# Investigation of Point-Contact Strategies for CFD Simulations of Pebble-Bed Reactor [PERSON]
## 3 Results
### Case 2: PLOFC
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Strategy** & \multicolumn{3}{c}{**Case 2a**} & \multicolumn{3}{c}{**Case 2b**} \\  & \(T_{max,fuel}\) & \(T_{avg,full}\) & \(T_{avg,fluid}\) & \(T_{max,fuel}\) & \(T_{avg,fuel}\) & \(T_{avg,fluid}\) \\ \hline
1a—Decrease 0.1\% & 13.8 & 6.5 & \(5.8\) & \(1.4\) & \(0.7\) & \(0.2\) \\
1b—Decrease 0.5\% & 20.2 & 9.7 & \(8.9\) & \(1.6\) & \(0.7\) & \(0.4\) \\
1c—Decrease 1.0\% & 25 & \(12.1\) & \(11.5\) & \(1.5\) & \(0.7\) & \(0.6\) \\ \hline
2a—Increase 0.1\% & N/A & N/A & N/A & N/A & N/A & N/A \\
2b—Increase 0.5\% & \(-1.5\) & \(-0.9\) & \(-1.2\) & \(0\) & \(0\) & \(-0.2\) \\
2c—Increase 1.0\% & \(-4.3\) & \(-2.5\) & \(-3.1\) & \(-0.6\) & \(-0.2\) & \(-0.4\) \\ \hline
**3a–Bridge 10\%** & **0.0** & **0.0** & **0.0** & **0.0** & **0.0** \\
3b—Bridge 15\% & \(-2.9\) & \(-1.4\) & \(-1.2\) & \(-0.7\) & \(-0.3\) & \(-0.1\) \\
3c—Bridge 20\% & \(-4.7\) & \(-2.3\) & \(-2\) & \(-1.3\) & \(-0.6\) & \(-0.2\) \\ \hline
4a—Cap 10\% & 15.6 & \(7.8\) & \(8.1\) & \(0.5\) & \(-0.5\) & \(-1.3\) \\
4b—Cap 15\% & 19.6 & \(9.6\) & \(9.7\) & \(0.6\) & \(-0.5\) & \(-1.3\) \\
4c—Cap 20\% & 22.8 & \(11.1\) & \(11.1\) & \(0.8\) & \(-0.4\) & \(-1.2\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Relative differences in plQOI computed from each pebble contact strategy for Cases 2a and 2b (%). to the baseline bridge 10% d\({}_{p}\) method. The temperatures computed using Strategy 4 varied from 0.4% to 0.8%. Figure 7 presents the Case 2c pebble surface temperatures along the views from the \(xz\)-plane (side view), \(xy\)-plane (top view), and \(xy\)-plane (bottom view) using Strategies 1a, 2b, 3a, and 4a. The pebble surface temperature contours from Strategy 2b were found to be lower than those from the baseline and other strategies. Table 6 highlights that the fluid velocity is the highest for Strategy 2, which explains the lower surface temperature. Nevertheless, the maximum and volume-averaged temperatures are nearly identical across the strategies. Figure 7: Case 2c PLOFC pebble surface temperatures (°C) for (row 1) Strategy 1a (diametral decrease of 0.1%) and (row 2) Strategy 2b (diametral increase of 0.5%), (row 3) Strategy 3a (bridge with 10% d\({}_{p}\)), and (row 4) Strategy 4a (cap with 10% d\({}_{p}\)). From left to right: Pebble surface temperatures along the \(xz\)-plane side view, \(xy\)-plane top view, and \(xy\)-plane bottom view (see Figure 2). Although the temperatures were not sensitive to the contact strategy, much larger variances were observed in the predictions of the maximum velocity--nearly 7.5% using Strategy 4c--and volume-averaged velocity, at 5.1% using Strategy 1c, within the bed. It can be concluded that any modification to the bed that increases or decreases the porosity, even slightly, results in larger differences in fluid velocity. This trend was also observed in the predictions of pressure drops from all the tested strategies. For instance, Strategy 1a (decrease 0.1%) with a 0.33% porosity error and Strategy 3a (decrease 1%) with a 3.26% porosity error yielded differences in pressure drop predictions of 1.2% and 19.1%, respectively, compared to the baseline. Similarly, varying the cap sizes from 10% (0.031% porosity error) to 20% (0.187% porosity error) caused the differences in the pressure drops to increase from 0.52% to 7.2%, respectively.