###### Abstract  
The gas-liquid Venturi injector has been widely applied in industrial production due to its advantages of high entrainment and low energy consumption. In this study, Computational Fluid Dynamics (CFD) was employed to investigate the effect of the gas-liquid interface structure within the mixing section on entrainment behavior by varying the geometry of the mixing section during gas-liquid coaxial flow. The simulation results indicate that along the jet direction, the gas-liquid interface generally transitions from a smooth cylindrical shape to a lobed structure in the mixing section. Surface waves mainly appear in the lobed region. Furthermore, lobed and surface wave structures reduce pressure loss and enhance entrainment. Additionally, the study found that longer mixing sections enhance entrainment under low flow resistance. This study provides valuable insights for achieving high jet entrainment and offers supplementary research on gas-liquid interface structures in jets constrained by solid boundaries. V 2  
chamber (<5%), and the length of the mixing tube has a more complex impact on gas entrainment behavior, with no clear maximum value. Due to the significant differences in research equipment sizes and nozzle outlet velocities, the results of the geometric parameters often vary widely. Therefore, the impact of the injector structure, particularly the configuration of the mixing section, on the entrainment rate warrants further investigation. Furthermore, the injector entrainment process can also be viewed as a jet entrainment process with a solid boundary. The behavior of surface waves at the gas-liquid interface has a significant effect on gas-liquid transfer [15; 16; 17]. Most studies have focused on the formation of surface waves. [PERSON] et al. [18; 19; 20] investigated the stability theory of jets in viscous fluids, suggesting that surface waves form at the gas-liquid interface due to disturbances from the surrounding gas. [PERSON] et al. [21; 22] found that jet surfaces are inherently unstable regardless of how disturbances at the gas-liquid interface evolve. [PERSON] et al. [23; 24] studied the transition of jets from absolute to convective instability under varying conditions, identifying a critical Weber number. [PERSON] et al. [25; 26; 27] experimentally determined that the condition for jet breakup within an injector is a gas-to-liquid volume ratio of less than 1.3 to 2, and the range of jet breakup increases with the rise in back pressure. However, little research has focused on the relationship between injector structure, surface wave characteristics, and entrainment. In this study, the Volume of Fluid (VOF) model is employed to simulate the gas-liquid coaxial flow, with a particular focus on the interaction between the two phases at their interface. The VOF model is well-suited for capturing clearly defined gas-liquid boundaries, which are typically observed in the annular flow regime [28; 29]. Although, in practical scenarios, the liquid phase may partially penetrate the gas core, forming an annular-mist flow regime, the jet in this study does not undergo significant breakup and predominantly retains the distinct gas-liquid interface characteristic of the annular flow. Other multiphase models, such as the Mixture or Eulerian-Eulerian models, are generally more applicable to flows involving substantial jet breakup and intense turbulence, which fall outside the scope of this investigation. A primary limitation of the VOF model is its sensitivity to grid resolution. Achieving the level of resolution required for highly accurate predictions of interface dynamics would necessitate a computationally expensive grid, comprising tens of millions of cells, which is not feasible with current computational resources. Nevertheless, the VOF model has demonstrated robust performance in simulating phenomena such as jet breakup [30], collision [31], and cavitation [32], even with moderate grid resolutions.