# Investigation of Point-Contact Strategies for CFD Simulations of Pebble-Bed Reactor [PERSON]
## 3 Results  
### Case 1: Full Power  
In Case 1, each contact strategy was investigated regarding the full-power condition. Table 4 presents the plQOI computed for each contact strategy, including the following: the pressure drop across the entire bed, \(\Delta P_{bed}\), the maximum velocity magnitude within the bed, \(v_{max,\,fluid}\), the volume-averaged velocity magnitude within the bed, \(v_{avg,\,fluid}\), the maximum pebble temperature, \(T_{max,\,fuel}\), the volume-averaged pebble temperature, \(T_{avg,\,fuel}\), and the volume-averaged fluid temperature within the bed, \(T_{avg,\,fluid}\). Strategy 3a (bridging 10%) was used as the baseline reference against which all the other contact strategies were compared because it had the smallest true porosity error of \(-0.01\%\). Figure 4 presents the pebble temperature, fluid temperature, and fluid velocity fields for the baseline Strategy 3a. In general, the relative agreement of the other contact strategies decreased as the porosity error increased. The hydraulic plQOI were more sensitive than the thermal plQOI. The sensitivity of the contact strategy on the hydraulic plQOI was approximately 10 times greater than the observed differences when compared to the \(\phi_{extrap}\) in the mesh convergence study. The sensitivity of the contact strategy on the thermal plQOI was approximately equal to the observed differences when compared to the \(\phi_{extrap}\) in the mesh convergence study. In Strategy 1, reducing the pebble volume, the Strategy 1a approach of a 0.1% diametral reduction performed the best: less than 1% differences were observed for all the plQOI when compared to the baseline. On the other hand, Strategy 1c, with the error of porosity at 3.264%, had the highest difference in plQOI; it significantly under-predicted the pressure drop and average velocity magnitude at 33.0% and 7.3%, respectively. For Strategy 2, this approach of increasing the pebble diameter was found to overestimate the pressure drop and average velocity magnitude. Only increasing the diameter by 0.1% yielded a small porosity error, but the simulations were not numerically stable unless a significantly finer mesh than required for the other strategies was used. A 1% diametral increase yielded a porosity error of \(-3.28\%\) and large relative differences in pressure drop (21.1%) and average velocity magnitude (5.1%). With respect to Strategy 3, changing the bridge diameter from 10% to 20% of the pebble diameter had a small impact. The largest differences were found to be less than 3.2%. Strategy 4 underestimated the pressure drop and slightly underestimated the average fluid velocity, but it still yielded very similar pebble and fluid temperatures compared to the baseline. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Strategies** & **Porosity Error [\%]** & \(\Delta P\) & \(v_{avg,\,fluid}\) & \(T_{max,\,fuel}\) & \(T_{avg,\,fuel}\) & \(T_{avg,\,fluid}\) \\ \hline
1a—Decrease 0.1\% & 0.3284 & \(-0.3\) & \(-0.4\) & \(-0.1\) & 0.0 & 0.0 \\
1b—Decrease 0.5\% & 2.0051 & \(-\)22.4 & \(-4.3\) & 0.2 & 0.2 & 0.0 \\
1c—Decrease 1\% & 3.2647 & \(-33.0\) & \(-7.3\) & 0.3 & \(-0.1\) & 0.0 \\ \hline
2a—Increase 0.1\% & N/A & N/A & N/A & N/A & N/A & N/A \\
2b—Increase 0.5\% & \(-1.6459\) & 12.4 & 2.8 & 0.2 & 0.0 & 0.0 \\
2c—Increase 1\% & \(-3.2844\) & 21.1 & 5.1 & 0.2 & 0.0 & 0.0 \\ \hline
**3a–Bridge 10\%** & \(-\)**0.0123** & **0.0** & **0.0** & **0.0** & **0.0** & **0.0** \\
3b—Bridge 15\% & \(-\)0.0578 & 1.8 & \(-0.2\) & 0.2 & 0.1 & 0.0 \\
3c—Bridge 20\% & \(-\)0.1850 & 3.1 & 0.5 & 0.6 & 0.3 & 0.0 \\ \hline
4a—Cap 10\% & 0.0309 & \(-\)13.1 & \(-1.9\) & 0.0 & 0.0 & \(-0.1\) \\
4b—Cap 15\% & 0.0597 & \(-\)22.8 & \(-3.6\) & 0.2 & \(-0.4\) & 0.0 \\
4c—Cap 20\% & 0.1869 & \(-\)27.3 & \(-4.4\) & \(-0.4\) & \(-0.3\) & 0.0 \\ \hline GCI (\%)—Bridge 10\% & N/A & 3 & 0.1 & 3.4 & 0.6 & 0.1 \\ \hline \end{tabular}
\end{table} 
Table 4: Relative differences in plQOI computed from various pebble contact strategies for Case 1 (%).