The head-on-collision coalescence dynamics for droplets on surfaces with different wettabilities was numerically investigated by using the method of coupled level set and volume of fluid, and a high-speed video experiment was used to validate the simulation results. The simulated coalescence behavior of droplets is consistent with the experiment results. We compared droplets with different Weber numbers (3.67 ≤ We ≤ 50) coalescing on solid surfaces of different wettabilities and contact angles (80° ≤ α ≤ 160°). The result is a regional division diagram that relates Weber number and contact angle based on different coalescence phenomena. The factors causing droplet deposition, air entrapment, bounce, and partial bounce during collision coalescence are described based on an analysis of interactions involving inertial forces, surface tension, and wall adhesion forces. Furthermore, the effect of Weber number and contact angle on droplet coalescence behavior is elaborated by analyzing the relationship between coalescence time and wettability radius under different Weber numbers and contact angles. Finally, through an energy analysis, we explain the maximum spreading radius and oscillation of droplets with different Weber numbers on solid surfaces with different wettabilities and in the process of head-on-collision coalescence.

De-oiling hydrocyclones are extensively applied in oilfield wastewater purification for fluid separation, because of their efficient and rapid processing performance. However, it is difficult for a hydrocyclone to separate tiny oil droplets owing to the inadequate centrifugal force. A type of coalescence hydrocyclone (CHC) is designed based on the principle of centrifugal separation and hydraulic droplet collision coalescence. The structure, coalescence theory, and separation principle of the CHC are introduced. Based on the Euler–Euler method, the population balance model (PBM) is applied to a continuous equation to simulate the distributions of the velocities, oil volume fraction, and oil droplet size in the CHC. The simulation method is verified by conducting particle image velocimetry tests in a polymethyl methacrylate hydrocyclone. The results indicate that the CHC can improve the separation performance by enlarging the oil droplet size by collision coalescence before entering the hydrocyclone. The results of separation efficiency experiment indicate that the CHC can not only improve the oil–water separation efficiency but also clearly enhance the processing capability. The optimal inlet flow rate is 5.5 m³·h^-1 and the split ratio is 25%. The feasibility of the structure design and the reliability of the separation performance are validated by experiments.

The dynamic behavior of compound droplets, which are made up of a millimeter-sized particle and distilled water, impacting substrates of different wettabilities is investigated via high-speed photography. The effects of the size of the particle within the compound droplet, substrate contact angle, and impact height on the deformation of the droplets and the characteristics of the impact are analyzed. It is found that the collisions of compound droplets with substrates can be classified into four categories based on the observed experimental phenomena that occur during the impact. These categories are referred to as adhesion collision, rebound collision, daughter-droplet collision (or partial rebound collision), and breakup collision. We consider both the impact of water droplets and compound droplets (with one of two different-sized particles) on substrates of different wettabilities. The effects of inertia, surface tension, and adhesion between the substrate and the liquid droplet, and adhesion between the particle and the liquid droplet are considered to explain the different collision phenomena of compound droplets and reveal the evolution mechanism of the droplet morphologies in the experiments. Furthermore, the effects of the height from which the droplet is released and the contact angle of the substrate (i.e., its wettability) on the maximum spreading diameter and maximum jet height of the droplet are presented quantitatively. The effect of the size of the particle within the compound droplet and the substrate contact angle on the dynamic behavior of the compound droplet subject to impact with the substrate is also described.

Directional transportation of droplets plays a crucial role in the fields of anti-fog, anti-icing, material transportation, and several other applications. Herein, the total directional transport of droplets on a surface with controllable wettability was numerically investigated by the level set method coupled with the volume of fluid method. The accuracy of the numerical simulation results was validated via high-speed photography experiments. The directional migration and morphological evolution mechanisms of droplets impacting on the superhydrophobic–hydrophobic interface were revealed. Moreover, the effects of the impacting position on the directional migration velocity, rebound height, and transport distance of droplets were systematically analyzed. Theoretical models were derived for predicting the transport distance and rebound height of droplets. The simulation results reveal that, for a droplet impacting on the superhydrophobic and hydrophobic interface, four stages exist, i.e., spreading, contraction, rebound, and directional migration, which differ from those for a droplet impacting on a normal surface. It is thus deduced that the adhesion length is a significant factor that affects the directional migration parameters. Moreover, there exists an optimal adhesion length for the impacting droplet, under which the transport distance can be maximized. The maximum transport distance of the droplet under the optimal adhesion length is 12 mm. Furthermore, the values predicted by using the theoretical models agree well with the actual ones, proving the feasibility of the prediction models. The results contribute to the fundamental theory of droplet directional migration and are valuable for related engineering applications.

Variable-diameter pipes are extensively used in the transportation of oil–water two-phase flow in long-distance pipelines. However, the lack of flow field characteristics and deformation of discrete-phase oil droplets in variable-diameter pipes makes them inefficient for efficient transportation. The variations of pressure, velocity, vorticity, and turbulent kinetic energy of the flow field in a sudden contraction and expansion pipe were studied using numerical simulation and experiment. Additionally, the effect of flow field characteristics on the movement and deformation of discrete oil droplets was investigated. The results showed that the larger the variable-diameter ratio, the greater the pressure drop in the sudden expansion region, and the turbulent kinetic energy decreased, while the sudden expanding shoulder vortex increased. Moreover, for the same droplet size and maximum inlet flow rate, the maximum deformation of an oil droplet gradually increased with the variable-diameter ratio. Meanwhile, with the same pipe diameter, the larger the inlet flow rate, the larger the maximum deformation of oil droplets in the thin pipe segment, the more discrete phase model particles generated in the pipe, and the smaller the critical breaking value. Furthermore, the diameter of discrete-phase oil droplets in the same pipe diameter did not affect the turbulent kinetic energy in the pipe. The reported results provide excellent theoretical guidance for improving the sequential transportation of oil products in pipe.

Investigating the motion of discrete oil droplets in a rotating flow field can provide a theoretical basis for optimizing the flow field and structural parameters of hydrocyclones and centrifugal separation equipment. In the present work, the particle image velocimetry (PIV) method was applied to study the velocity distribution of a three-dimensional axial-rotor-driven rotating flow field and the influence of the velocity distribution of different rotor speeds on the flow field. The radial migration of oil droplets with different particle sizes in the rotating flow field was visually analyzed using high-speed video (HSV). The results showed that the oil droplets with the same radial position had diameters of 2.677 and 4.391 mm, whereas the movement times to the axis were 0.902 and 0.752 s. The larger the oil droplet size, the shorter the time to move to the axial center of the rotating flow field. The radial velocities of oil droplets with diameters of 2.677 and 2.714 mm were 0.0221 and 0.02 m·s⁻¹, respectively. In addition, a mathematical expression was established between the radial migration time and the oil droplet size in the rotating flow field. The accuracy of the proposed expression was verified using experiments.