## 3 Results and Discussions
### Flow Pattern Evolutions in Evaporator and Condenser
#### 3.3.2 Flow Pattern Evolutions in the Condenser  
As depicted in Figure 7, the visualization results indicate that the condensation of vapor in the condenser is the major cause of the breakup of continuous vapor core in the OHPs. It can be seen  
Figure 6: Evolution of flow pattern in the evaporators of the OHPs with different inner diameters (\(Q\sim\)100 W). that, because of the continuous condensation of vapor, the vapor bonds emerge at several locations connecting the non-broken long vapor slug or the continuous vapor core of the annular flow (see long slugs A and B in Figure 7a and slug A\({}^{\prime}\) in Figure 7b). As the condensation further proceeds, the long vapor slugs are broken up into several shorter ones via the breakup of vapor bonds, and the produced shorter ones continue shrinking (see slugs C and D produced by slug A as well as slugs E and F produced by slug B in Figure 7a, slug B\({}^{\prime}\) and bubbles C\({}^{\prime}\), D\({}^{\prime}\) produced by slug A\({}^{\prime}\) in Figure 7b). Based on the comparison among the breakup of long vapor slugs in the OHPs with different inner diameters, it is indicated that the reduction in the inner diameter of OHP results in the increasing role of surface tension, which enhances the capillary instability and thus induces more irregular fluctuation on the vapor-liquid interface. Therefore, as indicated in Figure 7a, there are more collapses on long slugs in the OHP with \(D_{\rm i}\) = 1 mm, forming more short slugs than that under \(D_{\rm i}\) = 2 mm, 3 mm. Figure 7: Evolution of flow pattern in the vertical tube at the condenser of the OHPs with different inner diameters: (**a**) snapshots of flow pattern evolution under \(D_{\rm i}\) = 1 mm; (**b**) snapshots of flow pattern evolution under \(D_{\rm i}\) = 2 mm; and (**c**) snapshots of flow pattern evolution under \(D_{\rm i}\) = 3 mm. Additionally, to further clarify the influence of inner diameter on the vapor-liquid two-phase distributions, Figure 8 compares the distribution of dimensionless bubble/slug length (\(L/D_{\rm i}\)) at the condensers of the OHPs with different \(D_{\rm i}\) under S-O and B-O motions of working fluid at \(Q\) = 90 W, where \(L\) is the real length of bubble/slug (see Figure 5). Herein, the percentage of bubble/slug length, \(P_{i}\), is introduced to quantitatively represent the length distribution of bubbles/slugs  
\[P_{j}=\frac{n_{j}}{n_{\rm t}}\times 100\%, \tag{4}\]  
where \(n_{j}\) is the bubble/slug number within a certain range of dimensionless size, and \(n_{\rm t}\) is the total number of bubbles/slugs in a statistic duration of 10 s. As shown, the proportion of dispersed bubble (\(L/D_{\rm i}<1\)) increases as the tube diameter increases, implying that the nucleate boiling at the evaporator with bigger inner diameter produces more dispersed bubbles flowing into the condenser and thus increases their proportion there. Compared with dispersed bubbles, as the inner diameter increases, the proportion of short slugs (\(1\leq L/D_{\rm i}<10\)) experiences a decrease. As mentioned above, this is mainly attributed to the drop of capillary instability in the OHP, which reduces the probability of breakup from the long slugs to the shorter ones. Moreover, this decreasing capillary instability with increasing inner diameter also induces the larger proportion of long slugs (\(100\leq L/D_{\rm i}\)) under \(D_{\rm i}\) = 2 mm than that at \(D_{\rm i}\) = 1 mm. On the other hand, enlarging the inner diameter of the OHP also amplifies the absolute distance between vapor slugs, which decreases the probability of coalescences among vapor slugs into long vapor slugs, resulting in smaller proportion of long slugs (\(100\leq L/D_{\rm i}\)) under \(D_{\rm i}\) = 3 mm than that at \(D_{\rm i}\) = 2 mm.