## 4 Results and Discussion  
### Experimental Results  
Pressure fluctuation can be used to predict the initiation mechanism of slug flow. In this study, pressure fluctuation measurement is used to verify the characteristics of slug flow. The flow process of slug flow can be rigorously divided into three distinct stages, viz., the generation of slug flow, the eruption of slug flow, and the flow back of slug flow, as eloquently demonstrated in Figure 6. The initial phase of slug flow generation is also precisely illustrated in Figures 7-14, as shown in green and purple boxes in Figure 6. The subsequent slug eruption process can be visually perceived in Figures 15-18, as displayed in the orange box in Figure 6. Ultimately, the process of slug flow back has been elucidated meticulously in Figures 19-22, as presented in the black box in Figure 6.  
Figures 7 and 8 represent the pressure fluctuation of liquid accumulation initiation movement in the tube. The case corresponds to gas velocity (\(V_{\rm CS}\)) = 5.08 m/s, liquid accumulation of 200 mL, and angle \(\theta\) = 20\({}^{\circ}\). When the gas is introduced, and the liquid accumulation in the pipe begins to move, its pressure fluctuates without any regularity, showing a saw-tooth fluctuation signal, as displayed in Figures 7 and 8. Observing the figures, it can be inferred that the pressure peak at P3 is higher than at P1 and P2. It is attributed to P3 being located at the elbow connection, with the horizontal pipe section on the left and the inclined section on the right, as shown in Figure 9. As gas flows through the pipe, the static liquid accumulation surface experiences a progressive destabilization, leading to small amplitude waves (indicated by the red ellipse in Figure 9). This wave formation leads to a gradual increase in the volume of liquid accumulation at P3. Consequently, the reduced gas flow area results in higher pressure at P3 compared to pressures at P1 and P2. The cause of P1 being slightly larger than P2 is due to P1 being closer to the gas  
Figure 5: Paper Flowchart. Figure 6: The pressure signal at P3 in the experiment. inlet and P2 being farther away, with the same flow cross-sections of the two places, the gas has a slight pressure loss at P2. Comparing Figures 7 and 8, the pressure of P4 is less than P1, P2, and P3. It is because P4 is in the middle of the inclined pipe section, and no liquid slug appeared in this section, as shown in Figure 10. Due to the narrowing of the gas flow section at P3, P4 has a smaller gas volume and a larger cross-section in the pipe than P1 and P2, so its pressure is less than P1 and P2. Figure 11 shows the pressure fluctuations at liquid accumulation initiation. As gas ingress persists, the liquid in the pipe blocks the elbow, causing the pressure at P3 to increase sharply, and then the pressures at P1 and P2 also increase, as shown in Figure 11.  
Figure 8: The pressure fluctuation of P4 at initiation movement. Figure 6.0 The initiation movement image at P4. Figure 6.1 The pressure fluctuation at initiation movement. Figure 6.2 The initiation movement image at P3. It shows that the pressure fluctuation changes of P1, P2, and P3 are similar in Figure 6.3 The cause of the liquid level at the elbow rising and touching the top of the pipe is the inclined pipe blocks the downstream movement of the liquid slug, as can be seen from the red ellipse in Figure 6.4 With a fixed cross-section, the increase of gas velocity leads to increasing pressure, and the blockage at the elbow enhances this pressure rise. However, the pressure loss in the pipeline is also enlarged, which can be inferred by comparing P1 and P2 in Figures 7 and 11. There is an increase in average pressure for liquid accumulate initiation compared to initiation movement. It can be seen from Figures 11 and 12 that with the continuous influx of gas, the amplitude and frequency of pressure fluctuation in the pipeline increase. Similar to liquid accumulate initiation, the fluctuation of initiation movement is also an irregular saw-tooth wave.