| Most domestic crude oil has high pour points,high viscosity and high wax content,so it needs heating for gathering.The high wax content of crude oil results in wax deposition on the wellbore and the wall of gathering and transportation pipeline,which generally requires shut in and hot washing.How to save energy consumption of heating gathering and transportation and improve the efficiency of heat washing and wax removal under the premise of ensuring the safety of production is the key to the operation and management of oil and gas gathering a nd transportation system.When crude oil is heated,paraffin is the main component of its phase transformation.Therefore,the melting of paraffin in water is involved in the mixing of hot water for gathering and transporting crude oil and hot washing wax.The melting process can be regulated and the melting rate can be enhanced by studying the heat transfer and flow characteristics of this process.The melting of gelled crude oil and paraffin in gathering pipelines is an important scientific problem in the field of phase change heat transfer and multiphase flow.This research not only has theoretical value,but also has important guiding significance for energy conservation and consumption reduction in oil,petrochemical and construction fields.Experimental,theoretical and numerical methods are utilized to study the melting and flow characteristics of wax in gathering and transportation pipelines in this paper.The method and basis of regional divisiondivision for paraffin melting are studied,and a mathematical model which can describe the nonlinear characteristics between latent heat of phase change and liquid phase ratio is established by macro and microscale experiments.The melting process of paraffin in the constrained boundary is simulated on REV scale,and the numerical study of the melting and flow characteristics of paraffin is researched on macroscale by scale correlation.It is confirmed that there are solid area,fuzzy area and liquid area in the process of paraffin melting by the visual experiment of paraffin melting in a square cavity,and the moving process of phase interface is tracked by digital image and infrared thermal image.The change process of the porous structure in the fuzzy region at different temperatures is photographed by microscopic experiment,and it is revealed that there are two regions in the fuzzy region,porous medium region and multiphase flow region.The results show that the melting process of paraffin can be divided into four regions,liquid region,multiphase flow region,porous media region and solid phase region.The POM picture is statistically analyzed according to the relationship between the solid wax crystal and the liquid paraffin as the continuous phase and the discrete phase captured in the micrograph.The cr itical liquid phase ratio of the transition from porous media area to multiphase flow area under the experimental conditions is granted.The interval phase change heat which can reflect the nonlinear relationship between melting latent heat and temperature is defined.The relationship between the phase change heat and the temperature is fitted according to the DSC curve of differential calorimetry.The carbon number composition of paraffin is determined by gas chromatography,and the liquid ratio is obtained as a function of temperature.A novel model named phase change heat-porous medium suitable for each region is established in REV scale according to the results of melting region division and considering the nonlinear relationship between melting latent heat and liquid-phase ratio.The lattice Boltzmann model for the mixture melting in a square cavity is established.Melting characteristics of porous media in square cavity is studied.Double distribution function model of lattice Boltzmann method(DDF-LBM)is conducted to solve the phase change heat-porous medium model on mesoscale.The accuracy of DDF-LBM is verified by the reference solution of natural convection in square cavity.The correctness of the phase change heat-porous media model for the moving phase interface tracking and liquid phase ratio prediction is verified by visual experiments.The results show that the maximum relative error is 8.40%,and the average relative error is 3.72%.Numerical investigation on paraffin melting in square cavity with different Pr number,Ra number,transformation radius and average transformation temperature is conducted.Sensitivity of melting behavior characteristics to various influencing factors is analyzed with numerical results.The results show that the melting duration is most sensitive to Pr number,the average dimensionless temperature at the end of melting is most sensitive to Ra number,the liquid fraction is most sensitive to the average phase transition temperature,and the fuzzy region fraction is most sensitive to the phase transition radius.VOF coupled phase change heat-porous media model is developed.Melting and flow characteristics of paraffin in gathering pipeline are researched.Taking the physical parameters as a bridge,the VOF coupled phase change heat-porous media model is established through the correlation of mesoscale and macroscale,and the accuracy of the model is verified by experiments.The process of paraffin melting and flowing after melting under natural and forced convection i s simulated.The effects of melting water temperature,wax ball size,wax initial temperature and flow rate on wax ball center temperature and liquid phase ratio are analyzed.The melting stage is divided into irregular melting stage and normal melting sta ge based on the numerical results.The correlations between the dimensionless time Fo number and Ste number,Ra number,Re_p number and subcooling degree Ci are proposed based on the dimensionless analysis.Melting and flow characteristics of gelled crude oil in water are studied.A formula for calculating the dimensionless time of gelled crude oil melting in water is proposed.The effects of Bo number and We number on the two-phase flow are analyzed.The results show that the interfacial tension is the key factor affecting the fracture and deformation of the droplets.The deformation of the droplet in the continuous phase flow field is obviously reduced when the interfacial tension exceeds the critical value,and there is no fracture phenomenon.The two-phase flow can be effectively controlled by surfactant in a large range.The research in this paper aims to reproduce and characterize the melting and flow characteristics of paraffin in water.The research results can provide a theoretical basis and technical support for the phase transformation problems in the fields of petroleum,chemical industry and construction. |
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