| The pipeline is one of the important equipment in the process of seabed resource exploitation,and is used to transport the oil-gas mixture from the seabed to the offshore platform.The flow of oil and gas flow in the pipe is complicated.When the amount of gas and liquid conveyed changes or is small,severe slugging flow is likely to occur in the pipe.This periodic flow pattern will bring many problems to the stable operation of combined piping and downstream processing equipment.Although there are many studies on the gas-liquid severe slugging flow,there is still no perfect theoretical model to predict and optimize this flow pattern.Therefore,it is of great significance to study the mechanism of severe slugging flow that occurs during the gas-liquid mixed transportation in the pipeline system and to control and eliminate this flow pattern.In this paper,the numerical simulation method of CFD is used to study the flow characteristics and transient characteristics of the gas-liquid severe slugging flow in the combined declined-riser pipe system.The two-dimensional equivalent model was established using Fluent software,and the VOF phase interface tracking method was used to simulate the four flow patterns by changing the inlet gas velocity and liquid velocity in the combined pipeline system with a diameter of 0.051m.the severe slugging flow regime was formed in the pipe,and the hydrodynamic characteristics under severe slugging flow regime were investigated.The research in this paper has obtained the following conclusions and understanding:(1)By changing the gas-liquid conversion speed in the combined piping system,the four flow patterns in the piping system are numerically simulated,which are severe slugging? flow,severe slugging?,transition to severe slugging and stable flow.The severe slugging? flow has four cycles,which are the formation of liquid plug,the outflow of liquid plug,gas-liquid eruption and liquid fall.The periodic generation and shedding of gas-phase vortices are the incentives for the formation of severe slugging flow,and the bias flow phenomenon is likely to occur at the elbow when the gas-liquid eruption occurs.(2)By analyzing the standard deviation and power spectrum of the pressure fluctuation signal of the flow field,the gas-liquid flow pattern in the tube can be better identified,and the stability of the flow is analyzed.At the same time,the effects of gas velocity and gas-liquid ratio on the physical quantities such as severe slugging flow pressure,riser outlet mass flow rate,circulation period,gas-liquid eruption time,and liquid slug length are analyzed.(3)The analysis of the numerical simulation results of the severe slugging flow in the combined piping system shows that the average value of the mixed phase velocity in the decline pipe is significantly greater than riser.The maximum mass flow rate at the outlet of the riser is about 5.3 times than that of the inlet.The gas content indicates that the gas-liquid flow in the radial middle part of the decline is relatively stable,and the outlet of the riser is the place where the gas-liquid alternation is the most intense.(4)Liquid viscosity,density and surface tension will change the occurrence range of severe slugging flow and the formation process of long liquid slug,and have a greater impact on pressure fluctuation amplitude and outlet mass flow parameters. |
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