## 4 Results and Discussion
### Experimental Results
Figure 12 represents the pressure fluctuation of P4 at liquid accumulation initiation. Compared with initiation movement, the pressure fluctuation of P4 at this moment decreases sharply. At this time, there is no liquid slug formed in the inclined pipe section, but some liquid exists at the bottom of this section, see Figure 6.5 The critical state of slug flow in the pipe is presented in Figure 6.6 The main reason for the decrease of pressure at P4 is that the liquid slug at P3 is blocked, resulting in a small amount of gas flowing up the inclined section, which reduces the pressure. Figure 6.7 The pressure fluctuation at liquid accumulation initiation. Figure 6.8 The pressure fluctuation of P4 at liquid accumulation initiation. Figure 6.9 The liquid accumulation image at P3. The gas continued to flow in so that the liquid accumulated at the elbow was sprayed upward in the form of a slug, and the pressure change of the horizontal pipe is shown in Figure 6.0 The pressures at P1, P2, and P3 are smaller than the corresponding pressure for the liquid accumulation initiation case, with multiple pressure peaks. Compared with P1 and P2, the pressure wave peak of P3 is higher, and so is the frequency. There is a gap at the top of the pipe at P3, the appearance of the gap reduces the pressure in the horizontal section, as highlighted by the red ellipse in Figure 6.1 Compared with the initial blockage, after some liquid slug are discharged, the amount of liquid accumulation in the section decreases, and the blockage height is somewhat reduced. The pressure fluctuation at P4 at the appearance of slug flow is presented in Figure 6.2 With the eruption of slug flow, a large slug occurs in the inclined section, as highlighted by the yellow ellipse in Figure 6.3 Due to a large amount of gas and liquid being mixed in the pipe, and the occurrence of slug flow, the pressure at P4 increases sharply. It can be observed that the pressure fluctuation at P4 shows periodic regularity in Figure 6.4 Compared with the fluctuation amplitude at P1, P2, and P3, the fluctuation amplitude at P4 is larger, and the frequency is smaller. The reason is that the liquid accumulation at the elbow compresses the air and stores a larger pressure to carry a larger gas pressure when it leaves. Thus, the liquid slug rushes into the up-dip pipe. Occupying the pipe flow area in a short time, when compressing the air again, increases the pressure in the pipe. Figure 6.5 The pressure fluctuation of P4 at the eruption of slug flow. Figure 6.6 The liquid accumulation image at P4. Figure 6.7 The pressure fluctuation at the eruption of slug flow. Following slug flow eruption, only a fraction of the slug exits the pipeline while the rest flows back, leading to a subsequent rise in pressure at P1, P2, and P3, as depicted in Figure 6.8 As can be seen from Figures 14 and 18, the pressure fluctuations at P1, P2, and P3 in the eruption of slug flow are weaker than the flow back of slug flow. The liquid slug eruption causes a flow back, leading to accumulation at the elbow, represented by the red ellipse in Figure 6.9 Nonetheless, the level of accumulated liquid is lower compared to its previous state. After part of the slug flows back, the pressure fluctuation at P4 is displayed in Figure 6.0 At this time, the accumulated liquid at the elbow in the pipe is not enough to make the slug rush out of the pipe section. Thus, the regular fluctuation in the pipe section persists, as depicted in Figure 6.1 Since there is still a small liquid slug in the inclined tube, the  
Figure 6.2 The pressure fluctuation at the flow back of slug flow. Figure 6.3 The eruption of slug flow image at P4. Figure 6.4 The pressure fluctuation of P4 at the flow back of slug flow. pressure at P4 is greater than that at P1, P2, and P3. However, compared to the case with the eruption of slug flow, the pressure fluctuating amplitude is lower and the frequency is smaller. Figure 22 shows the liquid slug leaving at P4 and the remaining liquid slug flow backs along the pipe as highlighted by a yellow ellipse.