Large Eddy Simulation Study Of Surfactant And Water Ring Transport Flow Characteristics

Nowadays the reliance on oil resources remains strong,heavy oil with abundant reserves will become an important alternative resource in the future.As an efficient and energy-saving transfer technology,water ring transport of heavy oil has attracted more and more attention.It is inevitable to flow through complex local pipes in transportation process,the flow boundary changes,causing local losses,water ring flow structure is prone to destabilization,the stability of water ring loss will greatly reduce the transfer efficiency.Adding some surfactant into water can improve stability of water circulation,inhibit the generation and development of vortexes and reduce local losses.This subject focuses on the flow characteristics of annular flow in local pipe fittings,and studies the influence of surfactants on the flow characteristics and the mechanism of water ring stability.The main contents of this paper include:Oil-water ring flow experimental system is built to carry out experimental research and verify the simulation results.The simulation results agree roughly with the experimental results,which proves that the numerical calculation method selected in this paper is applicable to the research of this subject.LES model can better capture the vortex shape,oil-water annular flow in various local pipe fittings is studied through LES,the results show that oil-water flow distribution of annular flow is mainly affected by vortex,secondary flow and buoyancy,oil-water interface fluctuation is mainly caused by the interaction of two-phase velocity gradient and vortex.When the curvature ratio decreases or the inlet speed increases,the intensity of the vortex and double helical flow in the elbow increases.When the curvature ratio is 2,the stability and energysaving effect of the annular flow are better.Choosing the appropriate inlet speed for elbows with different curvature ratios can improve the stability and transmission capacity of the annular flow.The larger the collapse ratio of the sudden contraction pipe is,the more obvious the influence of the vortex on the annular flow is,the greater the interfacial fluctuation is and the dramatically lower the oil transfer efficiency is.At the same oil delivery rate,reducing the oil/water inlet speed ratio can reduce the eccentricity of the annular flow,and a higher oil delivery efficiency can be achieved when the inlet speed ratio is 1.33.At same speed,the smaller the diameter ratio of the sudden expansion pipe,the wider the return area and the lower the transfer efficiency.The change of inlet speed has a significant influence on the annular flow in the sudden expansion and contraction pipe.when inlet speed is 1.5?2m/s,annular flow has a good stability and high oil transfer efficiency,which is greater than or equal to 2.5m/s.Oilwater interface fluctuates greatly and oil contact adhesion occurs,when inlet is less than 1m/s.The annular flow is completely eccentric and unstable,and the oil transfer efficiency decreases sharply.The mechanism of surfactant solution on stability of water ring and its influence on flow characteristics of annular flow in local pipe fittings were further studied.The results show that surfactant can reduce local loss and oil-water interface fluctuation,affect the movement of secondary flow.In flow with high eccentricity of oil flow,it can prevent instability of water ring except for the second normal stress caused by viscoelastic fluid.Surfactants may prevent eccentric instability of annular flow by increasing the water phase viscosity and decreasing the oil-water interfacial tension.In the unstable flow of water ring structure,surfactant can increase the viscosity of solution and increase the loss along the flow path.Only in the fully developed turbulent flow can the flow loss be reduced and the transportation efficiency be improved.In elbow flow,the pressure difference in the elbow increases with the addition of surfactant,the double helical secondary flow motion caused by the pressure difference increases,and the circumferential flow in the elbow becomes more eccentric.Surfactants can prevent the instability of water ring and improve the oil transfer efficiency when the speed ratio of oil to water inlet is 2 in the abrupt tube flow.Surfactants have little effect on oil transfer efficiency when the speed ratio of oil to water inlet is 0.667?1.In the sudden expansion pipe flow,the distance from which the annular flow maintains stability becomes longer after adding surfactant,but the oil transfer efficiency decreases somewhat.The larger the pipe diameter ratio,the smaller the decrease in oil transfer efficiency.The higher the CTAC concentration is at 200 ppm,the higher the oil transfer efficiency is.Surfactants can greatly reduce the interfacial fluctuation,prevent the water ring from destabilizing and reduce the sticky wall of heavy oil,as well as the pressure drop in variable diameter section and outlet section during the sudden expansion and contraction pipe flow.When the inlet speed is greater than or equal to 2.5m/s,the annular flow has a higher stability and oil transfer efficiency when the CTAC concentration is 600 ppm.