| Heavy oil reserves are abundant in the world.Heavy oil transportation technology has also been studied under the rapid development of exploration and mining technology.Heavy oil has high viscosity and poor mobility.Single-phase transportation would cause excessive flow pressure drop and low transport efficiency.Therefore,it is proposed that water-annulus is used as lubricating layer to isolate the friction between oil and pipeline for achieving oil-water flow,which is known as oil-water core-annular flow(CAF)method.However,the stability of the water-annulus is the key point of this transportation,as well as the premise for the stable transport of heavy oil.Especially under the effect of the valve opening and closing,pipeline vibration and so on,the oilwater core-annular flow is easily disturbed and even ruptured.Thus,this topic focuses on the stability of core-annular flow as the valve element was taken into the pipeline system.Based on the existing research results of the annular flow,three research methods,which are the theoretical deduction,lab experiment and numerical simulation,are used to analyze the influence mechanism of the ball valve rotating on the stability of oil-water core-annular flow.The main research focuses on the following aspects:Due to the theoretical research of oil and water eccentric core-annular flow(E CAF),the exact solution of velocity profile,pressure drop gradient and shear stress of the oil and water two phases under the valve is rotating is deduced.The experimental platform of the oil and water annular flow is built and the experiment is carried out.Compared with the experimental data and the simulated data,and the feasibility of applying the numerical calculation to the model of this subject is illustrated in the case of good agreement.The dynamic simulation of core-annular flow during the valve opened and closed is carried out based on the sliding mesh model and the CLSVOF multiphase flow model,and the changes rules of the speed of ball valve and the thickness of water-annulus on the flow field characteristics of core-annular flow are analysed.The simulation results show that the larger starting speed could accelerate the recovery of the core-annular flow pattern,but the flow velocity in the flow field and the pressure fluctuation in the tube also increase,the stability of the annular flow is low;The wall-stick phenomenon is severe at the water-annulus of 1.0mm and 1.5mm,and the lower water holdup weakens the reconstruction of the oil-water two phase flow from the breakup to the CAF flow pattern.When the closing valve degree is less than 30%,the oil can be maintained by the water-annulus to achieve core-annular flow.The oil-water coreannular flow is unstable and cracked with the valve opening increasing.Combined with the CLSVOF multiphase flow model and fluid-structure interaction model,the pipe-ball valve and oil-water core-annular flow is simulated and analyzed.The flow field characteristics of oil-water annular flow under fluid-structure interaction are revealed.In addition,the simulation results show that the variation of oil-water core-annular flow flow field characteristics between the coupled model and the uncoupled model is basically the same.In addition,the displacement and shear stress of the tube wall affected by the annular flow field are relatively small. |
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