## 3 Results and Discussion
### Effect of Mixing Section Structure on Gas-Liquid Interface  
Altering the structure of the mixing section inevitably affects the structures of the suction chamber and diffuser section. Therefore, to minimize the impact of the structure of the mixing section, dimensionless parameters \(D_{M}/D_{N}\) and \(L_{M}/D_{M}\) are introduced, and the length and contraction angle of the suction chamber and diffuser section are kept constant.  
Figure 9 presents the three-dimensional instantaneous radius contours of the gas-liquid interface near the mixing section exit under different \(D_{M}/D_{N}\) and \(L_{M}/D_{M}\) to elucidate the effect of the mixing section structure on the gas-liquid interface. It shows that as \(L_{M}/D_{M}\) increases, the gas-liquid interface near the mixing section exit transitions from smooth to slug or punctiform. This transition is due to the friction between the lobed interface and the environment during jet flow, with increased mixing section length enhancing the frictional force between the gas and liquid, thereby reducing the stability of the jet surface. As \(D_{M}/D_{N}\) increases, the gas-liquid interface near the mixing section exit undergoes a transition from smooth to slug and then from slug to punctiform. This transition occurs because surface waves can be induced by shear gas flow parallel to the liquid surface. According to the Kelvin-Helmholtz instability mechanism, surface waves are generated when the velocity difference near the interface exceeds a certain threshold [36]. Moreover, the width of surface waves formed on the concave surface of the lobes increases with the gas-liquid velocity difference. Due to the limitations of the convex surfaces of the lobes, the surface waves become sharper when the width of the waves on the concave surface approaches the width of the convex surfaces. When the width of the surface waves exceeds that of the concave surface, the waves start to appear on the convex surface.  
Figure 10 shows the types of interfaces under different \(D_{M}/D_{N}\) and \(L_{M}/D_{M}\). It indicates that smooth interfaces primarily occur at lower \(D_{M}/D_{N}\) or \(L_{M}/D_{M}\) values. In fact, under these conditions, the friction between the lobed interface and the environment in the mixing section is weak, resulting in a relatively stable jet and a smooth interface surface. When \(D_{M}/D_{N}\leq 2.25\) and \(L_{M}/D_{M}\) reaches a critical value, slug interfaces start to appear, and the critical value decreases as \(D_{M}/D_{N}\) increases. Usually, with an increase in \(D_{M}/D_{N}\), the length of the mixing section increases under the same \(L_{M}/D_{M}\), leading to higher frictional resistance within the mixing section, thereby reducing the stability of the jet surface. Puntiform interfaces only appear when \(D_{M}/D_{N}\geq 2.5\) and \(L_{M}/D_{M}\geq 10\), as the formation of punctiform interfaces requires the surface wave width to exceed the width of the concave surface. Additionally, since the surface area of the convex surfaces is smaller than that of the concave surfaces, a longer mixing section is required to ensure sufficient frictional resistance to generate surface waves.  
Figure 8: The instantaneous gas–liquid interface in different mixing sections: (**a**) \(D_{M}/D_{N}=1.75\), smooth interface; (**b**) \(D_{M}/D_{N}=2.25\), slug interface; and (**c**) \(D_{M}/D_{N}=2.5\), punctiform interface.