Study On Non-Equilibrium Two-Phase Flow Model In Natural Gas Liquid Transmission Pipeline

Natural gas liquid(NGL)are multi-component mixtures mainly composed of low-carbon alkanes such as ethane,propane and butane.With the development of condensate gas fields in China,the production of NGL is increasing,and pipelines are an important way to transport NGL.However,due to temperature and pressure changes during the transfer process,NGL is prone to cause severe phase transitions.At the initial stage,bubbles are generated at the tiny cracks in the pipe wall,forming the bubble-boiling two-phase flow.As the bubble continues to accumulate,there is a distinct phase interface between the gas and liquid phases,forming the condensation and evaporation two-phase flow.Because of the heat and material transfer resistance between the two-phase,the model based on the assumption of isothermal instantaneous phase equilibrium can not accurately reflect the actual two-phase flow in the NGL pipeline.In order to depict the non-equilibrium two-phase flow for NGL pipeline,this paper carry out following researches.(1)At the bubble boiling stage,the growth process of NGL bubbles is analyzed.The heat transfer model between the pipe wall and the fluid is established based on the Fourier heat conduction law.Based on the bubble dynamics principle,the mass transfer model is established.For the bubble characteristic parameters in the mass transfer model:based on the bubble force balance principle,a calculation model of bubble take-off diameter suitable for various inclination angles is established.The comparison between the new model and the experimental data shows that the maximum relative deviation is 7.03%,and the average relative deviation is 5.1%in the vertical pipeline,the maximum deviation is 11.34%,and the average deviation is 10.07%in the horizontal pipeline.Preferably,the Mikic model and the Kirichenko model are used to calculate the bubble departure frequency and activation core density of the NGL fluid,respectively.(2)At the evaporation/condensation stage,Based on the Maxwell-Stefan equation,the gas-liquid mass transfer model under the action of chemical potential gradient is derived.The mass transfer model is solved based on the two-film theory.The mass transfer model was validated using experimental data:The maximum deviation of the mass transfer coefficient between the calculated value and the collected data is 11.7%,and the average deviation is 4.13%.The variation law of mass transfer rate was analyzed:the smaller the temperature gradient or pressure gradient between the initial time and the termination,the smaller the chemical potential gradient,and the smaller the mass transfer rate.Considering the influence of contact area in the gas-liquid interface,the calculation method of heat transfer coefficient is studied.(3)The mathematical model for judging non-equilibrium steady state is proposed based on the principle of minimum entropy generation and Peng Robinson equation of state.The solve method of the model is proposed and the model is verified.The results show that the compositions of gas-liquid phases is significant difference between the non-equilibrium steady state and the conventional thermodynamic equilibrium state,and when the difference between the initial ambient temperature and the final is greater,the difference of gas-liquid composition between the non-equilibrium steady state and the thermodynamic equilibrium state is greater.(4)According to the above achievements,based on the two-fluid model,the heat and mass transfer model of bubble boiling and evaporative/condensation stage,and the model for judging the non-equilibrium stationary state,the NGL non-equilibrium two-phase flow model is established.Considering the mathematical characteristics of the model,the model are discretized based on the central implicit difference method.The model is solved by the generalized Newton iteration method.Based on the field data of the Yingmaili-Yaha NGL pipeline,the model is verified by comparing the non-equilibrium two-phase flow model with the equilibrium two-phase flow model:The liquid holdup,temperature and pressure of the pipeline differ by 11.11%,4.24%and 0.42%.The influence of temperature and pressure changes on the NGL pipeline flow is analyzed.