Simulations On Multiscale Flow Behaviors And Heat Transfer Characteristics Of Waxy Crude Oil Transportation Through Horizontal Pipeline

In the petrochemical industry,wax deposition is a serious problem that will cause pipelines blockage of waxy crude oil transportation.Reasonable prediction of the mechanism and understanding the factors and conditions of wax deposition can reduce the cost and improve the safety of pipeline transportation for preventing or reducing wax deposition.The waxy crude oil transport in horizontal pipelines is a multiscale heterogeneous liquid-solid two-phase flow and heat transfer system,the paraffin layer adhered to the pipeline inner surface and formed liquid-solid stratification with waxy crude oil at the macroscale,the paraffin crystalline components in waxy crude oil precipitate and deposit to form heterogeneous structures of particle clusters at the mesoscale,and it appears that the interaction between particles and the surrounding waxy crude oil at the microscopic scale.Due to the mesoscale structure,it results in the heterogeneous distribution and affects interface drag force of liquid-solid two-phase flow in horizontal pipeline,and the flow mechanism and heat transfer differ from that of the homogeneous liquid-solid system.Thus,it is necessary to establish suitable drag models for heterogeneous liquid-solid system to study the process of transporting waxy crude oil in horizontal pipeline.It is revealed that the mesoscale heterogeneous structure of paraffin crystalline forming particle clusters is a key factor in multiscale flow behavior.In this paper,two multiscale methods are used to establish a liquid-solid two-phase flow model in horizontal pipelines,and the numerical simulation of the heterogeneous distribution flow characteristics of waxy crude oil transportation,which are the extreme model based on the EMMS model and the correlation model based on the filtered two-fluid model.The stability conditions are obtained by analyzing the relationship between different scales and the principle of minimum energy consumption through EMMS model.Considering the heterogeneous structure of paraffin crystalline particles clusters in the grid-scale,the EMMS drag model is constructed by combining the hydrodynamics conservation equations;The results obtained by the high-resolution simulation are filtered by the filtered two-fluid model,and the small-scale flow structure is filtered and separated by the filter length.According to the simulation results of the fully analytical mesoscale structures,the drag constitutive equation suitable for the coarse grid simulation is derived.The variables for the particle volume fraction,the filter length,and the slip velocity in the grid are correlated to construct the filtered drag models.In addition,a mathematical model of heat transfer in horizontal pipeline waxy crude oil transportation is established based on the Solidification & Melting model and convective heat transfer model.In view of the two stages of the paraffin solidification and the paraffin crystalline particles dispersed in liquid phase crude oil,the factors affecting the latent heat process of phase change and the heat transfer process between liquid and solid phases were studied respectively.Based on the Eulerian-Eulerian model combined with the EMMS drag model,the liquid-solid two-phase flow characteristics of waxy crude oil transportation in horizontal pipelines are numerically simulated.Under the condition of the different liquid velocities and initial particle volume fractions,the heterogeneous distribution in horizontal pipelines and the influence of particle agglomeration on flow are obtained.The effects of particle collision,particle cluster size and inhomogeneous structure factor on the energy dissipation of the mesoscopic heterogeneous structure of particle agglomerates in liquid-solid two-phase flow are studied.The results obtained by EMMS drag model are in good agreement with the experimental pressure drop data compared with the Gidaspow model.The mechanism of liquid-solid interaction in the EMMS model is different from that in the uniform model.The phenomenon of paraffin crystalline particles deposited at the bottom of the pipeline simulated by EMMS model is more obvious,and presents heterogeneous layered structures.In addition,the system of competition and coordination between the liquid waxy crude oil dominant mechanism and the paraffin crystalline particle dominant mechanism makes the dynamic particle agglomeration form a lower potential energy to obtain smaller resistance and minimum suspension transport energy to the bottom of the pipeline.Based on the filtered two-fluid model,considering the wall effect and slip effect respectively,three heterogeneous filtered drag models are established: the Filtered model I,the Filtered model II with wall correction and the Filtered model III with filtered slip velocity.The liquid-solid two-phase flow characteristics of waxy crude oil transportation in horizontal pipelines are numerically simulated.The filtered drag models reveal the heterogeneous distribution of paraffin crystalline particles,and the accuracy of the results obtained by using the filtered drag model to predict the pressure drop of pipeline waxy crude oil transportation is higher than that of the EMMS drag model;By comparing the results of the three filtered models,it is shown that the slip effect is the main reason for the agglomeration of paraffin crystalline particles deposited at the bottom of the pipeline.Above all,it is shown that the mesoscale flow behavior of particle clusters is detailedly reproduced,providing a new strategy for the prediction of wax aggregation and flow process during waxy crude oil transportation.Based on the solidification and the liquid-solid interphase heat transfer model of waxy crude oil,the heat transfer process of waxy crude oil transportation in horizontal pipe is numerically simulated.Aiming at the phase change heat transfer,the effects of the temperature of wall and crude oil difference and flow velocity parameters on the temperature field,liquid fraction and heat transfer coefficient are studied;Aiming at the liquid-solid heat transfer process of solid paraffin crystalline dispersed in liquid crude oil,the effects of average diameter of paraffin crystalline particles,wall temperature and flow velocity on the convective heat transfer between liquid-solid interphase in pipeline are studied.Furthermore,the filtered interphase heat transfer coefficient is used to study the effect of mesoscale paraffin crystalline particle clusters on the heat transfer between liquid and solid phases.In summary,this paper studies the multiscale flow and heat transfer characteristics of waxy crude oil transported in horizontal pipelines using numerical simulation methods,and that constructs the theoretical framework of mesoscale scientific research of liquid-solid system.The flow state of liquid-solid two-phase flow and the phenomenon of wax deposition in the pipeline are predicted,revealing the distribution law of liquid-solid two-phase flow in the process of transporting waxy crude oil,and providing the influence of the heterogeneous structure of particle agglomeration on flow and heat transfer at the mesoscale.The multiscale flow models and heat transfer models of heterogeneous liquid-solid two-phase flow in horizontal pipeline for waxy crude oil transportation are established,and the heterogeneous drag models considering the mesoscale structures of paraffin crystalline particles agglomeration can improve the prediction accuracy of numerical simulation results.