## 4 The Experimental Campaign
This condition affects the estimation of the emissivity and the transmission factor that change progressively throughout the day. Figure 15: Mock-up surface temperature measurement with an IR camera during a run with a 300 kW/m\({}^{2}\) surface loading: (**a**) snapshot during a total LOFA run (valve V-1 at 0%); the arrows indicate the coolant flow direction in the observed channel segments; (**b**) evolution of the averaged surface temperature during an experimental run. In addition, the recordings were disturbed by the local non-uniformities of the heated surface emissivity, such as the spot indicating temperatures above the defined scale (bright gray color) on top of channel 3. These spots became more and more evident as the experiment progressed because of the cooling and heating cycles. Adding up all the associated uncertainties the mean uncertainty of the surface temperature should be around \(\pm 15\)\({}^{\circ}\)C, making this measurement more qualitative then quantitative. Two batches of tests were performed: the first batch at 300 kW/m\({}^{2}\) with a total mass flow rate of 500 g/s (Table 4), and a second batch at 330 kW/m\({}^{2}\) and 650 g/s (Table 5). Because of the similarities found between the two batches, only the results of the tests at 300 kW/m\({}^{2}\) are discussed in the following section. Figure 16 shows the evolution of the flow rates in the two cooling branches during one experimental run, the same one as the one for which the surface temperature shown in Figure 15b is recorded. From Figure 15b as well as from Tables 4 and 5 it can be seen that, for 60% valve opening scenarios, the flow rate in the affected branch dropped to 61% from the steady-state condition and to 52% for a 40% valve opening, correspondingly. While the surface temperature of the mock-up increased, the remaining cooling capabilities of the channel 3 and 4 were sufficient to stabilize the surface temperature below 550 \({}^{\circ}\)C (Figure 15). The same behaviour can also be observed for the coolant temperature evolution presented in Figure 17. These plots were obtained by averaging over the several runs performed with the same parameters. The associated random uncertainties vary from 0.4 \({}^{\circ}\)C to 1.3 \({}^{\circ}\)C for the 60% opening case and from 0.3 \({}^{\circ}\)C to 1.1 \({}^{\circ}\)C, showing good reproducibility of the experiment. The total uncertainty (random and systematic), which is shown in Figure 17, varies from 2 \({}^{\circ}\)C to 2.9 \({}^{\circ}\)C, the maximum being obtained for the channel 3 outlet temperature for both cases. The situation was different once the valve V-1 opening was set at lower values as 40%. When closing the valve to 20%, 10%, or 5% the surface temperature approached 550 \({}^{\circ}\)C and showed no sign of settling at a value below this limit. For the 20% opening, the trend plotted in Figure 18a indicates that the coolant temperature at the outlet of channel 3 and 4 \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Parameter** & **Steady State** & \multicolumn{4}{c}{**Partial LOFA Runs**} & \multicolumn{2}{c}{**Full LOFA**} \\ \hline Valve V-1 opening (\%) & 100 & 60 & 40 & 20 & 10 & 5 & 0 \\ Valve V-2 opening (\%) & 0 & 34 & 44 & 54 & 63 & 64 & 67 \\ LOFA duration (s) & - & 120 & 120 & 23 & 12 & 6 & 3 \\ \hline Flow meter 1 (g/s) & 230 & 140 & 120 & 73 & 53 & 40 & 30 \\ Flow meter 2 (g/s) & 270 & 270 & 270 & 270 & 270 & 270 & 270 \\ \hline Max. outlet temp. (\({}^{\circ}\)C) & 325 & 337 & 340 & 348 & 346 & 343 & 334 \\ Avg. pressure drop—Circuit 1 (mbar) & 1730 & 650 & 490 & 195 & 105 & 80 & 50 \\ Avg. pressure drop—Circuit 2 (mbar) & 2140 & 2140 & 2140 & 2140 & 2140 & 2140 & 2140 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: First batch of tests performed at 300 kW/m\({}^{2}\).