# CFD Analysis of Subcooled Flow Boiling in 4 x 4 Rod Bundle
## 5 Results  
### Results  
Before the results of the six cases are shown, Figure 6 illustrates a stable flow falling by approximately 5.53 kPa of pressure from about 200 kPa from the inlet to the outlet. The pressure distribution is shown in Figure 6.  
For the parameters of the results, the meanings of the volume fraction and IAC are as follows. \[Volume\ fraction=\frac{Volume\ of\ vapor}{Volume} \tag{25}\]  
Figure 5: One-million-grid rod bundle: (**a**) 3-D near the outlet and (**b**) cross-section of the outlet. Figure 6: Pressure distribution of numerical analysis results in Case 1. \[IAC=6\frac{Volume\ fraction}{Interaction\ length\ scale} \tag{26}\]  
In Equation (26), the interaction length scale is defined as the bubble diameter. Figures 7-12 show the comparison of the results of Lines 1-5 in Cases 1-6. In each figure, (a) shows the volume fraction and (b) shows the IAC. In Case 1, it is considered that the VF and IAC were accurately predicted. Cases 2 and 3 (subcooled temperature of 20.2 \({}^{\circ}\)C or 25.2 \({}^{\circ}\)C) have different inlet temperature from Case 1 (subcooled temperature of 22.7 \({}^{\circ}\)C). The axis shows numerical values similar to experimental values, but has smaller values to the rod surface. There was no difficulty in interpreting the same tendency, such as the slight decrease in numerical results near the rod surface, as the experimental results. The result of Case 4 shows accurate prediction. Case 4 also indicates low VF and IAC, the same as the experiments, because of the low heat flux of 148 kw/m\({}^{2}\). In Case 5, which had a high heat flux of 200 kw/m\({}^{2}\), the IAC was low near the rod surface, but it can be said that it measures some reasonable value in the VF. Lastly, Case 6, which had a low flow rate, was under-interpreted near the rod surface except on Line 1, but it shows that it interprets the appropriate values near the axis and across Line 1. Figure 11: Case 5 numerical results of Lines 1–5 compared with experiment results: (**a**) volume fraction and (**b**) interfacial area concentration. Figure 10: Case 4 numerical results of Lines 1–5 compared with experiment results: (**a**) volume fraction and (**b**) interfacial area concentration. Figure 9: Case 3 numerical results of Lines 1–5 compared with experiment results: (**a**) volume fraction and (**b**) interfacial area concentration. Figure 13 shows the numerical analysis results for the volume fraction distribution contour of all cases for the cross-section with a height of 580 mm. In Figure 13c,d, the volume fraction distributions are characterized by a low value between the rod surface and the wall. However, in Figure 13a,b,e,f, the volume fraction distributions are characterized by a high value between the rod surface and the wall. Figure 12: Case 6 numerical results of Lines 1–5 compared with experiment results: (**a**) volume fraction and (**b**) interfacial area concentration. Figure 13: _Cont._  
This can be explained by the velocity and temperature of the liquid. Under the same conditions, Case 4 had the smallest heat flux (148 kW/m\({}^{2}\)) and Case 5 had the largest heat flux (200 kW/m\({}^{2}\)). The detailed analysis of this is shown in Figures 14 and 15, taking Cases 4 and 5 as examples. As shown in Figure 14, the liquid velocity decreased more as it approached the outlet in the wall edge. Therefore, the liquid temperature increased because of heat transfer. However, because Case 4 was a case with very low heat flux, the temperature at the wall edge excluding the rod surface did not rise sufficiently to the boiling point, as shown in \[Volume\ fraction=\frac{Volume\ of\ vapor}{Volume} \tag{25}\]0 Therefore, as shown in Figure 14, air bubbles were not generated at the corners.