Study On Solidification Heat Transfer Mechanism Of Crude Oil In Pipe Based On The Lattice Boltzmann Method

In the process of crude oil storage and transportation,there may be accident shutdown,planned shutdown and intermittent shutdown to save transportation cost.Once the shutdown time exceeds the allowable safe shutdown time of the pipeline,it may cause pipeline condensation accident and make it difficult to restart the pipeline.After the shutdown of the pipeline,as the crude oil continuously dissipates heat to the external environment,the wax crystals will precipitate from the crude oil and form a gelled network structure,and its fluidity and thermal characteristics will change.Therefore,it is of great significance to study the solidification heat transfer mechanism of crude oil after shutdown for reasonable control of shutdown time,scientific formulation of restart scheme and optimization of pipeline insulation structure design.In this paper,the Lattice Boltzmann Method is used to solve the governing equations of phase change heat transfer of crude oil for the first time.From the perspective of dimensionless number,the double distribution function model based on enthalpy method is established,the different heat transfer stages and modes of crude oil are defined,and the phase change heat transfer mechanism of crude oil is deeply analyzed.The main work and conclusions are as follows:(1)The physical and mathematical models of phase change heat transfer of waxy crude oil in shutdown pipeline are established.The mathematical model of shutdown heat transfer is solved by using the double distribution function model based on the Lattice Boltzmann Method.The density distribution function is used to describe the velocity field of crude oil,and the temperature distribution function is used to describe the temperature field of crude oil.The relaxation time of the evolution equation of temperature distribution function in the phase transition region varies with the liquid holdup or temperature.The evolution equations of density distribution function and temperature distribution function in the double distribution function model can be regressed to the governing equations of phase change heat transfer of crude oil by the Chapman Enskog Expansion.(2)The visualization experiment of paraffin melting in square cavity was carried out to verify the accuracy of the double distribution function model.Based on the double distribution function model,the liquid phase rate of paraffin in the lower cavity at different times was calculated by FORTRAN language programming,and compared with the liquid phase rate of paraffin in the experimental test.The results show that with the transition of paraffin phase change heat transfer from non steady state to steady state,the absolute error between numerical calculation and visualization experiment of paraffin liquid phase rate becomes smaller and smaller.The maximum absolute error is 2.71%,the minimum absolute error is 0.50%,and the average absolute error is 1.09%.Therefore,the double distribution function model meets the accuracy requirements.(3)Based on the mathematical model of shutdown heat transfer and double distribution function model,the phase change state of crude oil in the pipeline is divided into the solid phase region,phase change region and liquid phase region.The evolution law of temperature field,flow field and liquid holdup field of crude oil in the pipeline is calculated and analyzed by FORTRAN language programming.The heat transfer process of crude oil in the pipeline after shutdown is divided into three stages,namely,the dominant stage of convective heat transfer,convective heat transfer and pure heat transfer,and the pure heat transfer.The effects of Ra number,Pr number,phase transition radius and solid-liquid thermal diffusivity ratio on the temperature field,flow field and liquid holdup of crude oil were analyzed.The results show that with the increase of Ra number,the sinking speed of cold oil is more obvious than that of hot oil floating,and the increase of speed is larger;with the increase of Fo number,the influence of solid-liquid thermal diffusion coefficient ratio on the temperature field,flow field and liquid holdup field of crude oil is gradually significant;when the Pr number of crude oil is increased by increasing viscosity,the natural convection velocity slows down,the convective heat transfer intensity decreases,and the heat transfer efficiency increases;When the wax precipitation point is increased to increase the phase change radius,the natural convection velocity of crude oil is slightly accelerated,the release of latent heat is increased in the same time,and the temperature of crude oil in the phase change and liquid phase regions is increased,but the shape of temperature field and liquid holdup field has almost no change.