Numerical Study Of Turbulent Drag-Reducing Flow Characteristics Of Polymer Solutions

The turbulence drag reduction effect is a phenomenon in which the flow resistance of turbulent flow is significantly reduced by the addition of polymers,and research on turbulence drag reduction has been conducted since 1948,when Toms first discovered the phenomenon.Resistance reduction techniques have received much attention and research as an effective solution to energy efficiency.It has been found that when a polymer is added to a fluid,it not only produces a drag reduction effect,but also reduces the heat transfer performance.If polymer drag reduction is applied to cooling and heating systems,the flow resistance and heat loss in the transmission process can be greatly reduced,thus achieving energy savings.Although turbulence reduction technology has been applied in practice,there is still no definite conclusion about the mechanism of polymer resistance reduction because the study of turbulence is not thorough enough and polymer turbulence reduction is a cross-cutting issue involving several disciplines.Scholars have proposed many hypotheses for turbulent drag reduction in polymer solutions,trying to provide a reasonable explanation from the perspective of turbulence physics.In this thesis,we apply the equivalent viscosity model and BSL-Reynolds stress turbulence model,which reflect the effect of polymer drag reduction,based on CFD numerical simulation technology to study the flow characteristics of turbulent drag reduction in polymer solutions in detail,in order to provide a comprehensive understanding of the mechanism of turbulent drag reduction and promote the application of polymer turbulent drag reduction technology in a more intuitive way.Analysis of the effects of three factors,namely,drag reduction parameter,flow rate and fluid temperature,on the turbulent drag reduction effect of polymer solution reveals that,under the same working condition,the turbulent flow starts to drag reduction requires the polymer to reach a certain concentration value,i.e.,the drag reduction parameter reaches a certain value;continue to increase the concentration of polymer,the drag reduction effect will increase with the increase of concentration value,but when the drag reduction reaches the maximum value,continue to increase the concentration of polymer,the drag reduction The resistance reduction rate with the change of resistance reduction parameter shows a trend of slow growth,then rapid growth,then slow down again,and after reaching the maximum resistance reduction,it starts to decrease slowly;the greater the flow speed,i.e.,the higher the Reynolds number,the higher the turbulence intensity,the better the resistance reduction effect,the resistance reduction rate starts to grow rapidly and reaches the maximum resistance reduction later than when the flow speed is low;different polymers are suitable for different temperature ranges and different thermal stability.different temperature ranges and different thermal stability of different polymers.In order to study the flow characteristics of polymer turbulent reduction,the flow conditions under different dampening parameters at a flow velocity of 9m/s were simulated,and the vortex viscosity coefficient,turbulent kinetic energy,Reynolds stress and velocity distribution of turbulent dampening flow were analyzed,and the results showed that:(1)in the whole pipeline cross-section,the vortex viscosity coefficient,turbulent kinetic energy,turbulent vortex dissipation gradually decreased with the increase of dampening parameters,and reached the maximum dampening(2)The turbulent vortex coefficient is M-shaped in the pipe cross section,and the closer to the wall,the smaller the vortex coefficient,and the maximum is reached at the center of the pipe radius.The higher the vortex coefficient,the greater the turbulence intensity;The addition of polymeric dampening agent reduces the turbulent velocity pulsation intensity in the direction perpendicular to the wall and in the flow direction,and the maximum pulsation value of the velocity pulsation increases with the dampening parameter,with a slight shift to the left,relative to the low dampening away from the wall;(3)The simulation results of the velocity distribution in the near-wall region of Newtonian fluid wall turbulence,i.e.,without dampening,are consistent with Plante's theory of the mean velocity distribution of boundary layer flow.For polymer turbulent drag reduction flow,the average velocity distribution of polymer turbulent drag reduction flow is consistent with the three-layer velocity distribution theory proposed by Virk,but the thickness of the linear bottom layer and transition layer changes,and the transition layer and log-law layer move more obviously in the direction away from the wall;(4)the larger the drag reduction rate is,the closer the average velocity distribution is to the maximum drag reduction curve proposed by Virk;in turbulent flow,the flow direction vortex-induced The addition of polymer suppresses the downward sweeping motion of the strip and the induced effect of the flow vortex,reduces the energy dissipation in the buffer layer in the near-wall area,and greatly reduces the dissipation of kinetic energy of fluid flow,thus achieving turbulent drag reduction.Due to the large variation of the flow characteristic parameters near the wall,the effect of boundary layer changes on polymer drag reduction flow characteristics is studied by setting triangular ribs in the vertical flow direction on the wall of the pipe,and the analysis shows that the destruction of the relative stability of the boundary layer has a reducing effect on the drag reduction of the polymer.The higher the height of the ribs,the greater the effect on the drag reduction of the polymer,and after reaching the maximum drag reduction,the higher the ribs,the faster the drag reduction rate will be reduced by continuing to increase the polymer.The buffer layer in the boundary layer is the key area of polymer turbulence drag reduction,the greater the extent of the destruction of the buffer layer,the greater the impact on the drag reduction effect of polymer,when the destroyed boundary layer regains a stable state,the polymer continues to play a role,and due to the addition of polymer,the buffer layer has a thickening of the phenomenon.