| In the context of petroleum extraction and transportation,the phenomenon of entrainment between oil and water phases is commonly encountered.Existing research predominantly focuses on the investigation of intermixing phenomena in conventional horizontal or inclined pipelines.However,quantitatively analyzing the mechanisms of entrainment between oil and water phases remains a significant challenge due to its inherent randomness and uncertainty.Moreover,there is a noticeable gap in research regarding quantitative aspects,such as the characterization of two-phase flow fields and the development of numerical models for phase entrainment.This thesis aims to deepen the understanding of the mechanisms governing entrainment in oil-water two-phase flow by building upon experiments and theories.It also leverages state-of-the-art experimental facilities and employs computational fluid dynamics(CFD)numerical simulation techniques.Firstly,a small-scale experimental loop system for oil-water two-phase flow is employed.This system introduces disturbances to the oil-water interface by means of small cylinder,enabling the study the oil-water two-phase flow pattern and pressure drop under different flow conditions.Building upon the geometric characteristics in a bipolar coordinate system,a model for predicting the flow field and pressure drop of single-pressure stratified flow is established.The accuracy of this prediction model is meticulously validated using ultrasonic detection equipment and fluid mechanics turbulent theory.Different turbulence models are systematically compared to provide method foundations for the initial setup of numerical simulations for stratified flow.Secondly,based on the interface capturing method of the open-source computational fluid dynamics software Open FOAM,the factors influencing the numerical model of surface tension were investigated.Addressing the previous oversight of quantitatively analyzing the factors affecting the surface tension numerical model,on the foundation of Open FOAM’s algebraic Volume of Fluid(VOF)interface capturing method framework,an surface tension numerical model was developed,coupling different interface normal vectors and Heaviside functions.A unified discrete format and solving method were employed.Through simulations encompassing classic multiphase flow kinematics and dynamics,it was observed that the surface tension model coupled with interface normal vectors processed through the Levet-set method and a linear Heaviside function yielded better predictive accuracy.This conclusion provides model support for post-processing flow field data obtained through particle velocimetry observations.Thirdly,experiments are conducted coupling high-speed cameras and particle velocimetry measurements.These experiments investigate the flow field of single-phase pipe flow around a cylinder and the kinematics and dynamics of the droplet detachment process from the oil-water interface in two-phase pipe flow around a cylinder.Notably,it was observed that the shedding frequency of vortices in single-phase pipe flow exhibited a clear correlation with the Reynolds number.Furthermore,it distinctly established that the interface height is a primary factor influencing the development of interface waves leading to droplet generation in the excitation of oil-water two-phase flow around bluff body.The research on the flow field of oil-water two-phase cylinder flow reveals the similar frequency between interface waves and vortex shedding.Based on the observed flow field evolution in particle image velocimetry,combined with results from the surface tension numerical model,a force balance analysis of the droplet detachment process is conducted to explore the mechanisms of oil-water flow entrainment.Finally,with the help of the open-source CFD software Open FOAM,a three dimensional physical model of horizontal pipe with the same dimensions as the experiments is established.Based on the established stratified flow field and pressure drop prediction model,and the development patterns of interface waves,boundary conditions at the inlet are set to study the influence of different properties and flow conditions on oil-water twophase entrainment.The research indicates that lower oil-water interface heights,along with the cylinder placed in the higher-density and higher-viscosity phase,favor the mixing of oil-water two-phase flow.Interface height and density emerge as primary factors affecting the amplitude of interface waves,while viscosity influences the magnitude of the two-phase velocity difference.The phenomenon of oil-water droplet mixing is collectively determined by the velocity difference between the two phases and the amplitude of interface waves.In summary,this research delves into the mechanisms of oil-water two-phase flow entrainment through a combination of experiments and numerical simulations.It provides valuable insights into the factors influencing surface tension modeling and the dynamics of phase entrainment in oil-water flow,which can contribute to solving engineering problems in the design and operation management of mixed-transport pipelines. |
