Research On Drag Reduction And Flow Characteristics Of Polymer Solutions In Sudden Expansion Pipes

With the improvement of energy saving and environmental protection awareness in this world today,additive drag reduction technology is receiving more and more attention and research.As a kind of additive,high polymer can reduce the resistance and increase the transport volume in the pipeline transport system,so it has broad application prospects in petrochemical,HVAC and other fields.As a non-Newtonian fluid,polymer solution is also widely used in food,medicine and other fields.However,at present,most studies on polymer drag reduction focus on straight pipes,while there are few studies on local components in pipeline system,such as sudden expansion pipe.The backflow in the sudden expansion pipe leads to local resistance loss of flow and increases energy consumption during transportation.Therefore,the research on flow of polymer solution in a sudden expansion pipe can improve the understanding of drag reduction mechanism and flow characteristics,which has profound significance and important value in theoretical research and engineering application.In this thesis,the flow drag reduction studies of additive solutions at home and abroad are reviewed firstly.According to the characteristics of polymer solutions,Cross model is selected to describe the shear thinning property,and FENE-P model is selected to describe the viscoelasticity of solutions.Secondly,the tensor form of the viscoelastic constitutive equation FENE-P model is expanded in Cartesian coordinate system,and the method of compiling by UDF macros and functions is described in detail to realize the secondary development of CFD software and complete the coupling with the momentum equation.Finally,for the geometric model of sudden expansion pipes,meshing,boundary condition setting and calculation model verification are completed,and the numerical simulations of turbulent drag reduction and laminar flow in polymer solution are studied,so as to reveal the drag reduction performance and mechanism of the polymer and explore the flow characteristics of different flow states.Based on the shear thinning Cross model,the turbulent flow of water and PAM solution with different concentrations in tubes with a sudden expansion ratio of 1.5 and 1.8 are simulated.Drag reduction performance and flow field characteristics including velocity distribution,pressure distribution,recirculation zone and turbulent kinetic energy distribution are studied,and vortex distribution is analyzed by Q criterion vortex identification technology to explore drag reduction mechanism.Findings are as follows:(1)In the upstream and downstream sections,the drag reduction rate firstly increases and then decreases with the increase of Re.In the concentration range studied in this thesis,the drag reduction rate increases with the increase of concentration,and decreases with the increase of pipe diameter.(2)In the sudden expansion section,the drag reduction performance is significantly reduced compared with the upstream and downstream sections.The drag reduction rate decreases monotonically with the increase of Reynolds number,and decreases with the increase of the sudden expansion ratio,and the increase of concentration only increases the drag reduction rate slightly.(3)Compared with pure water,after the sudden-expansion surface,the velocity of polymer PAM solution decreases slowly,and the distance of recovery to full development flow increases;the pressure growth rate of the PAM solution slows down,and the position of the maximum pressure recovery lags behind.The addition of PAM also inhibits the increase of turbulent kinetic energy,making the maximum turbulent kinetic energy in the sudden expansion section decrease,and the turbulent kinetic energy near the wall in the straight pipe section decrease significantly.(4)Under the influence of PAM molecular chain movement,the length of the recirculation zone increases compared with that of pure water.In additon,the length of the recirculation zone increases and then no longer changes with the increase of Reynolds number,and increases with the increase of the expansion ratio.(5)The molecular chain structure of PAM affects the vorticity at the step,making the vorticity distribution increase along the axial direction and decrease along the radial direction,reducing the maximum vorticity value,which is the primary reason for drag reduction in sudden expansion section.Based on the FENE-P viscoelastic model,the laminar flow of water and polymer solution in the planar sudden expansion pipes with the sudden expansion ratio of 2.4 and 4 are simulated.The flow characteristics of velocity,pressure and in the recirculation zone,as well as rheological parameters of viscoelastic fluid on the recirculation zone in the three development stages are explored.Findings are as follows:(1)The laminar flow of the planar sudden expansion shows three development stages with the increase of Reynolds number,which are,the symmetric flow,the asymmetric flow with two recirculation zones and three recirculation zones.The decrease of the sudden expansion ratio and the addition of polymers both increase the two critical Reynolds number.(2)With the change of development stage,the downstream velocity of sudden expansion surface fluctuates more obviously under the influence of recirculation zone,and the recovery distance of full development increases.(3)The pressure drop of polymer solution in the piep is higher than water,which means there is no drag reduction effect in laminar flow.(4)The effects of different rheological parameters on the reflux region of the three developmental stages are different.With the increase of the molecular stretch length and the decrease of the viscosity ratio,the recirculation zone of developmental stage I is monotonically shortened;the length of the third recirculation zone and the asymmetry of developmental stage Ⅱ and Ⅲ are reduced,and flows can be restored to the previous stage of development.With the increase of Weissenberg number,the length of recirculation zone in the development stage I decreases to a certain value and does not change,and in development stage Ⅱ and Ⅲ,the asymmetry and the third recirculation zone shows a trend of decreasing first and then increasing.