The investigation of droplet impingement and liquid layer breaking behavior in swirling flow fields is of utmost importance. The intricate two-phase flow occurring in separators is a typical example of droplet and liquid film motion in swirling flow fields, which includes droplet impact on the wall, splashing, and entrainment of interface waves. Currently, the behavioral traits of droplets and liquid films in swirling flow fields, as well as the kinetic mechanisms governing these phenomena, remain unclear, necessitating further research.

This study aims to investigate the behavioral characteristics of droplets and liquid films in gas-liquid separators under two-phase swirling flow conditions using a combination of theoretical analysis, numerical simulation, and mechanism experiments. The research entails the establishment of a separation efficiency model for gas-liquid separators based on the bidirectional vortex model and secondary droplet entrainment phenomena, such as droplet splash and wave shear entrainment. The study is divided into three main aspects, which are discussed in detail in the following sections.

(1) By visualizing droplet impact on the wall and LBM numerical simulation of droplet impact on liquid film, the behavior characteristics of droplet impact on the wall and liquid film in a swirling flow field were obtained. A comprehensive experimental platform was built to visualize the droplet behavior in a swirling flow field, and tests were conducted to obtain the droplet impact wall in the working condition range of 2.7 m·s^-1<V_{g, in}<6.5m·s^-1. Four typical asymmetric spreading patterns exist: downward sliding spreading, horizontal shuttle sliding breaking, upward sliding breaking, and adhesion breaking [1]. The droplet impact drywall splash is not easy to occur and can be neglected. A semi-elliptical-hemispherical crown structure is established for asymmetric droplet spreading, and droplet energy conversion and dissipation are analyzed to obtain the maximum spreading diameter of the droplet impacting wall in a swirling flow field. At the same time, the numerical simulation model of LBM is established. The calculation is simplified to a single-liquid-phase calculation by using the free surface energy to simulate the action of the two phases in the gas-liquid interface, and adding a centrifugal force external force model is established to realize the simulation of swirling flow, high gas-liquid density ratio, and large scale droplet impact. The predicted droplet impact behavior on the drywall surface is in good agreement with the experiment, with an average error of 4.3% in the transverse spreading factor. The model prediction found that droplet impact liquid film behavior in the swirling flow field mainly includes droplet deposition, crown splashing, and prompt splashing. An increase in temperature will cause a more significant droplet splash. Further, the splash threshold and splash entrainment rate prediction relations for droplets impacting liquid film in a swirling flow field were obtained.

(2) The critical wave characteristics of liquid film instability in the swirling flow field and the law of droplet entrainment onset were obtained by theoretical force balance analysis. The Kelvin-Helmholtz instability analysis of the gas-liquid interface wave in the circumferential and axial directions in the swirling flow field was carried out to obtain the critical wavelengths of the liquid film in the axial and circumferential directions. The ellipsoidal crown crest model was developed for the interfacial waves to carry out the force analysis, and the predicted relationship between the shear droplet entrainment onset and the entrainment rate was obtained. Sensitivity analysis of operating and structural parameters found that flow rate, temperature, and swirl angle had strong effects on interfacial wave characteristics and droplet entrainment. As the two-phase flow rate increases, the liquid film instability increases and the shear wave height ratio increases, causing the droplet entrainment rate to increase. When the operating temperature increases, the circumferential and axial critical wavelengths decrease, and the surface tension decreases, which leads to an increase in droplet entrainment rate. With the increase of swirl angle, the circumferential wavelength increases, the droplet entrainment onset point shows the change law of increasing and then decreasing, and the required critical shear force increases and then decreases, so the droplet entrainment first decreases and then increases. The optimal swirling vane angle is between 40º-60º to minimize droplet entrainment rate.

(3) The present study aimed to establish a separation efficiency model for an axial-flow swirl separator by considering the effects of bidirectional vortex velocity field and secondary droplet entraining rate. To this end, the Bragg-Hawthorne equation is formulated using the convective function, and the theoretical analysis is conducted to obtain the bidirectional vortex gas flow field in the swirling flow regime. Furthermore, the gas phase field generated from the theoretical study in conjunction with the DPM (discrete particle model) is used to evaluate the droplet trajectories in the swirling field. The separation efficiency model is established by considering the droplet impacting liquid film splash and liquid film wave shear entrainment effects. The model accuracy is evaluated by comparing the coupled bidirectional vortex model with the separation efficiency model with secondary droplet entrainment rate and experimental data. According to the findings, there is an average inaccuracy of 1.5% between the separation efficiency model coupled bidirectional vortex model and secondary droplet entrainment rate and the experiment. Therefore, the proposed model offers improved prediction accuracy and an expanded range of applicability.

In conclusion, investigating droplet impingement and liquid film fragmentation in swirling flow fields holds significant academic importance for comprehending gas-liquid flow separation efficiency. The development of a separation efficiency model, which incorporates droplet dynamics, wave interface instability, and vortex motion theory, provides a theoretical framework for swirl vane separator optimize, and the effective and economical operation of nuclear power plants.