Heat Transfer Mechanism Of Waxy Crude Oil Based On Wide Phase Interface Partition Method

The liquid-solid phase transition of waxy crude oil covers many physical phenomena,such as latent heat absorption or release,liquid-solid/solid-liquid transformation,phase interface movement,fluid-solid coupling,and heat transfer mode transformation.The process of heat transfer and mass transfer is extremely complex,which is closely related to the existence of wax and the complexity of its components.How to accurately describe the phase change heat transfer characteristics of waxy crude oil at different stages by closely combining the aggregation behavior and phase state changes of wax crystals during phase transition has been the key to reasonably control the shutdown time,scientifically formulate the restart scheme,and optimize the pipeline insulation structure design in the field of oil and gas storage and transportation.In view of this,this paper adopted the method of laboratory experiment,theoretical analysis and numerical simulation to analyze the influence of phase evolution process of wax crystal on the heat transfer mode in view of the shutdown process of waxy crude oil in long-distance pipeline.On this basis,the heat transfer model of "wide phase interface partition" of the new partition was constructed to study the heat transfer mechanisms of solidification and melting of crude oil.In this thesis,polarizing microscope was used to carry out the microscopic experiment of wax-containing crude oil in cooling and dewaxing,with using the statistical principle to characterize the changes in the number of wax crystal particles,particle area,average wax diameter and other microscopic parameters.Based on the microscopic picture of wax precipitation and the changing characteristics of wax crystal parameters,the concept of "coalescence point" was put forward,and the influence of phase state change of wax crystal on heat transfer mode was analyzed.The physical model of "wide phase interface partition" of waxy crude oil pipeline was established,and the waxy crude oil in the pipeline was divided into liquid phase zone,liquid-solid mixed porous media fuzzy zone and solid phase zone based on the coalescence point and freezing point,with the governing equations being proposed for each zone.The latent heat in the phase transition process of crude oil was treated by the equivalentinstantaneous specific heat capacity,and the enthalpy method was used to characterize the nonlinear change of liquid phase ratio with temperature.An overhead pipeline shutdown test bench was set up to test the temperature change at 5 typical locations in the pipe.The simulation results were compared with the experimental data,and the maximum relative error was 3.81%,proving that the model and solution method were correct.Based on the model of "wide phase interface partition",the solidification evolution and heat transfer mechanism of crude oil were studied for typical pipelines such as overhead pipelines and buried pipelines.The change rules of crude oil temperature field,flow field and condensate layer in pipelines under different laying methods were clarified.The results showed that,compared with the overhead pipeline,there was no significant eddy current caused by convection in the pipeline at the initial stage of pipeline shutdown,so the temperature field and flow field were more evenly distributed.Natural convection caused the condensate layer to appear at the bottom of the pipeline first.Therefore,during the shutdown process,the occurrence of condensate layer in the pipeline can be determined by tracking the temperature at the bottom.According to the Ra numbers change in two overhead pipeline for Φ 219×6 mm and Φ 820×6 mm,the violent natural convective motion in the early stage of shutdown was in the transition zone of turbulent flow of the layer.By using the change trajectory of the highest temperature point of crude oil in the pipeline,the main four stages of phase change heat transfer of crude oil in the pipeline were clearly divided: intense natural convection heat transfer stage,natural convection heat transfer weakening stage,latent heat release stage and condensate reservoir growth stage.The cross-influence mechanisms of phase transition heat transfer of waxy crude oil in shutdown pipeline were studied.The effects of porous media characteristics,physical properties and other external factors(such as atmospheric environment,soil physical properties and initial temperature of shutdown)on phase transition heat transfer of waxy crude oil were discussed.In view of the above main influencing factors,the orthogonal test method was used to analyze the sensitivity of the factors based on the total coagulation time of crude oil in the pipeline.After crude oil exhibited the characteristics of porous medium,the temperature drop rate was significantly slowed down.The higher the coalescence point,the faster the liquid phase rate decreased in the early stage,but slowed down in the later stage.The influence of viscosity difference of different kindsof crude oil can be neglected for shutdown pipeline,and the initial temperature of crude oil was the most significant external factor affecting the heat transfer under the shutdown condition of long-distance pipeline.The results showed that,different from the formation of a large eddy in the pipeline during solidification,a small eddy was formed at the bottom of the pipeline after all the crude oil in the pipeline had melted during heating and melting.Due to the permeability of porous media,the temperature rise curve in the melting process had a steep increase interval.