The effect of variable-diameter ratio on the deformation characteristics of discrete oil droplet in the sudden contraction and expansion round pipe

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.