| In China,solid wastes(dense paste)is encouraged to use and dispose through combustion,power generation,land use and landfill in recent years.Pipeline transportation system was introduced to this area based on its advantages like clean(avoid exposure of the materials in the open air),high efficiency,labour saving,etc.However,dense paste has high solids content,high viscosity and non-fluidity under normal atmosphere condition and behaves as the non-Newtonian fluids in pipeline flow.Therefore,it is of great practical significance to change the flow state of the dense paste,thus increased the conveying distance,meanwhile,reduced the resistance during transportation and the wear rate of the inner pipe wall.reduce the wear speed of the paste and the pipeline,improve the transportation efficiency and save the energy,and promote the industrial development and clean development.In order to study the pressure loss reduction methods,some experimental studies and theoretical analysis were carried out.The injected liquid near the pipe inner wall increased the thickness of the boundary layer and promote mobility.The main research contents are as follows:(1)Many quantitative indexes were developed to better describe the behaviour of dense paste,which are applied to describe the particle population characteristicsThe definition of dense paste is given according to its physical properties and a quantitative scope is described.Dense paste is Bingham fluid in pipe,based on which summarizes the reason of pressure loss.It is found that the frictional resistance loss is the most important factor,relatively that other factors can be neglected.Dense paste is mostly composed of fine particles.A strong surface force is formed,resulting in the flow condition changing from the two-phase flow into structural flow.The characteristics of structural flow is detailed in paper,the flow pattern of the pump flow and the microcosmic dynamic change of the structure during the cutting process are also introduced.The influence factors,such as pipe diameter,concentration,flow rate,particle density(specific gravity),particle shape and particle distribution are analyzed,which provide the basis for the following study.(2)Many physical and chemical properties of dense paste are measured and proved the existence of the wall slip.Analyzing the basic physical characteristics of dense paste,and put forward the corresponding quantitative indexes and parameters.Many factors such as concentration,viscosity and yield stress of the dense paste were systematically tested.Among them,the measurement method of yield stress was proposed innovatively.Due to the high concentration of coal slime,electromagnetic flowmeter is not effective many times.The feasibility of the electromagnetic flowmeter to measure the flow rate of dense paste is urgently to be studied.It is found that the concentration and conductivity of dense paste can be reckoned as parameters to judge the availability of electromagnetic flowmeter.When it is applied to coal slime,roughly estimated the upper limit of mass concentration is to 63%,the lower limit of conductivity is 0.035 mS/cm.Wall slip is existed near the pipe inner wall.For the same sample,the two yield stresses calculated by rheometer model are always larger than the curve extrapolation yield stress.This is associated with the phenomenon that the existence of the wall slip along the pipeline.When the pastes flow in pipeline,particles migrations lead to the formation of the liquid layer with low viscosity on the inner surface of the pipe wall,which would lubricate the inner wall.The lubrication along the wall facilitates the flow of the paste by reducing the conveying resistance and the shear stress.And the higher concentration is,the greater wall slip influences.(3)Developing the test rig.On the basis of the previous ‘small-scale rheological and resistance characteristics test system' device,liquid injection equipment is designed and installed between valve and test pipe.It was designed by injecting liquid through several points to form a lubricate annulus adjacent to the pipe wall,which increased the thickness of the boundary layer and promoted mobility.So it is possible to maintain a low concentration at the wall surface.Liquid injection equipment was designed on the basis of the phenomena of wall slip near the inner wall of pipe,and its precision test proved that the device can be employed to control the thickness of boundary.(4)Taking coal slime as an example,the pressure loss of dense paste in pipe was carried out.Single factor and multi-factor comprehensive analysis method were adopted to analyse the pipe pressure loss law.For the same pipeline,the boundary layer thickness is one of the highest magnitude of drag reduction,and when it was changed,the pressure loss is most influential.It indicates that the influence of the boundary layer on the pipeline resistance is very important in the pipe transportation,and the boundary layer can be used to reduce the friction.The pipe diameter is one of the influential factors on drag-reduction performance of coal slime,and the pressure loss reduced more significantly with the decrease of pipeline diameter.The smaller the diameter,the more obvious the thickness of the same boundary layer is.The thickness of the boundary layer is larger,the relationship between the unit length and the pipe diameter is less obvious.When the thickness of the boundary layer is less than 1mm,the relative drag reduction effect is the most obvious,the pressure difference per unit length decreases fastest.With the increase of the thickness of the boundary layer,the drag reduction value increases but the relative drag reduction effect decreases.In other cases,at any concentration,when the boundary liquid is PEO,the degree of the drag reduction is more obvious than water.(5)Taking coal slime as the test sample,the pipeline characteristics and the pressure loss reduction efficiency due to the change of the boundary layer property was studied for the first time.According to the test data,keep the other conditions constant,the value of the drag reduction decreases with the increase of the thickness of the boundary layer,and the thickening of the boundary layer has a significant effect on the decrease of the resistance of the fluid.When the thickness of the sliding boundary layer is increased,the pressure drop per unit length is more uniform and the pressure drop curve is smoother when the concentration is low.With the increase of pipeline diameter,the thickness of the boundary layer increased.Although the transmission speed decreased,the unit length of the decline in the rate of pressure reduced gradually.Thus,the smaller the diameter,the more obvious boundary layer's drag reduction effect is.When the other parameters are unchanged,the drag reduction rate of the boundary layer decreased with the increasing of the conveying speed.(6)Through orthogonal test and significance analysis,the influence of each factor on drag reduction was analyzed.The order of parameters influencing pipeline transportation efficiency is: boundary layer thickness ?> paste concentration > pipe diameter (9.The thickness of the boundary layer has the greatest influence on the transmission resistance.If this characteristic of the boundary layer can be used in the pipeline transportation process,it is of great significance for the drag reduction in the pipeline transportation process.Optimize the structure of the pipeline and the rational arrangement of the location of the drag reduction device provide a reasonable basis for the pipeline design process and extension of delivery distance.The basic technical problem of dense paste pipeline transportation is to choose the appropriate pipeline diameter,mass concentration and transport flow rate under the same solid material conveying quantity and conveying distance,witch,in the end,achieves reliable operation and good economic effect. |
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