Stdudy On The Coalescence Characteristics Of Oil-water Emulsion Droplets Under The Combination Of Flow Around A Cylinder And Electric Field

Water-in-oil(W/O)emulsions with small particle size droplets greatly hinder oil-water separation and affect the quality of oil products.Applying an electric field is a commonly used method to remove small particle size water droplets.A single electric dehydration technology can not efficiently and stably achieve oil-water separation.The combined method of chemical demulsifier and electric dehydration is mostly used,and combined dehydration supplemented by other physical fields technology.Adding a cylindrical flow around the emulsion flow treated by a DC electrostatic field can generate a Karman vortex street in the wake area to generate a shear force on the two-phase flow,and increase the probability of deformation,collision,and coalescence of the dispersed phase droplets.By studying the coalescence efficiency of the dispersed phase under different influencing factors,it provides new ideas and theoretical support for improving the dehydration efficiency of oil-water emulsions.The CFD software was used to simulate the process of the emulsion in the flow field around the cylinder and the combined effect of the flow around the cylinder and the DC electrostatic field.The two-phase flow level set model and the mixture phase transfer model were used to compare and analyze the influencing factors such as the characteristic length of the cylinder,the flow velocity,the electric field strength,and the initial volume fraction of the water phase.(1)The level set model of W/O emulsion flow around a cylinder is established.By analyzing the change of fluid volume fraction caused by the change of cylinder diameter,fluid flow velocity,and electric field intensity,the effect of the surrounding flow field on the deformation and coalescence of water droplets is studied.The results show that under this model,among the changes of cylinder diameter D=0.1m,0.075m,0.05m,the vortex shedding phenomenon caused by a cylinder with a diameter of D=0.075m is the most uniform,and the vortex exists behind the column for the longest time.The vortex velocity and pressure are both suitable for droplet coalestion.Through the analysis of the parameters such as the maximum volume fraction at the exit,the number of droplets in the analysis area,the average particle size of droplets in the analysis area,and the proportion of identifiable droplets in the analysis area,the results show that the droplet aggregation effect is the best at D=0.075m,and the proportion of identifiable droplets in the analysis area is more than 6%in the flow rate range of 0.5-1.5m/s,and the dispersive aggregation effect is better.The optimum working condition is 1m/s,reaching 7%.Seven electric field strengths were applied at a speed of 1m/s,and the coalescence effect of the dispersed phase under the combined action of the surrounding flow field and the electric field was analyzed.The results showed that 6,7k V/cm had a better effect on the coalescence of droplets,of which 6k V/cm accounted for The ratio is the highest,reaching 8%.Comparing the optimal working conditions under the four physical fields of natural flow,electrostatic flow,flow around a cylinder,and flow around the electrostatic combined field,it is obtained that the water phase coalescence effect of the flow field around the cylinder is increased by 4.7 times than that of the natural flow,and2.4 times higher than that of the electrostatic flow.The combined field is 6.1 times higher than the natural flow and 2.7 times higher than the electrostatic flow effect.In general,the order of coalescence efficiency from large to small is:combined field>flow field around a cylinder>electrostatic flow>natural flow.(2)A mixture phase transfer model was established to study the conditions of different flow rates,different field strengths,and different initial volume fractions of the dispersed phase.The results show that:for the mixture physical model,the vortex shedding is gradually obvious when the velocity is greater than 0.75m/s.The flow velocity increases within 0.75-3.0m/s,and the coalescence effect shows a trend of increasing first and then decreasing.The coalescence effect is best at 2.0m/s.The particle size of the water droplets is distributed into five sizes,d₁=12.5 microns,d₂=25 microns,d₃=50 microns,d₄=100 microns,d₅=200 microns,and the average particle size is d=50microns.Working condition 1,the volume fraction of dispersed phase is 0.05,and the proportion of the number of water droplets with diameters of d1 and d₂ is set to 80%and20%,and the number of other particle sizes is 0,that is,the volume fraction of d1 water droplets s₁=0.0167,and the volume fraction of d₂ water droplets s₂=0.033.Under working condition 1,the proportion of dispersed phase in s1 is reduced from 33%to 9%in the process of 0.75-2.0m/s,indicating that the flow around the cylinder has fully coalesced the water droplets of d1 diameter.In working condition 2,s₁ was set as 0.05,and the proportion of large water droplets at 2.0m/s was 46%,which was less than 50%of working condition 1.It shows that for emulsion with more serious emulsification,the flow around the cylinder could still promote the coalescence of water droplets,but the efficiency would decrease.In working condition 3,s₅ was set to 0.05.In the process of0.75-2.0m/s,the proportion of droplets in diameter of d₄ and d₅decreased by 28%and22%,it shows that for emulsified liquids with a relatively high proportion of large droplets,the cylindrical flow around model had poor stability effect on large droplets.In working condition 4,when the proportion of dispersed phase increases from 0.05 to0.1,the coalescence efficiency increases by 5%on the whole,it shows that the dispersion coalescence efficiency will be improved with the increase of the proportion of dispersed phase,which proves the stability of the model to the coalescence of water droplets.Applying the DC electrostatic field,the variation process of the field intensity from 0 to 9k V/cm was studied at 1.5m/s,and it shows that the coalescence effect was the best at 6k V/cm.In working condition 5,the dispersed phase accounted for 0.05,s₁=0.0167,s₂=0.033,and the electric field was applied.At this time,the proportion of droplets with diameter of d₅ in working condition 1 was 6%,and that in working condition 5 was 17.9%,it shows that the electrostatic field could promote the aggregation of large diameter droplets and keep them stable.In working condition 6,s₁was set to 0.05,and the difference between the proportion of large water droplets in0k V/cm and 6k V/cm was about 18.9%.In working condition 5,the data was 23.2%,which proved that the electric field was not obvious in promoting the process of d₁coalescence to d₂.In working condition 7,s₅ was set to 0.05,and finally s₅ reached 53%,much higher than 12%without applied electric field,it shows that the DC electrostatic field could maintain the stability of large water droplets.When the field intensity was greater than 7k V/cm,large water droplets would divergent,increasing the number of water droplets with diameter d₃ and below.In working condition 8,the proportion of dispersed phase is increased to 0.1,and the coalescence efficiency of the combined field reaches 75%,which is 28%higher than that of the flow field around the cylinder,it shows that the stability and effectiveness of the combined field.By comprehensive comparison,under the conditions of the volume fraction of dispersed phase is 0.05,the flow rate is 1.5m/s,and the field intensity is 6k V/cm,the agglomeration efficiency of large water droplets is 9%,15%,46%and 69%under the four conditions of natural flow,electrostatic flow,cylindrical flow and the combined electric field around the flow.Compared with the natural flow,the coalescing efficiency of the cylindrical flow field is increased by 5 times,and the coalescing efficiency of the combined field is increased by 7.7 times.The coalescence efficiency of the combined field increases by4.6 times for electrostatic flow and 1.5 times for cylindrical flow.(3)Both the level set method and the mixture model prove that the flow around the cylinder has a significant effect on promoting the coalescence of water droplets,and the DC electrostatic field has a significant effect on maintaining the stability of large water droplets.If only the cylinder participates in the flow around the flow field,it is easy to cause the large water droplets to coalesce to break up again.If only the DC electrostatic field participates in the flow,the coalescence process will be slow and the processing efficiency will be affected.The flow field around the cylinder combined with the DC electrostatic field can achieve the maximum coalescence efficiency for the coalescence of the water phase in the oil-water emulsion with low water content.