in value after the sharp increase associated with the 180\({}^{\circ}\) turn. When looking at pressure evolution, for the two-channel model, the pressure loss due to the 180\({}^{\circ}\) bend was about 6 kPa, which represents almost half of total calculated pressure loss (14 kPa). In comparison with the one-channel model, it can be seen that the pressure in the center of the channel was lower at about 0.1 MPa when the inlet and outlet nozzles are considered, but there was almost no influence on the fluid velocity. Figure 4: Surface temperature distribution for the counter-flow configuration and comparison with the 1-channel model. Looking at these two figures one can say that, even though the two models show differences, the trends for the temperature, the pressure, and the velocity are very similar. In particular, for the maximum surface temperature, a parameter of primary interest for the present investigation, the difference was below 1%, the one-channel model temperature being slightly higher (464 \({}^{\circ}\)C as compared with 460 \({}^{\circ}\)C). As such, one can conclude that the two-channel model introduces only minor deviations as compared with a more realistic, but more computational demanding model that includes the inlet and outlet nozzles. This relevancy of the simpler model is expected to hold also for the simulation of the LOFA transient of interest, the temperature increase being expected to occur over the channel with no or reduced flow, in the area where the transient is dominated mostly by conduction.